logging in or signing up Cell Division chhabra61 Download Post to : URL : Related Presentations : Share Add to Flag Embed Email Send to Blogs and Networks Add to Channel Uploaded from authorPOINT lite Insert YouTube videos in PowerPont slides with aS Desktop Copy embed code: (To copy code, click on the text box) Embed: URL: Thumbnail: WordPress Embed Customize Embed The presentation is successfully added In Your Favorites. Views: 394 Category: Education License: All Rights Reserved Like it (2) Dislike it (0) Added: July 11, 2010 This Presentation is Public Favorites: 4 Presentation Description No description available. Comments Posting comment... By: chhot555 (7 month(s) ago) nice ppt............................. Saving..... Post Reply Close Saving..... Edit Comment Close By: harpalsingh268 (8 month(s) ago) nice ppt Saving..... Post Reply Close Saving..... Edit Comment Close By: asmaazaki86 (18 month(s) ago) thanxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx very much Saving..... Post Reply Close By: lokash (16 month(s) ago) can you help me to download this ppt.please i need it.i have to refer it for my project Saving..... Edit Comment Close Premium member Presentation Transcript Cell Division AK CHHABRA and Jayeeta Chakraborty : Cell Division AK CHHABRA and Jayeeta Chakraborty DISCLAIMER All copyrights of the figures used in the present presentation lie with the original developers, The information has been gathered here for educational purpose and not for sale. Cell Division ‘ Flash back : Cell Division ‘ Flash back Discovery of cell division Rudolf Virchow (1855, 1859) ,first suggested new cells are formed from division of pre-existing cells. Strasburger (1873) proposed that nuclei are formed from pre-existing ones. Boveri (1879) and Fleming (1879,1880) studied details of somatic cell division. Fleming coined the term Mitosis in 1882. Meiosis was termed by Farmer and Moore (1905). Cell division : Cell division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in prokaryotes is known as binary fission Or amitosis. In another type of cell division present only in eukaryotes, called meiosis, a cell is permanently transformed into a gamete and cannot divide again until fertilization. Right before the parent cell splits, it undergoes DNA replication. Cell Division Significance of cell division : Cell Division Significance of cell division . Cell multiplication Continuity Asexual reproduction Multicellular organisms Growth Cell size Genetic similarity Repair Regeneration Sexual reproduction Reshuffling of genetic traits Mutations Cell Division Factors controlling cell division : Cell Division Factors controlling cell division Cell size Kernplasma or karyoplasmic ratio Mitogens Mitogens are agents that trigger cell division. The common plant mitogen is hormone cytokinin. Cell Division Generation time : Cell Division Generation time Daughter cells formed after division may grow in size and divide again. Period between two successive divisions is called genaration time. The series of changes which involves the growth and division of a cell is called cell cycle. Generation time varies from a few minutes to a few days depending upon the type of cell and its enviromental condition. Cell Division Cell cycle : Cell Division Cell cycle Cell Division Interphase-divided into 3 stages : Cell Division Interphase-divided into 3 stages 1) G1- phase: RNA and proteins are synthesised. Duration of G1 phase is variable. It is longer for cells dividing infrequently and shorter in cells undergoing frequent division. In G1 phase the cell has three options: To continue cell cycle and enter S-phase. Stop cell cycle and enter Go phase. Get arrested in G1 phase whereas it may enter G0 phase or re-enter cell cycle. Cell Division Interphase-divided into 3 stages continued… : Cell Division Interphase-divided into 3 stages continued… 2 ) S- phase: the chromosome replicates. The DNA content doubles and duplicate set of genes are formed. It is also called invisible phase of M-stage since in this phase the chromosomes prepare themselves for equitable distribution later on. 3) G2- phase: synthesis of DNA stops, however formation of RNAs and protein continues. Cell Division Mitotic phase (M phase) : Cell Division Mitotic phase (M phase) M-phase is the final phase of cell cycle. It represents the phase of actual division. It consists of karyokinesis (division of nucleus) and cytokinaesis ( division of cell protoplast) . After M-phase a cell may re-enter fresh cycle or pass into Go-phase. Go-phase is the stage when cell cycle is arrested, therefore further divisions stop. Cell Division Amitosis/ Binary fission : Cell Division Amitosis/ Binary fission It was discovered by Remak (1841, 1855) and described by Fleming (1882) Here there is no differentiation of chromosomes and spindle. The nucleus elongates and constricts in the middle to form two daughter nuclei. Cell Division Mitosis : Cell Division Mitosis Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle - the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. Cell Division Mitosis consists of the following: : Cell Division Mitosis consists of the following: Interphase Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis Cell Division Interphase : Cell Division Interphase It is the resting stage or non mitotic portion of the cell cycle. It is comprised of G1, S and G2 stages of the cell cycle. DNA is replicated during the S-phase of Interphase. Cell Division Pre- prophase : Cell Division Pre- prophase In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and actin filaments (called preprophase band) underneath the plasma membrane around the equatorial plane of the future mitotic spindle. Cell Division Pre- prophase Continued… : Cell Division Pre- prophase Continued… This band marks the position where the cell will eventually divide. The cells of higher plants (such as the flowering plants) lack centrioles: with microtubules forming a spindle on the surface of the nucleus and then being organized into a spindle by the chromosomes themselves, after the nuclear membrane breaks down. The preprophase band disappears during nuclear envelope disassembly and spindle formation in prometaphase. Cell Division Prophase : Cell Division Prophase Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome. Since the genetic material has already been duplicated earlier in S phase, the replicated chromosomes have two sister chromatids, bound together at the centromere by the cohesion complex. Chromosomes are visible at high magnification through a light microscope. Close to the nucleus are structures called centrosomes, which are made of a pair of centrioles. Cell Division Prophase continued… : Cell Division Prophase continued… The centrosome is the coordinating center for the cell's microtubules. A cell inherits a single centrosome at cell division, which replicates before a new mitosis begins, giving a pair of centrosomes. The two centrosomes nucleate microtubules (which may be thought of as cellular ropes or poles) to form the spindle by polymerizing soluble tubulin. Molecular motor proteins then push the centrosomes along these microtubules to opposite sides of the cell. Although centrioles help organize microtubule assembly, they are not essential for the formation of the spindle, since they are absent from plants, and centrosomes are not always used in meiosis. Cell Division Pro-metaphase : Cell Division Pro-metaphase The nuclear envelope disassembles and microtubules invade the nuclear space. This is called open mitosis, and it occurs in most multicellular organisms. Fungi and some protists, such as algae or trichomonads, undergo a variation called closed mitosis where the spindle forms inside the nucleus or its microtubules are able to penetrate an intact nuclear envelope. Each chromosome forms two kinetochores at the centromere, one attached at each chromatid. Cell Division Pro-metaphase continued… : Cell Division Pro-metaphase continued… A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome.Although the kinetochore structure and function are not fully understood, it is known that it contains some form of molecular motor.When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to "crawl" up the tube toward the originating centrosome. This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome's two chromatids. Cell Division Metaphase : Cell Division Metaphase As microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes convene along the metaphase plate or equatorial plane, an imaginary line that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug-of-war between people of equal strength Cell Division Metaphase continued… : Cell Division Metaphase continued… In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. Metaphase comes from the Greek μετα meaning "after." Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules (spindle fibres), it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned Cell Division Anaphase : Cell Division Anaphase When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase (from the Greek ανα meaning “up,” “against,” “back,” or “re-”). Two events then occur; First, the proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids, which have now become distinct sister chromosomes, are pulled apart by shortening kinetochore microtubules and move toward the respective centrosomes to which they are attached. Cell Division Anaphase continued… : Cell Division Anaphase continued… Next, the nonkinetochore microtubules elongate, pulling the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. The force that causes the centrosomes to move towards the ends of the cell is still unknown, although there is a theory that suggests that the rapid assembly and breakdown of microtubules may cause this movement. Cell Division Anaphase continued… : Cell Division Anaphase continued… These two stages are sometimes called early and late anaphase. Early anaphase is usually defined as the separation of the sister chromatids, while late anaphase is the elongation of the microtubules and the chromosomes being pulled farther apart. At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations. Cell Division Telophase : Cell Division Telophase Telophase (from the Greek τελος meaning "end") is a reversal of prophase and prometaphase events. It "cleans up" the after effects of mitosis. At telophase, the nonkinetochore microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. Cell Division Telophase continued… : Cell Division Telophase continued… A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Mitosis is complete, but cell division is not yet complete Cell Division Cytokinesis : Cell Division Cytokinesis Cytokinesis is often mistakenly thought to be the final part of telophase; however, cytokinesis is a separate process that begins at the same time as telophase. Cytokinesis is technically not even a phase of mitosis, but rather a separate process, necessary for completing cell division Cell Division Cytokinesis continued… : Cell Division Cytokinesis continued… In animal cells, a cleavage furrow (pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei.In both animal and plant cells, cell division is also driven by vesicles derived from the Golgi apparatus, which move along microtubules to the middle of the cell. In plants this structure coalesces into a cell plate at the center of the phragmoplast and develops into a cell wall, separating the two nuclei Cell Division Cytokinesis continued… : Cell Division Cytokinesis continued… The phragmoplast is a microtubule structure typical for higher plants, whereas some green algae use a phycoplast microtubule array during cytokinesis. Each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase. Cell Division Common features of all cell : Cell Division Common features of all cell Cell Division significance of mitosis : Cell Division significance of mitosis Mitosis is important for the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell. Transcription is generally believed to cease during mitosis, but epigenetic mechanisms such as bookmarking function during this stage of the cell cycle to ensure that the "memory" of which genes were active prior to entry into mitosis are transmitted to the daughter cells. Cell Division Meiosis : Cell Division Meiosis In biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original cell is replicated during S-phase of the cell cycle. Two cell divisions separate the replicated chromosomes into four haploid gametes or spores. Cell Division Meiosis continued… : Cell Division Meiosis continued… During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell. Because the chromosomes of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Cell Division Meiosis process : Cell Division Meiosis process Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. Cell Division Meiosis process continued… : Cell Division Meiosis process continued… Interphase is followed by meiosis I and then meiosis II. Meiosis I consists of separating the pairs of homologous chromosome, each made up of two sister chromatids, into two cells. One entire haploid content of chromosomes is contained in each of the resulting daughter cells; the first meiotic division therefore reduces the ploidy of the original cell by a factor of 2. Cell Division Meiosis process continued… : Cell Division Meiosis process continued… Meiosis II consists of decoupling each chromosome's sister strands (chromatids), and segregating the individual chromatids into haploid daughter cells. The two cells resulting from meiosis I divide during meiosis II, creating 4 haploid daughter cells. Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle Cell Division Meiosis includes the stages of : Cell Division Meiosis includes the stages of meiosis I prophase I, metaphase I, anaphase I, telophase I, meiosis II prophase II, metaphase II, anaphase II, telophase II. Cell Division Meiosis I : Cell Division Meiosis I Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. Cell Division Meiosis I : Cell Division Meiosis I However, after meiosis I, although the cell contains 46 chromatids, it is only considered as being N, with 23 chromosomes. This is because later, in Anaphase I, the sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. Cell Division Meiosis II : Cell Division Meiosis II In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) per daughter cell from the first division. Cell Division Meiosis Prophase I : Cell Division Meiosis Prophase I During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. Cell Division Meiosis Prophase I continued… : Cell Division Meiosis Prophase I continued… The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma Cell Division Meiosis Leptotene : Cell Division Meiosis Leptotene The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads". During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another. Cell Division Meiosis Zygotene : Cell Division Meiosis Zygotene The zygotene stage, also known as zygonema, from Greek words meaning "paired threads", occurs as the chromosomes approximately line up with each other into homologous chromosomes. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place Cell Division Meiosis Pachytene : Cell Division Meiosis Pachytene The pachytene stage, also known as pachynema, from Greek words meaning "thick threads",contains the following chromosomal crossover. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology Cell Division Meiosis Pachytene continued… : Cell Division Meiosis Pachytene continued… Exchange takes place at sites where recombination nodules (the chiasmata) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope Cell Division Meiosis Diplotene : Cell Division Meiosis Diplotene During the diplotene stage, also known as diplonema, from Greek words meaning "two threads", the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. Cell Division Meiosis Diplotene continued… : Cell Division Meiosis Diplotene continued… The chiasmata remain on the chromosomes until they are severed in Anaphase I. In human fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. Cell Division Meiosis Diakinesis : Cell Division Meiosis Diakinesis Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through". This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible Cell Division Meiosis Diakinesis continued… : Cell Division Meiosis Diakinesis continued… Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. Cell Division Meiosis Metaphase-I : Cell Division Meiosis Metaphase-I Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. Cell Division Meiosis Anaphase I : Cell Division Meiosis Anaphase I Kinetochore(bipolar spindles) microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome has only one functional unit of a pair of kinetochores, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center. Cell Division Meiosis Telophase- I : Cell Division Meiosis Telophase- I The last meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I. Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Cell Division Meiosis II : Cell Division Meiosis II The four main steps of Meiosis II are: Prophase II Metaphase II Anaphase II Telophase II. Cell Division Meiosis Prophase II : Cell Division Meiosis Prophase II In prophase II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division Cell Division Meiosis Metaphase II : Cell Division Meiosis Metaphase II In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. Cell Division Meiosis Anaphase II : Cell Division Meiosis Anaphase II This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. Cell Division Meiosis Telophase II : Cell Division Meiosis Telophase II The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells. Cell Division significance of meiosis : Cell Division significance of meiosis Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count. In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities or lethality . Cell Division significance of meiosis continued… : Cell Division significance of meiosis continued… Polyploidy is poorly tolerated in most animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation. Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring. You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.
Cell Division chhabra61 Download Post to : URL : Related Presentations : Share Add to Flag Embed Email Send to Blogs and Networks Add to Channel Uploaded from authorPOINT lite Insert YouTube videos in PowerPont slides with aS Desktop Copy embed code: (To copy code, click on the text box) Embed: URL: Thumbnail: WordPress Embed Customize Embed The presentation is successfully added In Your Favorites. Views: 394 Category: Education License: All Rights Reserved Like it (2) Dislike it (0) Added: July 11, 2010 This Presentation is Public Favorites: 4 Presentation Description No description available. Comments Posting comment... By: chhot555 (7 month(s) ago) nice ppt............................. Saving..... Post Reply Close Saving..... Edit Comment Close By: harpalsingh268 (8 month(s) ago) nice ppt Saving..... Post Reply Close Saving..... Edit Comment Close By: asmaazaki86 (18 month(s) ago) thanxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx very much Saving..... Post Reply Close By: lokash (16 month(s) ago) can you help me to download this ppt.please i need it.i have to refer it for my project Saving..... Edit Comment Close Premium member Presentation Transcript Cell Division AK CHHABRA and Jayeeta Chakraborty : Cell Division AK CHHABRA and Jayeeta Chakraborty DISCLAIMER All copyrights of the figures used in the present presentation lie with the original developers, The information has been gathered here for educational purpose and not for sale. Cell Division ‘ Flash back : Cell Division ‘ Flash back Discovery of cell division Rudolf Virchow (1855, 1859) ,first suggested new cells are formed from division of pre-existing cells. Strasburger (1873) proposed that nuclei are formed from pre-existing ones. Boveri (1879) and Fleming (1879,1880) studied details of somatic cell division. Fleming coined the term Mitosis in 1882. Meiosis was termed by Farmer and Moore (1905). Cell division : Cell division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in prokaryotes is known as binary fission Or amitosis. In another type of cell division present only in eukaryotes, called meiosis, a cell is permanently transformed into a gamete and cannot divide again until fertilization. Right before the parent cell splits, it undergoes DNA replication. Cell Division Significance of cell division : Cell Division Significance of cell division . Cell multiplication Continuity Asexual reproduction Multicellular organisms Growth Cell size Genetic similarity Repair Regeneration Sexual reproduction Reshuffling of genetic traits Mutations Cell Division Factors controlling cell division : Cell Division Factors controlling cell division Cell size Kernplasma or karyoplasmic ratio Mitogens Mitogens are agents that trigger cell division. The common plant mitogen is hormone cytokinin. Cell Division Generation time : Cell Division Generation time Daughter cells formed after division may grow in size and divide again. Period between two successive divisions is called genaration time. The series of changes which involves the growth and division of a cell is called cell cycle. Generation time varies from a few minutes to a few days depending upon the type of cell and its enviromental condition. Cell Division Cell cycle : Cell Division Cell cycle Cell Division Interphase-divided into 3 stages : Cell Division Interphase-divided into 3 stages 1) G1- phase: RNA and proteins are synthesised. Duration of G1 phase is variable. It is longer for cells dividing infrequently and shorter in cells undergoing frequent division. In G1 phase the cell has three options: To continue cell cycle and enter S-phase. Stop cell cycle and enter Go phase. Get arrested in G1 phase whereas it may enter G0 phase or re-enter cell cycle. Cell Division Interphase-divided into 3 stages continued… : Cell Division Interphase-divided into 3 stages continued… 2 ) S- phase: the chromosome replicates. The DNA content doubles and duplicate set of genes are formed. It is also called invisible phase of M-stage since in this phase the chromosomes prepare themselves for equitable distribution later on. 3) G2- phase: synthesis of DNA stops, however formation of RNAs and protein continues. Cell Division Mitotic phase (M phase) : Cell Division Mitotic phase (M phase) M-phase is the final phase of cell cycle. It represents the phase of actual division. It consists of karyokinesis (division of nucleus) and cytokinaesis ( division of cell protoplast) . After M-phase a cell may re-enter fresh cycle or pass into Go-phase. Go-phase is the stage when cell cycle is arrested, therefore further divisions stop. Cell Division Amitosis/ Binary fission : Cell Division Amitosis/ Binary fission It was discovered by Remak (1841, 1855) and described by Fleming (1882) Here there is no differentiation of chromosomes and spindle. The nucleus elongates and constricts in the middle to form two daughter nuclei. Cell Division Mitosis : Cell Division Mitosis Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle - the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. Cell Division Mitosis consists of the following: : Cell Division Mitosis consists of the following: Interphase Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis Cell Division Interphase : Cell Division Interphase It is the resting stage or non mitotic portion of the cell cycle. It is comprised of G1, S and G2 stages of the cell cycle. DNA is replicated during the S-phase of Interphase. Cell Division Pre- prophase : Cell Division Pre- prophase In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and actin filaments (called preprophase band) underneath the plasma membrane around the equatorial plane of the future mitotic spindle. Cell Division Pre- prophase Continued… : Cell Division Pre- prophase Continued… This band marks the position where the cell will eventually divide. The cells of higher plants (such as the flowering plants) lack centrioles: with microtubules forming a spindle on the surface of the nucleus and then being organized into a spindle by the chromosomes themselves, after the nuclear membrane breaks down. The preprophase band disappears during nuclear envelope disassembly and spindle formation in prometaphase. Cell Division Prophase : Cell Division Prophase Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome. Since the genetic material has already been duplicated earlier in S phase, the replicated chromosomes have two sister chromatids, bound together at the centromere by the cohesion complex. Chromosomes are visible at high magnification through a light microscope. Close to the nucleus are structures called centrosomes, which are made of a pair of centrioles. Cell Division Prophase continued… : Cell Division Prophase continued… The centrosome is the coordinating center for the cell's microtubules. A cell inherits a single centrosome at cell division, which replicates before a new mitosis begins, giving a pair of centrosomes. The two centrosomes nucleate microtubules (which may be thought of as cellular ropes or poles) to form the spindle by polymerizing soluble tubulin. Molecular motor proteins then push the centrosomes along these microtubules to opposite sides of the cell. Although centrioles help organize microtubule assembly, they are not essential for the formation of the spindle, since they are absent from plants, and centrosomes are not always used in meiosis. Cell Division Pro-metaphase : Cell Division Pro-metaphase The nuclear envelope disassembles and microtubules invade the nuclear space. This is called open mitosis, and it occurs in most multicellular organisms. Fungi and some protists, such as algae or trichomonads, undergo a variation called closed mitosis where the spindle forms inside the nucleus or its microtubules are able to penetrate an intact nuclear envelope. Each chromosome forms two kinetochores at the centromere, one attached at each chromatid. Cell Division Pro-metaphase continued… : Cell Division Pro-metaphase continued… A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome.Although the kinetochore structure and function are not fully understood, it is known that it contains some form of molecular motor.When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to "crawl" up the tube toward the originating centrosome. This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome's two chromatids. Cell Division Metaphase : Cell Division Metaphase As microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes convene along the metaphase plate or equatorial plane, an imaginary line that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug-of-war between people of equal strength Cell Division Metaphase continued… : Cell Division Metaphase continued… In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. Metaphase comes from the Greek μετα meaning "after." Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules (spindle fibres), it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned Cell Division Anaphase : Cell Division Anaphase When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase (from the Greek ανα meaning “up,” “against,” “back,” or “re-”). Two events then occur; First, the proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids, which have now become distinct sister chromosomes, are pulled apart by shortening kinetochore microtubules and move toward the respective centrosomes to which they are attached. Cell Division Anaphase continued… : Cell Division Anaphase continued… Next, the nonkinetochore microtubules elongate, pulling the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. The force that causes the centrosomes to move towards the ends of the cell is still unknown, although there is a theory that suggests that the rapid assembly and breakdown of microtubules may cause this movement. Cell Division Anaphase continued… : Cell Division Anaphase continued… These two stages are sometimes called early and late anaphase. Early anaphase is usually defined as the separation of the sister chromatids, while late anaphase is the elongation of the microtubules and the chromosomes being pulled farther apart. At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations. Cell Division Telophase : Cell Division Telophase Telophase (from the Greek τελος meaning "end") is a reversal of prophase and prometaphase events. It "cleans up" the after effects of mitosis. At telophase, the nonkinetochore microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. Cell Division Telophase continued… : Cell Division Telophase continued… A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Mitosis is complete, but cell division is not yet complete Cell Division Cytokinesis : Cell Division Cytokinesis Cytokinesis is often mistakenly thought to be the final part of telophase; however, cytokinesis is a separate process that begins at the same time as telophase. Cytokinesis is technically not even a phase of mitosis, but rather a separate process, necessary for completing cell division Cell Division Cytokinesis continued… : Cell Division Cytokinesis continued… In animal cells, a cleavage furrow (pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei.In both animal and plant cells, cell division is also driven by vesicles derived from the Golgi apparatus, which move along microtubules to the middle of the cell. In plants this structure coalesces into a cell plate at the center of the phragmoplast and develops into a cell wall, separating the two nuclei Cell Division Cytokinesis continued… : Cell Division Cytokinesis continued… The phragmoplast is a microtubule structure typical for higher plants, whereas some green algae use a phycoplast microtubule array during cytokinesis. Each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase. Cell Division Common features of all cell : Cell Division Common features of all cell Cell Division significance of mitosis : Cell Division significance of mitosis Mitosis is important for the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell. Transcription is generally believed to cease during mitosis, but epigenetic mechanisms such as bookmarking function during this stage of the cell cycle to ensure that the "memory" of which genes were active prior to entry into mitosis are transmitted to the daughter cells. Cell Division Meiosis : Cell Division Meiosis In biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original cell is replicated during S-phase of the cell cycle. Two cell divisions separate the replicated chromosomes into four haploid gametes or spores. Cell Division Meiosis continued… : Cell Division Meiosis continued… During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell. Because the chromosomes of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Cell Division Meiosis process : Cell Division Meiosis process Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. Cell Division Meiosis process continued… : Cell Division Meiosis process continued… Interphase is followed by meiosis I and then meiosis II. Meiosis I consists of separating the pairs of homologous chromosome, each made up of two sister chromatids, into two cells. One entire haploid content of chromosomes is contained in each of the resulting daughter cells; the first meiotic division therefore reduces the ploidy of the original cell by a factor of 2. Cell Division Meiosis process continued… : Cell Division Meiosis process continued… Meiosis II consists of decoupling each chromosome's sister strands (chromatids), and segregating the individual chromatids into haploid daughter cells. The two cells resulting from meiosis I divide during meiosis II, creating 4 haploid daughter cells. Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle Cell Division Meiosis includes the stages of : Cell Division Meiosis includes the stages of meiosis I prophase I, metaphase I, anaphase I, telophase I, meiosis II prophase II, metaphase II, anaphase II, telophase II. Cell Division Meiosis I : Cell Division Meiosis I Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. Cell Division Meiosis I : Cell Division Meiosis I However, after meiosis I, although the cell contains 46 chromatids, it is only considered as being N, with 23 chromosomes. This is because later, in Anaphase I, the sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. Cell Division Meiosis II : Cell Division Meiosis II In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) per daughter cell from the first division. Cell Division Meiosis Prophase I : Cell Division Meiosis Prophase I During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. Cell Division Meiosis Prophase I continued… : Cell Division Meiosis Prophase I continued… The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma Cell Division Meiosis Leptotene : Cell Division Meiosis Leptotene The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads". During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another. Cell Division Meiosis Zygotene : Cell Division Meiosis Zygotene The zygotene stage, also known as zygonema, from Greek words meaning "paired threads", occurs as the chromosomes approximately line up with each other into homologous chromosomes. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place Cell Division Meiosis Pachytene : Cell Division Meiosis Pachytene The pachytene stage, also known as pachynema, from Greek words meaning "thick threads",contains the following chromosomal crossover. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology Cell Division Meiosis Pachytene continued… : Cell Division Meiosis Pachytene continued… Exchange takes place at sites where recombination nodules (the chiasmata) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope Cell Division Meiosis Diplotene : Cell Division Meiosis Diplotene During the diplotene stage, also known as diplonema, from Greek words meaning "two threads", the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. Cell Division Meiosis Diplotene continued… : Cell Division Meiosis Diplotene continued… The chiasmata remain on the chromosomes until they are severed in Anaphase I. In human fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. Cell Division Meiosis Diakinesis : Cell Division Meiosis Diakinesis Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through". This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible Cell Division Meiosis Diakinesis continued… : Cell Division Meiosis Diakinesis continued… Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. Cell Division Meiosis Metaphase-I : Cell Division Meiosis Metaphase-I Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. Cell Division Meiosis Anaphase I : Cell Division Meiosis Anaphase I Kinetochore(bipolar spindles) microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome has only one functional unit of a pair of kinetochores, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center. Cell Division Meiosis Telophase- I : Cell Division Meiosis Telophase- I The last meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I. Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Cell Division Meiosis II : Cell Division Meiosis II The four main steps of Meiosis II are: Prophase II Metaphase II Anaphase II Telophase II. Cell Division Meiosis Prophase II : Cell Division Meiosis Prophase II In prophase II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division Cell Division Meiosis Metaphase II : Cell Division Meiosis Metaphase II In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. Cell Division Meiosis Anaphase II : Cell Division Meiosis Anaphase II This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. Cell Division Meiosis Telophase II : Cell Division Meiosis Telophase II The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells. Cell Division significance of meiosis : Cell Division significance of meiosis Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count. In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities or lethality . Cell Division significance of meiosis continued… : Cell Division significance of meiosis continued… Polyploidy is poorly tolerated in most animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation. Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring.