Chapter 4: Cell Reproduction

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Chapter 4: Cell Reproduction Section 3: DNA Section 1: Cell Division and Mitosis Section 2: Sexual Reproduction and Meiosis

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Many organisms start as just one cell. That cell divides and becomes two, two become four, four become eight, an so on. Many-celled organisms, including you, grow because cell division increases the total number of cells in an organism. Why is cell division important?

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Even after growth stops, cell division is still important. Every day, billions of red blood cells in your body wear out and are replaced. During a few seconds, your bone marrow produced about six million red blood cells. Why is cell division important? Cell division is important because it increases the number of cells and causes many celled organisms to grow.

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The cell cycle is a series of events that takes place from one cell division to the next. The time is takes to complete a cell cycle is not the same in all cells. Length of Cycle

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A living organism has a life cycle. A life cycle begins with the organism’s formation, is followed by growth and development, and finally ends in death. Cells have periods of formation, grow and development, and death called life cycles. The Cell Cycle

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Most of the life of any eukaryotic cell—a cell with a nucleus—is spent in a period of growth and development called interphase. Cells in your body that no longer divide, such as nerve and muscle cells, are always in interphase. Interphase During interphase, a cell duplicates its chromosomes and prepares for cell division.

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Before a cell divides, a copy of the hereditary material must be made so that each of the two new cells will get a complete copy. Each cell needs a complete set of hereditary material to carry out life functions. Interphase

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Interphase After interphase, cell division begins. The nucleus divides, and then cytoplasm separates to form two new cells.

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Mitosis (mi TOH sus) is the process in which the nucleus divides to form two identical nuclei. Each new nucleus also is identical to the original nucleus. Mitosis Mitosis is described as a series of phases, or steps. The steps of mitosis in order are named prophase, metaphase, anaphase, and telophase.

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A chromosome (KROH muh sohm) is a structure in the nucleus that contains hereditary material. Steps of Mitosis

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During interphase, each chromosome duplicates. Steps of Mitosis

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Steps of Mitosis

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During prophase, the pairs of chromatids are fully visible when viewed under a microscope. The nucleolus and the nuclear membrane disintegrate. Steps of Mitosis: Prophase Two small structures called centrioles (SEN tree olz) move to opposite ends of the cell.

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Between the centrioles, threadlike spindle fibers begin to stretch across the cell. Plant cells also form spindle fibers during mitosis but do not have centrioles. Steps of Mitosis

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In metaphase, the pairs of chromosomes line up across the center of the cell. The centromere of each pair usually becomes attached to two spindle fibers—one from each side of the cell. Steps of Mitosis: Metaphase

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In anaphase, each centromere divides and the spindle fibers shorten. Each pair of chromatids separates, and chromatids begin to move to opposite ends of the cell. Steps of Mitosis: Anaphase The separated chromatids are now called chromosomes.

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In the final step, telophase, spindle fibers start to disappear, the chromosomes start to uncoil, and a new nucleus forms. Steps of Mitosis: Telophase

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For most cells, the cytoplasm separates after the nucleus divides. Division of the Cytoplasm In animal cells, the cell membrane pinches in the middle, like a balloon with a string tightened around it, and the cytoplasm divides.

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In plant cells, the appearance of a cell plate tells you that the cytoplasm is being divided. Division of the Cytoplasm New cell walls form along the cell plate, and new cell membranes develop inside the cell walls.

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Results of Mitosis Mitosis is the division of the nucleus, and it produces two new nuclei that are identical to each other and the original nucleus.

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Results of Mitosis Each new nucleus has the same number and type of chromosomes. Every cell in your body, except sex cells, has a nucleus with 46 chromosomes—23 pairs.

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Each of the trillions of cells in your body, except sex cells, has a copy of the same hereditary material. Results of Mitosis Allows growth and replaces worn out or damaged cells.

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In asexual reproduction, a new organism (sometimes more than one) is produced from one organism. Asexual Reproduction The new organism will have hereditary material identical to the hereditary material of the parent organism.

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Organisms with eukaryotic cells asexually reproduce by cell division. Cellular Asexual Reproduction However, bacteria do not have a nucleus so they can’t use mitosis. An organism with no nucleus divides into two identical organisms by fission.

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Budding is a type of asexual reproduction made possible because of cell division. Budding and Regeneration When the bud on the adult becomes large enough, it breaks away to live on its own.

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Some organisms can regrow damaged or lost body parts. Budding and Regeneration Regeneration is the process that uses cell division to regrow body parts. Sponges, planaria, sea stars, and some other organisms can use regeneration for asexual reproduction.

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Section 2: Sexual Reproduction During sexual reproduction, two sex cells, sometimes called an egg and a sperm, come together which is called fertilization.. Sex cells are formed from cells in reproductive organs. Sperm are formed in the male reproductive organs. Eggs are formed in the female reproductive organs.

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Sexual Reproduction The joining of an egg and a sperm is called fertilization, and the cell that forms is called a zygote (ZI goht). Following fertilization, cell division begins. A new organism with a unique identity develops.

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Diploid Cells A typical human body cell has 46 chromosomes. Each chromosome has a mate that is similar to it in size and shape and has similar DNA. Human body cells have 23 pairs of chromosomes. Human body cells are diploid because they have 23 pairs of similar chromosomes.

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Haploid Cells Human sex cells are haploid, because they have 23 single chromosomes. They have only half the number of chromosomes as body cells. Haploid means “single form.” Human sex cells have only 23 chromosomes—one from each of the 23 pairs of similar chromosomes.

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Meiosis and Sex Cells A process called meiosis (mi OH sus) produces haploid sex cells.

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Meiosis and Sex Cells Meiosis ensures that the offspring will have the same diploid number as its parent.

Meiosis Key Facts : 

Meiosis Key Facts Meiosis reduces the number of chromosomes in the body cells in half to make sex cells. This is needed because when the egg and sperm combine they have the correct total number of chromosomes.

Meiosis Key Facts : 

Meiosis Key Facts One body cell ends up as 4 sex cells Each sex cell has half the number of chromosomes as a body cell. Human body cells have 46 chromosomes, sex cells have 23.

Meiosis Key Facts : 

Meiosis Key Facts Any sex cells have half the number of chromosomes that the body cell has. A bee might have 10 chromosomes, its sex cells would have 5.

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Meiosis and Sex Cells During meiosis, two divisions of the nucleus occur. These divisions are called meiosis I and meiosis II. The steps of each division have names like those in mitosis and are numbered for the division in which they occur.

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Meiosis and Sex Cells

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Summary of Meiosis In meiosis II, the nuclei divide and the chromatids separate, producing four cells with half the number of chromosomes of the original nucleus. In meiosis I, the nucleus divides and produces two new cells with on duplicated chromosome each.

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Mistakes of Meiosis Meiosis occurs many times in reproductive organs. Mistakes can produce sex cells with too many or too few chromosomes.

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Mistakes of Meiosis Sometimes, zygotes produced from these sex cells die. If the zygote lives, every cell in the organism that grows from that zygote usually will have the wrong number of chromosomes. Organisms with the wrong number of chromosomes may not grow normally.

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Section 3: What is DNA? A chemical that contains information that an organism needs to grow and function. A cell uses this code in its hereditary material. The code is called deoxyribonucleic (dee AHK sih ri boh noo klay ihk) acid, or DNA.

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What is DNA? DNA is stored in cells that have a nucleus. When a cell divides, the DNA code is copied and passed to the new cells. In this way, new cells receive the same coded information that was in the original cell.

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DNA’s Structure In 1952, scientist Rosalind Franklin discovered that DNA is two chains of molecules in a spiral form. In 1953, scientists James Watson and Francis Crick made a model of a DNA molecule. The structure of DNA is similar to a twisted ladder.

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A DNA Model According to the Watson and Crick DNA model, each side of the ladder is made up of sugar-phosphate molecules. Each molecule consists of the sugar called deoxyribose (dee AHK sih ri bohs) and a phosphate group.

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A DNA Model The rungs of the ladder are made up of other molecules called nitrogen bases. The bases are represented by the letters A, G, C, and T.

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A DNA Model The amount of cytosine in cells always equals the amount of guanine, and the amount of adenine always equals the amount of thymine. Adenine always pairs with thymine, and guanine always pairs with cytosine.

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Copying DNA When chromosomes are duplicated before mitosis or meiosis, the amount of DNA in the nucleus is doubled. The two sides of DNA unwind and separate.

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Copying DNA Each side then becomes a pattern on which a new side forms. The new DNA has bases that are identical to those of the original DNA and are in the same order.

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