DNA replication

THE STRUCTURE AND REPLICATION DNA : 

THE STRUCTURE AND REPLICATION DNA Created by : SEPTI SETIYAWATI (3315106763)

DNA : 

DNA stands for deoxyribose nucleic acid This chemical substance is present in the nucleus of all cells in all living organisms DNA controls all the chemical changes which take place in cells The kind of cell which is formed, (muscle, blood, nerve etc) is controlled by DNA The kind of organism which is produced (buttercup, giraffe, herring, human etc) is controlled by DNA DNA

DNA molecule : 

DNA is a very large molecule made up of a long chain of sub-units The sub-units are called nucleotides Each nucleotide is made up of a sugar called deoxyribose a phosphate group -PO4 and an organic base DNA molecule

Ribose & deoxyribose : 

Ribose is a sugar, like glucose, but with only five carbon atoms in its molecule Deoxyribose is almost the same but lacks one oxygen atom Both molecules may be represented by the symbol Ribose & deoxyribose

The bases : 

The most common organic bases are The bases

Nucleotides : 

The deoxyribose, the phosphate and one of the bases Combine to form a nucleotide Nucleotides

Joined nucleotides : 

A molecule of DNA is formed by millions of nucleotides joined together in a long chain Joined nucleotides

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In fact, the DNA usually consists of a double strand of nucleotides The sugar-phosphate chains are on the outside and the strands are held together by chemical bonds between the bases

2-stranded DNA : 

2-stranded DNA

Bonding 1 : 

The bases always pair up in the same way Adenine forms a bond with Thymine and Cytosine bonds with Guanine Bonding 1

Bonding 2 : 

Bonding 2

Pairing up : 

Pairing up

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The paired strands are coiled into a spiral called A DOUBLE HELIX

THE DOUBLE HELIX : 

sugar-phosphate chain bases THE DOUBLE HELIX

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DNA Double Helix DNA consists of two polynucleotide chains wound around each other to form a double helix. The helix twists in the right-handed direction—think of two strands of rope twisted around each other in the clockwise direction. This twisting is an inherent property of DNA, and it is the same when the helix is looked at from either end.

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One end of the chain ends with a phosphate linked to the 5' carbon of the sugar and is called the 5' end. The other end of the chain ends with an hydroxyl group linked to the 3' carbon of the sugar and is called the 3' end. Each polynucleotide chain consists of a string of nucleotides linked by phosphodiester bonds between phosphate and sugar

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After Watson and Crick proposed the double helix model of DNA, three models for DNA replication

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In this model the two parental DNA strands are back together after replication has occurred. That is, one daughter molecule contains both parental DNA strands, and the other daughter molecule contains DNA strands of all newly-synthesized material. Conservative Model

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Semiconservative Model In this model the two parental DNA strands separate and each of those strands then serves as a template for the synthesis of a new DNA strand. The result is two DNA double helices, both of which consist of one parental and one new strand.

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Dispersive Model In this model the parental double helix is broken into double-stranded DNA segments that, as for the Conservative Model, act as templates for the synthesis of new double helix molecules. The segments then reassemble into complete DNA double helices, each with parental and progeny DNA segments interspersed.

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How Antiparallel DNA Strands Are Replicated STEP 1.

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STEP 2.

Step 3. : 

Step 3.

Step 4. : 

Step 4.

Step 5. : 

Step 5.

ALL STEP. : 

ALL STEP.

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A large number of enzymes and other proteins are involved in the synthesis of new DNA at a replication fork. Enzymes and Proteins in DNA Replication

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This DNA polymerase replaces the RNA primer with DNA. This is a different type of DNA polymerase from the main DNA polymerase which synthesises DNA on a DNA template. In E. coli the main enzyme is DNA polymerase III and the enzyme that replaces the RNA primer with DNA is DNA polymerase I. When the RNA primer has been replaced with DNA, there is a gap between the two Okazaki fragments and this is sealed by DNA ligase. Another DNA polymerase:

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DNA ligase seals the gap left between Okazaki fragments after the primer is removed. As the Okazaki fragments are joined, the new lagging strand becomes longer and longer. DNA ligase: Location: At the replication fork. Function: Unwinds the DNA double helix. Helicase:

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Location: On the template strands. Function: Synthesizes new DNA in the 5' to 3' direction using the base information on the template strand to specify the nucleotide to insert on the new chain. Also does some proofreading; that is, it checks that the new nucleotide being added to the chain carries the correct base as specified by the template DNA. If an incorrect base pair is formed, DNA polymerase can delete the new nucleotide and try again. In E. coli the enzyme used for all new DNA synthesis except for the replacement of the RNA primers is DNA polymerase III. DNA polymerase I replaces the primers. DNA polymerase:

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The new DNA strand made discontinuously in the direction opposite to the direction in which the replication fork is moving. The new DNA strand made continuously in the same direction as movement of the replication fork. Lagging Strand: Leading strand:

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Location: On the template strand which dictates new DNA synthesis away from the direction of replication fork movement. Function: A building block for DNA synthesis of the lagging strand. On one template strand, DNA polymerase synthesizes new DNA in a direction away from the replication fork movement. Because of this, the new DNA synthesized on that template is made in a discontinuous fashion; each segment is called an Okazaki fragment. Okazaki fragment:

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The direction of replication i.e., the direction in which the replication fork moves as the DNA double helix unwinds. Overall direction of replication (movement of replication fork):

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The parental DNA double helix that will be unwound and used as the template for new DNA synthesis. Parent DNA:

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Location: Wherever the synthesis of a new DNA fragment is to commence. Function: DNA polymerase cannot start the synthesis of a new DNA chain, it can only extend a nucleotide chain primer. Primase synthesizes a short RNA chain that is used as the primer for DNA synthesis by DNA polymerase. Primase:

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Location: On single-stranded DNA near the replication fork. Function: Binds to single-stranded DNA to make it stable. Single-strand binding (SSB) proteins

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Step 1A. The DNA is already partially unwound to form a replication fork.B. On the bottom template strand, primase synthesizes a short RNA primer in the 5' to 3' direction.C. Primase leaves, and DNA polymerase adds DNA nucleotides to the RNA primer in the 5' to 3' direction. In E. coli the enzyme used is DNA polymerase III. This new DNA is called the leading strand because it is being made in the same direction as the movement of the replication fork.

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Step 2A. On the top template strand, primase synthesizes a short RNA primer in the 5' to 3' direction.B. Primase leaves, and DNA polymerase adds DNA nucleotides to the RNA primer in the 5' to 3' direction. In E. coli the enzyme used is DNA polymerase III. This new DNA is called the lagging strand because it is being made in the direction opposite to the movement of the replication fork. The segment produced is also called an Okazaki fragment.

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Step 3The DNA unwinds some more and the leading strand is extended by DNA polymerase adding more DNA nucleotides. Thus, the leading strand is synthesized continuously.

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Step 4A. On the top template strand, a new RNA primer is synthesized by primase near the replication fork and is DNA is added to it by DNA polymerase. This produces the second Okazaki fragment. Thus, the lagging strand is synthesized discontinously.B. DNA ligase joins the two Okazaki fragments to produce a continuous chain.C. The process repeats as the DNA continues to unwind. Because one new DNA strand is synthesized continuously and the other is synthesized discontinuously, this model is called the semidiscontinuous model for DNA synthesis

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Step 5A. A different type of DNA polymerase removes the RNA primer and replaces it with DNA. In E. coli the enzyme used is DNA polymerase I.B. DNA ligase joins the two Okazaki fragments to produce a continuous chain.C. The process repeats as the DNA continues to unwind. Because one new DNA strand is synthesized continuously and the other is synthesized discontinuously, this model is called the semidiscontinuous model for DNA synthesis.