Dna Replication 2.5

Topic 2.5: DNA Replication 2.5.1 Semi-conservative DNA replication 

The copies of the DNA molecule each contain half the original DNA molecule.

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The double helix unwinds and a new complementary polynucleotide forms on each of the original strands.

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Therefore in each of the copies. One strand is from the original DNA molecule and the other has newly formed.

2.5.2 Mechanism of DNA replication

1. The original double helix molecule. 2. Helicase enzyme breaks the hydrogen bonds between complementary base pairs. This unzips the double helix at a position called the replication fork. 3. There is an abundant supply of nucleotides in the nucleus for the formation of the new polynucleotides.

4. Nucleotides base pair to the bases in the original strands. 5. DNA polymerase joins together the nucleotides together with strong covalent phosphodiester bonds To form a new complementary polynucleotide strand. 6. The double strand reforms a double helix under the influence of an enzyme. 7 Two copies of the DNA molecule form behind the replication fork. These are the new daughter chromosomes. Speed of replication: DNA replication can take a few hours and this limits the speed of cell division. Bacteria can replicate quickly because of the relatively small amount of DNA. Eukaryotic organism's accelerate DNA replication by having thousands of replication forks along the length of the DNA molecule.

2.5.3 Conservation of base sequence The DNA base sequence is usually read down one side of the molecule or the other. The sequence is usually read with reference to the bases and their corresponding identifying letter. e.g.

ATG CTC ATT TTA GGG CCC ATA CTC

= 24 bases thus we can write the complementary sequence of the other helix as:

Q TAC GAG TAA AAT CCC GGG TAT GAG In DNA replication (1) will act as the template for a new complementary sequence of bases (2): copy 1: (1)

ATG CTC ATT TTA GGG CCC ATA CTC

(2)

TAC GAG TAA AAT CCC GGG TAT GAG

and (2) will act as a template for the other new complementary sequence (1)

Copy 2: (2)

TAC GAG TAA AAT CCC GGG TAT GAG

(1)

ATG CTC ATT TTA GGG CCC ATA CTC

therefore after replication the base sequence of copies 1 (1)(2) and copy 2 (1) (2) are identical to each other. Importantly they are also identical in base sequence to the original base sequence of (1)(2). 

The DNA base sequence of the double helix is conserved from one replication to another.

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As cells divide the DNA is copied so that each new cell possess a copy of each of the original DNA molecules