2.3.2 (DNA replication)

What is DNA polymerase?

• An enzymes which is used in DNA replication which catalyses the formation of phosphodiester bonds between adjacent nucleotides

What is Helicase?

• An enzyme that catalyses the breaking of hydrogen bonds between nitrogenous pairs of bases in the DNA molecule

Describe the process of semi-conservative DNA replication

1. Unwind: The double helix is untwisted by a gyrase enzyme

2. Unzips: Hydrogen bonds between the nucleotides are broken by DNA helicase enzymes, which results in 2 single strands of DNA with exposed nucleotide nitrogenous bases

3. The phosphorylated nucleotides in the nucleoplasm of the nucleus are bonded to the exposed nitrogenous bases, following complementary base-pairing rules

4. The enzyme DNA polymerase catalyses the addition of the new nucleotide bases(in the 5’ to 3’ direction) to the single strands of DNA; it uses each single strand of unzipped DNA as a template)

5. The leading strand is synthesised continuously, whereas the lagging strand is in fragments that are later joined, catalysed by ligase enzymes

6. Hydrolysis of the activated nucleotides, to release extra phosphate groups, supplies energy to make phosphodiester bonds between the sugar residue on one nucleotide and the phosphate group on the next nucleotide

7. 2 DNA molecules form, identical to each other and the parent molecule

Explain why it is called semi-conservative replication?

• Because in the daughter molecules, they both contain one strand from the parent molecule and one new strand

Describe the process of semi-conservative DNA replication in a summarised manner.

1) Double helix is untwisted by gyrase.

2) Hydrogen bends between bans are broken by helicase.

3) DNA thus, unzips to expose nitrogenous bares.

4) Each strand outs a template to the new strand

5) Free phosphorylated DNA nucleotides added in the 5' to 3' direction

6) Hydrogen bonds form between bases.

7) Phosphodiester bands form between phosphate sugar phosphate backbone as catalysed by DNA polymerase. It checks for mistakes to avoid mutations

How can mutations be checked for during DNA replication?

• There are enzymes which proof read and edit out incorrect nucleotides, which reduces the rate of mutation production.