DNA replication

When does DNA replication occur?

During the synthesis phase of interphase

Why does DNA replication occur?

To create more/new cells

What is semi-conservative replication?

Each new DNA molecule is made of 1/2 the old strand and 1/2 the new strand.

How accurate is DNA replication?

3 mistakes per 3 million bases

What are the main steps of DNA replication

1. Unwinding
2. Polymerization
3. Quality Control

Describe the unwinding stage.

DNA unwraps from nucleosomes, DNA gyrase unwinds DNA. DNA Helicase breaks H bonds between nucleotides. SSBP attaches to stabilize and to prevent reformation.

Describe the polymerization and quality control stages.

RNA primer attaches to the strands, it is used as a starting point. DNA polymerase(III) builds new strand in a 5’-3’ direction. Polymerase(III) builds the new strands of DNA and adds nucleotides to template strands. DNA polymerase(I) removes RNA primer and adds DNA nucleotides. It also checks for mistakes. DNA ligase creates p-diester links between Okazaki fragments

What is the leading strand?

5’-3’. Polymerase works continuously towards the replication fork.

What is the lagging strand?

3’-5’. Polymerase builds discontinuously away from the replication fork. This creates Okazaki fragments.

Describe DNA

Double stranded molecule with anti-parallel strands. 5’ end contains phosphate group and 3’ end contains OH group.

What are p-diester links?

These links occur between the phosphate group on the 5’ carbon and the OH group on the 3’ carbon.

What is a purine?

Adenine and Guanine, larger double ring structure

What is a pyrimidine?

Thymine and Cytosine, smaller single ring structures

Explain H-bonding

Occurs between nitrogenous bases. Adenine and Thymine have 2 bonds. Cytosine and Guanine have 3 bonds.

Why is there a leading and lagging strand?

The strands are antiparallel and DNA polymerase only works in a 5’-3’ direction.

How is the lagging strand replicated discontinuously?

Lagging strand is 3’-5’ so it cannot be replicated continuously because polymerase only moves 5’-3’. The lagging strand replicates in sections called Okazaki fragments as polymerase replicates away from the fork.

How are mistakes fixed?

DNA polymerase(I) checks for mistakes. The mistakes are cut out and replaced with correct code.

How do large polymers fit in tiny nucleus?

DNA coils around nucleosomes to fit.

What happens if SSBP malfunctions?

Strands will reconnect with H-bonds

What happens if primase is inactive?

DNA polymerase would not have a starting point to attach to and would not be able to replicate

What happens if DNA polymerase(I) is inactive?

RNA primers would not get cut out and mistakes will not be checked