DNA Replication

In which part of the cell cycle does DNA replication occur?

DNA replication occurs during the S phase (synthesis phase) of the cell cycle.

What did Meselson and Stahl’s experiment demonstrate?

Meselson and Stahl showed that DNA replication is 'semi-conservative'.

How did Meselson and Stahl track DNA replication?

They tracked normal and 'heavy' nitrogen isotopes during replication, showing that the percentage of light nitrogen increases with each replication.

What phrase describes the method of DNA replication?

The phrase used to describe DNA replication is 'semiconservative'.

What happens during semi-conservative replication?

In semi-conservative replication, strands unwind, each strand serves as a template, new DNA nucleotides are added according to complementary base pairs, resulting in a new DNA molecule that has one original and one new strand with the same base pair order.

What is the difference in origins of replication between prokaryotes and eukaryotes?

Prokaryotes have one replication bubble (circular DNA) and replication occurs in both directions, whereas eukaryotes have thousands of bubbles (linear DNA) that all eventually meet.

In what direction does DNA replication occur?

DNA replication occurs in the 5’ to 3’ direction.

What is the role of primase in DNA replication?

Primase creates an RNA primer which serves as an attachment point for DNA polymerase III.

What does DNA polymerase III do during replication?

DNA polymerase III adds complementary DNA nucleotides in the 5’ to 3’ direction, always adding to the 3’ end of the new strand.

What are the two newly created strands during DNA replication called?

The two newly created strands are called the leading strand and the lagging strand.

Describe the leading strand of DNA.

The leading strand runs 5’ to 3’, its template strand runs 3’ to 5’, and it is created continuously in the 5’ to 3’ direction.

Describe the lagging strand of DNA.

The lagging strand runs 3’ to 5’, its template strand runs 5’ to 3’, and it is created discontinuously using Okazaki fragments.

What is the function of DNA polymerase I?

DNA polymerase I replaces RNA primers with DNA.

What does DNA ligase do?

DNA ligase links all unbonded segments of DNA.

What is the end result of DNA replication?

The end result is 2 new molecules of DNA, each containing one parent strand and one new strand.

Explain the process of DNA replication in brief (8 marks).

DNA replication is semi-conservative. DNA helicase separates the strands, single stranded binding proteins prevent rewinding. On the leading strand, primase adds an RNA primer, allowing DNA polymerase III to continuously add nucleotides in the 5’ to 3’ direction. The lagging strand requires multiple primers and forms Okazaki fragments. DNA polymerase I removes RNA and replaces it with DNA, while DNA ligase links unbonded nucleotides.

What is the purpose of PCR?

PCR (Polymerase Chain Reaction) produces large quantities of a small specific sequence of DNA in the laboratory, useful when only a small amount of DNA is available.

Outline the steps of the PCR process.

1. Denaturation: Heat DNA sample to separate strands. 2. Annealing: DNA primers attach to opposite ends. 3. Elongation: DNA Taq polymerase copies the strands.

What functions do the enzymes have in DNA replication?

• DNA helicase: unwinds DNA by breaking hydrogen bonds. - Primerase: creates RNA primer. - DNA Polymerase III: adds DNA nucleotides in 5’ to 3’ direction. - DNA polymerase I: replaces RNA primers with DNA. - DNA ligase: links all DNA nucleotides. - DNA gyrase: straightens the strand post unwinding. - Single Strand Binding Proteins: prevent DNA strands from rewinding.

What are Okazaki fragments?

Okazaki fragments are short 5’ to 3’ segments of DNA that are about 100-200 bases long, added by DNA polymerase III in the 5’ to 3’ direction.

Compare the leading and lagging strand

Leading Strand: runs 5’ to 3’, continuously made, single primer. Lagging Strand: runs 3’ to 5’, discontinuously made, has Okazaki fragments, multiple primers.