IB Biology: DNA Replication

IB Biology: DNA Replication

DNA Replication

Key Statements

• DNA replication can only occur in a 5 to 3 prime direction.
• The two polynucleotides that make up a DNA strand are anti-parallel to each other.
• All polymerization in the leading and lagging strands all begin with the addition of an RNA Primer, which the DNA polymerase can bind to.

State that DNA replication occurs in a 5′ to 3′ direction.

Leading Strands

• The LEADING STRAND is the strand of unwound DNA that is synthesized towards the replication fork in a 5′ to 3′ direction.
• Replication can be carried out continuously.

Lagging strands

• The LAGGING STRAND is the strand of unwound DNA that is synthesized away from the replication fork.
• Replication is not continuous.
• RNA Primase lays down an RNA primer at intervals along the lagging strand and DNA polymerase attaches to the template and lays down new DNA in a 5 to 3 prime direction forming an Okazaki fragment.
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RNA primer + Nucleotides= Okazaki fragment

Explain the role of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates.

Steps In Synthesizing the DNA Strand

1. Helicase unwind the DNA double helix that act as the template strands for DNA replication.
2. Single strand binding proteins stabilize the unwound DNA.
3. The leading strand of DNA is synthesized continuously in a 3’ to 5’ direction by DNA polymerase.
4. The lagging strand is synthesized discontinuously. RNA Primase synthesizes a short RNA primer, which is extended by DNA polymerase III to form an Okazaki fragment.
5. The RNA primer is replaced with DNA by DNA polymerase I and DNA ligase forms a covalent phosphodiester bond joining two adjacent Okazaki fragments.

State that DNA replication is initiated at many points in eukaryotic chromosomes.

• DNA replication is initiated at many points in eukaryotic chromosomes.
• Many sites of replication on the DNA strand, otherwise known as replication bubbles.

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