MOL 114: DNA Replication and Repair

DNA Replication and Repair

• DNA replication is the process of duplicating chromosomes before cell division, ensuring each daughter cell receives an accurate copy of genetic information.
• The primary structure of DNA is a polymer of nucleotides linked by 3'→5' phosphodiester bonds.

Eukaryotic DNA Replication

• Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication "bubble".
• A eukaryotic chromosome may have hundreds or thousands of origins of replication.
• Replication proceeds in both directions from each origin, until the entire molecule is copied.

Formation of the Replication Fork

• Proteins required for DNA strand separation: DNA helicase and single-stranded DNA-binding protein.
• Unwinding at the replication fork causes supercoiling, which is relieved by DNA topoisomerases.

Action of DNA Topoisomerases

• They have strand-cutting and strand-resealing activities.
• Type I DNA topoisomerases and Type II DNA topoisomerases (DNA gyrase in prokaryotes) are used to relieve supercoiling.

DNA Polymerase

• The enzyme responsible for copying DNA templates, reading parental nucleotide sequences in the 3'→5' direction, and synthesizing new DNA strands only in the 5'→3' direction.
• DNA polymerases catalyze the elongation of new DNA at a replication fork and have proofreading capabilities, correcting mistakes through 3'→5' exonuclease activity.

RNA Primer and Primase

• DNA polymerases require an RNA primer, which is a short, double-stranded region consisting of RNA base-paired to the DNA template.
• The primer is short (5–10 nucleotides long), and the 3'end serves as the starting point for the new DNA strand.
• RNA primer is synthesized using primase enzyme.

Leading and Lagging Strands

• Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork.
• The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase.

Telomeres

• Telomeres are non-coding and repetitive sequences at the ends of linear DNA molecules in eukaryotic chromosomes.
• With each round of replication, telomeres are shortened because DNA polymerase cannot complete synthesis of the 5' end of each strand.
• This contributes to the aging of cells, as eventually the telomeres become so short that the chromosomes cannot function properly and the cells die.

Mismatch Repair

• Replication errors can cause a mismatch of one to several bases, which can be corrected by identifying the strand containing the mismatch and removing the mismatched nucleotide by an exonuclease.
• Mutation to the proteins involved in mismatch repair in humans is associated with hereditary nonpolyposis colorectal cancer (HNPCC), also known as Lynch syndrome.

This quiz covers the process of DNA replication, causes of DNA damage, and repair mechanisms. It also explores the effects of DNA repair failure and its connection to antivirals and cancer treatments.