DNA Replication and Structure Overview

DNA Replication Overview

• DNA replication is the process where a cell duplicates its DNA to prepare for cell division.
• It occurs in eukaryotic cells' nucleus during the S phase of the cell cycle.
• It involves numerous enzymes, complexes, and regulatory mechanisms to ensure accuracy and speed.
• Understanding replication is crucial for advancements in cell therapies and cancer research.

DNA Structure

• DNA consists of a 5-carbon deoxyribose sugar, phosphate, and a nitrogenous base (purine/pyrimidine).
• Double-stranded DNA forms a double helix with two sugar-phosphate backbones on the outside and bases paired on the inside.
• DNA wraps around histone proteins to form nucleosomes, which condense into chromatin fibres and ultimately chromosomes, facilitating compact storage within the nucleus.
• Accessing DNA requires unwinding the chromatin structure.

Initiation

• Replication begins at origins of replication, typically A-T rich regions (2 hydrogen bonds easier to separate than G-C bonds with 3 hydrogen bonds).
• Each origin is used only once per cell cycle, ensured by licensing factors (pre-replication complex).
• Helicases unwind the DNA double helix, creating replication forks.
• Single-strand binding proteins prevent separated strands from re-annealing.
• Topoisomerase relieves DNA supercoils ahead of the replication fork.
• Primase synthesizes short RNA primers to provide a 3' hydroxyl group for DNA polymerase.

Elongation

• DNA polymerase adds complementary nucleotides to the existing strands.
• Replication proceeds bidirectionally from the origin.
• The leading strand replicates continuously in the 5' to 3' direction.
• The lagging strand replicates discontinuously, forming Okazaki fragments.
• DNA polymerase I removes RNA primers and replaces them with DNA.

Termination

• RNA primers on the lagging strand are removed by exonucleases and replaced with DNA.
• DNA ligase seals nicks between Okazaki fragments to create a continuous strand.
• Proofreading exonucleases correct errors in the newly synthesized DNA strand.
• Telomerase adds telomere sequences to chromosome ends in order to continue copying strands without loss.
• The process results in two identical DNA molecules, each with one old and one new strand.

This quiz explores the fundamentals of DNA replication, its structure, and the complex processes involved in the lifecycle of a cell. Participants will learn about the importance of enzymes, the formation of the double helix, and the role of chromatin in DNA accessibility. Perfect for students studying cell biology and genetics!