DNA Replication Overview

DNA Replication

• DNA replication is the process of creating two identical copies of a DNA molecule
• DNA replication occurs before cell division to ensure each daughter cell receives a complete set of DNA
• Both prokaryotes and eukaryotes replicate DNA semi-conservatively, meaning each new DNA molecule contains one original strand and one newly synthesized strand

Prokaryotic and Eukaryotic Differences

• Prokaryotic DNA is circular and located in the cytoplasm
• Eukaryotic DNA is linear and located in the nucleus
• Prokaryotes have a single origin of replication, while eukaryotes initiate replication at multiple origins

DNA Replication Steps

• DNA replication occurs in three steps: initiation, elongation, and termination
• The process starts with the unwinding of the DNA double helix, which is carried out by the enzyme helicase
• Single-strand binding proteins (SSBs) stabilize the separated strands, preventing them from re-annealing
• DNA polymerase adds nucleotides to the newly synthesized strand, following the base pairing rules (A with T, and G with C)
• DNA polymerase I proofreads the newly synthesized strands, correcting any errors
• DNA ligase joins the newly synthesized fragments together

Leading and Lagging Strands

• Replication occurs in both directions from the origin, creating two replication forks
• One strand, the leading strand, is synthesized continuously in the 5’ to 3’ direction
• The other strand, the lagging strand, is synthesized discontinuously in short fragments called Okazaki fragments

Telomeres

• Telomeres are repetitive sequences at the ends of linear eukaryotic chromosomes
• Telomerase, an enzyme with an RNA component, adds these repeated sequences, preventing chromosome shortening during replication

Prokaryotic vs. Eukaryotic Replication

• Prokaryotic replication occurs in the cytoplasm, while eukaryotic replication occurs in the nucleus
• Prokaryotic replication is faster than eukaryotic replication
• Prokaryotic DNA is circular, while eukaryotic DNA is linear
• Prokaryotes use DNA polymerase I and III, while eukaryotes use DNA polymerase α, δ, and ε

Replication of Circular DNA

• In some prokaryotes, replication begins at a single origin and continues in two directions until the entire chromosome is copied
• The resulting structure looks like the Greek letter theta (θ)

Rolling Circle Model

• This model is common in bacteriophages and uses a nick at the origin of replication to initiate DNA synthesis
• The 5' end of the molecule is displaced and acts as a primer, allowing for continuous replication and multiple copies of the genome

Eukaryotic Chromosome Replication

• Eukaryotes replicate their chromosomes during the S phase of the cell cycle
• The process requires multiple origins of replication to ensure efficient replication of the large eukaryotic genome

Checkpoints in Eukaryotic Replication

• Several checkpoints ensure proper DNA replication before cell division
• These checkpoints include proteins called cyclins and enzymes called cyclin-dependent kinases (Cdks)
• These ensure the cell is large enough, the environment is favorable, all DNA is replicated, and chromosomes are attached to the mitotic spindle

Nucleosome Assembly

• During replication, nucleosomes must disassemble and reassemble
• New histones and associated chromatin proteins must be synthesized for the newly formed DNA molecules

This quiz covers the essential processes of DNA replication, including the differences between prokaryotic and eukaryotic DNA. Learn about the three main steps of replication: initiation, elongation, and termination. Test your understanding of how DNA ensures accurate genetic transmission during cell division.