DNA Replication

What is the primary function of DNA replication?

T o ensure each daughter cell receives an identical copy of the genetic material during cell division.

What are the three challenges of DNA replication?

1. Accessing genetic information 2. Synthesizing two antiparallel strands 3. Preventing errors in replication

What is semiconservative replication?

Each daughter DNA molecule consists of one original strand and one newly synthesized strand.

What experiment demonstrated semiconservative replication?

The Meselson and Stahl experiment using CsCl density gradient centrifugation.

What is a replication fork?

The Y-shaped structure where DNA replication occurs as the strands separate.

How does bidirectional replication work?

DNA synthesis proceeds in both directions from a single origin.

What is the role of DNA polymerase?

It synthesizes new DNA strands by adding nucleotides to the growing chain.

In which direction does DNA polymerase synthesize DNA?

5' to 3' direction.

What is the leading strand?

The DNA strand synthesized continuously in the direction of the replication fork.

What is the lagging strand?

The DNA strand synthesized discontinuously as Okazaki fragments.

What enzyme joins Okazaki fragments?

DNA ligase.

What are Okazaki fragments?

Short DNA fragments synthesized on the lagging strand during replication.

What is the function of RNA primers in DNA replication?

They provide a starting point for DNA polymerase to initiate synthesis.

Which enzyme synthesizes RNA primers?

Primase.

How are RNA primers removed?

DNA polymerase I removes them and fills the gaps with DNA.

What is the function of helicase?

It unwinds the DNA double helix at the replication fork.

What is the function of single-strand binding proteins (SSBs)?

They stabilize single-stranded DNA and prevent it from reannealing.

What is the role of DNA gyrase (topoisomerase II) in replication?

It relieves supercoiling ahead of the replication fork.

What is the function of DNA polymerase III in prokaryotes?

It is the main enzyme responsible for DNA synthesis.

How does proofreading ensure replication accuracy?

DNA polymerase has 3' to 5' exonuclease activity that removes mismatched bases.

What is the role of Tus proteins in E. coli replication?

They bind to Ter sites and terminate replication.

What is the function of the Origin Recognition Complex (ORC) in eukaryotes?

It binds to replication origins and initiates replication.

What is the function of the MCM complex in eukaryotic replication?

It acts as a helicase to unwind DNA.

How do eukaryotic cells prevent multiple rounds of replication?

By regulating pre-replicative complexes and licensing factors.

What is the end-replication problem?

The inability to fully replicate the ends of linear DNA due to primer removal.

How do eukaryotic cells solve the end-replication problem?

Using telomerase to extend the ends of chromosomes.

What is the sequence of human telomeres?

TTAGGG.

What enzyme extends telomeres?

Telomerase, a ribonucleoprotein with an RNA template.

What is the function of PCNA in eukaryotic replication?

It acts as a sliding clamp to enhance polymerase processivity.

What are the main DNA polymerases in eukaryotic replication?

DNA polymerase α (initiates replication), δ (lagging strand), and ε (leading strand).

What is a topoisomerase?

An enzyme that relieves DNA supercoiling.

What are the two types of topoisomerases?

Type I (cuts one strand) and Type II (cuts both strands).

What is the role of DNA ligase in replication?

It seals nicks in the sugar-phosphate backbone.

What are the differences between prokaryotic and eukaryotic replication?

Prokaryotic replication has a single origin, fast replication, and long Okazaki fragments, while eukaryotic replication has multiple origins, slower speed, and short Okazaki fragments.

What are the checkpoints in the eukaryotic cell cycle?

G1/S (start checkpoint), G2/M (DNA damage checkpoint), and spindle checkpoint.

What is the function of cyclins in DNA replication?

They regulate cell cycle progression by activating cyclin-dependent kinases (CDKs).

What is the Hayflick limit?

The number of times a normal human cell can divide before telomere shortening leads to senescence.

How do cancer cells bypass the Hayflick limit?

By reactivating telomerase to maintain telomere length.

What is DNA mismatch repair?

A repair mechanism that corrects errors missed by proofreading during replication.

How do quinolone antibiotics (e.g., ciprofloxacin) target bacterial replication?

By inhibiting bacterial topoisomerase II (DNA gyrase), preventing DNA supercoiling.

How do eukaryotic topoisomerase inhibitors (e.g., doxorubicin) work?

By stabilizing DNA-topoisomerase cleavage complexes, preventing DNA replication.

What are the two types of replication forks?

Unidirectional (one fork) and bidirectional (two forks).

What is the role of RNase H1 in eukaryotic replication?

It removes RNA primers from Okazaki fragments.

What is the sliding clamp in eukaryotic replication?

PCNA (Proliferating Cell Nuclear Antigen), which increases DNA polymerase efficiency.

What is the primary difference between bacterial and human telomerase activity?

Most bacterial chromosomes are circular and do not require telomerase, whereas human cells use telomerase to extend linear chromosome ends.