DNA Replication, Repair & Telomeres

What prevents DNA strands from reforming hydrogen bonds?

Single-strand binding proteins

What enzyme adds DNA nucleotides in the 5′ to 3′ direction?

DNA polymerase III

What enzyme removes RNA primers and replaces them with DNA?

DNA polymerase I

What enzyme unwinds DNA strands?

Helicase

What enzyme relieves strain as DNA unwinds?

Topoisomerase

What enzyme seals gaps in DNA backbones?

DNA ligase

What enzyme makes RNA primers?

RNA primase

What happens first in DNA replication?

Helicase unwinds DNA at the origin

What happens second in DNA replication?

Primase adds RNA primers

What happens third in DNA replication?

DNA polymerase III adds nucleotides

What happens fourth in DNA replication?

DNA polymerase I replaces RNA primers with DNA

What happens last in DNA replication?

Ligase seals gaps in the DNA backbone

What happens during replication initiation?

DNA unwinds and replication begins

What happens during replication elongation?

DNA polymerase builds new DNA strands

What happens during replication termination?

Replication finishes

What is the leading strand?

DNA strand made continuously

What is the lagging strand?

DNA strand made in short Okazaki fragments

Why is the lagging strand made in fragments?

DNA polymerase can only build 5′ to 3′

What are Okazaki fragments?

Short DNA pieces made on the lagging strand

What are the final products of DNA replication?

Two identical DNA molecules

What is semiconservative replication?

Each new DNA molecule contains one original strand and one new strand

What is mismatch repair?

Fixes incorrect nucleotides added during DNA replication

What is excision repair?

Removes and replaces damaged DNA sections

When does mismatch repair occur?

During or shortly after DNA replication

When does excision repair occur?

When DNA is damaged

What can cause DNA damage?

Radiation and other damaging agents

What enzyme group removes damaged DNA sections?

Nucleases

What enzyme seals gaps during DNA repair?

DNA ligase

What are telomeres?

Repetitive DNA sequences that protect chromosome ends

What is the function of telomeres?

Protect important DNA as chromosomes shorten during replication

Why do telomeres shorten?

The ends of lagging strands cannot be fully copied

What is telomerase?

An enzyme that extends telomeres

Which cells typically have telomerase?

Germ cells and certain stem cells; many cancer cells also activate it

Why are telomeres linked to aging?

They generally shorten as cells divide over time

What happens if DNA replication enzymes malfunction?

DNA may be copied incorrectly or incompletely

What happens if DNA repair fails?

Mutations can accumulate and disrupt cell function