DNA replication & tolemers

What does DNA stand for?

Deoxyribonucleic acid.

What is the overall structure of DNA?

A double helix consisting of two complementary strands.

What are the building blocks of DNA?

Nucleotides.

What are the three components of a DNA nucleotide?

A sugar, a phosphate group, and a nitrogenous base.

What sugar is found in DNA?

Deoxyribose.

What are the four nitrogenous bases in DNA?

Adenine, thymine, cytosine, and guanine.

What are the complementary DNA base-pairing rules?

Adenine pairs with thymine (A–T), and cytosine pairs with guanine (C–G).

What holds complementary DNA bases together?

Hydrogen bonds.

What forms the DNA sugar-phosphate backbone?

Alternating sugar and phosphate groups.

What does antiparallel mean in DNA?

The two strands run in opposite directions.

What is DNA replication?

The process of copying DNA before cell division.

Why must DNA replicate before cell division?

So each daughter cell can receive a copy of the genetic information.

What is helicase?

An enzyme that unwinds DNA and separates the strands by disrupting hydrogen bonds between paired bases.

What is a DNA replication fork?

The Y-shaped region where DNA strands separate and new strands are synthesized.

What is a DNA replication bubble?

A region where the DNA double helix has opened so replication can occur.

What is primase?

An enzyme that synthesizes short RNA primers to provide starting points for DNA synthesis.

What is an RNA primer?

A short RNA sequence that provides a starting point for DNA polymerase.

What is DNA polymerase?

An enzyme that adds complementary DNA nucleotides to a growing strand.

In which direction does DNA polymerase synthesize new DNA?

5′ to 3′.

What is the leading strand?

The new DNA strand synthesized continuously toward the replication fork.

What is the lagging strand?

The new DNA strand synthesized discontinuously in short fragments away from the replication fork.

What are Okazaki fragments?

Short DNA fragments synthesized on the lagging strand.

What is DNA ligase?

An enzyme that joins DNA fragments by sealing breaks in the sugar-phosphate backbone.

What is semiconservative DNA replication?

Replication in which each new DNA molecule contains one original strand and one newly synthesized strand.

What are the basic steps of DNA replication?

Unwind and separate the DNA; add RNA primers; synthesize complementary DNA; join the fragments on the lagging strand.

What is a telomere?

A protective region of repetitive DNA at the end of a linear chromosome.

What is the repetitive DNA sequence of human telomeres?

TTAGGG, repeated many times.

What is the end-replication problem?

The difficulty of completely copying the ends of linear DNA molecules, which can cause telomeres to shorten.

What happens to telomeres during repeated cell divisions?

They generally shorten in many somatic cells.

What is cell senescence?

A state in which a cell permanently stops dividing but remains alive and metabolically active.

What is the Hayflick limit?

The approximate limit on how many times many normal human cells can divide before entering senescence.

Approximately how many divisions is the Hayflick limit?

Roughly 40–60 divisions for many normal human cells.

Why do telomeres shorten during DNA replication?

The ends of linear chromosomes cannot be completely copied by the usual DNA replication machinery.

What is telomerase?

An enzyme that uses its own RNA template to add repetitive DNA sequences to chromosome ends.

How does telomerase work?

It binds to a chromosome end, uses its RNA template to add telomeric repeats, and allows complementary DNA synthesis.

How does telomerase help protect chromosomes?

It helps counteract telomere shortening and reduces the loss of important DNA near chromosome ends.

Why are telomeres important?

They help protect chromosome ends from deterioration and inappropriate DNA repair.

What is the relationship between telomeres and cellular aging?

Telomere shortening can contribute to the signals that cause some cells to enter senescence.

Does telomerase completely prevent cellular aging?

No. Telomere maintenance is only one factor affecting cellular aging.