Chapter 14 - DNA Structure and Function

What did Griffith discover (1928)?

The transforming principle—non‑virulent bacteria became virulent when exposed to heat‑killed virulent bacteria.

What did Avery, MacLeod, and McCarty show (1944)?

DNA is the transforming principle responsible for heredity.

What did Hershey & Chase prove (1952)?

DNA (radioactive phosphorus) enters bacterial cells, not protein (radioactive sulfur).

What are Chargaff's Rules?

A=T and G=C; purines pair with pyrimidines.

Which bases are purines?

Adenine and Guanine (double‑ring).

Which bases are pyrimidines?

Cytosine and Thymine (single‑ring).

What did Rosalind Franklin contribute?

X‑ray diffraction images revealing DNA's double helix.

What are the components of a nucleotide?

Deoxyribose sugar, phosphate group, nitrogenous base.

What bonds form the sugar‑phosphate backbone?

Phosphodiester bonds.

What bonds hold nitrogenous bases together?

Hydrogen bonds.

How many hydrogen bonds between A-T?

Two.

How many hydrogen bonds between G-C?

Three.

What does antiparallel mean?

One DNA strand runs 5'→3', the other 3'→5'.

When does DNA replication occur?

During S‑phase.

What is semiconservative replication?

Each daughter DNA has one old strand and one new strand.

Function of helicase?

Unwinds DNA at the replication fork.

Function of topoisomerase?

Relieves overwinding tension ahead of the fork.

Function of primase?

Synthesizes RNA primers.

Function of DNA polymerase III?

Adds nucleotides in the 5'→3' direction.

Function of DNA polymerase I?

Removes RNA primers and replaces them with DNA.

Function of ligase?

Seals gaps between DNA fragments.

Leading vs lagging strand?

Leading is continuous; lagging is discontinuous with Okazaki fragments.

What are Okazaki fragments?

Short DNA segments on the lagging strand.

What is proofreading?

DNA polymerase corrects errors during replication.

What is mismatch repair?

Enzymes fix incorrectly paired bases after replication.

What is nucleotide excision repair?

Removes damaged DNA (e.g., thymine dimers).

What are telomeres?

Repetitive non‑coding ends of chromosomes that protect genes.

What does telomerase do?

Extends telomeres in eukaryotic cells.

Prokaryotic vs eukaryotic chromosome shape?

Prokaryotes - circular; Eukaryotes - linear.

Number of origins of replication?

Prokaryotes - one; Eukaryotes - many.

Presence of histones?

Prokaryotes - none; Eukaryotes - yes.

Presence of telomeres?

Only in eukaryotes.