Chapter 11

Question 1

What did Griffith’s experiment show?

Answer 1

A “transforming principle” could transfer genetic material from dead S bacteria to live R bacteria, making them virulent.

Question 2

What did Avery, MacLeod & McCarty conclude?

Answer 2

DNA is the molecule that causes transformation; destroying DNA prevented it, proving DNA is genetic material.

Question 3

What are Chargaff’s rules?

Answer 3

%A = %T and %G = %C; base composition varies among species.

Question 4

What did Rosalind Franklin’s X-ray work reveal?

Answer 4

DNA is a helical molecule with two strands and specific spacing between bases.

Question 5

What did Watson & Crick propose?

Answer 5

The double-helix model: antiparallel sugar-phosphate backbones with complementary base pairing.

Question 6

What are the three parts of a nucleotide?

Answer 6

Phosphate group, deoxyribose sugar, nitrogenous base.

Question 7

What bases are in DNA?

Answer 7

Adenine, Thymine, Cytosine, Guanine.

Question 8

What bases pair together and how?

Answer 8

A–T (2 H-bonds); G–C (3 H-bonds) via hydrogen bonding.

Question 9

What are phosphodiester bonds?

Answer 9

Covalent bonds between the 3′ OH of one nucleotide and 5′ phosphate of the next, forming the backbone.

Question 10

What does ‘antiparallel’ mean in DNA?

Answer 10

One strand runs 5′→3′, the other 3′→5′.

Question 11

What’s at the 5′ and 3′ ends?

Answer 11

5′ = phosphate group; 3′ = hydroxyl (-OH) group.

Question 12

Why is polarity important in replication?

Answer 12

DNA polymerase adds nucleotides only to the 3′ end, so synthesis is 5′→3′.

Question 13

How is information stored in DNA?

Answer 13

In the linear sequence of nucleotide bases.

Question 14

What does ‘semiconservative replication’ mean?

Answer 14

Each new DNA molecule has one old strand + one new strand.

Question 15

Which experiment proved semiconservative replication?

Answer 15

Meselson–Stahl experiment using ¹⁵N and ¹⁴N E. coli DNA.

Question 16

Where does replication start?

Answer 16

At an origin of replication (ori) where the helix unwinds.

Question 17

What is a replication fork?

Answer 17

The Y-shaped region where parental strands separate and new DNA is made.

Question 18

Function of helicase?

Answer 18

Unwinds the double helix by breaking hydrogen bonds.

Question 19

Function of topoisomerase?

Answer 19

Relieves supercoiling by cutting, swiveling, and rejoining DNA.

Question 20

Function of single-strand binding proteins?

Answer 20

Keep template strands separated and stable.

Question 21

Function of primase?

Answer 21

Builds short RNA primers to start DNA synthesis.

Question 22

Function of DNA polymerase III?

Answer 22

Main enzyme that adds nucleotides 5′→3′ to the growing strand.

Question 23

Function of DNA polymerase I?

Answer 23

Removes RNA primers and replaces them with DNA.

Question 24

Function of DNA ligase?

Answer 24

Seals sugar-phosphate gaps between fragments.

Question 25

What happens on the leading strand?

Answer 25

Continuous DNA synthesis toward the fork using one primer.

Question 26

What happens on the lagging strand?

Answer 26

Discontinuous synthesis away from the fork, forming Okazaki fragments, each needing its own primer.

Question 27

How is replication kept accurate?

Answer 27

1) Correct base pairing (A–T, G–C) 2) DNA polymerase active-site selectivity 3) Proofreading (3′→5′ exonuclease activity).

Question 28

What are the three levels of DNA compaction?

Answer 28

1) DNA wrapping around histones → nucleosomes 2) 30-nm fiber (zigzag of nucleosomes) 3) Radial loop domains anchored to nuclear matrix.

Question 29

What is heterochromatin vs euchromatin?

Answer 29

Heterochromatin = tightly packed, inactive DNA; Euchromatin = loosely packed, active DNA.

Question 30

How compact are mitotic chromosomes compared to relaxed DNA?

Answer 30

About 10,000 times more compact during metaphase.