Lecture 19

Question 1

How many chromosomes do you have?

Answer 1

46 chromosomes

Question 2

How many different types of chromosomes does a human male have? A human female?

Answer 2

24 for males and 23 for females.

Question 3

How many chromosomes did you inherit from your father? From your mother?

Answer 3

23 from each parent.

Question 4

What are the major landmarks on a chromosome (there are two) and what do they do?

Answer 4

Centromere, Telomere

Question 5

How does the packing of a chromosomes change at different stages of the cell cycle?

Answer 5

Middle of mitosis - most compact (condense chromosomes)
G1 - loose
S -

Question 6

What is a DNA supercoil?

Answer 6

DNA supercoil is the coiling of DNA due to tension.

Question 7

Is DNA in the cell generally underwound or overwound? What does this mean?

Answer 7

Generally unwound, it causes tension in the DNA. In order to relieve the tension, the DNA would supercoil.

Question 8

How does this effect the running of a circular DNA on an agarose gel?

Answer 8

It is slower to move through the agarose gel as it is larger and harder to move through the holes in the gel.

Question 9

What does a topoisomerase enzyme do to the structure (base pairs) of DNA? To the shape?
To the size?

Answer 9

It helps unwind the structure of DNA

Question 10

What is the general mechanism (no detail required) by which Type II topoisomerase acts?

Answer 10

It relieves the tension in the DNA by passing an intact DNA through another broken DNA molecule and then mends the broken one.

Question 11

What is a nucleosome?

Answer 11

A nucleosome is a subunit of chromatin. It contains less than two turns of DNA wrapped around an eight set histone. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4 and 164 bases.

Question 12

What is chromatin?

Answer 12

Chromatin is a set of nucleosomes.

Question 13

Where are the 5 types of histones found in chromatin?

Answer 13

H3, H4, H2B, H2A and H1

Question 14

What is the (overall) structure of a

nucleosome?

Answer 14

~2 Turns of DNA around histone.

Question 15

What is important about the histone N‐terminal tails?

Answer 15

The tail (N-terminal) protrudes from the nucleosomes, accessible for modification and interacts with neighbouring histones. Modification (covalent) changes packing of chromatins.

Question 16

Name three types of histone modification that can be used to regulate chromatin packing.

Answer 16

Methylation, Acetylation and phosphorylation.

Question 17

What is meant by the ‘histone code’?

Answer 17

Pattern of modification has information and can be read by proteins called code reader complexes. These proteins then interacts with other proteins to carry out different functions.

Question 18

What is a 30nm fiber?

Answer 18

A 30nm fiber is a condensed chromatin produced as a result of acetylation of the histone tails. (HDAC - histone deacetylase). Not accessed by DNA binding proteins. (100 fold)

Question 19

What is the difference in chromatin packing between a part of the DNA that is not being accessed by proteins, and a part of the DNA that is being actively used (eg. actively transcribed)?

Answer 19

The chromatin packing between a part of the DNA that is not accessed is more compact than parts of DNA that is actively used.

Question 20

Name three examples of changes to the histones that bring about changes in the chromatin packing.

Answer 20

Chromatin remodelling complex can loosen up the compaction, histones can be removed or histone can be modified to loosen up the compaction

Question 21

What DNA base is chemically modified in the regulation of gene expression? Is gene expression increased or decreased because of it?

Answer 21

Cytosine is chemically modified (methylation) to 5 methyl cytosine. It does not affect the base pairing. (Important function in promoter regions - eukaryotes; form clusters called CpG island)