Histones and DNA packaging

Question 1

What size is one complete turn of DNA?

Answer 1

10 bases

3.4nm long

Question 2

How much DNA is there in a human diploid cell?

Answer 2

~2m

Question 3

What is the composition of chromatin?

Answer 3

DNA
RNA
Proteins

Question 4

What is chromatin?

Answer 4

What chromosomes are made of

Question 5

What are histones?

Answer 5

Proteins found in chromatin
They are the most common nuclear protein
Account for almost half of nuclear protein

Question 6

Describe the basic features of histones

Answer 6

Small molecular weight

Positively charged

Question 7

Name the core histones

Answer 7

H2A
H2B
H3
H4

Question 8

Name the linker histone

Answer 8

H1

Question 9

What is the width of double stranded DNA?

Answer 9

2nm

Question 10

What is the basic unit of chromatin?

Answer 10

A nucleosome

Question 11

How much DNA is in one nucleosome?

Answer 11

200bp

Question 12

How many turns of DNA make one nucleosome?

Answer 12

1.8 (2)

Question 13

List the components of a nucleosome

Answer 13

200bp of DNA
2x H2A
2x H2B
2x H3
2x H4
H1

Question 14

How do H3 and H4 interact?

Answer 14

They form a heterodimer called the histone handshake motif

Question 15

What is formed from the H3/H4 heterodimer?

Answer 15

A tetramer that is horse shoe like in structure

Question 16

How do H2A and H2B interact?

Answer 16

They form a dimer

Question 17

Where does the H2A/H2B dimer go?

Answer 17

One goes below and one goes above the H3/H4 tetramer

Question 18

What would happen if a histone was altered?

Answer 18

The DNA packaging around the octamer would be altered

Question 19

Name four variations of H2A

Answer 19

H2A.Z
macroH2A
H2A.Bbd
H2A.X

Question 20

Compare H2A and H2A.Z

Answer 20

H2A.Z has 60% identity to H2A
It alters the stability between H2A and H2B
Alters interaction between the dimer and tetramer
Alters the canonical nucleosome
Associated with transcriptionally active chromatin

Question 21

What is the role of histone variants?

Answer 21

Differentiates chromatin at centromeres, active genes and heterochromatin

Question 22

What does H3.3 do?

Answer 22

It marks actively transcribed loci by replication independent nucleosome activity

Question 23

What is level two of chromatin structure?

Answer 23

The 10nm fibre
“beads on a string”
Packing ratio: 6-7

Question 24

What is level three of chromatin structure?

Answer 24

The 30nm solenoid
Further coiling of the 10nm fibre
Packing ratio: ~40

Question 25

What is H1s role in the solenoid?

Answer 25

It forms the centre of the rings of the 10nm fibre

Question 26

How many nucleosomes make a 30nm solenoid?

Answer 26

6

Question 27

What is the fourth level of chromatin structure?

Answer 27

Loops of 60-100kb of DNA tethered by non histone scaffold proteins Packing ratio: 680

Question 28

What is level five of chromatin structure?

Answer 28

700nM fibre Coiled coil Condensed fibre Packing ratio: 10^4

Question 29

What is level six of chromatin structure?

Answer 29

A metaphase chromosome

Question 30

What is the packing ratio?

Answer 30

How much smaller the folded version is than the extended length of DNA

Question 31

Describe the metaphase chromosome

Answer 31

Telomeres at each end A centromere Sections of heterochromatin and euchromatin

Question 32

Where is the centromere?

Answer 32

At the primary constriction

Question 33

What is the purpose of the centromere?

Answer 33

Site that spindles attach to | Chromosome segregation occurs here

Question 34

What are the two types of chromatin?

Answer 34

Heterochromatin | Euchromatin

Question 35

Describe heterochromatin

Answer 35

Highly condensed Transcriptionally inactive Repetitive DNA sequences

Question 36

What is constitutive heterochromatin?

Answer 36

All cells of a given species package the same regions of DNA into constitutive heterochromatin Genes contained within constitutive heterochromatin will be poorly expressed

Question 37

What is facultative heterochromatin?

Answer 37

DNA packaged into facultative heterochromatin is not consistent throughout the cells of a species Facultative heterochromatin is regulated and associated with morphogenesis and differentiation In different cells it may be packaged in euchromatin

Question 38

When is heterochromatin replicated?

Answer 38

Late in the S phase

Question 39

Describe euchromatin

Answer 39

More open configuration Replicated early in S phase Contains both transcriptionally active and inactive genes