Genome

Question 1

The human genome contains _ nucleotides packaged into _ chromosomes

Answer 1

3.2 x 10^9

24

Question 2

Diameter of the nucleus

Answer 2

~6 micrometres

Question 3

Chromatin

Answer 3

DNA and its associated proteins

Question 4

What causes DNA to bind to histones

Answer 4

• ionic bonding
• DNA is neg charged due to phosphates
• histones are pos charged due to high lys arg content

Question 5

Nucleosome

Answer 5

-148 bp of DNA wound in sequence around histone octamer

Question 6

Linker DNA

Answer 6

54bp DNA segment that joines nucleosomes together

Question 7

Experimental evidence for nucleosomes

Answer 7

1. ‘Beads on a string’ appearance of chromatin in EMs

2. Micrococcal nuclease digestion of chromatin

Question 8

What determines nucleosome position on DNA

Answer 8

1. DNA flexibility

2. Nearby bound proteins

Question 9

Nuclear matrix

Answer 9

insoluble network of non-histone proteins which forms a scaffold onto which specific DNA sequences attach to form long loops

Question 10

Euchromatin

Answer 10

Loosely packed form of DNA containing genes that are actively transcribed
(92% of genome)

Question 11

Heterochromatin

Answer 11

Very tightly packed DNA that is inaccessible to polymerases, thus is not actively transcribed

Question 12

Chromatin remodelling complexes

Answer 12

Large protein complexes that modify nucleosome structure using ATP, leaving DNA less tightly bound to histone proteins

Question 13

Euchromatin packing ratio

Answer 13

750

Question 14

Heterochromatin packing ratio

Answer 14

20,000 - 50,000

Question 15

Evidence for the association between chromosome decondensation and transcription

Answer 15

When nuclei are digested with DNase I, regions more prone to cleavage are those near actively transcribed genes

Question 16

Proteins that make up the histone octamer

Answer 16

H2A
H2B
H3
H4

Question 17

First level of DNA packing

Answer 17

Nucleosomes

Question 18

Second level of DNA packing

Answer 18

30nm fibre

Question 19

30nm fibre

Answer 19

structure formed when histone H1 pulls the nucleosomes together into a regular repeating array

Question 20

Two ways the nucleosomes structure is altered to allow DNA access

Answer 20

1. Chromatin-remodelling complexes

2. Modification of histone tails

Question 21

Histone tail modification

Answer 21

• acetyl, phosphate or methyl groups are added to the tails
• histone stabilisation isn’t altered but overall 30nm fibre is
• modified histone will recruit and bind different proteins causing different effects

Question 22

Heterochromatin is mostly found….

Answer 22

around the centromere and telomeres (mammals)

Question 23

Chromatin assembly factor

Answer 23

adds the new H3 and H4 tetramers to the newly synthesised DNA

Question 24

Five histone modification enzymes

Answer 24

1. Histone acetyltransferase
2. Histone deacetylase
3. Methyltransferase
4. Kinase
5. Ubiquitin transferase

Question 25

How does histone acetlyation affect DNA?

Answer 25

- modifies the transcription of DNA - HATs make DNA more accessible - HDACs make DNA less accessible

Question 26

Transcriptional activators

Answer 26

- hyperacetylate histones near the activator binding site by interacting with HATs - facilitate promoter-TF interaction

Question 27

Transcriptional repressors

Answer 27

- deacetylate histonesnear the repressor binding site by interacting with HDACs - inhibit promoter-TF interaction

Question 28

Centromere

Answer 28

- Part of DNA linking sister chromatids | - site of assembly of the kinetochore

Question 29

Kinetochore

Answer 29

set of proteins at the centromere that provide a site of attachment for the spindle fibres

Question 30

Replication origin

Answer 30

- First point of opening of the double helix during replication - A-T rich and with unique short repeating sequences that multimeric origin binding proteins recognise

Question 31

Telomeres

Answer 31

- end structures of eukaryotic chromos - G rich and with unique short repeating species-specific sequences (TTAGGG in humans) - forms 15 residue 3'-terminal overhangs which are looped back into stable t-loops

Question 32

Two roles of telomeres

Answer 32

1. Stabilise chromo ends | 2. Prevent loss of genetic info after each replication

Question 33

Two diseases involving abnormal telomeres

Answer 33

1. Down's syndrome: shorter telomeres, accelerated ageing, x3 rate telomere shortening 2. Dyskeratosis congenita (DKC): telomerase mutation, short telomeres in stem cells

Question 34

SIR protein chromatin modification

Answer 34

bind to H3/H4 tails thus deacetylating them and inactivating transcription

Question 35

Three examples where DNA methylation is | associated with transcriptional silencing

Answer 35

1. X-xhromosome inactivation 2. Genomic imprinting 3. Repression of transposable element proliferation