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Flashcards in Genome Deck (35):
1

The human genome contains _ nucleotides packaged into _ chromosomes

3.2 x 10^9
24

2

Diameter of the nucleus

~6 micrometres

3

Chromatin

DNA and its associated proteins

4

What causes DNA to bind to histones

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

5

Nucleosome

-148 bp of DNA wound in sequence around histone octamer

6

Linker DNA

54bp DNA segment that joines nucleosomes together

7

Experimental evidence for nucleosomes

1. 'Beads on a string' appearance of chromatin in EMs
2. Micrococcal nuclease digestion of chromatin

8

What determines nucleosome position on DNA

1. DNA flexibility
2. Nearby bound proteins

9

Nuclear matrix

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

10

Euchromatin

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

11

Heterochromatin

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

12

Chromatin remodelling complexes

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

13

Euchromatin packing ratio

750

14

Heterochromatin packing ratio

20,000 - 50,000

15

Evidence for the association between chromosome decondensation and transcription

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

16

Proteins that make up the histone octamer

H2A
H2B
H3
H4

17

First level of DNA packing

Nucleosomes

18

Second level of DNA packing

30nm fibre

19

30nm fibre

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

20

Two ways the nucleosomes structure is altered to allow DNA access

1. Chromatin-remodelling complexes
2. Modification of histone tails

21

Histone tail modification

-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

22

Heterochromatin is mostly found....

around the centromere and telomeres (mammals)

23

Chromatin assembly factor

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

24

Five histone modification enzymes

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

25

How does histone acetlyation affect DNA?

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

26

Transcriptional activators

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

27

Transcriptional repressors

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

28

Centromere

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

29

Kinetochore

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

30

Replication origin

-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

31

Telomeres

-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

32

Two roles of telomeres

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

33

Two diseases involving abnormal telomeres

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

34

SIR protein chromatin modification

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

35

Three examples where DNA methylation is
associated with transcriptional silencing

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