G6

Question 1

What is epigenetics

Answer 1

heritable changes in gene expression which persist through cell division and arent caused by changes in nt sequence

Question 2

what are two examples of epigentic modifications

Answer 2

histone modifications - modifies chromatin to reguulate genes
DNA modification - methylation to repress expression of particular gnees.

Question 3

what is downsyndrome an example of

Answer 3

trisomy of chromosome 21

Question 4

what bases are methylated in mammals and prokaryotes

Answer 4

cs in mamamls

some As in prokartotes

Question 5

what is chromatin

Answer 5

compaction of DNA via interactions with proteins, histones, forms chromatin, template for epigenetic modifications.

Question 6

what is euhromatin

Answer 6

actively transcribed extended structure

Question 7

what is heterochromatin

Answer 7

often highly repetative, condensed adn not transcribed, near centromeres and teleomeres (not 100% true as eg. y chromosome contains big chunks of heterochromatin

Question 8

Where is DNA methylated in euks

Answer 8

methyl to fifth carbon of cytosine ring to form 5-methyl cytosine, mostly on CpG dinucleotide sites

Question 9

how can methylation be potentially harmful if there is an elevated mutational rate

Answer 9

as CG >TG which could form CGA -> TGA which is a stop codon.

Question 10

what percent of CPGS are methylated in mice

Answer 10

4%

Question 11

what are CpG islands

Answer 11

regions with high frequency of CpG sites. - uneven distribution - clusters of CpGs (typically unmethylated) mark housekeeping active genes.

Question 12

what is kind of mark is CpG methylation

Answer 12

generally repressive

reducing DNA binding of many proteins, binding site for methyl binding proteins, (MBD domain containing MeCP2)

Question 13

what roles does DNA methylation play in development?

Answer 13

reg gene expression
imprinting
x inactivation
differentation
aging and cancer
defence against transposons, viruses - downreg of expression of endogenous retrovirsues
counteract recombination of repetative DNA
important in chromosome segregation

Question 14

What are the 5 DNA methytransferances

Answer 14

Dnmt1: maintainance methylation - counteracts dilution of methyl groups
Dnmt2- very little activity
Dnmt3a - de novo methylation: imprinting pericentric region satellites
Dnmt3b: de novo methylation: CpG islands and X chromosome
DnmttL: no enzymatic activity, reg subunit: imprinting

Question 15

what id demethylation

Answer 15

direct removal of methyl group.

- via BER or NER

Question 16

what is the intermediate before 5-methyl cytosine removal

Answer 16

5-hydroxy methyl cytosine

Question 17

What is the CXXC domain

Answer 17

Binuclear Zn chelating domain found in several chromatin association proteins, which can discriminate between unmethyalted and methylated CpGs

Question 18

Describe a nucelosome

Answer 18

147bp of DNA wrapped around an octamer of histones H2a, H2B, H3 and H4
assemebled as H3/H4 tetramere followed by addition of two H2A/H2B dimers.

Question 19

where do you get most modification of histones

Answer 19

on linkers H1

- high levels of H1 = increasingly repressive

Question 20

when is paternal imprinting often involved?

Answer 20

in the early stages of embryogenesis - maximises growth rate of embryo and resources it takes from the mother.

Question 21

what is hypomethylation

Answer 21

inhibition of methylation

Question 22

Why are tortoise shell cats always female

Answer 22

because ginger and black colours are on one x chromosome each, inactivation continues in descendent cells to give mottled pattern.

Question 23

are all X linked genes subject to X inactivation on Xi?

Answer 23

no some genes are still active.

Question 24

where does x inactivation occur?

Answer 24

somatic cells

but embryonic cells are XaXa