chromatin

Question 1

what is chromatin?

Answer 1

highly ordered DNA-protein-RNA complex containing RNA, non histone proteins and histones

Question 2

what are the 5 types of histones?

Answer 2

H1
H2A
H2B
H3
H4

Question 3

what is a nucleosome?

Answer 3

the fundamental subunit of chromatin, composed of a little less than 2 turns of DNA wrapped around a histone octamer

Question 4

what were the results of experiments using chromatin and micrococcal nuclease?

Answer 4

a 200 bp fragment seen and multiples of this indicating a regular arrangement of nucleosomes
a 160 bp fragment
a 146 bp fragment which does not decrease in size, this is the dna wound around the nucelosome

Question 5

where does H1 bind?

Answer 5

binds to linker DNA close to the nucleosome

Question 6

what joins adjacent nucleosomes?

Answer 6

linker DNA

Question 7

what is the rough rule about the length of linker DNA?

Answer 7

the more highly transcribed the gene sequence is the longer the linker region

Question 8

what is a solenoid?

Answer 8

nucleosomes are then condensed into a 30nm fiber called a solenoid which loop to give further condensation
a chromosome scaffold is needed

Question 9

what level of condensation is present at interphase?

Answer 9

loosely packaged euchromatin

Question 10

what level of condensation is present at metaphase?

Answer 10

highly condensed heterochromatin

Question 11

what sequences are most commonly methylated?

Answer 11

CpG sequences

Question 12

what effect does methylation have on chromatin condensation?

Answer 12

methylation of cytosine leads to heterochromatin formation

Question 13

what effect does acetylation have on chromatin condensation?

Answer 13

acetylation of lysine on N terminal tail leads to euchromatin condensation

Question 14

what is euchromatin?

Answer 14

chromatin that is transcriptionally active and in an open configuration
H1 binds weakly and RNA polymerase can access the gene
extensive acetylation of core nucleosomal histones
stains poorly

Question 15

what is heterochromatin?

Answer 15

chromatin that is transcriptionally inactive
H1 binds strongly
stains darkly
replicated in S phase

Question 16

what are the two types of heterochromatin?

Answer 16

constitutive

facultative

Question 17

what us constitutive heterochromatin?

Answer 17

permanently irreversibly condensed
genetically inactive in somatic cells
repetitive DNA sequences that could never be transcribed

Question 18

what is facultative heterochromatin?

Answer 18

heterochromatin that can be decondensed
genes may be expressed elsewhere in the genome
can encompass and entire chromosome

Question 19

what is X inactivation?

Answer 19

a mechanism for switching off an entire X chromosome

Question 20

why does X inactivation occur?

Answer 20

to prevent a potentially toxic double dose of X linked genes

Question 21

where is X inactivation initiated?

Answer 21

X-inactivation centre

Question 22

what does the X-inactivation centre code for?

Answer 22

a large non coding RNA

X-inactivation specific transcript

Question 23

how does X-inactivation occur?

Answer 23

XIST coats the whole chromosome
it recruits repressive proteins
the chromosome then becomes highly condensed and transcriptionally inactive

Question 24

what is DNase I?

Answer 24

an enzyme used to degrade DNA
secreted by exocrine cells
found most abundantly in pancreas

Question 25

what is a chromosome scaffold?

Answer 25

comprised of non-histone proteins in low abundance

help maintain the integrity of the chromosome

Question 26

what is the nuclear matrix?

Answer 26

a filamentous structure seen on the inside face of the nuclear membrane
maintains the shape of the nucleus