Lecture 2 - Chromsomes & Chromatin

Question 1

how is the genome broken up in eukaryotes?

Answer 1

in eukaryotes, the genome is broken up into long, linear, double-stranded structures called chromosomes

human genomes have 24 chromosomes [22 autosomes and 2 sex chromosomes]

human cells are diploid (2n = 46)

Question 2

eukaryote genomes are often larger and more complex because:

Answer 2

• there are more genes that contain introns
• there is more regulation (cell type specific expression)
• more ‘junk’ DNA - repeated sequences; transposons, simple repeat and duplications

Question 3

what is the function of telomeres?

Answer 3

telomeres are there to stabilise the ends of chromosomes

Question 4

replication origins:

Answer 4

where the duplication process of the DNA starts

Question 5

function of centromere:

Answer 5

to segregate the sister chromosomes

Question 6

sister chromosomes are held together until…

Answer 6

… chromosome segregation during mitosis

Question 7

what does a replicated chromosome consist of?

Answer 7

a replicated chromosome consists of two sister chromatids

Question 8

organisation of human centromeres:

Answer 8

alpha satellite DNA sequences repeated thousands of times

Question 9

the kinetochore consists of:

Answer 9

an inner and outer plate, formed by a set of kinetochore proteins

Question 10

how does chromosome organisation change during the cell cycle?

Answer 10

in interphase, the chromosomes are distributed throughout the nucleoplasm whilst during mitosis the chromosomes condense and individual chromosomes can be seen with a light microscope

Question 11

when are chromosomes organised during the cell cycle and what does this organisation do?

Answer 11

chromosomes are organised during the interphase, where they are not visible but still partially resolved from one another across the nucleus (chromosome territories), this arrangement minimises tangles between chromosomes and helps condensation and segregation during mitosis

Question 12

the Rabl conformation:

Answer 12

some cell types, especially in plants, chromosomes adopt the Rabl configuration with the centromeres clustered at one end, and the telomeres abutting the nuclear envelope

Question 13

up to what extent can mitotic chromosomes be more compact than the length of DNA?

Answer 13

mitotic chromosomes can be up to 10,000x more compact than the length of DNA

Question 14

what is the fundamental unit of chromatin?

Answer 14

the fundamental unit of chromatin is the nucleosome

Question 15

DNA isolated from an interphase nucleus appears as:

Answer 15

30nm thick fibre

Question 16

how are individual nucleosomes arranged on the DNA?

Answer 16

individual nucleosomes are arranged on the DNA like beads on a string

Question 17

how is DNA compacted?

Answer 17

DNA is compacted by association with proteins - chromatin (association of protein + DNA)

Question 18

what does the nucleosome core particle consist of?

Answer 18

the nucleosome core particle consists of 147 base pairs of DNA wound around a protein core which is composed of highly basic histone proteins

Question 19

the octomeric histone core contains:

Answer 19

two molecules of each of the four types of histones: H2A, H2B, H3 & H4

Question 20

how much and how does DNA war around the histone octometer?

Answer 20

147bps of DNA is wrapped around the octamer in a sequence non-specific manner

Question 21

how does histone H1 bind?

Answer 21

histone H1 binds to both the DNA and nucleosome in the area where the DNA exits the nucleosome - stabilising the chromatin fibre

Question 22

function of histone H1:

Answer 22

an additional histone, histone H1 arranged nucleosomes into the 30nm fibre

Question 23

how is the 30nm chromatin fibre further condensed during mitosis?

Answer 23

the 30nm fibre is further condensed at two additional levels to achieve the lacking that is observed in mitotic chromosomes

Question 24

chromatin in the nucleus appears in two forms:

Answer 24

a diffuse & open configuration called euchromatin and a condensed electron dense form called heterochromain

Question 25

euchromatin:

Answer 25

transcriptionally active - relatively loose nucleosome arrangement

Question 26

heterochromatin:

Answer 26

is transcriptionally inactive associated chromatin conformations that are highly compacted - inactive genes, long repeat sequence arrays or regions adjacent to repeats

Question 27

interphase chromatin is very dynamic

• decondensed or open chromatin (= euchromatin) is associated with:
• whilst closed condensed chromatin (=heterochromatin) is associated with:

Answer 27

transcriptionally active regions of the genome

transcriptionally repressed regions of the genome

Question 28

histone remodellers:

Answer 28

they are ATP-dependent chromatin-remodelling complex proteins which, through ATP hydrolysis, catalyse the sliding of nucleosomes

Question 29

how does chromosome condensation exist along the length of a chromosome?

Answer 29

chromatin condensation can vary along the length of the chromosome

Question 30

describe the DNA structure in telomeres and centromeres:

Answer 30

telomeres and centromeres are typically heterochromatic, these regions also being gene poor

Question 31

describe the DNA structure in gene rich regions of the chromosome:

Answer 31

the gene rich regions are euchromatic

Question 32

what do regional modifications produce in chromosomes?

Answer 32

regional modifications produce blocks of different degrees of compaction along the chromosome

Question 33

what does each histone also contain and what is significant about it?

Answer 33

each histone has a ‘tail’ that extends out of the nucleosome core - these tails are significant as they can be modified via methylation and acylation to influence chromatin structure and thus gene expression

Question 34

how can histone modifications alter gene expression:

Answer 34

Histone modifications can change overall chromatin structure by either directly affecting how nucleosomes pack together or by promoting or inhibiting the binding of proteins that can alter chromatin organisation

Question 35

how can heterochromatic chromatin modifications spread and what does this cause?

Answer 35

heterochromatic chromatin modifications can spread from the telomere into telomeric adjacent genes - frequently silencing them (as seen in practical though how when you move the red pigment gene close to the telomeres in yeast, this gene becomes silent and produces a more pink/white colouration)

Question 36

X-chromosome inhibition:

Answer 36

to ensure equal expression in XY & XX individuals the extra X chromosome is transcriptionally silenced in early development - this silenced heterochromatic state is then inherited throughout the rest of development