Chromosome Structure and Chromatin

Question 1

What is the spacing between base pairs on DNA?

Answer 1

3.4 Angstroms

Question 2

How many base pairs are there in the human genome?

Answer 2

3.2 billion

Question 3

How many protein coding genes are present in the human genome?

Answer 3

~ 22000

Question 4

What is the simplest level of DNA organisation?

Answer 4

Chromatin in a double stranded helix.

Question 5

What makes up the nucleosome?

Answer 5

DNA complexed with histones

Question 6

What is the chromatasome?

Answer 6

The nucleosome and H1 histone protein

Question 7

How is the nucleosome arranged?

Answer 7

It is folded up to produce a 30nm fibre with a loop of length 300 nm. The 300nm fibre is compressed to 250nm and becomes a chromatid.

Question 8

How are chromosomes arranged at metaphase?

Answer 8

Highly condensed.

Question 9

How are chromosomes arranged in interphase?

Answer 9

Chromosomes are less distinct with variable levels of compaction depending on activity and function of particular regions.

Question 10

What is euchromatin?

Answer 10

Chromatin that is highly de-compacted and is potentially active active in gene expression.

Question 11

What is heterochromatin?

Answer 11

Highly compacted chromatin that’s transcriptionally inactive.

Question 12

What is the function of heterochromatin that contains very few genes?

Answer 12

To form key structures such as centromeres or telomeres.

Question 13

What do the beads of the beads on a string analogy represent?

Answer 13

Nucleosomes

Question 14

What does the string of the beads on a string analogy represent?

Answer 14

DNA

Question 15

What are the histones in a nucleosome?

Answer 15

H2A, H2B, H3 and H4

Question 16

What is condensation?

Answer 16

A series of higher order structures adopted by chromatin.

Question 17

What are territories?

Answer 17

Areas that individual chromosomes concentrate in.

Question 18

Where does euchromatin concentrate?

Answer 18

In the middle of the nucleus

Question 19

Where does heterochromatin concentrate?

Answer 19

In the periphery of the nucleus.

Question 20

Why is regulating DNA accessibility helpful?

Answer 20

As it regulates gene expression.

Question 21

Why is gene expression regulated?

Answer 21

To ensure correct timing and location of protein production.

Question 22

What is chromatin remodelling?

Answer 22

Alteration of chromatin structures to increase DNA accessibility.

Question 23

What is chromatin remodelling required for?

Answer 23

Transcription.

Question 24

What is the function of histone modifying proteins?

Answer 24

To modify chromatin structure and ATP dependent chromatin remodelling complexes to permit DNA replication and transcription.

Question 25

What is the charge on a histone?

Answer 25

It has a positively charged tail at the N terminus.

Question 26

How do histone acetyltransferases change the chemical nature of histones?

Answer 26

By adding acetyl groups to lysine residues on histones making them neutral.

Question 27

What is the function of HDACs?

Answer 27

To remove acetyl groups from lysine residues.

Question 28

How does the function of HATs activate transcription?

Answer 28

By opening the nucleosome which increases DNA accessibility, facilitating transcription factor binding and recruiting chromatin remodellers.

Question 29

How are HDACs involved in cancer?

Answer 29

Some cancers are caused by tumour suppressors that have been excessively deacetylated.

Question 30

What are the mechanisms used to increase local access to DNA?

Answer 30

- nucleosome sliding - nucleosome displacement - partial histone displacement - replacement of octamer subunits with histone variants.

Question 31

What is epigenetics?

Answer 31

The study of changes in gene expression prompted without changes in the DNA sequence.

Question 32

What catalyses methylation of cytosine?

Answer 32

DNA methyltransferase

Question 33

What catalyses the removal of a methyl group from cytosine?

Answer 33

Demethylase

Question 34

Why are heavily methylated genes inactive?

Answer 34

As they attract proteins that repress transcription.