Genomes II - Lecture 8

Question 1

How were nucleosomes discovered?

Answer 1

Digesting of chromatin with endonucleases

Question 2

What is DNA extracted from the nucleus called?

Answer 2

Chromatin

Question 3

What is chromatin

Answer 3

A DNA-protein complex with proteins spaced at regular intervals along the DNA (200bp or multiples of 200bp)

Question 4

What are the proteins in chromatin called?

Answer 4

Histones

Question 5

What are nucleosomes made up of?

Answer 5

○ An octamer
○ Two each of histones H2A, H2B, H3, H4

Question 6

What is histone H1?

Answer 6

○ Linker histone
○ Brings linker DNA closer together
○ Attached outside the nucleosome

Question 7

What is a chromatosome?

Answer 7

Nucleosome + DNA + linker histone

Question 8

What is 30nm chromatin fibre?

Answer 8

○ Nucleosomes are stacked only with non-neighbours
○ Zig-zag back and forth connected by a straight DNA linker

Question 9

How is the chromatin fibre formed and stabilised?

Answer 9

○ Formation: Interaction between H4 acidic N-terminal tail and H2A/H2B acidic patch
○ Stabilised: Off-axis asymmetric binding of H1

Question 10

What is the importance of histones/nucleosomes?

Answer 10

○ DNA packaging into nucleosomes is dynamic
○ Nucleosomes detach or shift to allow transcription of DNA
○ Nucleosomes must detach for replication of DNA
○ Nucleosome presence or modification can control gene expression

Question 11

What is euchromatin?

Answer 11

Contains active genes as 30nm fibres

Question 12

What is nucleolus?

Answer 12

Contains ribosomal DNA genes and site of ribosome biogensis

Question 13

What is heterchromatin?

Answer 13

Contains inactive genes and more densely packed

Question 14

What is constitutive heterchromatin?

Answer 14

Contains DNA that is always tightly packed in all cells e.g. X chromosome in females

Question 15

What is facultative heterchromatin?

Answer 15

Contains DNA that is tightly packed only in some cells

Question 16

What is DNA attached to in euchromatin?

Answer 16

○ Nuclear matrix (inside nucleus) and nuclear lamina (around edge of nucleus) so the DNA does not get tangled up
○ Maintains the shape of the nucleus

Question 17

What are some examples of nuclear matrix and nuclear lamina associated diseases?

Answer 17

○ Progeria (disfigured nuclei) - premature ageing
○ Down’s Syndrome
○ Huntington disease

Question 18

What is the highest level of packaging and where is it found?

Answer 18

○ Metaphase chromosome
○ Only found in dividing cells

Question 19

What are the centromeres?

Answer 19

○ Holds the daughter chromsomes together
○ Contains histone CENP-A instead of H3
○ Attachment points for microtubles

Question 20

What do telomeres do?

Answer 20

Protects the ends:
○ From exonuclease attack
○ From being mistaken for chromosomes breaks and joined together by DNA repair mechanisms

Question 21

What are features of a karyogram?

Answer 21

○ All bands have names used for mapping positions of genes
○ Centromeres have variable positions
○ Constitutive heterchromatin has a darker stain
○ Ribosomal DNA repeats together