MBB 267 Week 4: Barnes 6

Question 1

What is a centromere?

Answer 1

the specialized chromosomal region upon which kinetochores assemble and direct the equal segregation of chromosomes during mitosis and meiosis
(doesn’t have to be in the centre of the chromosome)

Question 2

What is a general centromere like?

Answer 2

Chromatin structure is conserved – and maintained through the cell cycle
-but specific DNA sequences are not conserved throughout the eukaryotes.

Question 3

What is the structure of a yeast centromere?

Answer 3

Very highly conserved in regions I and III.
-Very AT-rich region II.
-Only 120 bp suffices to direct microtubule
attachment and mitotic segregation!

Question 4

What is the structure of a human centromere?

Answer 4

Alphoid satellite DNA: AT- rich sequences. Each repeat is 171bp.

• Higher-order structure of several repeats with slightly divergent sequences – forms a larger repeating unit.
• In tandem arrays at the centromeres of all human chromosomes.

Question 5

What is Alphoid satellite DNA?

Answer 5

a family of different tandems repeated in a particular patterns which is found in the genome.

Question 6

What is a nucleosome?

Answer 6

A nucleosome is the basic structural unit of DNA packaging in eukaryotes, A nucleosome is a section of DNA that is wrapped around a core of proteins.

Question 7

What is the structure of a nucleosome?

Answer 7

A standard nucleosome comprises eight histones – 2 of them are called H3.

• CENP-A (centromere protein A) replaces H3 at the eukaryotic centromere.
• This specialised histone marks the nucleosome as different to dictate kinetochore binding – recognised by the kinetochore proteins.

Question 8

What is the structure of a nucleosome in a human?

Answer 8

This is also how rhe inactive heterochromatin is cnosereved

• At human centromeres – additional modified nucleosomes, eg
• H3 is methylated: H3K4me2
• Around the centromere – repression of transcription:“pericentric heterochromatin”.
• Specific methylation of histones at these nucleosomes.

Question 9

What is a kinetochore?

Answer 9

A complex of many proteins, where the microtubles attach to the centromere.

Question 10

Function of kinetochores?

Answer 10

The kinetochore is an elaborate protein complex that

• Recognises centromeric epigenetic markers such as alternative histones and methylation
• Attaches the centromere to the microtubules, allowing segregation at mitosis.
• -One important part of the kinetochore is the Ndc80 complex.

Question 11

What is a holocentric chromosome?

Answer 11

In C. elegans, cenH3 histones are distributed throughout the chromosome and attach to kinetochore “holocentric” = these attachments are along the whole chromosome

• They dont have a centromere
• So the whole chromosome is seperated.