Lecture 4 - Eukaryotic Cell Division

Question 1

Functions of microtubules

Answer 1

1. Form part of the cytoskeleton
2. Transport vesicles through the cytoplasm
3. Form the mitotic spindle

Question 2

What can cause the MTs to shrink and collapse?

Answer 2

If the rate of GTP hydrolysis exceeds that of the addition of new GTP-bound subunits then the MT will shrink and collapse from the plus end (the ‘GTP cap’ end)

Question 3

What does the centrosome consist of?

Answer 3

A pair of centrioles embedded in the associated pericentriolar matrix

Question 4

Explain growth and duplication of centrosomes

Answer 4

1. During G1 phase the two centrioles separate by a few microM.
2. During S phase a daughter centriole begins to grow near the base of each mother centriole at a right angle to it. Growth of the daughter is completed in G2.

Question 5

Explain migration of duplicated centrosomes

Answer 5

1. MPF (cyclin B-Cdk1) complexes can phosphorylate kinesin motor proteins early on in M phase and stimulate them to drive centrosome separation
2. The new pairs now migrate to opposite poles of the cell
3. Each individual centrosome now begins to assemble its own array of microtubules (growing at their - end which is located at the centrosome)

Question 6

What is MT flux?

Answer 6

The constant loss of tubulin subunits at the - end and addition at the + end

Question 7

What drives separated sister chromatids towards opposite poles?

Answer 7

1. MT depolymerisation from the + end (MT flux)

2. Motor proteins

Question 8

What are the variable levels of condensation exhibited in Interphase chromosomes?

Answer 8

1. Euchromatin = largely decondensed and potentially active in gene expression. Contains the majority of genes.
2. Heterochromatin: highly condensed and inactive in gene expression. Almost exclusively consists of highly repetitive DNA sequences

Question 9

What is the chromosome territory?

Answer 9

The discrete space inside a nucleus that they occupy

Question 10

Explain the process of DNA condensation

Answer 10

INTERPHASE
1. DNA coiling around a protein core to form a nucleosome. 11nm wide
2. These are then coiled with 6 nucleosomes per turn to form a thicker fibre. Diameter is 30nm
3. This coiled coil is then gathered into bundles approximately 300nm wide. Certain regions interact with a scaffold.
MITOSIS
4. They are then condensed and are packaged into chromosomes to form arms approximately 700nm wide

Question 11

Chromatin

Answer 11

Term used to describe the specific association of DNA and protein that constitutes a functional eukaryotic chromosome

Question 12

What are the proteins that make up the core of a nucleosome?

Answer 12

Histones 2A, 2B, 3, and 4.

Question 13

Explain histone tails

Answer 13

Histone proteins have long tails that are very rich in positively charged amino acids. They can interact with the -vely charged phosphate backbone of DNA

Question 14

What does Histone H1 do?

Answer 14

It enables DNA compaction by neutralizing DNA’s negative charge

Question 15

Which chromatin is not accessible for expression?

Answer 15

Condensed chromatin

Question 16

Which chromatin is accessible for expression?

Answer 16

Decondensed