Mitosis and meiosis

Question 1

Name two core differences between meiosis and mitosis.

Answer 1

1. Mitosis occurs all throughout the body whilst meiosis only occurs in the germ cells.
2. Meiosis requires two divisions to produce four viable haploid daughter cells.

Question 2

What’s the order of the cell cycle

Answer 2

M
G1
S
G2

Question 3

What characterizes the M-phase?

Answer 3

Split into five distinct sub-phases.
1. Prophase : Chromatin condenses.
2. prometaphase: MT spindles are constructed and attach to organelles and chromatids.
3. Metaphase : The chromosomes are organised in the cells’ equatorial plane.
4. Anaphase : MT machinery segregates chromatids / organelles.
5. Telophase + Cytokinesis : New nuclear envelopes are constructed, the cell divides.

Question 4

What’s the importance of MAR-regions on chromosomes? How do they relate to chromosomal territories?

Answer 4

1. MAR regions interact with the nuclear matrix which consists of various proteins and fibers. The interactions can be static or dynamic.
2. During interphase (I), chromosomes are often found in certain locations, referred to as chromosomal territories. Peripherally located chromosomes are often downregulated, and no causality has been established.

Question 5

Describe what SMC proteins are.

Answer 5

SMC = Structural maintenance of chromosomes.

SM proteins consist of two coil domains, each domain hosting two heads (total 4). The heads are bound together via a flex region. One end of the complex is coiled, the other end is bound to some associated protein.

Two examples further discussed in other slides.
1. Cohesins :
2. Condensins

Question 6

What’s the difference(s) between condensin 1 and 2?

Answer 6

Function:
Both condensins work by looping around chromatids and compressing themselves upon binding ATP –> Chromatin condensation.

Differences:
1. Condensin 1 is present in the cytoplasm and can thus only act on chromatids after the nuclear envelope has been broken (telophase in the metaphase).

2. Condensin 2 is present inside the nucleus.

Question 7

What are cohesins?

Answer 7

Function: Cohesins hold chromatids (!!) together during the metaphase. They may also have a regulatory effect as other proteins cannot act on the domains to which the cohesins bind.

Cohesins are heterodimers of SMC1/3. Condensins stop their action when the anaphase-promoting complex (APC) + CDC20 ubiquitiases the securin-separase complex which frees separase. Separase will in turn degrade the cohesins.

Question 8

What’s the synaptonemal complex?

Answer 8

Condensins hold chromatids together, the synaptonemal complex holds chromosomes together.

Before the chromatids are separated, there will be two copies of each chromosome present at the cell’s equatorial plane. Between the two most central chromatids, zip proteins anchor with their C-terminals and interact with each other via their N-terminals.

For unknown reasons, when you KO homologous recombination proteins, the synaptonemal complex won’t form.

Question 9

Fun question:
a. What fraction of egg cells are aneuploid?
b. What fraction of embryos are spontaneously aborted?

Answer 9

a. 20%
b. 25%