Lecture 1: Cell cycle 1

Question 1

Learning objectives

Answer 1

• Describe the key events in each of the phases of the eukaryotic cell cycle, including quiescence
• understand the cytoskeletal changes underlying M phase
• give a detailed description of the regulation of cell cycle progression including the roles cyclins, cyclin-dependent kinases, the Retinoblastoma protein, E2F and the anaphase promoting complex
• understand how growth factors can regulate the restriction point.

Question 2

What are the traditional ideas of the cell cycle, as known since the 1800s (ie the order of phases)?

Answer 2

Interphase (most of the time), prophase, metaphase, anaphase, telophase

Question 3

How often does the typical mammalian cell divide?

Answer 3

Every 24 hours

Question 4

How much of the dividing time is spend in interphase and how much is spent in mitosis?

Answer 4

Interphase = 23 hours
Mitosis = 1 hour

Question 5

What are the modern ideas of the cell cycle?

Answer 5

G1, S, G2 = Interphase
M = Mitosis and cytokinesis

Lengths:
G1 - 12 hours
S - 6 hours
G2 - 5 hours
M - 1 hour

Question 6

What happens during G1?

Answer 6

During G1 phase the cell takes the decision as to whether conditions are favourable for division. The cell assesses the conditions/surroundings: space, nutrients, growth factors, etc.
Once the decision has been made, cannot stop cell division. If decision wrong, this can lead to diseases like cancer.
G = Gap or Growth

Question 7

How many base pairs are there in the human genome?

Answer 7

4 x 10^9

Question 8

What happens during S phase?

Answer 8

DNA synthesis. The genome is duplicated.
Required:
- DNA polymerase (very accurate, very few errors)
- large amounts of nucleotides
The duplication must be accurate. A mistake/mutation can lead to cancer.

Question 9

What does S phase result in?

Answer 9

Sister chromatids held together by cohesin complexes. In the middle of the sister chromatids is the centromere, which is a constriction where there is less DNA and is important is segregating the chromosomes in the mitotic phase.

Question 10

What happens in prophase?

Answer 10

Chromosome condensation. The sister chromatids are condensed. Normally, the chromosomes should not be condensed, because gene transcription can’t take place when the chromosomes are condensed as the transcription factors and RNA polymerase can not access the genes.

The spindle begins to form, co-ordinated by the centrosomes.

Question 11

What is the centrosome?

Answer 11

The micro-tubule organising centre. In the centre of the centrosome, there are two centrioles, which are at right angles to each other. The surface of the centrosome is covered in nucleating sites (gamma-tubulin ring complexes), from which microtubules grow.

Question 12

What is the spindle made of?

Answer 12

Microtubules with emanate from the centrosome at each pole of the cell.
There are three types of microtubule:
- astral microtubules: attach the spindle to the cell membrane, which elongates/stretches the membrane
- kinetochore microtubules: chromosomes are attached to these from both directions (the microtubules pull the chromosomes)
- overlap microtubules: overlap with microtubules coming from opposite direction. They push against each other to elongate/stretch the spindle

Question 13

Describe prometaphase.

Answer 13

• Chromosomes fully condensed
• Centrosomes at opposite sides of the cell
• Spindle formation largely complete
• Nuclear envelope breakdown

Question 14

Describe the nuclear breakdown.

Answer 14

Occurring prometaphase, nuclear breakdown happens in seconds. A break appears in the nuclear membrane, which then disintegrates, releasing the nuclear material into the cytoplasm. This means there is much more space for cell division.

Question 15

What happens during metaphase?

Answer 15

• Chromosomes align at the equator (there is a balance of forces from both sides of the spindle)
• Kinetochores attached to microtubules.
• If a chromosome falls off the equator, a microtubule grabs the centromere and pulls the chromosome back into the centre with the others

Question 16

What are kinetochores?

Answer 16

A kinetochore is the protein complex assembled at the centromere that binds to microtubules of the spindle.

Question 17

What triggers anaphase?

Answer 17

• The balance of forces on the chromosomes from the spindle.

- Anaphase promoting complex (APC)

Question 18

What occurs during anaphase?

Answer 18

• Sister chromatids separate
• Daughter chromosomes migrate to opposite poles (microtubules still pulling)
  This process is well regulated.

Question 19

Give the names for the chromatids/chromosomes before and after separation during anaphase.

Answer 19

Before: sister chromatids
After: daughter chromosomes

Question 20

What does securin do?

Answer 20

Inhibits separase by binding to it, preventing the separase from separating the chromatids. Separase is a protease enzyme.

Question 21

What does separase do when not inhibited by securin?

Answer 21

Cleaves cohesin (which holds the chromatids together)

Question 22

What happens at the end of metaphase when the chromosomes are aligned?

Answer 22

When the chromosomes are aligned, Anaphase promoting complex (APC) is activated and destroys securin. These frees separase, which cleaves cohesin, releasing the sister chromatids so they are no longer bound together.

Question 23

What happens once the chromatids have been separated by separase?

Answer 23

The microtubules (especially kinetochore microtubules) undergo dynamic instability (catastrophe) after losing the GTP cap and having the less stable GDP-tubulin region of the microtubules exposed. The microtubules shrink rapidly towards the poles during anaphase, pulling the chromosomes very rapidly to the poles of the cell.

Question 24

How are the poles of the cell pushed/pulled apart by the growing microtubules during prophase and metaphase?

Answer 24

• A sliding force is generated between overlap microtubules from opposite poles to push the poles apart
• A pulling force acts directly on the poles to move them apart.

Question 25

What happens during telophase?

Answer 25

• daughter chromosomes arrive at poles
• daughter chromosomes de-condense (allows gene expression/normal function again)
• nuclear envelope reformation
  This concludes nuclear division.

Question 26

What happens during cytokinesis?

Answer 26

The cytoplasm is divided when the actin ring forms and contracts, squeezing the cytoplasm apart. Common organelles such as mitochondria and chloroplasts are randomly distributed (stochastic mechanism), because there are so many of them in the cell. Rarer organelles such as the Golgi and ER undergo active distribution. As there are fewer of these organelles in the cell, which new cell each organelle goes into must be controlled.