The cell cycle and its control

Question 1

What is contact inhibition of cell growth?

Answer 1

Inhibition of mitotic activity as cells come into contact with each other as they occupy a given amount of space

Question 2

What happens during the three stages of interphase?

Answer 2

G0 - general functioning of the cell, where cell cycle machinery is dismantled
G1 - the cell makes sure that it has everything that is necessary for duplication
S - DNA replication, protein synthesis and replication of organelles
G2 - the cell checks that everything is ready to enter mitosis

Question 3

What are centrosomes and what do they consist of?

Answer 3

Centrosomes are organelles near the nucleus of a cell, which consist of a mother and daughter centriole (barrels of nine triplet microtubules) arranged perpendicular to each other.
They are a microtubule organising centre responsible for forming the mitotic spindle

Question 4

What happens to the centrosomes during G1 and S?

Answer 4

• The mother and daughter centrioles separate in G1
• Then the mother produces another daughter and the daughter produces another mother, resulting in the formation of 2 centrosomes (the duplication takes place during S phase)

Question 5

What are the points around the centrosome from which microtubules arise?

Answer 5

Nucleating sites (gamma-tubulin ring complexes)

NOTE: nucleation is the assembly of microtubules, which project out in all directions

Question 6

Describe the condensation of chromatin that takes place during prophase.

Answer 6

• The double helices wrap around histones to form “beads-on-a-string”
• This is then further compacted to form 30nm fibres
• 30nm fibres are compacted and associate with a chromosome scaffold
• Further wrapped/condensed to form a chromosome (1400nm)

Question 7

What is a kinetochore?

Answer 7

Condensed chromosomes each consist of 2 sister chromatids (identical, due to DNA replication)

Kinetochores are protein complexes associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach

Question 8

Describe the arrangement of centrosomes at the end of prophase/beginning of prometaphase.

Answer 8

They are on opposite sides of the nucleus

Question 9

Summarise the process of spindle formation

Answer 9

• Radial microtubule arrays (ASTERS) form around each centrosome
• Radial arrays that meet (in the middle) => polar microtubules to form

Question 10

What are the three main types of microtubles?

Answer 10

1. Kinetochore microtubule – attach to kinetochores
2. Polar microtubule – interdigitate at the spindle mid-zone and push the spindle poles apart via motor protein
3. Astral microtubule – microtubule that does not connect to a kinetochore, but anchors the spindle pole to the cell membrane

Question 11

Describe what happens in early and late prometaphase

Answer 11

Early:

• breakdown of nuclear membrane
• attachment of chromosomes to spindle via kinetochores

Late:
- Upon capture, the chromosomes slide rapidly towards centre

Question 12

What important protein holds sister chromatids together?

Answer 12

Cohesin

Question 13

What happens in anaphase A?

Answer 13

Cohesin is broken down and the microtubules get shorter so the chromatids/daughter chromosomes are pulled toward opposite spindle poles

Question 14

What happens in anaphase B?

Answer 14

Daughter chromatids continue to migrate towards the poles

The centrosomes also migrate apart

Question 15

Describe what happens in telophase.

Answer 15

• Daughter chromatids arrive at the pole and the nuclear envelope reassembles
• Assembly of a contractile ring of actin and myosin filaments that squeezes the cell at the cleavage furrow so it divides into two daughter cells

Question 16

What is the midbody?

Answer 16

A structure of microtubule bundles and other proteins involved in cytokineses found at the point where the actin-myosin contractile ring is formed

Question 17

Describe how the spindle assembly checkpoint works.

Answer 17

The kinetochore has proteins that emit a signal when the kinetochore is
NOT attached to microtubules
When a microtubule attaches to the kinetochore, it stops emitting the signal
At the end of metaphase, you want all the kinetochores to stop sending signals before you can proceed to anaphase

Question 18

Name two proteins that allow the kinetochores to detect spindle attachment.

Answer 18

1. Centromere protein E/CENP-E (kinetochore tension sensing)
2. BUB-protein kinase (dissociates from the kinetochores when the chromatids are properly attached to the spindle – when all dissociated, anaphase proceeds.

Question 19

What can cause aneuploidy?

Answer 19

Mitotic checkpoint defect
=> Mis-attachment of the spindles (so chromatids end up at different poles to the ones that they should be at or they get lost at cytokineses - merotelic)

Aberrant mitosis (production of an abnormal number of centrosomes leads to a multipolar spindle => abnormal division of the chromatids, and abnormal cytokinesis)

Question 20

Describe the four different types of spindle attachment.

Answer 20

Amphitelic – normal spindle attachment
Syntelic – both kinetochores of the sister chromatids are attached to spindles from one centrosome
Merotelic – one kinetochore of one of the sister chromatids is attached to spindles from both centrosomes
Monotelic – one kinetochore of one of the sister chromatids is attached to a spindle, the other is unattached

Question 21

Broadly speaking, explain how cell cycle checkpoints can be a target for anti-cancer therapy.

Answer 21

By targeting the cell cycle checkpoints, the cancer cells can be arrested in mitosis
Cells are very vulnerable when they are in mitosis and are more easily killed

Question 22

What do checkpoint kinases do, hence making them a target for anti-cancer drugs?

Answer 22

Checkpoint kinase (CHKE1 and CHKE2) – Serine threonine kinase activation holds cells in G2 phase until all is ready.

Inhibition by a CKI leads to untimely cell transition to mitosis

Question 23

Give examples of anti-cancer drugs which work differently

Answer 23

Taxanes and Vinca alkaloids (particularly for breast and ovarian cancers)
These alter microtubule dynamics and produce unattached kinetochores, which leads to long-term mitotic arrest

Question 24

What can happen to cells that are held up at a checkpoint, i.e. if something goes wrong during the cell cycle?

Answer 24

• They can undergo DNA repair and then proceed through the cell cycle (temporary cell cycle arrest)
• If the damage is irreparable, they can undergo apoptosis

Question 25

How are the cell cycle checkpoints affected when in a cancerous state?

Answer 25

• Increased growth factor production progress passed the G1 checkpoint and into S phase
• Impaired G2 checkpoint (checking for DNA damage)
• Impaired metaphase checkpoint (checking sister chromatid alignment)
• Prevention of cell entering G0 phase

Question 26

Exit from G0 to begin the cell cycle is highly regulated and requires growth factors to initiate intracellular signalling cascades. Explain the signalling pathways that result in activation of the extracellular signal-related kinase (ERK) cascade

Answer 26

• When a ligand (e.g. Epidermal growth factor (EGF); Platelet-derived growth factor (PDGF))
binds to a Tyrosine Kinase Receptor*, they cause neighbouring TKRs to associate with each other, forming cross-linked dimers => each TKR in the dimer phosphorylates multiple tyrosines on the other TKR (cross-phosphorylation).
• Intracellular proteins that have a particular domain (SH2 domain) bind to phosphorylated tyrosines in the cytoplasmic TKR tails.
• Ultimately, TKR activation brings about changes in gene transcription due to crosstalk between intermediates in various signalling pathways from the membrane to the nucleus; kinase cascades characterised by successive kinase activation/phosphorylation by the next kinase (phosphorylation reversed by phosphatases

*Can also have a serine/threonine kinase receptor; amino acid side chains also have the necessary -OH group.