Cell Cycle Flashcards
To cover: - the events occurring at each phase of the cell cycle. - how the cell controls its passage through the cell cycle using a series of checkpoints. - the molecular mechanisms underlining these checkpoints
What is the cell cycle? [2 marks]
- The duplication of cell contents
- The division of cells into 2 new daughter cells.
What is mitosis? [1 mark]
The division of somatic cells into 2 identical diploid daughter cells.
What are the stages of mitosis? [6 marks]
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
What is prophase? [3 marks]
When:
- Chromosomes condense
- Centrosomes move to opposite poles
- Mitotic spindle forms at each pole
What is prometaphase? [2 marks]
When:
- Nuclear envelope breaks down
- Chromosomes attach to mitotic spindle
What is metaphase? [3 marks]
When:
- Centrosomes are at opposite poles
- Chromosomes line up at the equator of the mitotic spindle
- Chromsomes are at their most condensed
What is anaphase? [2 marks]
When:
- Sister chromatids separate
- Daughter chromosome move to the opposite spindle pole
What is telophase? [3 marks]
When:
- Chromosomes arrive at the spindle poles
- Chromosomes decondense
- Nuclear envelope reforms
What is cytokinesis? [2 marks]
- The division of cytoplasm and organelles.
- A contractile ring of actin and myosin II constricts the cell.
How does cytokinesis occurs? [2 marks]
- Actin and myosin bundles accumulate between poles
- Ring contracts and forms an indentation
Stages of interphase [4 marks]
- G1 phase
- S phase
- G2 phase
- G0 phase
What does G1 check for? [4 marks]
Checks:
- Extracellular environment
- Growth factors
- Mitogenic signals
- DNA damage
What does G2 check for? [2 marks]
Checks:
- If DNA has replicated properly
- DNA damage
What occurs in G phases? [1 mark]
Growth and preparation for the subsequent phase.
Why can’t cells in G0 phase re-enter the cell cycle? [3 marks]
- Re-entry is not possible (i.e. nerve cells)
- Only re-enter the cell cycle after being stimulated (i.e. hepatocytes).
- Constantly in cell cycle (i.e. epithelial cells of the gut, haematopoietic cells)
What is the mitotic spindle? [3 marks]
- Bipolar array of microtubules (positive = growing end, negative = shrinking end)
- Attaches to chromosomes via kinetochore
- The 3 types of spindle mictotubules are astral, kintochore and interpolar
Astral microtubules [3 marks]
- Acts as the framework
- Radiates outwards from pole
- Contracts cell cortex to position spindle
Kinetochore microtubules [1 mark]
Attachs to kinetochore
Interpolar microtubules [2 marks]
- Extends across equator
- Interacts with positive end of microtubule
What is a kinetochore? [3 marks]
- A protein structure formed on a chromatid
- Where spindle fibres attach
- There are 92 kinetochores present in the human cell
What is the centromere? [1 mark]
The part of the chromosome connected to the spindle fibre
What is the structure of the centrosome? [2 marks]
- Pair of centrioles
- Surrounded by pericentriolar matrix (cloud of amorphous material)
What is nondisjunction? [2 marks]
- When homologous chromosomes can’t separate
- Either at meiotic division I or division II
Autosomal disjunctions [3 marks]
- Down’s syndrome (trisomy 21)
- Edward’s syndrome (trsomy 18)
- Patau syndrome (trisomy 13)
Allosomal (sex chromosomes) disjunctions [3 marks]
- Turner’s syndrome (XO)
- Triple XX syndrome
- Klinefelter’s syndrome (XXY)
Function of cyclin dependent kinases (CDKs) [3 marks]
- Phosphorylate the target proteins
- Become active when bound to cyclins
- Activating a CDK triggers the next step in the cell cycle
Function of cyclins [3 marks]
- Regulate CDKs
- Different cyclins are made at each stage of the cell cycle
- Cyclin degradation terminates CDK activity
Levels of CDKs and cyclins throughout the cell cycle [2 marks]
- CDK levels stay stable throughout
- Cylin levels vary throughout the cell cycle
What is the retinoblastoma protein (pRB)? [2 marks]
- Tumour suppresor protein
- Target proteins for cyclins
What is p53? [3 marks]
- Tumour suppresor protein
- Promotes transcription of genes that induce cell cycle arrest
- Binds to promoter of p21 and stimulates its expression
What does p21 do? [1 mark]
Binds and inhibits G1/S-Cdk and S-Cdk complexes
Inhibitor of kinase 4 family (INK4) [1 mark]
Inhibits G1 CDKs
CDK inhibitory protein (CIP)/kinase inhibtory protein (KIP) [3 marks]
- Inhibits all other CDK-cyclin complexes (in late G1, G2 and M)
- Gradually sequestered by G1 CDKs
- This allows the activation of later CDKs
What genes are often mutated in human cancers? [2 marks]
- p53
- pRB
