The Cell Cycle Flashcards

1
Q

How is the cell cycle useful in uni- and multicellular organisms?

A
UNICELLULAR ORGANISMS (eg. bacteria, yeast):
-Each cell cycle gives rise to two new organisms.

MULTICELLULAR ORGANISMS:

  • a single-celled zygote must undergo many rounds of the cell cycle to make a new fully grown organism
  • the organism must also constantly replace any cells that die during the lifetime of that organism
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2
Q

What are the 3 different situations with cell cycle re-entry?

A
  • cell cycle re-entry in NOT possible (eg. nerve cells)
  • cells are maintained in G0 until stimulated to divide (eg. hepatocytes)
  • cells are constantly in the cell cycle (eg. epithelial cells of the gut, haematopoietic cells in bone marrow)
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3
Q

What are the different phases of the cell cycle in eukaryotes?

A

-GAP PHASE 1 (G1): growth and preparation for S phase
- SYNTHESIS PHASE (S): chromosome duplication
- GAP PHASE 2 (G2): growth and preparation for M phase
(these three phases are all under Interphase)

  • MITOTIC PHASE (M): mitosis + cytokenisis
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4
Q

Describe the M Phase in detail.

A

PROPHASE:

  • Chromosomes condense
  • Centrosomes move to opposite poles
  • Mitotic spindle forms

PROMETAPHASE:

  • Breakdown of nuclear envelope
  • Chromosomes attach to mitotic spindle

METAPHASE:

  • Centrosome are at opposite poles
  • Chromosomes are at their most condensed & line up at the equator of the mitotic spindle

ANAPHASE:

  • Sister chromatids separate synchronously
  • Each new daughter chromosome moving to the opposite spindle pole

TELOPHASE:

  • Chromosome arrives at the spindle poles
  • Chromosomes de-condense
  • Nuclear envelope reforms

CYTOKINESIS:
- Cytoplasm divides

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5
Q

Describe the mitotic spindle, and how it works.

A

It’s a bipolar array of microtubules.
It starts to assemble during prophase from the centrosomes at each pole. It attaches to the chromosomes via the kinetochore. It pulls apart the sister chromatids.

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6
Q

What are the three types of spindle microtubules?

A
  • ASTRAL MICROTUBULES: they anchor the spindle poles to the cell membrane
  • KINETOCHORE MICROTUBULES: they help in lining up the chromosomes
  • INTERPOLAR MICROTUBULES: they interdigitate with each other from opposite poles - extending across the equator - to provide stability to the bipolar spindle
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7
Q

What is the kinetochore?

A

It is a protein structure formed on the centromere, where the spindle fibres attach to pull the chromatids apart during cell division.

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8
Q

What is the centromere?

A

It is a part of the chromosome connected to the spindle fibre.

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9
Q

What are chromatids?

A

They are the two chromosomes that have been replicated and are linked through the centromere.

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10
Q

What are centrosomes?

A

They’re the microtubule organising centres (MTOCs) in somatic cells. They consist of a pair of centrioles surrounded by a pericentriolar matrix (a cloud of amorphous material).
It’s duplicated during interphase, and they migrate to opposite poles in preparation for the M phase.

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11
Q

Describe Cytokinesis.

A

It’s the final step in the cell cycle. It divides the cytoplasm into two daughter cells.
The contractile ring is a cytoskeletal structure composed of actin and myosin bundles. It accumulates between the poles of the mitotic spindle beneath the plasma membrane. The ring contracts and forms an indentation/cleavage furrow, dividing the cell into two.
All the cell’s organelles must be redistributed between the two daughter cells, as organelles cannot spontaneously regenerate.

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12
Q

List some difference between mitosis and meiosis.

A

MITOSIS:

  • two cells made at the end
  • diploid (2n) DNA
  • the cell divides once
  • no recombination between homologous chromosomes (no exchange of DNA sequences between tightly linked chromosomes)

MEIOSIS:

  • four cells made at the end
  • haploid (n) DNA
  • the cell divides twice
  • homologous recombination occurs (“chiasm” structure, allows exchanged of DNA between mother and father chromosomes)
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13
Q

Describe Meiosis.

A

Meiosis is a specialised cell division that starts with 1 diploid cell & ends with 4 haploid cells
Its purpose is to produce gametes: sperm & egg.
One round of DNA replication during S phase & 2 rounds of cell division

MEIOSIS I: homologous chromosomes line up on the spindle & separate to opposite spindle poles

MEIOSIS II: sister chromatids line up on the spindle & separate to opposite spindle poles

Recombination occurs between homologous chromosomes.

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14
Q

What happens when meiosis goes wrong?

A

Nondisjunction is the failure of homologous chromosomes to separate from one another either at meiotic I or meiotic II division.

In autosomes, it’s usually fatal, exceptions being:

  • Trisomy 21 (Down’s syndrome)
  • Trisomy 18 (Edward’s syndrome)
  • Trisomy 13 (Patan syndrome)

In sex chromosomes:

  • XO (Turner’s syndrome)
  • XXX (Triple X syndrome)
  • XXY (Klinefelter’s syndrome)
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15
Q

How does the cell cycle have to be regulated?

A
  • Each phase must occur only once per cell cycle
  • Phases must be in the correct order, G1-S1-G2-M
  • Phases must be non-overlapping
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16
Q

What are the 3 checkpoints in the cell cycle?

A

G1 CHECKPOINT (end of G1 before S phase)

  • check DNA damage so no mutated or damaged cells replicate
  • check the extracellular environment
  • check for room and nutrients for growth

G2 CHECKPOINT (end of G2, before M phase)

  • checks for DNA damage
  • checks if DNA replicated properly
METAPHASE CHECKPOINT (during metaphase) 
- checks if all chromosomes are aligned on the mitotic spindle
17
Q

What are the cell cycle regulators?

A

Cdks:

  • enzymes that phosphorylate the target proteins
  • they become active when bound to the corresponding cyclin

Cyclins:

  • regulators of Cdks
  • different cyclins are produced at each phase of the cell cycle
18
Q

What are the basic principles of cell cycle control?

A
  • Cdk levels are fairly stable throughout the cell cycle
  • Cyclin levels vary as part of the cell cycle
  • Cdk bound to Cyclin is active and phosphorylates a ‘target’ protein
  • Cdk activation triggers the next step in the cell cycle, such as the entry into the S or M phase
  • Cyclin degradation terminates Cdk activity
19
Q

What normally happens at the G1 checkpoint to allow the cell to enter the S phase?

A
  • induction/expression of Cyclin D
  • binding/ activation of Cdk4
  • phosphorylation of pRB
  • release and activation of E2F
  • S phase gene transcription
20
Q

What happens at the G1 checkpoint if there is damage to the DNA?

A
  • normally, p53 is degraded quickly as it is unstable, and maintained at very low levels
  • phosphorylated (active) p53 is not degraded
  • active p53 promotes transcription of genes that induce cell cycle arrest, in particular, it binds to the promoter region of the p21 gene and stimulates p21 expression
  • p21 binds and inhibits G1/S-Cdk and S-Cdk complexes
  • cell arrests in G1 (allowing time to repair the damaged DNA)
21
Q

What are the two families of Cdk Inhibitors (CKIs)?

A

Inhibitor of the Kinase 4 family (INK4)
- they specifically inhibit G1 Cdks (eg. CDK4)

CDK Inhibitor Protein/ Kinase Inhibitory Protein (CIP/KIP)

  • they inhibit all other CDK-Cyclin complexes (late G1, G2 and M)
  • they’re also gradually sequestered by G1 CDKs, thus allowing activation of later CDKs
22
Q

How does the misregulation of the cell cycle cause cancer?

A
  • cells escape the normal cell cycle checkpoint, leading to uncontrolled progression through the cell cycle
  • many genes that regulate the cell cycle (eg. p53, p21) are often mutated in human cancers