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Fundamentals of the Living Cell > The Cell Cycle > Flashcards

The Cell Cycle Flashcards

1
Q

What is the cell cycle?

A

The duplication of cell contents (DNA, organelles, cytoplasm) and their subsequent segregation and division into two new cells. This effectively passes on the genetic material to the next generation.

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2
Q

How many organisms is created in the cell cycle of a unicellular organism?

A

Each cell cycle gives rises to 2 new organisms.

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3
Q

How many many organisms are created in the cell cycle of a multicellular organism?

A

In a multicellular organism, such as humans:
-The fertilized egg (zygote) must undergo many cell
cycles to make a fully grown organism
-Must also constantly replace any cells that die during
the lifetime of the organism

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4
Q

What are the cell cycle phases?

A

The cell cycle is composed of 4 stages, G1, S, G2 and M phase.

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5
Q

What is the G1 phase?

A

The cell increases in size and prepares to copy its DNA

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6
Q

What is the S phase?

A

The cell’s DNA is copied in DNA replication (chromosome duplication).

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7
Q

What is the G2 phase?

A

Proteins required for the M phase of the cell cycle are synthesised (e.g microtubules for mitotic spindle and membrane proteins for the two new daughter cells).

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8
Q

What is the M phase?

A

The cells divide by first dividing the DNA content in mitosis followed by cytokinesis. Mitosis can be further broken down into prophase, prometaphase, metaphase, anaphase and telophase.

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9
Q

What phase a majority of the cell in?

A

Many of the cells in our body are in the G0 phase and not all can re-enter the cell cycle.
Cells can halt the cell cycle and become arrested in the G0 phase which is the resting state.
Some cells such as neurons are permanently in the G0 phase and this accounts for the inability to regenerate (hence why nerve damage is difficult to repair)
Other cells are constantly in the cell cycle such as epithelial cells of the gut and haematopoietic cells (blood making cells) found in the bone marrow.

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10
Q

What happens during prophase?

A
  • Chromosomes condense
  • Centrosomes move to opposite poles
  • Mitotic spindle forms
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11
Q

What happens during prometaphase?

A
  • Break down of the nuclear envelope

- Chromosomes attach to mitotic spindle

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12
Q

What happens during metaphase?

A
  • Centrosomes are at opposite poles

- Chromosomes are at their most condensed form and line up at the equator of the mitotic spindle

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13
Q

What happens during anaphase?

A
  • Sister chromatids separate

- Each new daughter chromosomes move to the opposite spindle pole

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14
Q

What happens during telophase?

A
  • Chromosomes arrive at the spindle poles
  • Chromosomes decondense
  • Nuclear envelope reforms
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15
Q

What happens after telophase?

A

Cytokinesis which the the cytoplasmic division:
-At the position of the metaphase plane
-Contractile ring of actin and myosin II constrict the cell into 2 daughter cells.
*Contractile ring:
-Cytoskeleton structure composed of actin and
myosin bundles
-Accumulates between the poles of the mitotic spindle
beneath the plasma membrane
-Ring contracts and forms an indentation or cleavage
furrow, dividing the cell in two

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16
Q

What is the mitotic spindle?

A

It is the bipolar (have a positive and negative end) array of microtubules.
They start to assemble during prophase from the centrosomes at each pole.
They attach to the chromosomes via the kinetochore )a large protein structure assembled on the centromere).
They pull apart the sister chromatids.

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17
Q

What are the three different types of microtubules?

A
  • Astral microtubules
  • Kinetochore microtubule
  • Interpolar microtubules
18
Q

What is a kinetochore?

A

A protein structure formed on a chromatid where the spindle fibres attach to pull the chromatids apart during cell division.

19
Q

What is a centromere?

A

A part of the chromosomes connected to the spindle fibre.

20
Q

What is a chromatid?

A

Chromatids are the 2 chromosomes that have been replicated and are linked through the centromeres.

21
Q

What is a centrosome?

A

The centrosome is a Microtubule-organising centre in somatic animal cells.
Centrosome consists of a pair of centrioles surrounded by pericentriolar matrix (a cloud of amorphous – without a clear defined shape, material).
Duplicated during interphase, specifically during the S phase
Migrating to opposite poles in preparation for M phase.

22
Q

Why is it assumed that the cell organelles are ready in the new daughter cells?

A

As cell organelles cannot spontaneous so all cell organelles must be redistributed between the 2 new daughter cells.

23
Q

What is the difference between mitosis and meiosis?

A

MITOSIS:

  • Produce two new daughter cells
  • Diploid (2n) DNA
  • Cells divide once
  • Daughter cells identical to the parent diploid cell.
  • No recombination between homologous chromosomes (no exchange between tightly linked chromosomes)

MEIOSIS:

  • Produce four new cells
  • Haploid (1n) DNA - Each containing half the amount of genetic material of the parent cell
  • Cell divides twice
  • Homologous recombination occurs (chiasm structure allows exchange of DNA between father and mother).
24
Q

What is the purpose of meiosis?

A

To produce gametes for sexual reproduction. In this process the parent cell duplicates its DNA as usual but then undergoes two successive rounds of cell division (meiosis 1 and meiosis 2) to produce four haploid cells.

25
Q

What happens during meiosis 1?

A

The homologous chromosomes line up on spindle and separate to opposite spindle poles.

26
Q

What happens during meiosis 2?

A

The sister chromatids lines up on the spindle and separate to opposite spindle poles.

27
Q

What happens when meiosis goes wrong?

A
  • Nondisjunction – failure of homologous chromosomes to separate from one another, either at meiotic division I or meiotic division II
  • Autosomes – usually fatal, exceptions are:
    a) Trisomy 21 (down’s syndrome)
    b) Trisomy 18 (Edward’s syndrome)
    c) Trisomy 13 (Patau syndrome)
  • No viable autosomal monosomies
  • Sex chromosomes:
    a) XO (Turner’s syndrome)
    b) XXX (Triple X syndrome)
    c) XXY (Klinefelter’s syndrome)
28
Q

How is the cell cycle regulated?

A

A sophisticated control system is used to ensure that the cell cycle is tightly regulated ensuring that:
-each phase only occurs once per cell cycle
-each phase is carried out in the correct order
-phases must be non-overlapping
To do this, the cell has a number of checkpoints which are used by the cell to monitor the progress of the cell cycle and decide whether it should go to the next phase or not, examples of checkpoints being:
-Extracellular environment – is it favourable for the cell
to enter the cell cycle?
-DNA damage checkpoints – check the cell DNA is not
damaged before DNA replication occurs and before
the cell enters M phase
-G2 checkpoint – check that the cell DNA is fully
replicated before entry into M phase
-Metaphase checkpoint - check that all chromosomes
are aligned on the mitotic spindle before chromosome
separation is initiated

29
Q

How are these checkpoints regulated?

A

They are regulated by negative pathways that arrest the cell cycle rather than through the removal of positive signals telling it to proceed through the cell cycle. It is controlled by cyclin dependant kinases.

30
Q

What is cyclin?

A

It is a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependant kinase enzymes.

31
Q

How does cyclin dependant kinases work?

A
  • Cdk must be complexed with cyclin in order to be activated.
  • Cyclin levels are cyclical (hence their name) during the cell cycle.
  • Once bound, the kinase ability of the Cdk is turned on and it is able to phosphorylate its target protein and allow the cell to proceed to the next stage of the cell cycle.
  • Cyclin degradation terminates Cdks activity.
32
Q

How does the cyclin controls different between simple eukaryotes such as yeast and more complex eukaryotes?

A

-In lower eukaryotes, such as yeast, a single class of
Cdk protein binds successively to different classes of
cyclins in order to trigger the different cell cycle
phases. This concept is illustrated in the diagram
below.
-However in higher eukaryotes, the cell cycle controls
are more complex and different classes of Cdks act at
each stage of the cell cycle. Below is a summary of the
different classes of cyclins and Cdks corresponding to
each of the cell cycle checkpoints.

33
Q

LOOK AT CELL CYCLE NOTES TO SEE THE TABLE FOR THE DIFFERENT CYCLINS AND CDKS AND THEIR ROLES.

A

LOOK AT CELL CYCLE NOTES TO SEE THE TABLE FOR THE DIFFERENT CYCLINS AND CDKS AND THEIR ROLES.

34
Q

How does cyclin D initiate the cell cycle and entry into the S phase?

A

-Extracellular signals (mitogens) leads to the expression of cyclin D (a G1 cyclin)
-Cyclin D binds to its specific Cdk molecule (Cdk4), activating Cdk4 kinase activity
-Cyclin D-Cdk4 phosphorylates pRb
-Phosphorylated pRB dissociates from the E2F protein
-E2F is now active and can stimulate gene expression
of cyclin E (G1/S cyclin) and cyclin A (S cyclin)
-S phase entry can now proceed

35
Q

What happens at the G1 Checkpoint?

A

The DNA is checked for damage and the extracellular enviroment is also checked.

36
Q

How does the DNA damage repair system work at G1?

A
  • The G1 DNA damage checkpoint inhibits progression into S phase via the activation of p53.
  • Usually, cell levels of p53 are low because it is constantly being degraded by the proteasome.
  • Phosphorylated (active) p53 is not degraded
  • DNA damage results in decreased p53 degradation and this allows p53 to stimulate the expression of p21.
  • So basically, active p53 promotes transcription of genes that induce cell cycle arrest, in particular it binds to promotor region of p21 gene and stimulates p21 expression
  • p21 then binds to and inhibits G1/S-Cdk and S-Cdk complexes, leading to cell cycle arrest.
  • The cell arrest allows time to repair the damaged DNA.
  • If DNA repair not possible cycle will undergo apoptosis.
37
Q

How does the G2 checkpoint differ from the G1 checkpoint?

A

The G2 damage checkpoint relies on a different set of proteins to inactivate M-Cdk and block the cell from entering M phase

38
Q

What are the two families of cyclin-dependant kinase inhibitors?

A
  • Inhibitor of kinase 4 family (INK4): specifically inhibits G1 CDKs (e.g.: CDK 4)
  • CDK inhibitory protein/ kinase inhibitory protein (CIP/KIP) family: inhibit all other CDK-cyclin complexes (late G1, G2 & M) gradually sequestered by G1 CDKs thus allowing activation of later CDK.
39
Q

How is cancer related to the cell cycle?

A
  • In a multicellular organism, an adult no longer requires cells to grow and divide to increase the overall number of cells, instead cell death must be balanced by cell proliferation to maintain cell numbers.
  • When the checkpoints regulating the cell cycle malfunction, cell escapes normal cell cycle checkpoint —-> uncontrolled progression through the cell cycle.
  • This is because there are no barriers to prevent the cell from uncontrolled entry into the progression through the cell cycle and this can result in cancer.
  • In human cancers, many genes that regulate cell cycle (e.g.: p53 and pRB) are often mutated.
40
Q

What is an example of a barrier that stops uncontrolled cell division?

A
  • One such barrier is the retinoblastoma protein pRB.
  • pRB is an inhibitor of the cell cycle, it binds to E2F and its loss therefore results in deregulation of the cell cycle.
  • This is particularly noticeable in the immature cells of the retina, in which loss of pRB function (both copies of the gene no longer functioning), leads to a childhood cancer.
  • pRB is therefore known as a tumour suppressor gene.
41
Q

What are the symptoms that predispose cancer?

A 
DNA repair defects:
-ataxia telangiectasia
-Bloom's syndrome
-Fanconi's anaemia
-Li-Fraumeni syndrome
-Lynch type II (HNPCC)
-Xeroderma pigmentosum
Chromosomal instability:
-Down's syndrome
-Klinefelter's syndrome
42
Q

LOOK AT NOTES TO SEE THE TABLE AT THE END TO SEE THE DIFFERENT DNA DAMAGES.

A

LOOK AT NOTES TO SEE THE TABLE AT THE END TO SEE THE DIFFERENT DNA DAMAGES.

Fundamentals of the Living Cell (34 decks)

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