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Cell and molecular biology > Mitosis and meiosis > Flashcards

Mitosis and meiosis Flashcards

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1
Q

What are the different cell phases in order after cytokinesis up to mitosis and their conditions.

A

G1- cellular contents, excluding the chromosomes, are duplicated
S - Each of the 46 chromosomes are duplicated
G2 - The cell checks the duplicated chromosomes for errors and makes any needed repairs.

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2
Q

What happens in G1 (1st gap)

A

cells recover from previous division
cells grow and increase in volume
high protein synthesis
organelles duplicated
materials for DNA duplication accumulated
G1 checkpoints take place

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3
Q

What happens in S (synthesis)

A

DNA is replicated
Cell continues to grow, organelles and centrosome is duplicated
Cohesin protein complexes are made to attach sister chromatids.
S checkpoint to ensure DNA replication is complete

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4
Q

What happens in G2?

A

Growth continues, proteins required for chromosome manipulation are produced
Cytoskeletal filaments change
G2 checkpoints check DNA

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5
Q

What are the stages of mitosis in order

A

Prophase
Prometaphase
Metaphase
Anaphase
Telophase

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6
Q

What happens at prophase

A

Replicated chromosomes condense
Mitotic spindle assembles between the two centrosomes that have begun moving apart.

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7
Q

What is a kinetochore and a centrosome

A

Kinetochore: protein complex attached to the centromere
Centrosome: the organelle upon which microtubules form

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8
Q

What happens at prometaphase?

A

Starts abruptly with the break down of the nuclear envelope
Kinetochores attach to spindle microtubules

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9
Q

What happens during metaphase

A

Chromosomes align at the equator of the spindle midway between the poles

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10
Q

What happens during anaphase

A

Sister chromatids separate and each is pulled towards their respective spindle pole
Kinetochore microtubules shrink and the spindle poles move apart both contributing to chromosome segregation.

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11
Q

What happens during telophase

A

The two sets of chromosomes arrive at the spindle poles and start to decondense.
A new nuclear envelope reassembles around each set completing the formation of two nuclei
The division of the cytoplasm begins the assembly of the contractile ring.

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12
Q

What happens during cytokinesis

A

Cytoplasm divided in two by actin and myosin contractile ring creating two daughter cells.

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13
Q

How are chromosomes condensed.

A

Condensin SMC complexes bind to chromosomes at the beginning of mitosis and folds them into consecutive cis-loops until they are highly compacted. Condensin activity is upregulated by CDK kinases.
SMC complexes use ATP hydrolysis to manipulate DNA as facilitated by CDK kinase. Cohesin SMC complex brings together two different regions of DNA to cohese them together. This in then followed by looping.

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14
Q

How is the nuclear envelope broken down and reformed

A

When mitotic CDK complexes (a kinase btw) are active: nuclear lamin and pore proteins are phosphorylated inhibiting their interaction the inner nuclear membrane causing the membrane/envelope to breakdown.
When CDK complexes are inactive at the end of mitosis the nuclear envelope reforms from the constituent parts from the degradation.

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15
Q

What is a kinase

A

an enzyme that catalyses the transfer of a phosphate group from ATP to a specified molecule

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16
Q

What does the spindle checkpoint monitor

A

Whether each kinetochore has two microtubule attachments, one from each pole. When this condition is met chromosome segregation can start. This ensures that only copy of each of chromatid goes to each pole.

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17
Q

How are microtubules captured by kinetochores?

A

At prometaphase microtubules are seeking out kinetochores. Part of the microtubule (not the end) will interact with the kinetochore Making astral microtubule kinetochore. Motor proteins on the microtubule will move the kinetochore along the microtubule to make an end-on connection forming kinetochore microtubules.

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18
Q

How are sister chromatids separated?

A

Anaphase A: microtubules shrink pulling chromatids towards their poles.
Anaphase B: microtubules from each kinetochore push on each other and the kinetochores are themselves pulled pole-wards. Both helping to move the chromatids further apart and widen the distance between the poles

19
Q

How is anaphase initiated

A

At this point in metaphase the chromatids are aligned and the microtubules are pulling on their respective kinetochores towards the poles. For separation to occur cohesin complexes need to break down allowing for DNA strands that bound to their replicated counterpart to be separated. Cohesin degradation is facilitated by the enzyme separase. Separase is inhibited when bound to the inhibitory protein securin. Signals given by the spindle checkpoint being satisfied increase the activity of M-CDk which in turn activates APC which degrades securin releasing separase.

20
Q

How does APC help regulate the cell cycle?

A

It degrades cyclin which in turns deactivates the CDK’s (cyclin dependent kinases) which are driving the cell into mitosis.

21
Q
A
22
Q

Haploid vs diploid

A

Haploid = 1 set of chromosomes
Diploid = 2
sperm + egg = zygote
2 x haploid = diploid ^

23
Q

Why does meiosis make haploids and mitosis diploids?

A

Meiosis undergoes one S phase then two cell divisions:
2y x 2 = 4y
4y / 2 = 2y
2y / 2 = y -> haploid (theres 4 of them tho)

24
Q

What are the differences between meiosis and mitosis at different phases?

A

Prophase: in meiosis chromosomes condense and homologous pairs synapse forming tetrads instead of just sister chromatids.
Metaphase: Tetrads align at metaphase plate.
Anaphase: the tetrad splits the homologous sister chromatid pairs are pulled to opposite poles to each other.
Meiosis I ^
Meiosis II: After the first cell division the second follows similarly to mitosis and the sister chromatids now separate.

25
Q

How does meiosis produce genetic diversity during prophase I.

A

Sister chromatids become synapsed with their corresponding genes on homologous chromosomes. At the synapses the DNA crosses over and now each chromosome has recombinant chromosomes composed of both maternal and paternal DNA.

26
Q

How does meiosis produce diversity at metaphase I?

A

At the metaphase plate chromosomes cross over and shuffle genetic material. Chromosomes are orientated randomly at the centre so this provides a huge variety of combinations of DNA.

27
Q

How does down syndrome / other similar problems occur?

A

Non-disjunction errors during meiosis I or II
Chromatids can be segregated into the same gamete which can lead to too many or too little chromosomes in a sperm or egg cell.

28
Q

Why is the gene cdc 2 so important?

A

It is the ‘master’ cell regulator. Mutants of this gene will block S phase, gap 1 and gap 2. Cdc2 is a cyclin dependent kinase and as such is also called CDK1

29
Q

How is transition from G2 to mitosis regulated.

A

Gene Cdc2 causes the transition
Wee1 inhibits cdc2 to slow the transition down so it occurs at the right time.
Cdc25 upregulates cdc2 allowing cells to transition into mitosis.

30
Q

What protein is required for activating Cdc2?

A

Cyclins

31
Q

What is present in cytoplasm in mitotic cells but not cells in interphase?

A

All cell nuclei have mitotic promoting factor which pushes cells from interphase into mitosis. It is a homolog of cdc2.

32
Q

What is MPF and Cdc2

A

Mitotic promoting factor which is a cyclin dependent kinase (CDK) cyclin complex
Cdc2 is a CDK

33
Q

How were cdc 2 , mpf and cyclins discovered in terms of there role in cell cycle control?

A

cdc2 :Identified via mutants of fission yeast
mpf: by injecting cytoplasm from a mitotic cell into a cell in interphase.
cyclins: could observed at large quantities during specific cell cycle phases and were destroyed afterwards.

34
Q

How does CDK affect mitotic actions?

A

The kinase phosphorylates other proteins in a cell cycle dependent manner
Specifically
1. Condensin loading
2. Mitotic spindle assembly
3. Nuclear envelope breakdown
4. Activation of anaphase promoting complex (APC) which breaks down cohesin and degrades cyclin -> mitotic exit

35
Q

What processes require MPF protein and what type of reaction does MPF facilitate for these processes to occur?

A
  1. Chromatin compaction (condensin loading)
  2. Assembly of the mitotic spindle
  3. Nuclear envelope breakdown.
  4. Activation of the APC (anaphase promoting complex) - targets proteins for destruction including cohesin proteins between sister chromatids. (m phase -> a phase) and the cyclin that promotes mpf activation.
    Phosphorylation of key targets.
36
Q

How does cyclin concentration vary throughout the cell cycle?

A

Mitosis: high concentration followed by degradation at the end of mitosis
Interphase: progressively increases following degradation until mitosis.

37
Q

What are the two main groups of cyclins?

A
  1. G1/S: Control entry into S phase
    - CDK4/6, cyclin D
    -CDK2, cyclin E
    -CDK2, Cyclin A (S phase)
  2. G2/M control entry into mitosis
    -cyclin B/CDK1
38
Q

What is the nurse model?

A

the term given to the regulation of cdks to ensure ordered progression through the cycle.
Inhibitory phosphorylation of m-CDK by wee1 keeps m cdks inactive until a critical threshold is reached (theres enough CDKs bound to cyclins) has been passed. This is followed by the rapid removal of the phosphorylation on m-cdks by cdc25 promoting a switch like change.

39
Q

How does the activity of cyclin ensure the cell cycle travels in one direction?

A

Cyclin degradation is facilitated by APF which is in itself activated when a threshold level of cyclins is reached .

40
Q

What is S-CDK involved in / what does it do?

A

Cells must be licensed to enter S phase in order to prevent replicated DNA being replicated twice. Licensing means loading of the inactive core of the replication machinery and requires low S-CDK.
S-CDK activates replication machinery initiating DNA replication while it also inhibits new DNA replication machinery being loaded preventing DNA being replicated twice.

41
Q

Describe CDK and cyclin activity through the cell cycle.

A

G1
1. Low cdk activity allows for the loading of licensing/replication factors
2. S- cyclin increases, inhibiting the loading the replication machinery and the cell is licensed and travel into S phase
3. S-CDK activates replication G1 -> S
4. S-CDK activity is high preventing relicensing
S -> G2
5. M-cyclin increase
6. m-cyclin threshold reached - > M-CDK activated -> triggers mitosisby inhibition of wee1
M
7. After spindle checkpoint is satisfied APC is activated and cyclins destroyed resetting the cycle.

42
Q

How do cell cycle checkpoints work generally? What are the main checkpoints?

A

Cells delay the cycle progression in response to external signals e.g. DNA damage
In interphase the main checkpoints are licensing in G1 -> S and the transition from G2 to mitosis.
The direction of travel through the cycle is ensured by CDKs that form complexes with cyclins. Cyclin are periodically expressed and destroyed.
CDK/cyclin complexes are inhibited by wee1 until activated by cdc25.

43
Q

How does the spindle assembly checkpoint work?

A

This checkpoint monitors whether cells assemble their chromosomes and microtubules correctly by monitoring
- The occupancy of kinetochores
- Microtubule tension (tension is created by two microtubules pulling in opposite directions).
This essentially ensures all the chromosomes are lined up at the metaphase plate and are attached to microtubules to both poles before initiation of anaphase.

44
Q

What does MAD2 protein do?

A

It is bound to kinetochores forming a protein complex and inhibits the APC complex, stopping chromatids from separating. When kinetochores become bioriented (both microtubules attached) MAD2 dissociates allowing APC to activate and separate the chromatids.

Cell and molecular biology (9 decks)

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