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REPRO > Mitosis and meiosis > Flashcards

Mitosis and meiosis Flashcards

1
Q

What is the function of mitosis?

A
  • Growth

- Replacement of dead cells

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2
Q

What are the 2 main phases of mitosis?

A
  • Mitotic (M)

- Interphase

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3
Q

What are the phases of interphase?

A
  • 1st gap or growth phase (G1)
  • Synthesis (S)
  • Second gap or growth phase (G2)
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4
Q

What are the functions of the mitotic and interphase phases?

A
  • Mitotic: true cell division phase

- Interphase: preparation for next cell division event

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5
Q

What is the G0 phase?

A
  • Between M and G1 phases
  • Cells are dormant/quiescent
  • Not dividing or doing anything else
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6
Q

Which chromosomes are involved in mitosis?

A
  • All chromosomes

- Make 2 homologous copies of each chromosome

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7
Q

Describe what happens in the G1 phase

A
  • First part of interphase
  • Normal cell activities
  • Increase in cell organells
  • Doubling of cell size
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8
Q

Describe what happens in the S phase

A
  • Synthesis
  • DNA synthesis
  • 2 copies of each chromosome (sister chromatids) are available
  • Linked at centromere
  • Centrioles replicate
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9
Q

What is the function of the centrioles?

A
  • Part of cytoskeleton apparatus

- Pull chromosome halves to each end

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10
Q

Describe what happens in the G2 phase

A
  • Cell grows in size
  • Each cell now has 2 pairs of centrioles
  • Mitotic phase starts again
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11
Q

What are the stages within the M phase of mitosis?

A
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
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12
Q

What happens in prophase?

A
  • Chromatin condenses, distinct chromosomes form
  • Each chromosome visible as 2 sister chromatids, joined by centromere
  • Nucleolus shrinks and disappears
  • Cytoplasmic microtubules of cytoskeleton disassemble then reasseble to form spindle
  • One pair of centrioles migrates to each pole of the spindle
  • Nuclear membrane disappears
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13
Q

Describe metaphase

A
  • Chromoseoms arrange on equator of spindle
  • Homologous chromosomes do not associate
  • Ensures daughter cell gets exact copy of parent cell’s DNA
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14
Q

In what phase can we see cell diving in tissue?

A

Metaphase

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15
Q

Describe anaphase

A
  • Chromatids separate
  • Chromatids migrate, led by centromeres, to opposite poles of cell
  • Eventually reach poles to make 2 daughter chromosomes
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16
Q

Describe telophase

A
  • Chromosomes unravel
  • Nuclear membranes reform
  • Nucleoli reform
  • Encase in one cell membrane at this point
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17
Q

Describe cytokinesis

A
  • Cleavage furrow forms and deepens until cytoplasm is halved
  • Ring of actin filaments attached to inner aspect of cell membrane
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18
Q

Why is there need for control over the cell cycle?

A

To ensure events are properly times and in correct order

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19
Q

In terms of individual cells, why is it necessary to have control over the cell cycle?

A
  • Damaged DNA must not be replicated
  • DNA replication must be complete and just once per cell cycle
  • Chromosomes must be positioned on spindle correctly
  • Phases of cycle must be synchronous
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20
Q

In terms of the whole animal, why is it necessary to have control over the cell cycle?

A
  • Cell division in tissue/animal coordinated by intercellular signals according to requirements of animal
  • E.g. proliferation of lymphocytes in infection or over-proliferation in cancer
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21
Q

What are the 2 levels of control of the cell cycle?

A
  • Intercellular

- Intracellular

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22
Q

What are the points of intracellular control of the cell cycle? What do these require?

A
  • Check points at end of G1, G2 and M phases

- Cyclin dependent protein kinases (Cdks) and cyclins

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23
Q

What is the first checkpoint and what would cause the cell cycle to arrest here?

A
  • G1 to S
  • If no mitogenic signal received or if DNA is damaged
  • If no mitgenic signal is received but there is no DNA damage, enters G0
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24
Q

What is the second checkpoint and what would cause the cell cycle to arrest here?

A
  • G2 to M

- Arrested if DNA damaged or incompletely duplicated

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25
Q

What is the third checkpoint and what would cause the cell cycle to arrest here?

A
  • M to G1

- Arrested if chromatids are not properly attached to spindle fibres

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26
Q

What happens if the DNA is incorrect?

A

Apoptosis

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27
Q

Describe Cdks in intracellular control of mitosis

A
  • Cyclin dependent protein kinases
  • Activities rise and fall as cell progrgesses through different phases
  • Acts as chekpoints
  • Cyclical changes in Cdks regulated by cyclins
  • Needs to be phosphrylaed to be activated
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28
Q

What does the nature of action of Cdks depend on?

A

The cyclin protein

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29
Q

What leads to the waves in activity of cyclin through each phase of the cell cycle?

A
  • At start, cyclin genes activated
  • Failure of this results in cessation of cycle
  • Cyclin proteins destroyed by proteasomes
  • Genes encoding cyclins for next cycle are upregulated and expressed
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30
Q

How is Cdks activity regulated?

A
  • Cyclins bind to Cdks (different types of each of these)
  • Attachment of ubiquitin molecules to cyclin (bound to the Cdk)
  • Once tail of ubiquitin, goes to proteasome complex in cell and protein degraded
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31
Q

Describe the fine regulation of Cdk

A
  • Can phosphorylate active Cdk further
  • Makes inactive
  • Can switch between active and inactive
  • can also control activity through binding of cell cycle inhibitors
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32
Q

What are the extracellular (intercellular) factors that control cell division, growth and apoptosis?

A
  • Mitogens/growth factors (stimulate cell growth)

- Surival factors (inhibit apoptosis and promote cell survival)

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33
Q

What are growth factors?

A
  • Signalling molecules

- Originate from neighbouring cells

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34
Q

What is the function of growth factors?

A
  • Activate pathways which control genes
  • Mitogenic
  • Maintain correct cell numbers in an organ/system
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35
Q

How do growth factors carry out their function?

A
  • Stimulate cells to enter G1 from G0
  • Activate RTKs, Ras and MAPK pathway
  • Results in activation of Myc
  • Myc increases expression of G1 cyclin kinases: stimulation of DNA synthesis
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36
Q

What signalling cascade do most growh factors use?

A

MAPK

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37
Q

What is Myc?

A

Transcription factor

38
Q

How do abnormal proliferation and DNA damage stimulate cell cycle arrest or apoptosis?

A
  • Excessive Myc activates Arf
  • Arf binds to p53 inhibitiory protein Mdm2
  • P53 now active, activates p21 and this causes cell cycle arrest or apoptosis
39
Q

When does apoptosis occur?

A
  • Damaged beyond repair
  • Infected with virus
  • Undergoing stressful conditions e.g. starvation
40
Q

Where does the decision for apoptosis come from?

A
  • Cell itself
  • Surrounding tissue
  • Cells of immune system
41
Q

Give examples of errors in cell division

A
  • Somatic mutations (damaged cell, make cells cancerous)

- P53 mutation implicated in squamous cell carcinoma

42
Q

What is meiosis?

A

A form of nuclear division occuring in germ cells to give rise to mature gametes

43
Q

What is the product of mitosis?

A

2 diploid daughter cells with 2 homologous copies of each chromosome

44
Q

What is the product of meiosis?

A

4 haploid cells that only carry one copy of each chromosome

45
Q

What are the 3 major phases of meiosis?

A
  • Meiotic S phase
  • Meiosis I
  • Meiosis II
46
Q

What occurs in meiosis I?

A

Segregation of homologous chromosomes (and crossing over)

47
Q

What occurs in meiosis II?

A

Segregation of non-homologous chromosomes (like mitosis)

48
Q

Describe the first meiotic division in meiosis

A
  • DNA replication
  • Consists of interphase and mitotic phase
  • Mitotic phase can be divided into prophase I, metaphse I, anaphase I and telophase I
49
Q

Describe the second meiotic division of meiosis

A
  • No DNA replication

- Consists of prophase II, metaphase II, anaphase II, telophase II

50
Q

Describe prophase I

A
  • Nucleolus shrinks
  • Centrioles migrate to opposite poles
  • Spindle forms
  • Chromosomes condense
  • 5 stages: leptotene, zytogene, paachytene, diplotene, diakenesis
51
Q

Describe chromosomal condensation in prophase I of meiosis

A
  • Homologous chromosomes undergo synapsis (synaptonemal complex) and lie together to form bivalent structure
  • Homologous chromosomes interlink
  • Chiasmata form
52
Q

Describe the leptotene phase of prophase I

A

Long slender chromosomes visible

53
Q

Describe the zytogene phase of prophase I

A

Homologous chromosomes begin to pair up (synapsis)

54
Q

Describe the pachytene phase of prophase I

A
  • Synapsis complete

- Crossing over occurs forming chiasmata

55
Q

Describe the diplotene phase of prophase I

A
  • Chromosomes move apart a little

- Each bivalent remains attached at 1 or more chiasmata

56
Q

Describe the diakenesis phase of prophase I

A
  • Transition to metaphase

- 4 chromatids in each bivalent visible

57
Q

Describe metaphse I

A
  • Nuclear enveope disappears
  • Bivalent chromosomes moe to equator of spindle
  • Centromeres of each bivalent chromosome oriente to opposite poles
58
Q

Describe anaphase I

A
  • Homologous chromosomes separate and migrate to opposite poles of spindle
  • One chromosome with 2 chromatids arrives at each pole
59
Q

Describe telophase I

A
  • Cytoplasmic division starts
  • Spindle breaks down
  • Nuclear envelope and nucleoli form
60
Q

Describe prophase II

A
  • Centrioles replicate, migrate to poles
  • New spindle forms at right angles to previous one
  • Non DNA replication
61
Q

Decribe metaphase II

A
  • Chromosomes move to equator of spindles

- Chromatids orientate to opposite poles

62
Q

Describe anaphase II

A

Chromatids separate and move apart to poles

63
Q

Describe telophase II

A
  • Spindle disappears
  • Nuclear envelope and nucleoli form
  • Chromosomes become thread like
64
Q

What cell type do the spermatogonia and oogonia develop from?

A
  • Primordial germ cells in the gonads
65
Q

What happens to the spermatogonia at sexual maturity?

A
  • Proliferate by mitosis
  • Some remain undifferentiated (spermatogonia stem cells)
  • Others differentiate by mitosis
66
Q

When do the spermatogonia stem cells enter meiosis?

A

Continually from puberty onwards (influence by androgens)

67
Q

When does the differentiation to mature sperms occur?

A

After meiosis is complete

68
Q

Give a basic overview of spermatogenesis

A
  • primary spermatocytes in testis undergo 1st meiotic division = secondary spermatocytes
  • 2nd meiotic division to form spermatids
  • Differentiate into spermatozoa
69
Q

Give a basic overview of oogenesis

A
  • Primary oocyte by first meiotic division to produce secondary oocyte and 1st polar body
  • Second meiotic divison = single haploid ovum and second polar body
70
Q

What are polar bodies?

A

Packages of chromosomes produced by oogenesis that will not be used any further

71
Q

Describe the proliferation of oogonia

A

Proliferate by mitosis inside ovary until enter meiosis

72
Q

When does meiosis I and II occur in oogonia development

A
  • Meiosis I starts before birth
  • Primary oogonia remain in prophase I until puberty
  • Resumed at each oestrus cycle to give secondary oocyte
  • Arrested at metaphase II
73
Q

When is meiosis II of oogonia completed?

A

After fertilisation

74
Q

What is the importance of exchanging alleles?

A
  • Variation in each generation

- Avoids genetic copies and thus inbreeding

75
Q

How can genetic reassortment take place?

A
  • Independent assortment of chromosomes
  • Recombination (crossing over)
  • Mutation
  • Segregation of genes (alleles)
  • Differences in gene frequency (chance/selection_
76
Q

Describe the independent assortment of chromosomes in genetic reassortment

A
  • Random orientation of homologous chromosomes at metaphase of meiosis I
  • Different possibilities for alignement of homologous chromosomes
  • Method of alignment affects outcome
  • Increases variation
77
Q

Describe recombination during crossing over in genetic reassortment

A
  • Recombinant chromosomes carry genes from 2 parents
  • also affects linked genes
  • Increased by random fertilisation
  • Crossing over occurs only between short segments at end of X and Y in sex chromosomes (pseudoautosomal region)
78
Q

How does random fertilisation increase gneetic variation on top of recombination?

A

Different gametes carry different combinations which are then rearranged again

79
Q

Explain gene dosage compensation in females

A
  • Inherit 2 X chromosomes
  • One randomly inactivated in embryo
  • Only one active copy of X linked genes
80
Q

How does X chromosome inactivation take place?

A
  • Methylation of DNA

- Expression of antisense transcript (XIST - X-inactive-specific-transcript)

81
Q

What is the result of X-chromosome inactivation?

A
  • Only one active X in each cell
  • Have 2 cell types: half from active X of father, half from active X of mother
  • Different alleles present on the X chromosomes reults in mosaic of cells expressing different alleles
82
Q

Explain dosage compensation and genomic imprinting

A
  • some sections of autosomes inactivated for dosage compensation
  • = Genomic imprinting, genes involved are “imprinted genes”
  • Due to epigenetic regulation: DNA methylation, non-coding RNA, histone modifications
  • imprinted genes are monoallelic expressed (either from maternal or patenral allele)
83
Q

What are imprinted genes involved in?

A
  • Embryonic growth
  • Placental development
  • Metabolism
84
Q

Compare somatic and germ line mutations

A
  • Somatic can cause disease (e.g. cancer) but cannot be passed to next generation
  • Germ line are transmitted to future generations
85
Q

Give an example of germ line chromosomal abnormalities

A
  • Chromosomes remain linked (non-disjunction)
  • Can happen at meiosis I or meiosis II
  • Leads to abnormal number of chromosomes in germ cells
86
Q

What is aneuploidy and how does it occur?

A
  • Fertilisation of gametes in which non-disjucntion has occured
  • Abnormal number of a particular chromosome
87
Q

What is a monosomic zygote?

A

Has one copy of a particular chromosome

88
Q

What is a trisomic zygote?

A

Has 3 copies of a particular chromosome

89
Q

What is polyploidy?

A

A condition in which an organism has more than 2 complete sets of chromosomes

90
Q

What are the 4 types of changes that can occur due to breakage of a chromosome

A
  • Deletion removes chromosomal segment
  • Duplication repeats a segment
  • Inversion reverses a segment within a chromosome
  • Translocation moves a segment from one chromosome to a another
91
Q

Give examples of disorders due to chromosomal defects

A
  • Downs syndrome (aneuploid condition, 3 copies of chromosome 21)
  • Klinefelter syndrome
  • Tortoiseshell males (XXY)
  • Monosomy X, aka Turner syndrome in humans, X0 females who are sterile

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