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Meiosis Flashcards

(63 cards)

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1
Q

What are the stages and substages of meiosis?

A

1) Interphase
2) (Meiosis 1)
3) Prophase 1
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakenesis
4) Metaphase 1
5) Anaphase 1
6) Telophase 1
7) Interphase 1
8) (Meiosis 2)
9) Prophase 2
10) Metaphase 2
11) Anaphase 2
12) Telophase 2

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2
Q

What happens in interphase?

A

The cell is carrying out
its normal activities. It is
not clear if the cell is in
G1, S-phase or G2.

Cells in S-phase are
replicating their DNA

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3
Q

Interphase is the […] stage

A

Interphase is the longest stage

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4
Q

Interphase has 3 stages:

A

G1, S, G2

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5
Q

What is G1 phase?

A
  • housekeeping
  • before S phase each chromosome is present as one DNA molecule (chromatid).
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6
Q

What is S phase?

A
  • synthesis of DNA
  • after S phase each chromosome is present as two DNA molecules
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7
Q

What is G2 phase?

A
  • preparation for cell division
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8
Q

Substages of prophase 1?

A

Leptotene
Zygotene
Pachytene
Diplotene
Diakenesis

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9
Q

What happens in prophase 1?

A
  • Zygotene
  • Pairing takes place between homologous chromosomes.
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10
Q

What does Homologous Chromosomes mean?

A
  • This term means ‘the same’
  • Here it refers to chromosomes that generally have the same genes and similar DNA sequences but these DNA sequences are not identical.
  • In the case of human chromosomes each ‘pair’ of chromosomes is homologous e.g. the two chromosome 21’s
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11
Q

In prophase 1, a protein structure synaptonemal complex) forms between …

A

…the homologous chromosomes holding them together

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12
Q

In prophase 1, the pairing begins at …

A

…a few sites along the chromosomes and brings corresponding parts together

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13
Q

Describe Prophase 1 Pachytene

A

The homologous chromosomes are fully paired and contracted

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14
Q

Whats the name of the protein structure which forms in prophase 1 between the homologous chromosomes holding them together?

A

synaptonemal complex

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15
Q

What occurs in pachytene?

A
  • Crossing over / recombination occurs.
  • For each crossing over event the DNA breaks in each of two non- sister chromatids followed by crosswise rejoining of the broken ends.
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16
Q

Define Chiasmata

A
  • Sites of crossing over, occur in synapsis.
  • Exchange of genetic material between non-sister chromatids.
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17
Q

Crossing over produces…

A

… recombinant chromosomes.

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18
Q

What happens in Diplotene?

A
  • The synaptonemal complex breaks down
  • The homologous chromosomes are held together by the crossovers.
  • (Each chromosome at this stage comprises two sister chromatids)
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19
Q

What happens in Chiasma?

A
  • The homologous chromosomes now ‘repel’ each other and reorganise themselves in three dimensional space.
  • The result is often a cross shape called a chiasma
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20
Q

There is at least one and usually more crossovers for …

A

…‘pair’ of homologous chromosomes

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21
Q

Crossovers have two roles:

A
  • to hold the homologous chromosomes together after the synaptonemal complex has broken down; this ensures that the homologous chromosomes separate into the new cells correctly (segregate)
  • to generate new arrangements of DNA (genetic variation)
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22
Q

Homologous chromosomes usually carry the same gene but …

A

…different versions (alleles) e.g. A and a

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23
Q

After crossovers, the different alleles in homologous chromosomes are …

A

…physically recombined into new orders

  • Resulting in genetic variation
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24
Q

What happens in Diakinesis?

A
  • The chromosome ‘pairs’ continue to condense
  • If the chromosomes are held as a pair they are called bivalents
  • If the chromosomes are held in threes they are called a trivalent.
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25
What happens in Metaphase 1?
1) The chromosome ‘pair’ is attached to the spindle fibres by their centromeres 2) The chromosome ‘pairs’ move towards the equator
26
How many chromosomes and chromatids does a bivalent have?
- 2 chromosomes - 4 chromatids
27
In Metaphase, the centromeres line up in the...
...centre of the cell (the equator)
28
What happens in anaphase 1?
- The attraction between the sister chromatids lapses. - The chromosomes move apart to opposite poles - At this stage in a diploid organism each new cell becomes haploid
29
What happens in Telophase 1?
- The chromosomes relax and become long and thin again - In some species a cell wall forms between the two products (sometimes called interkinesis) - These two cells now progress through the rest of meiosis independently, though often in synchrony
30
The interphase stage does not usually occur in the...
...middle of meiosis
31
Where there is an interphase ...
...no DNA replication occurs (no S Phase)
32
There is no [...] phase in meiosis when there is an [...]
There is no S phase in meiosis when there is an interphase.
33
What would the outcome from Meiosis be if the organism was tetraploid?
4 diploid cells
34
WHat happens in prophase 2
- Early prophase 2 looks similar to prophase 1 - But each visible structure is one chromosome
35
What happens in Metaphase 2?
Each chromosome attaches to the spindle by its centromere and is drawn to the equator.
36
How many chromosomes and chromatids in metaphase 2?
One chromosome, two chromatids
37
What happens in anaphase 2?
The chromatids separate and move to opposite poles
38
How many chromatids in anaphase 2?
One chromatid
39
What happens in telophase 2?
- Four new cells with nuclear membranes form - Each cell usually has half the original number of chromosomes
40
Outcome from Meiosis?
- Four cells (compared to two in mitosis) - Not genetically identical: each crossover results in two recombined chromatids
41
What would happen if the organism was triploid? (three copies of each chromosome)
The chromosome number may be halved
42
in triploids (3x), pentaploids (5x) etc. ...
...aberrations (e.g. non disjunction) are common.
43
During normal cell growth, mitosis produces ...
...daughter cells identical to parent cell (2n to 2n)
44
Meiosis results in...
...genetic variation
45
Meiosis results in genetic variation ...
...by shuffling of maternal and paternal chromosomes and crossing over.
46
No daughter cells formed during meiosis are ...
...genetically identical to either mother or father
47
During sexual reproduction, fusion of the unique haploid gametes produces ...
...truly unique offspring.
48
How many possible combination in independent assortment?
4 possible combinations
49
How many possible combinations in haploid humans?
8 million possible combinations!
50
64 trillion combinations for ...
...a diploid zygote!!!
51
When does dna replication occur in mitosis?
Occurs during interphase before nuclear division begins
52
When does dna relplication in meiosis occur?
Occurs once, during the interphase before meiosis I begins.
53
Number of divisions in mitosis?
One, including prophase, metaphase, anaphase, and telophase
54
Number of divisions in meiosis?
Two, each including prophase, metaphase, anaphase, and telophase.
55
Synapsis of homologous chromosomes in mitosis?
Does not occur
56
Synapsis of homologous chromosomes in meiosis?
Synapsis is unique to meiosis: During prophase 1, the homologues chromosomes join along their length, forming tetrads (groups of four chromatids)l synapsis is associated with crossing over between non sister chromatids.
57
Number of daughter cells and genetic compostion of mitosis?
Two, each diploid (2n) and genetically identical to the parent cell.
58
Number of daughter cells and genetic composition of meiosis?
Four, each haploid (n), containing half as many chromosomes as the parent cell, genetically non identical to the parent cell and to each other.
59
Role of mitosis in the animal body?
Enables multicellular adult to arise from zygote; produces cells for growth and tissue repair
60
Role of meiosis in the animal body.
Produces gametes; reduces chromosome number by half and introduces genetic variability among the gametes.
61
Define life cycle
sequence of stages in organisms reproductive history; conception to reproduction.
62
Define somatic cells
any cell other than gametes, most of the cells in the body.
63
Gametes are produced by...
... meiosis

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