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Meiosis Flashcards

1
Q

What is meiosis?

A

A type of nuclear division where a nucleus divides to produce four daughter nuclei that is genetically different from parents
- each daughter nuclei contains half the number of chromosomes of the original nucleus

Occurs in formation of gametes in animals and spores in plants

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2
Q

What are the features of a homologous chromosome?

A
  • have same size and centromeres in the same position
  • have identical sequence of gene loci (order of genes)
  • pair with each other during prophase I of meiosis
  • contain same genes but need not be identical (different alleles)
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3
Q

What is karyotype?

A

A display of chromosome pairs of a cell arranged by shape and size

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4
Q

What are the four stage of meiosis I/II ?

A

(PMAT)
Prophase I/II
Metaphase I/II
Anaphase I/II
Telophase I/II

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5
Q

What happens in interphase?

A
  • DNA replicates during S phase of internarse
  • DNA are in the form of thread like chromatin
  • centrioles replicate just right before meiosis I
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6
Q

What happens in prophase I ?

A
  • chromatin begins to condense to form chromosomes
  • homologous chromosomes pair up through a process called synapsis
  • each pair of homologous chromosomes make up bivalent / tetrad
  • nucleolus and nuclear envelope disintegrates
  • centrioles move to opposite poles and spindle fibres form
  • chiasmata (singular : chiasma) forms enabling crossing over to occur between the two non-sister chromatids
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7
Q

What are chiasmata?

A

Sites where non-sister chromatids of homologous chromosomes physically break and rejoin - a portion of one non-sister chromatid exchanges places with an equivalent portion of the other non-sister chromatid

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8
Q

What does the chiasmata do?

A
  • enables exchange of genetic materials between the non-sister chromatids of homologous chromosomes through crossing over
  • help hold the two homologous chromosomes together as they position themselves during metaphase I
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9
Q

What is crossing over?

A

The physical breakage and rejoining of two non-sister chromatids of the pair of homologous chromosomes at the chiasmata
- it involves the exchange of genetic materials between homologous chromosomes resulting in the formation of new combination of alleles

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10
Q

What happens during metaphase I ?

A
  • pairs of homologous chromosomes are arranged along the equator of the metaphase plate (horizontal)
  • spindle fibres are attached to the centromeres of the chromosomes via the kinetochore
  • independent assortment : arrangement of chromosomes of each bivalent ( left or right ) is independent of the orientation of other bivalents
    - total possible orientation = 2 power n (where n = no. of homologous pairs)
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11
Q

How to find the total number of possible orientations of homologous chromosomes in metaphase I (independent assortment) ?

A

Total possible orientation = 2 to the power of n (n= no. of homologous pairs)

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12
Q

What happens in anaphase I ?

A
  • homologous chromosomes separate and move towards the opposite poles due to shortening of the centromere-to-pole spindle fibres
  • sister chromatids remain attached at the centromere
  • characterised by distinct V-shape of the chromosomes with centromere leading
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13
Q

What happens in telophase I ?

A
  • homologous chromosomes reach opposite poles of the spindle
  • spindle fibres disintegrate
  • nuclear envelope and nucleolus reform (one at each pole)
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14
Q

Does telophase I always happen?

A

No
- in many plants there is no telophase I or interphase, the cell passes straight from anaphase I to prophase II where replication of the centrioles occur

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15
Q

What happens after telophase I ?

A

In some cells the nucleus enters interphase II (chromosomes uncoil) but there is no DNA replication

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16
Q

What happens in prophase II?

A
  • in cells where telophase I and interphase occured, the nucleolus and nuclear envelope disintegrate
  • centrioles (if present move to opposite poles)
  • spindle fibres form at right angles to the spindle axis of meiosis I
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17
Q

Why are spindle fibres in prophase II not formed along the same axis / why are the chromosomes not lines up along the same axis as in metaphase I (horizontal) ?

A

Ensures that the cell doesn’t keep dividing at the same equator which will result in distortion of cell shape

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18
Q

What happens in metaphase II?

A
  • chromosomes arrange themselves at the equator of the new spindle (vertical)
  • centromere of each chromosome is attached to the spindle fibre
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19
Q

Are the sister chromatids in meiosis II genetically identical ?

A

If crossing over took place in meiosis I, the two sister chromatids will not be genetically identical

20
Q

What happens in anaphase II?

A
  • centromere of each chromosome divide and separate
  • sister chromatids of each chromosome separate and move toward opposite poles showing a distinct V-shape with centromeres leading
  • the separated chromatids of each chromosome are now known as two individual chromosomes
21
Q

What happens in telophase II ?

A
  • chromosomes reach opposite poles of the spindle
  • chromosomes uncoil, lengthen and become indistinct
  • spindle fibres disintegrate
  • nuclear envelope and nucleolus reform
22
Q

What is the result of meiosis (I and II)?

A

4 daughter cells each being haploid and genetically different from the parent and other daughter cells

23
Q

What are the significances of meiosis?

A
  1. Preventing doubling of chromosomes in sexual reproduction
  2. Generates genetic variation
24
Q

How does meiosis prevent doubling of chromosomes in sexual reproduction?

A

During fertilisation, haploid gametes fuse to produce a zygote and restores the diploid number of chromosomes
- in human haploid sperm (23 chromosomes) fuses with haploid ovum (23 chromosomes) to form diploid zygote (23 pairs of chromosomes)

Meiosis (reduction division) involves the formation of haploid gametes which are a result of the separation of homologous chromosomes during anaphase I
- if meiosis did not occur prior to fertilisation, fusion of male and female gametes would result in a doubling of the number of chromosomes for each successive generation

25
How does meiosis generate variation?
Meiosis results in new combination of alleles in gametes through two mechanisms : 1. Chiasmata formation and crossing over between non-sister chromatids of homologous chromosomes 2. Independent assortment of homologous chromosomes
26
How does chiasmata formation and crossing over between non-sister chromatids of homologous chromosomes result in genetic variation?
Formation of chiasmata between homologous chromosomes during prophase I allows for crossing over to occur between non-sister chromatids = exchange of generic materials (alleles) between the pair of homologous chromosomes = new combination of alleles on the chromosomes
27
How does independent assortment of homologous chromosomes lead to genetic variation?
- orientation of each pair of homologous chromosomes at the equator during metaphase I are random - subsequent separation of homologous chromosomes at anaphase I results in random assortment of maternal and paternal chromosomes in gametes
28
Other than meiosis, what else contributes to genetic variation ?
Random fusion of gametes during fertilisation/sexual reproduction - fusion of male and female gamete from each parent is random - total possible variation = 2 power n power 2
29
How are changes DNA content (C) monitored?
Through incorporation of radioactive nitrogenous bases into the cells during S phase and subsequent detection by autoradiography
30
What is the DNA content (C) and chromosome number (n) of diploid cells?
DNA content : 2C Chromosome number : 2n
31
How to count the number of chromosomes?
Number of chromosomes = number of centromere - increase only when sister chromatids separate in anaphase of mitosis and A II of meiosis II - decrease only when cell divides
32
How to count number of DNA molecules?
Number of DNA molecules = number of chromatids - increase only when DNA replicates (S phase) - decrease only when cell divides
33
What does the normal human karyotype consist of ?
46 chromosomes - 22 pairs of autosomes (non-sex chromosomes) arranged by decreasing length - 1 pair of sex chromosomes (XX in female and XY is male)
34
What are the two different types of chromosomal aberrations?
1. Change in structure of chromosomes 2. Change in number of chromosomes
35
When does structural aberration occur?
During prophase I of meiosis, homologous chromosomes become intertwined providing opportunity for various kinds of structural aberration to occur
36
What are the four different structural aberrations?
1. Deletion : loss of a portion of chromosome 2. Insertion : portion of chromosomes detaches and reattaches in inverted positions 3. Translocation : portion of chromosome detaches and rejoins at different point on the same or different chromosome 4. Duplication : portion of chromosome replicates so that a set of genes is repeated
37
How does a change in chromosome number occur?
Non-disjunction that occurs in one, several or all the chromosomes within a nucleus resulting in change in chromosome number Abnormal events during mitosis can also cause a change
38
What is non-disjunction?
The failure of sister chromatids to separate during anaphase of mitosis or failure of homologous chromosomes to separate during anaphase I or failure of sister chromatids to separate during anaphase II of meiosis
39
What is a euploid?
An individual with the appropriate number of chromosomes for the species - human : 22 pair autosomes and 1 pair sex chromosomes
40
What is aneuploidy ?
Condition of the nucleus where there are one or several chromosomes more than or less than the diploid number of chromosomes
41
How does aneuploidy occur?
Non-disjunction of sister chromatids of one or several chromosomes during anaphase of mitosis OR non-disjunction of homologous chromosomes during anaphase I of of meiosis OR non-disjunction of chromatids during anaphase II of meiosis The gamete with the abnormal number of chromosomes will fuse with a normal haploid gamete and the zygote that results will contain abnormal number of chromosomes
42
What happens when non-disjunction occurs during meiosis I ?
Homologous chromosomes fail to separate : two gametes that lack that particular chromosome and two gametes with two copies of chromosomes each
43
What happens when non-disjunction occurs in meiosis II ?
Sister chromatids fail to separate : one gamete that lacks the particular chromosome, two normal gametes and one gamete with two copies of the chromosome
44
What is an example of aneuploidy?
Down syndrome aka trisomy 21 : extra chromosome 21 - physical growth delay - characteristic facial features (downward sloping eyes) - intellectual disability
45
What is polyploidy?
Condition of the nucleus where there are three or more times the haploid number of chromosomes
46
How does polyploidy occur ?
Non-disjunction of sister chromatids of all chromosome during anaphase of mitosis OR non-disjunction of all homologous chromosomes during anaphase I of meiosis OR non-disjunction of all chromatids during anaphase II of meiosis Fusion of normal haploid gamete with abnormal gamete results in a cell containing abnormal number of chromosomes (eg triploid) OR Fusion of two abnormal diploid gametes results in tetraploid cell (4n)

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