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Meiosis Flashcards

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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
Q

How does meiosis generate variation?

A

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
Q

How does chiasmata formation and crossing over between non-sister chromatids of homologous chromosomes result in genetic variation?

A

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
Q

How does independent assortment of homologous chromosomes lead to genetic variation?

A
  • 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
Q

Other than meiosis, what else contributes to genetic variation ?

A

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
Q

How are changes DNA content (C) monitored?

A

Through incorporation of radioactive nitrogenous bases into the cells during S phase and subsequent detection by autoradiography

30
Q

What is the DNA content (C) and chromosome number (n) of diploid cells?

A

DNA content : 2C

Chromosome number : 2n

31
Q

How to count the number of chromosomes?

A

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
Q

How to count number of DNA molecules?

A

Number of DNA molecules = number of chromatids
- increase only when DNA replicates (S phase)
- decrease only when cell divides

33
Q

What does the normal human karyotype consist of ?

A

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
Q

What are the two different types of chromosomal aberrations?

A
  1. Change in structure of chromosomes
  2. Change in number of chromosomes
35
Q

When does structural aberration occur?

A

During prophase I of meiosis, homologous chromosomes become intertwined providing opportunity for various kinds of structural aberration to occur

36
Q

What are the four different structural aberrations?

A
  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
Q

How does a change in chromosome number occur?

A

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
Q

What is non-disjunction?

A

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
Q

What is a euploid?

A

An individual with the appropriate number of chromosomes for the species
- human : 22 pair autosomes and 1 pair sex chromosomes

40
Q

What is aneuploidy ?

A

Condition of the nucleus where there are one or several chromosomes more than or less than the diploid number of chromosomes

41
Q

How does aneuploidy occur?

A

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
Q

What happens when non-disjunction occurs during meiosis I ?

A

Homologous chromosomes fail to separate : two gametes that lack that particular chromosome and two gametes with two copies of chromosomes each

43
Q

What happens when non-disjunction occurs in meiosis II ?

A

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
Q

What is an example of aneuploidy?

A

Down syndrome aka trisomy 21 : extra chromosome 21
- physical growth delay
- characteristic facial features (downward sloping eyes)
- intellectual disability

45
Q

What is polyploidy?

A

Condition of the nucleus where there are three or more times the haploid number of chromosomes

46
Q

How does polyploidy occur ?

A

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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