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Biology SL > 3.3 Meiosis > Flashcards

3.3 Meiosis Flashcards

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1
Q

What is meiosis?

A
  • The process by which sex cells (gametes) are made in the reproductive organs
  • the reduction division of a diploid germ line cell into 4 genetically distinct haploid nuclei
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2
Q

What are the two processes of cellular division in meiosis?(2)

A
  • the first meiotic division separated pairs of homologous chromosomes number (diploid -> haploid)
  • the second meiotic division separates sister chromatids (created by replication of DNA during interphase)
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3
Q

What comes before meiosis?

A

Interphase

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4
Q

What occurs in interphase?(3)

A
  • DNA is replicated to produce 2 genetically identical copies
  • 2 identical DNA molecules are identifies as sister chromatids and are held together by a single centromere
  • sister chromatids are separated during meiosis II following the separation of homologous chromosomes in meiosis I
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5
Q

What is the growth stage that may occur between meiosis I and II?

A

Interkinesis

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6
Q

What is the process of meiosis I? (5)

A

it is a reduction division (diploid -> haploid) where homologous chromosomes are separated
- PI: chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
- MI: spindle fibres from opposing centromeres connect to bivalents (@centromeres) and align them along the middle of the cell
- AI: spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
- TI: chromises decondense, nuclear membrane may reform, cell divides (cytokinesis) to form 2 haploid daughter cells

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7
Q

What is the process of meiosis II? (5)

A

The second division separates sister chromatids (these sister chromatids may not be identical due to crossing in PI)
- PII: chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
- MII: spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
- AII: spindle fibres contract and separate the sister chromatids, chromatids (called chromosomes) move to opposite poles
- TII: chromosomes decondense, nuclear membranes reforms, cells divide (cytokinesis) to from four haploid daughter cells

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8
Q

What is the final outcome of meiosis?

A

The production of 4 haploid genetically distinct daughter cells (if crossing occurs in PI)

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9
Q

What process do homologous chromosomes undergo?

A
  • Synopsis - whereby they pair up to from a bivalent (or tetrad)
  • homologous chromosomes are held at chismata
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10
Q

Where does crossing over of genetic material occur between non-sister chromatids?

A

Chismata

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11
Q

What is the result of genetic exchange in crossing over?

A
  • resulting in the exchange of genetically material , new gene combinations are formed on chromatids (recombination)
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12
Q

What occurs once chismata are formed? (2)

A
  • homologous chromosomes condense as bivalents and then separates in meiosis
  • If crossing occurs then all 4 haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)
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13
Q

What are the arrangements of homologous chromosomes metaphase I line up at the equator?

A
  • maternal copy left /paternal copy right
  • paternal copy left / maternal copy right
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14
Q

What is random assortment? (2)

A
  • the orientation of pairs of homologous chromosomes as the subsequent assortment of chromosomes into gametes
  • the final gametes will differ depending on whether they got the maternal or paternal copy of a chromosomes in anaphase I
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15
Q

how to find the gamete combinations?

A

Gametes combination = 2^n

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16
Q

What will the fertilisation of 2 haploid gametes result in?

A

Result in the formation of a diploid zygote that can grow via mitosis

17
Q

What will occur is the chromosome number is not halved?

A

The total chromosomes numbers would double each generation (polyploidy)

18
Q

What are the 3 main sources of genetic variation from sexual reproduction?

A
  • crossing over (PI)
  • random assortment of chromosomes (MI)
  • random fusion of gametes from different parent
19
Q

What is crossing over? (2)

A
  • involves the exchange of segments of DNA between homologous chromosomes during PI
  • the exchange of genetic material occurs between non-sister chromatids called chiasmata
20
Q

What is consequence of recombination? (3)

A
  • all four chromatids that comprise the bivalent will be genetically different
  • chromatids that consist of a combination of DNA derived from both homologous chromosomes (recombinant)
  • offspring with recombinant chromosomes will have unique combinations that are not present in either parent
21
Q

What is random orientation? (2)

A
  • When homologous chromosomes line up in MI their orientation towards the opposing poles is random
  • the orientation of each bivalent occurs independently meaning different combinations of mat/pat chromosomes can be inherited when bivalents separate in AI
22
Q

What is random fertilisation?

A
  • the fusion of haploid gametes results in the formation of a diploid zygote
  • the zygote can be divided by mitosis and differentiate to form a developing embryo
23
Q

Why will random fertilisation always generate different zygotes?

A

As meiosis results in genetically distinct gametes

24
Q

what is non-disjunction?

A

the chromosome failing to separate correctly resulting in games with 1 extra or missing chromosome (aneuploidy)

25
Q

how does the failure of chromosomes to separate occur? (2)

A
  • failure of homologues to separate in AI (resulting in 4 affected daughter cells)
  • failure of sister chromatids to separate in AII (resulting in only 2 daughter cells being affected)
26
Q

what are 5 examples of diseases which result due to a zygote from a gamete from a non-disjunction event?

A
  • patau’s syndrome (trisomy 13)
  • edward’s syndrome (trisomy 18)
  • down syndrome (trisomy 21)
  • klinefelter syndrome (XXY)
  • turner’s syndrome (monosomy X)
27
Q

what does it mean if someone has down syndrome?

A
  • they have 3 copies of chromosome 21 (from non-disjunction)
  • 1 of the parental gametes had 2 copies of chromosome 21 due to non-disjunction
  • other parental gamete is random was normal and had a single copy of chromosome 21
  • when the 2 gametes fused during fertilisation the resulting zygote had three copies of chromosome 21
28
Q

what did studies show about the chance of non-disjunction and age? (3)

A
  • chances of non-disjunction increase as the age of parent increase
  • especially maternal age (30)
  • due to developing oocytes being arrested in PI until ovulation as part of the process of oogenesis
29
Q

what else do studies suggest about non-disjunction?

A

there is higher incidence of chromosomal errors in offspring as a result of non-disjunction in MI

30
Q

what is karyotying?

A

the process by which chromosomes are organised and visualised for inspection

31
Q

what is the process of karyotyping? (4)

A
  • cells are harvested from the foetus before being chemically induced to undertake cell division (so chromosomes are visible)
  • the stage during which mitosis is arrested will determine whether chromosomes appear with sister chromatids
  • chromosomes are stained and photographer before being organised according to structure
  • the visual profile generated = karyogram
32
Q

what is chronic villi sampling? (2)

A
  • removing a sample of the placental tissue (chorionic villus) via a tube inserted through the cervix
  • can be done at -11 weeks of pregnancy (slight risk of inducing miscarriage)
33
Q

what is aminocentesis?

A
  • the exrtaction of a small amount of amniotic fluid (fetal cells) with a needle
  • can be done at -16 weeks (lower risk of miscarridge)
34
Q

what are 2 ways that cells can be harvested?

A
  • amniocentesis
  • chronic villi sampling

Biology SL (38 decks)

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