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IB Biology > 3.3 Meiosis > Flashcards

3.3 Meiosis Flashcards

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1
Q

What is meiosis?

A

Meiosis is the process by which sex cells (gametes) are made in the reproductive organs

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2
Q

What does meiosis involve?

A

It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei

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3
Q

What does the first meiotic division involve?

A

The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)

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4
Q

What does the second meiotic division involve?

A

The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)

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5
Q

What is mitosis preceded by and what is its purpose?

A

Meiosis is preceded by interphase, during which DNA is replicated (in the S phase) to produce two genetically identical copies

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6
Q

What are the two identical DNA molecules termed as and what are they held together by

A

The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere

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7
Q

When are sister chromatids separated?

A

The sister chromatids are separated during meiosis II, following the separation of homologous chromosomes in meiosis I

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8
Q

What does meiosis consist of?

A

Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)

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9
Q

What phase occurs between meiosis I and meiosis II?

A

A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage

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10
Q

What is the first meiotic division termed as?

A

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

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11
Q

What occurs in P-1? (prophase in meiosis I)

A

P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs

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12
Q

What occurs in M-1?

A

M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell

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13
Q

What occurs in A-1?

A

A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell

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14
Q

What occurs in T-1?

A

T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

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15
Q

What is meiosis II termed as?

A

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

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16
Q

What is P-II?

A

P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)

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17
Q

What is M-II?

A

M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator

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18
Q

What is A-II?

A

A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles

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19
Q

What is T-II?

A

T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells

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20
Q

What is the final outcome of meiosis?

A

The final outcome of meiosis is the production of four haploid daughter cells

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21
Q

What is the difference between the daughter cells produced by meiosis?

A

These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)

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22
Q

What process fo homologous chromosomes undergo in prophase I (P-I)?

A

In prophase I, homologous chromosomes undergo a process called synapsis, whereby they pair up to form a bivalent (or tetrad)

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23
Q

What points are homologous chromosomes held together at?

A

The homologous chromosomes are held together at points called chiasmata (singular: chiasma)

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24
Q

Where can crossing-over occur?

A

Crossing over of genetic material between non-sister chromatids can occur at these chiasmata

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25
Q

What does crossing over lead to?

A

As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination)

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26
Q

What happens once chiasmata are formed?

A

Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis

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27
Q

How does crossing over affect daughter cells?

A

If crossing over occurs then all four haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)

28
Q

What do homologous chromosomes do in metaphase I?

A

During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:

Maternal copy left / paternal copy right OR paternal copy left / maternal copy right

29
Q

What is random in metaphase I?

A

This orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes

30
Q

How will random assortment affect gametes?

A

The final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome following anaphase I

31
Q

What will affect the number of possible gamete combinations?

A

As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs

Gamete combinations = 2n (where n represents the haploid number)

32
Q

Why do organisms need to make gametes?

A

Most sexually reproducing organisms are diploid, meaning they have two copies of every chromosome (maternal / paternal copy)

In order to reproduce, these organisms need to make gametes that are haploid (one copy of each chromosome)

33
Q

What will the fusion of two haploid gametes result in?

A

Fertilisation of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis

34
Q

What can happen if chromosome number was not halved in gametes?

A

If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)

35
Q

What is the advantage of meiotic division?

A

The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring

36
Q

What are the 3 main sources of genetic variation?

A

Crossing over (in prophase I)
Random assortment of chromosomes (in metaphase I)
Random fusion of gametes from different parents

37
Q

What is crossing over?

A

Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I

38
Q

Where does the exchange of genetic material occur?

A

The exchange of genetic material occurs between non-sister chromatids at points called chiasmata

39
Q

What is a consequence of crossing over, how do chromosomes differ?

A

As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different

40
Q

What are recombinants?

A

Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants

41
Q

What will offspring with recombinant chromosomes posses?

A

Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent

42
Q

What is random orientation?

A

When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random

43
Q

What does the orientation of each bivalent occurring independently mean?

A

The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I

44
Q

What does the orientation of each bivalent occurring independently mean?

A

The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I

45
Q

What is the total number of combinations that can occur in gametes?

A

The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes

46
Q

How many chromosomes do humans have and therefore how many gametes combos can be formed?

A

Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (2^23) by random orientation

47
Q

How many chromosomes do humans have and therefore how many gametes combos can be formed, considering crossing over?

A

If crossing over also occurs, the number of different gamete combinations becomes immeasurable

48
Q

What happens to the zygote?

A

This zygote can then divide by mitosis and differentiate to form a developing embryo

49
Q

What will generate different zygotes?

A

As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes

50
Q

When are identical twins formed?

A

Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses

51
Q

What is non-disjunction?

A

Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)

52
Q

What can be two causes of non-disjunction?

A

Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)

Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)

53
Q

What will happen if a zygote is formed from a gamete that has experienced a non-disjunction event?

A

If a zygote is formed from a gamete that has experienced a non-disjunction event, the resulting offspring will have extra or missing chromosomes in every cell of their body

54
Q

What are examples of conditions that arise from non-disjunction effects?

A

Patau’s Syndrome (trisomy 13)
Edwards Syndrome (trisomy 18)
Down Syndrome (trisomy 21)
Klinefelter Syndrome (XXY)
Turner’s Syndrome (monosomy X)

55
Q

What chromosomal abnormality do individuals with down syndrome present?

A

Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)

56
Q

How does a zygote therefore present trisomy 21 (what events lead to it0?

A

One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
The other parental gamete was normal and had a single copy of chromosome 21
When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21

57
Q

What may the chance of non-disjunction increase alongside?

A

There is a particularly strong correlation between maternal age and the occurrence of non-disjunction events

This may be due to developing oocytes being arrested in prophase I until ovulation as part of the process of oogenesis

58
Q

What is karyotyping?

A

Karyotyping is the process by which chromosomes are organised and visualised for inspection

59
Q

What can karyotyping be used for?

A

Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities

60
Q

How is karyotyping done in fetuses?

A

Cells are harvested from the foetus before being chemically induced to undertake cell division (so chromosomes are visible)

61
Q

What will determine whether chromosomes appear wth sister chromatids in karyotyping?

A

The stage during which mitosis is arrested will determine whether chromosomes appear with sister chromatids

62
Q

What is done to chromosomes so they’re visible on a karyogram?

A

Finally, chromosomes are stained and photographed, before being organised according to structure

The visual profile generated is called a karyogram

63
Q

What does chorionic villi sampling (CVS) involve?

A

Chorionic villi sampling involves removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix

It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)

64
Q

What does amniocentesis involve?

A

Amniocentesis involves the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle

65
Q

Does amniocentesis or CVS have a higher risk of miscarriage?

A

It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)

AMNIOCENTESIS
It is usually conducted later than CVS (~16 weeks of pregnancy) with a slightly lower risk of miscarriage (~0.5%)

66
Q
A

IB Biology (38 decks)

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