3.3 Meiosis

Question 1

What is meiosis?

Answer 1

• 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

Question 2

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

Answer 2

• 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)

Question 3

What comes before meiosis?

Answer 3

Interphase

Question 4

What occurs in interphase?(3)

Answer 4

• 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

Question 5

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

Answer 5

Interkinesis

Question 6

What is the process of meiosis I? (5)

Answer 6

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

Question 7

What is the process of meiosis II? (5)

Answer 7

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

Question 8

What is the final outcome of meiosis?

Answer 8

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

Question 9

What process do homologous chromosomes undergo?

Answer 9

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

Question 10

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

Answer 10

Chismata

Question 11

What is the result of genetic exchange in crossing over?

Answer 11

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

Question 12

What occurs once chismata are formed? (2)

Answer 12

• 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)

Question 13

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

Answer 13

• maternal copy left /paternal copy right
• paternal copy left / maternal copy right

Question 14

What is random assortment? (2)

Answer 14

• 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

Question 15

how to find the gamete combinations?

Answer 15

Gametes combination = 2^n

Question 16

What will the fertilisation of 2 haploid gametes result in?

Answer 16

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

Question 17

What will occur is the chromosome number is not halved?

Answer 17

The total chromosomes numbers would double each generation (polyploidy)

Question 18

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

Answer 18

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

Question 19

What is crossing over? (2)

Answer 19

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

Question 20

What is consequence of recombination? (3)

Answer 20

• 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

Question 21

What is random orientation? (2)

Answer 21

• 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

Question 22

What is random fertilisation?

Answer 22

• 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

Question 23

Why will random fertilisation always generate different zygotes?

Answer 23

As meiosis results in genetically distinct gametes

Question 24

what is non-disjunction?

Answer 24

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

Question 25

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

Answer 25

- 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)

Question 26

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

Answer 26

- patau's syndrome (trisomy 13) - edward's syndrome (trisomy 18) - down syndrome (trisomy 21) - klinefelter syndrome (XXY) - turner's syndrome (monosomy X)

Question 27

what does it mean if someone has down syndrome?

Answer 27

- 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

Question 28

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

Answer 28

- 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

Question 29

what else do studies suggest about non-disjunction?

Answer 29

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

Question 30

what is karyotying?

Answer 30

the process by which chromosomes are organised and visualised for inspection

Question 31

what is the process of karyotyping? (4)

Answer 31

- 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

Question 32

what is chronic villi sampling? (2)

Answer 32

- 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)

Question 33

what is aminocentesis?

Answer 33

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

Question 34

what are 2 ways that cells can be harvested?

Answer 34

- amniocentesis - chronic villi sampling