Meiosis

Question 1

What is meiosis?

Answer 1

• Process by which sex cells (gametes) are produced in the reproductive organs
• The gametes contain diploid nucleus (46 chromosomes) which divides by meiosis to produce four genetically distinct haploid nuclei

Question 2

What are the two cellular divisions in meiosis?

Answer 2

1. Meiotic division separates pairs of homologous chromosomes in half (diploid —> haploid) (46 —> 23)
2. Meiotic division separates sister chromatides (which were formed during interphase of DNA replication)

Question 3

What process follows meiosis and what happens?

Answer 3

• Interphase, during which DNA is replicated (in S phase) to produce 2 genetically identical copies
• The two identical DNA molecules are sister chromatids and held together by a centromere
• The identical sister chromatides are separated during meiosis 2, after which the homologous chromosomes are separated in meiosis 1

Question 4

What stages do meiosis 1 and 2 follow?

Answer 4

• Stages: prophase, metaphase, anaphase, telophase
• After meiosis, the DNA undergoes interphase in which the DNA is replicated to produce 2 sister chromatides per chromosome

Question 5

What is the phase called that may occur between meiosis 1 and 2?

Answer 5

• Interkinesis, no DNA replication occurs here

Question 6

Explain prophase of Meiosis 1.

Answer 6

• Cells begin with 2 copies of each chromosome (one mother, other father)
• Replicated chromosomes form homologous pairs called bivalents (2 chromosomes).
• Non-sister chromatids cross over at points called chiasmata and exchange segments of DNA.
• Spindle fibres grow from each pole of the
• Nuclear membrane dissolves.

Question 7

Explain metaphase of Meiosis 1.

Answer 7

• Homologous pairs align half-way between the 2 poles. Chromosomes show random orientation at the poles
• The spindle fibres attach to the centromeres of each chromosome and align them along the equatorial metaphase plate

Question 8

Explain anaphase of Meiosis 1.

Answer 8

• Spindle fibres contract and split the bivalent and the homologous chromosomes move to opposite poles of the cell
• The sister chromatids remain connected at the centromere and move to same pole

Question 9

Explain telophase of Meiosis 1.

Answer 9

• The first meiotic division ends when the chromosomes arrive at the poles. The chromatides partially uncoil and a nuclear membrane reforms around each nucleus (since they were dissolved before)
• Cytokinesis occurs and results in 2 haploid daughter cells

Question 10

Explain prophase of Meiosis 2.

Answer 10

• Separates sister chromatids (may not be identical due to crossing over)
• Chromosomes condense, centrioles migrate to opposite poles and spindle fibres start to form
• Nuclear membrane dissolves

Question 11

Explain metaphase of Meiosis 2.

Answer 11

• Spindle fibres attach to the centromere and connect each centromere to both poles
• Exert pressure to align sister chromatides at equator

Question 12

Explain anaphase of Meiosis 2.

Answer 12

• Spindle fibres contract and separate sister chromatids and move to opposite poles
• The separated sisters chromatides now called chromosomes

Question 13

Explain telophase of Meiosis 2.

Answer 13

• Sister chromatids reach opposite poles and uncoil. Nuclear membrane formation and cytokinesis follow.
• Meiosis is complete, product: four haploid daughter cells and are genetically distinct (due to crossing over is prophase 1)
• Meiosis 2 is same to Meiosis 1, just double amount undergoes meiosis to result in 4 haploid cells

Question 14

When does crossing over happen? Explain the process.

Answer 14

• In prophase 1
• Homologous chromosomes undergo synapsis, they pair up to form bivalent
• There are two sets of sister chromatids, each set is identical and joined at the centromere
• Crossing over occurs when equivalent portions of the non-sister chromatids are exchanged
• Crossing over occurs at the chiasmata

Question 15

What is the result of crossing over?

Answer 15

• This results in new combinations of alleles
• This contributes to genetic variation among the gametes. It also ensures that every gamete produced is genetically unique. Recombinant
• The bivalent undergo condensation and move toward the equatorial plate. Results in highly organised chromosomes.

Question 16

What happens during metaphase 1 in regard to random assortment?

Answer 16

• The pairs of homologous chromosomes (bivalents) after crossing over, align along the equatorial plate of the cell
• Either maternal copy left/paternal copy right OR paternal copy left/maternal copy right
• The orientation of the chromosomes is random. The final gametes will differ depending on whether they got the maternal or paternal copy of the chromosome
• This random assortment occurs for each homologous pair

Question 17

What parts of a sexual life cycle need to be haploid and which need to be diploid?

Answer 17

• The organisms are diploid, have two copies of every chromosome
• To reproduce, the organisms need to make gametes that are haploid
• The fertilization of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote
• If chromosome number was not halved in gametes, the total chromosome numbers would double each generation, not good

Question 18

What promotes genetic variation?

Answer 18

Through sexual reproduction due to:
- Crossing over
- Random assortment of chromosomes
- Random fusion of gametes from different parents

Question 19

Explain how crossing over promotes genetic variation.

Answer 19

• Exchange of genetic material between non-sister chromatids
• This recombination causes all four chromatids to be genetically different
• A mixture of the two homologous chromosomes are called recombinants
• Offspring with recombinant chromosomes have unique gene combinations, different to parents

Question 20

How does random orientation promote genetic variation?

Answer 20

• When the homologous chromosomes line up, their orientation towards the opposing poles is random
• Different combinations of maternal/paternal chromosomes can be inherited when bivalents separate in anaphase 1
• Number of combinations in gametes is 2^n (n= haploid no. of chromosomes). If 12 chromo., then there are 6 haploid chromo.

Question 21

How does random fertilisation promote genetic variation?

Answer 21

• Fusion of two haploid gametes results in a diploid zygote
• The zygote divides by mitosis and differentiate to form an embryo
• Since the gametes are genetically distinct, random fertilisation will always generate different zygotes

Question 22

What is non-disjunction?

Answer 22

• Chromosomes failing to separate correctly resulting in gametes with one extra or one missing chromosome
• Failure may occur in separating chromosomes in anaphase 1 (4 affected daughter cells)
• OR failure of sister chromatides to separate in anaphase 2 (two daughter cells affected)

Question 23

What are chromosomal abnormalities?

Answer 23

• If a gamete had a non-disjunction event and formed a zygote, the offspring will have extra or missing chromosomes in every cell
• E.g. Edwards Syndrome, Turner’s Syndrome
• Other syndromes might cause the embryo to not develop, hence down syndrome is the most common

Question 24

What is Down Syndrome?

Answer 24

• Have three copies of chromosome 21 (trisomy 21)
• Trisomy: having an extra copy of a single chromosome
• One parental gamete had two copies of chromosome 21, the other has normal

Question 25

Explain how studies show the influence of parental age on chances of non-disjunction.

Answer 25

• When age of parents increase, chances of non-disjunction increases
• Strong correlation between maternal age and non-disjunction
• Risk of chromosomal abnormalities increase significantly after maternal age 30
• Since maternal age is increasing, number of Down Syndrome offspring increase
• Non-disjunction in meiosis 1

Question 26

What is karyotyping?

Answer 26

• Chromosomes are organised and visualized for inspection, to determine the gender of an unborn or test for chromosomal abnormalities
  Types of karyotyping?
• Amniocentesis
• Chorionic villus sampling

Question 27

What is Amniocentesis? What are the risks?

Answer 27

• Preformed between weeks 14 and 20 of pregnancy
• Extracting amniotic fluid using a syringe needle, cells are cultured and karyotyped
  Risks:
• Infection, fetal trauma from the needle and miscarriage 0.5%

Question 28

What is chorionic villus sampling (CVS)? What are the risks?

Answer 28

• During weeks 10-13
• Fetal cells sampled by inserting a suctioning tool through the vagina or abdomen to reach the fetal cells in the chorion (later develops into placenta)
  Risks:
• Bleeding, infection and miscarriage 1% (higher than amniocentesis)