3.3 Meiosis

Question 1

Meiosis

Answer 1

a form of nuclear division that produces four haploid nuclei from one diploid nucleus
- Involved two cycles of division meiosis I and II
- Meiosis I separated pairs of homologous chromosomes to halve the chromosome number (diploid -> haploid)
- Meiosis II separates sister chromatids (created by the replication of DNA during interphase)

Question 2

Meiosis I Summary

Answer 2

• Prophase I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
• Metaphase I: Spindle fibers from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
• Anaphase I: Spindle fibers contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
• Telophase I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

Question 3

Meiosis II Summary

Answer 3

• Prophase II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
• Metaphase II: Spindle fibers from opposing centromeres attach to chromosomes (at centromere) and align them along the cell equator
• Anaphase II: Spindle fibers contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
• Telophase II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells

Question 4

Crossing Over

Answer 4

when equivalent portions of the non-sister chromatids are exchanged between homologous chromosomes and the points at which crossing over occurs are called chiasmata
- Can occur multiple times in the same tetrad
- As a result new gene combinations are formed on chromatids that were not present in either original chromosome
- If crossing over occur all four haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)

Question 5

Random Orientation/Random Assortment

Answer 5

The orientation of pairs of homologous chromosomes (paternal and maternal) is random and so is the subsequent assortment of chromosomes into gametes
The final gametes will differ depending on whether they get the maternal or paternal copy of a chromosome

Question 6

Reduction Division

Answer 6

Daughter cells will contain only half of the chromosomes present in the parent cell. Parent cells have four chromosomes and the daughter cells have two
The parent cells is a diploid and the daughter cell will be a haploid with only one version of each chromosome

Question 7

Sexual Life Cycle

Answer 7

• 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)
• Fertilization of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis
• If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)

Question 8

Three mechanism by which meiosis promotes genetic variation

Answer 8

Crossing over between non-sister chromatids (prophase I)
Random assortment of homologous chromosomes (metaphase I)
Fusion of gametes from different parents

Question 9

Crossing Over Genetic Variation

Answer 9

• As a consequence of crossing over all four chromatids that comprise the bivalent will be genetically different
• Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
• Offspring with recombination chromosomes will have unique gene combinations that are not present in either parent

Question 10

Random Orientation Genetic Variation

Answer 10

• The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes
• Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (223) by random orientation
• If crossing over also occurs, the number of different gamete combinations becomes immeasurable

Question 11

Random Fertilisation Genetic Variation

Answer 11

the fusion of two haploid gametes results in the formation of a diploid zygote
- As meiosis results in genetically distinct gametes random fertilization by egg and sperm will always generate different zygotes

Question 12

Non-Disjunction

Answer 12

chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing chromosome
- Failure of homologous 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)

Question 13

Chromosomal Abnormalities

Answer 13

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

Question 14

Down Syndrome

Answer 14

individuals will have three copies of chromosome 21
- One of the parental gametes had two copies of chromosome 21 as a result of nondisjunction
- The other parental gamete was normal and had a single copy of chromosome 21
- When the two gametes fuse during fertilization, the resulting zygote had three copies of chromosome 21

Question 15

Karyotyping

Answer 15

the process by which chromosomes are organized into karyograms
- Can be used to identify non-disjunction events leading to aneuploidy
- Cells are harvested from a fetus and induced to undergo division
- Chromosomes are then stained and photographed to produce a karyogram

Question 16

Techniques to harvest fetal cells

Answer 16

Chorionic villi sampling (cells are extracted from the placental tissue)
Amniocentesis (cells are extracted from the amniotic fluid)