Topic 12 - Meiosis and Sexual Reproduction

Question 1

Under what environmental conditions is sexual reproduction advantageous?

Answer 1

Sexual reproduction produces offspring with new, unique combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments.

Question 2

Under what environmental conditions is asexual reproduction advantageous?

Answer 2

In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment.

Question 3

What is meiosis?

Answer 3

A form of cell division that produces 4 haploid cells known as gametes

Question 4

Gametes

Answer 4

Specialized sex cells (i.e. eggs or sperm) that carry half the number of chromosomes of the parent cell, which combine during fertilization to form new organisms

Question 5

Sexual reproduction

Answer 5

The fusion of gametes (fertilization) to produce a diploid zygote

Question 6

Homologous chromosomes

Answer 6

Matched chromosome pairs containing the same genes in identical locations along their lengths (but are NOT genetically identical)

Question 7

True or false: diploid organisms inherit one copy of each homologue from each genetic contributor (mother/father)

Answer 7

True

Question 8

Autosome

Answer 8

Non-sex chromosome

Question 9

Sex chromosomes

Answer 9

Determine sex of the offspring; XX or XY in humans

Question 10

What happens to the chromosome number of a cell after meiosis? Describe briefly how the process of meiosis ensures that offspring have the same amount of DNA as their parents.

Answer 10

Chromosome number is reduced by half in meiosis. During sexual reproduction, gametes fuse to produce a diploid zygote, ensuring the offspring have the same amount of DNA as their parents.

Question 11

What are the key events in meiosis I and meiosis II?

Answer 11

Meiosis I
- Synapsis and crossing over occurs during prophase I
- Independent assortment occurs during metaphase I
- Homologous chromosomes separate during anaphase I
Meiosis II
- Sister chromatids separate during anaphase II
- The cell becomes haploid and the chromosome number is reduced by half during anaphase II

Question 12

Describe what happens in prophase I.

Answer 12

• As the nuclear envelope begins to break down, the proteins associated with the homologous chromosomes bring the pair closer together in a synaptonemal complex - this process is known as synapsis
• Crossing over: exchange of chromosomal segments between two homologues’ non-sister chromatids
• Chiasmata: sites of crossing over
• Crossing over events are the first source of genetic variation in the nuclei produced by meiosis

Question 13

Describe what happens in metaphase I.

Answer 13

• Homologues are held together at chiasmata
• Microtubules from opposite poles attach to each homologue
• Homologues align at metaphase plate randomly (independent assortment)
• Independent assortment is the second mechanism that introduces variation in gametes

Question 14

Describe what happens in anaphase I.

Answer 14

• Chiasmata are broken in anaphase I as homologues are pulled apart by microtubules
• Homologues are separated while sister chromatids remain tightly bound together
• Segregation of alleles occurs at this phase

Question 15

Describe what happens in telophase I.

Answer 15

• Nuclear envelope reforms around each set of chromosomes
• At each pole, there is only one chromosome set (while each chromosome still consists of two sister chromatids)
• Sister chromatids are no longer identical due to crossing over
• Cell is prepared for meiosis II

Question 16

Describe what happens during meiosis II

Answer 16

Meiosis II resembles a mitotic division.
Prophase II
- Nuclear envelope dissolves and spindle apparatus forms
Metaphase II
- Chromosomes align at metaphase plate
Anaphase II
- Sister chromatids are separated
- Cell becomes haploid and chromosome number is reduced by half
Telophase
- Nuclear envelope reforms; cytokinesis follows, resulting in 4 haploid (n) cells

Question 17

Compare mitosis and meiosis.

Answer 17

Function
- Mitosis: growth and development
- Meiosis: gamete (sex cell) production
Location in body
- Mitosis: all body tissues except gonads
- Meiosis: gonads
Generates variation (Y/N?)
- Mitosis: N
- Meiosis: Y
Rounds of cell division
- Mitosis: 1
- Meiosis: 2
n → n or 2n → n?
- Mitosis: n → n
- Meiosis 2n → n
Number of resulting cells
- Mitosis: 2 genetically identical cells
- Meiosis: 4 genetically distinct cells

Question 18

Why is genetic variation advantageous for a population of organisms?

Answer 18

Genetic variability allows for the possibility of some offspring to survive certain environmental conditions through natural selection. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success.

Question 19

Explain how crossing over, independent assortment, and random fertilization create genetic diversity.

Answer 19

Crossing over/recombination
- During prophase I, the homologous pairs all line up with each other and form a tetrad (bundle of four chromosomes.) The non-sister chromatids (maternal/paternal) exchange portions of their chromosome. The maternal chromatid will have some genetic information from the paternal chromatid and vice versa. This helps to increase genetic variation as it allows for new combinations of alleles to be inherited by the offspring.
Independent assortment
- During metaphase I, homologous chromosomes all line up at the metaphase plate of the cell. Each pair will sort independently of every other pair (maternal/paternal chromosomes will face opposite poles in various combinations independent of one another.) The resulting nucleus will therefore have a combination of maternal and paternal chromosomes in it before it moves into meiosis II.
- Random fertilization
Alleles are rejoined at random, one from each parent, during fertilization to produce a diploid zygote. For each chromosome pair, one homologous chromosome came from each parent. Genes are arranged in the same order, but with small variations, contributing to genetic diverstiy.