Lecture IX: Meiosis

Question 1

What is asexual reproduction?

Answer 1

The creation of an offspring through cell division of a single parent. No new genetic variations in next generation.
Prokaryotes and some single celled Eukaryotes reproduce asexually through binary fission. Binary fission in Eukaryotes involves mitosis.

Question 2

What are the 4 types of asexual reproduction in multi-cellular eukaryotes?

Answer 2

• Budding
• Regeneration
• Rhizomes
• Parthenogenesis

Question 3

Briefly Describe Budding reproduction

Answer 3

Offspring grows out from the parent and eventually separates, becoming independent. All mitotic division.

Question 4

Briefly Describe Regeneration reproduction

Answer 4

Growing a new organism from a surviving piece of the parent through mitotic cell division.

Question 5

Briefly Describe Rhizomes reproduction

Answer 5

Modified stems from the parent plant that travel horizontally under the soil from which new plants can grow. All mitotic division.

Question 6

Parthenogenesis

Answer 6

Females produce offspring from unfertilized eggs.
Some of these organisms reproduce solely by parthenogenesis while others switch between sexual reproduction and parthenogenesis.

Question 7

What is sexual reproduction?

Answer 7

Reproduction involving the production of gametes (sperm and eggs) through meiosis followed by fertilization.
Involves two parents
Offspring are genetically unique from each other and from their parents.

Question 8

What is Meiosis?

Answer 8

Gametes (sperm and egg) are required for sexual reproduction.
Meiosis makes the gametes.

Question 9

Why is Meiosis important?

Answer 9

Human somatic cells contain 46 chromosomes
If at conception:
egg with 46 chromosomes + sperm with 46 chromosomes
= child with 92 chromosomes/cell

Number of chromosomes / cell would double each generation!

Meiosis prevents this.

Question 10

Haploid / Diploid?

Answer 10

A cell can be classified as haploid or diploid depending on the number complete sets of unique chromosomes it has in its nucleus.

The number of unique chromosomes is given the notation n.

Human somatic cells have 2 sets of unique chromosomes, one inherited from mom and one from dad.

Cells with two sets are called Diploid

The number of chromosomes in a diploid cell = 2n

Cells with only one set (a.k.a. egg and sperm cells) are called haploid.

The number of chromosomes in a haploid cell = n

In humans n = 23

Question 11

What are homologous chromosomes?

Answer 11

Copies of the same chromosome are called homologous chromosomes because they share the same genes.
They have the same genes, but may have variations in those genes.
Different variations of genes are called alleles.

Question 12

What are alleles?

Answer 12

Related to homologous chromosomes, different variations of genes are called alleles.

Question 13

What is the job of Meiosis?

Answer 13

Convert diploid precursor cells (stem cells) in the ovary or testis into haploid sex cells (egg or sperm). (2n –> n)

Question 14

Give the details of Meiosis:

Answer 14

A much longer process than mitosis. Meiosis takes several days in males and years to complete in females, whereas mitosis typically takes less than one day.

Meiosis involves 2 cell divisions: Meiosis I and Meiosis II

Daughter cells are genetically different than the original cell.

Bacteria can reproduce asexually through binary fission very quickly, generally within 1-3 hours. Some are known to reproduce every 20 minutes under ideal conditions.

Question 15

Difference between Meiosis I/II?

Answer 15

In 1, there are still sisteer chromatids attached to chromososes, in 2, there are no longer.

Question 16

Stages of Meiosis I?

Answer 16

Stages:

Prophase I
Metaphase I
Anaphase I
Telophase I

Cytokinesis

Question 17

Prophase I?

Answer 17

Same as in mitosis:

Centrosomes migrate to opposite poles.
Nuclear membrane break down.
Spindle fibers attach to centromeres of replicated chromosomes.

Homologous chromosomes form tetrads and exchange segments of DNA between non-sister chromatids.
Non-Sister chromatids exchange equivalent pieces of DNA. This is called crossing-over.

Genetic Variation is caused by the crossing over, sister chromatids are no longer identical on each replicated chromosome.

Question 18

Metaphase 1?

Answer 18

sister chromatids are no longer identical on each replicated chromosome. Homologous chromosomes remain side by side. One chromosome on either side of the equator.
Instead of chromosomes lining up, it’s tetrads (XX) that line up as homologous pairs

Question 19

Anaphase I?

Answer 19

Spindle fibers pull tetrads apart. Chromosomes are pulled to opposite poles, and Sister chromatids remain attached at their centromeres.

Question 20

Telophase I and Cytokinesis

Answer 20

Same as Mitosis

Because, only one of each unique type of chromosome is in each daughter cell not 2 as is the case in Mitosis.

Question 21

Result of Meiosis I

Answer 21

2 haploid daughter cells
Each cell is now haploid because it has only 1 set of unique chromosomes (which are still replicated).
However, sister chromatids are no longer identical due to crossing over in Prophase I.
*DNA does not replicate again before Meiosis II begins since it is still in a replicated state. Centrosomes must be replicated.

Question 22

Meiosis II

Answer 22

The second cell division of Meiosis
Similar as Mitosis, Sister chromatids are separated into daughter cells.

Stages:

Prophase IIMetaphase IIAnaphase IITelophase II

Cytokinesis

Question 23

Propahse II

Answer 23

Centrosomes migrate to opposite poles
Nuclear envelope breaks down
Spindle fibers attach to centromeres of replicated chromosomes

Question 24

Metaphase II

Answer 24

Chromosomes align in single file on the midline of the cell, just like in mitosis.
One sister chromatid on either side of the line.

Differences from Mitosis:
Sister chromatids are not identical due to crossing over during Prophase I of Meiosis.
There are half the number of chromosomes aligned compared to mitosis.

Question 25

Anaphase II

Answer 25

Spindle fibers pull sister chromatids apart at their centromeres.
Sister chromatids go to opposite poles of the cell.

Question 26

Telophase II and Cytokinesis

Answer 26

Chromosomes amass at each pole.
Nuclear membranes reform.
Cytokinesis yields 4 haploid daughter cells that are genetically unique.

Question 27

How does meiosis increase genetic variation?

Answer 27

Meiosis and fertilization are responsible for most of the variation that arises each generation.

Three mechanisms contribute to genetic variation within sexually reproducing organisms:

1. Crossing over
2. Independent assortment of chromosomes
3. Recombination of genes during fertilization