CHAPTER 11: MEIOSIS

Where does meiosis occur?

in germ-line cells (diploid cells) in the gonads (testes/ovaries)

What is gametogenesis?

The production of gametes (sperm/ova) - only have 1 set of chromosomes (haploids)

What happens in fertilization?

The nuclei of an egg and sperm cell fuse, producing a zygote (chromosome # is restored)

What is the goal of meiosis?

To reduce genetic material by half & simplify it (from 2n, diploid —> n, haploid)

What is asexual production?

Reproduction that generates genetically identical offspring as the product of mitotic division

What happens in meiosis I?

Homologous chromosomes pair and nonsister chromatids phsysically exchange chromosome segments, known as crossing-over

Produces 2 cells, each with half the diploid # of chromosomes, with each chromosome consisting of 2 chromatids

What happens in meiosis II?

Sister chromatids separate, daughter chromosomes segregate into 4 different cells, each with the haploid # of chromosomes

What is synapsis?

Homologues pair along their length

What is homologous recombination?

Genetic exchange (crossing over) occurs between homologous chromosomes

What is reduction division?

Meiosis involves 2 successive divisions with no replication of genetic material between them

What are alleles?

Alleles are versions of each gene present in the members of homologous pairs (ex: gene that encodes proteins have different alleles that might encode different versions of the protein or no protein at all)

Individuals (except identical siblings) have unique combinations of alleles in each homologous pair

When does recombination/crossing over occur?

Prophase I of Meiosis I - homologous chromosomes exchange genes, creates new chromosome with new combinations of alleles for genes A-F

Also creates a chiasmata (point of contact between non-sister chromatids of homologous chromosomes during meiotic recombination)

What is the first meiotic division called, and what does it result in?

“Reduction division” - result in daughter cells that contain 1 homologue from each chromosome pair

Are there DNA replication BETWEEN MEIOTIC DIVISION?

No, because there is no S phase after meiosis I

Does the 2nd meiotic division reduce # of chromosomes?

No, only separates the sister chromatids for each homologue

What happens in prophase I?

Sister chromatids fold and condense in the nucleus. Then 2 chromosomes of each homologous synapse, forming tetrads. Then, chromatids of homologous chromosomes cross over. Chiasmatas are marked by thickened spots.

What happens in prometaphase I?

• Nuclear envelope breaks down

• Spindle enters the former nuclear area

• 2 chromosomes of each pair attach to kinetochore microtubules leading to opposite spindle poles

• Sister chromatids remain attached to each other

Also, not all chromosomes exchange materials

What happens in metaphase I?

• Chiasmata holds homologous pair together

  • Spindle microtubules attach to kinetochore proteins on the outside of each centromere

• Joined pairs of homologues line up on metaphase plate

  • Orientation of each pair is random

  • Orientation = 2^n, where n is # of chromosome pairs

What is the 2nd major source of genetic variability is meiosis?

Independent assortment

What is independent assortment?

A pair of homologous chromosomes consists of 1 maternal & 1 paternal chromosome.

In metaphase I, 1 member of each homologous pair is randomly attached to the spindle from one pole. The other member of each pair is attached to the spindle of the opposite pole.

What happens in anaphase I?

• Spindle fibers shorten & pull whole centromeres toward poles

• 2 chromosomes of each homologous pair separate and move to opposite spindle poles as the spindle microtubules contract

• Each chromosome still has 2 sister chromatids

• Random orientation results in independent assortment.

What happens in telophase I?

Chromosomes are segregated into 2 clusters; one at each pole.

• Nuclear membrane re-forms around each daughter cell

• Sister chromatids are no longer identical due to crossing over

What happens after telophase I?

Interkinesis - the single spindle of the first 2 meiotic division disassembles and microtubules reassemble into 2 new spindles for the 2nd division (*no DNA is replicated)

What happens in prophase II?

Chromosomes condense and a spindle forms

What happens in prometaphase II?

Nuclear envelope breaks down; spindle enters the former nuclear area; spindle microtubules attach to the 2 kinetochores of each chromosome

What happens in metaphase II?

Spindle microtubules align chromosomes on the metaphase plate

What happens in anaphase II?

• Spindles separate 2 chromatids of each chromosome and pull them to opposite spindle poles

• Chromatids (chromosomes) segregate to the 2 poles

What happens in telophase II?

• Chromatids decondense to extended interphase state

• Spindles disassemble, new nuclear envelopes form around the chromatin

• 4 haploid cells are produced

What is nondisjunction?

When the spindle fails to separate the homologous chromosomes (in meiosis I) or the sister chromatids (meiosis II)

• One pole receives BOTH chromosomes, the other pole has no copies of that chromosome

• Zygotes receive an extra chromosome have 3 copies of one chromosome instead of two

What are sex chromosomes?

Pairs of chromosomes that are different in male and female individuals of the same species (ex: XX, XY)

How do sex chromosomes work in meiosis?

• 2 X chromosomes are fully homologous in females

• In males, the X and Y are partly homologous

• X pairs with either X or Y from dad

• Egg can get either mom or dad’s X, sperm gets either X or Y

What are the differences between meiosis and mitosis?

What are the 3 sources of genetic variability?

1. Crossing-over - prophase I. When homologous chromosomes cross over, new allelic combinations in a chromatid are produced. After the 2nd meiotic division, 4 nuclei are produced, each receiving 1 of the 4 chromatids

2. Independent assortment of chromosomes RANDOMLY segregated to the poles - anaphase I

3. Particular sets of male and female gametes that unite in fertilization

Crossing over is a mechanism for __

Genetic recombination - produces genetic recombinants

• 1+ crossing-over events may occur in the same chromosome pair, involving the same or different chromatids

What is the random joining of male and female gametes?

When metaphase I occurs, 1 of each homologous pair is randomly attached to the spindle of each polel; the 23 chromosome pairs allow 2²3 different combinations of mat/pat chromosomes.

Because whichever sperm can fertilize whichever egg, there’s an increase in the variability of sexual production.

• Only exception is identical twins, which arise from mitotic division of a single fertilized egg

Is the diploid or haploid phase dominant?

Animals: Dominate diploid phase, reduced haploid phase, meiosis followed directly by gamete formation

Plants (+ some algae & fungi): haploid and diploid generations alternate

Fungi + algae: diploid phase is limited to a single cell (the zygote) via fertilization