Meiosis

Producing Gametes (eggs & sperm)

Another form of cell division?!

Mitosis produces two cells with the same number of chromosomes as the parent cell.

So -- Mitosis of a diploid cell (2n) produces two diploid daughter cells.

If two diploid cells went on to participate in sexual reproduction, they would produce a tetraploid (4n) zygote (which would be a problem!).

The Solution…

Meiosis is a process of cell division in eukaryotes characterized by: – Two divisions in a row: meiosis I and meiosis II – no DNA synthesis / replication (no S phase)

between the two divisions

Result: 4 cells with half the number of chromosomes as the starting cell (2n → n)

Animation – Mitosis vs. Meiosis

Gamete + Gamete = Zygote

Fusion of two n (haploid) cells produces a 2n zygote.

So – Meiosis is used to produce the gametes: sperm and eggs

Interphase

The parent cell is diploid (2n) – it has one copy of a chromosome from “mom” and another copy of that chromosome from “dad”.

Before Meiosis I, the cell prepares for division (G1, S, G2); most importantly, chromosomes are copied in S phase.

After S phase, the parent cell is still diploid, but now with replicated chromosomes.

Homologous Chromosomes

Homologous chromosomes are similar but not identical; a pair may carry different versions of the same genetic information. – Ex: A pair carries information about hair color;

one homologous chromosome may be for blond hair while the other homologous chromosome may be for black hair.

One chromosome was inherited from “mom”, the other from “dad”

Chromosomes and Chromatids

As a cell prepares to enter meiosis, each of its chromosomes has duplicated, as in mitosis.

Each chromosome thus consists of two sister chromatids, joined at the centromere.

Meiosis I

At the beginning of Meiosis I, a human cell contains:– 46 chromosomes, or – 92 chromatids.

This is the same number as during mitosis. So, it starts with one diploid (2n) cell.

Prophase I

Early Prophase Late Prophase

Video clip – 1:09

Prophase I (cont’d)

Nuclear membrane dissolves, spindle fibers form

Chromatids shorten and thicken, become visible

Synapsis occurs -- the two homologous chromosomes come near each other. – So -- there are actually four chromatids aligned

next to one another. This combination of four chromatids is called a tetrad

Prophase I (cont’d)

Crossing over occurs– segments of DNA from one chromatid in the

tetrad pass to another chromatid in the tetrad.

– result = a genetically new chromatid.

Metaphase I

Tetrads / pairs of homologous chromosomes line up at equator

Spindle fibers attach to centromeres

Video clip – 0:17

Anaphase I

Homologous chromosome pairs separate. One homologous chromosome (consisting of

two chromatids) moves to one side / pole of the cell, while the other homologous chromosome (consisting of two chromatids) moves to the other side of the cell.

Video clip – 0:24

Anaphase I

Result in humans = 23 chromosomes (each consisting of two chromatids) move to one pole, and 23 chromosomes (each consisting of two chromatids) move to the other pole. – Essentially, the chromosome number of the cell is

halved. – So the cells are considered haploid (n).– For this reason the process is a reduction-

division.

Telophase I

New nuclear membrane forms around each set of chromosomes (still consisting of 2 sister chromatids)

Cytokinesis occurs -- each daughter cell (with 23 chromosomes each consisting of two chromatids) then enters interphase, during which there is no duplication of the DNA

Video clip – 0:13

Meiosis II

occurs in essentially the same way as mitosis.

a cell containing 46 chromatids undergoes division into two cells, each with 23 chromosomes.

Starts with two haploid cells; ends with four haploid cells.

Prophase II

chromosomes condense each chromosome contains two chromatids

attached by the centromere

Video clip – 0:30

Metaphase II

chromatid pairs (chromosomes) gather at the center of the cell

identical to metaphase in mitosis

Video clip – 0:13

Anaphase II

Spindle fibers pull chromosomes apart at the centromeres

Spindle fibers move sister chromatids to opposite poles (sides) of the cell

Video clip – 0:14

Telophase II

Chromatids – now called chromosomes again - gather at the poles of the cells

A new nuclear envelope forms around each set of chromosomes

Usually, cytokinesis happens.

Video clip – 0:44

At end: animation

The Results of Meiosis

During meiosis II, each cell containing 46 chromatids produces two cells, each with 23 chromosomes.

Originally, there were two cells that underwent meiosis II; therefore, the result of meiosis II is four cells, each with 23 chromosomes.

Each of the four cells is haploid; that is, each cell contains a single set of chromosomes.

The Results of Meiosis (cont’d)

The 23 chromosomes in the four cells from meiosis are not identical because crossing over has taken place in prophase 1.

The crossing over creates variation, so that each of the four resulting cells from meiosis differs from the other three.

The Results of Meiosis (cont’d)

Thus, meiosis provides a mechanism for producing variations in the chromosomes.

Also, it accounts for the formation of four haploid cells from a single diploid cell.

Video clip – The Importance of Meiosis – 1:19

Animation – Cells Alive

Spermatogenesis

Meiosis that produces sperm (male animals) Occurs in testes The four haploid cells at the end of meiosis II

will all become sperm

Oogenesis

Meiosis that produces eggs (ovum = singular; ova = plural)

Occurs in ovaries Only one of the four haploid cells at the end

of meiosis II becomes an egg; the other three cells – all smaller than the egg because they get less cytoplasm – are called polar bodies

Video clip – 0:28

Errors in Meiosis

It is estimated that 10–20% of all human fertilized eggs contain chromosomal abnormalities, and these are the most common cause of pregnancy failure / miscarriage / spontaneous abortion (35% of miscarriages).

About Chromosomal Abnormalities

arise from errors in meiosis, usually meiosis I;

occur more often (90% of the time) during egg formation than during sperm formation;

become more frequent as a woman ages.

Chromosomal Abnormalities (cont’d)

Aneuploidy — the gain or loss of whole chromosomes — is the most common chromosome abnormality. It is caused by nondisjunction, the failure of chromosomes to correctly separate: – homologues during meiosis I or – sister chromatids during meiosis II

Video clip – 3:50

Chromosomal Abnormalities (cont’d)

Zygotes missing one chromosome ("monosomy") cannot develop to birth (except for females with a single X chromosome).

Three of the same chromosome ("trisomy") is also lethal except for chromosomes 13, 18, and 21 (trisomy 21 is the cause of Down syndrome).

Three or more X chromosomes are viable because all but one of them are inactivated.

Video clip – 4:19

Klinefelter Syndrome (XXY)

Trisomy of sex chromosomes Male Infertile (sterile)

Klinefelter’s Syndrome (cont’d)

Turner Syndrome (XO)

Monosomy of sex chromosomes Female (“O” indicates no second X

chromosome) Infertile

Turner Syndrome (cont’d)

Down Syndrome (Trisomy 21)

Trisomy of 21st pair of chromosomes Mental retardation Characteristic facial features Physiological problems – esp. with the heart

Down Syndrome (cont’d)