Meiosis is similar in many ways to mitosisMeiosis is similar in many ways to mitosisHowever, it is also very different…However, it is also very different…

•Meiosis results in the Meiosis results in the formation of haploid (n) cells.formation of haploid (n) cells.

–In Humans, these are the Ova In Humans, these are the Ova (egg) and sperm.(egg) and sperm.–Ova are produced in the Ova are produced in the ovaries in femalesovaries in females

–Sperm are produced in the Sperm are produced in the testes of males.testes of males.

–Involves 2 cell divisionsInvolves 2 cell divisions

–Each parent cell results in 4 Each parent cell results in 4 cells with 1/2 the normal cells with 1/2 the normal genetic information, genetic information, notnot identical daughter cellsidentical daughter cells

• Process called oogenesis

•Process called spermatogenesis

•Meiosis occurs in 2 phases: Meiosis occurs in 2 phases: Meiosis 1 & Meiosis & Meiosis 22

• Prior to the first division, the amount of DNA Prior to the first division, the amount of DNA doublesdoubles

What stage of the cell cycle would this “doubling” take place in?

What stage of interphase would this doubling take place?

the largest differences between mitosis and meiosis occur in prophase I.

Synapsis is time consuming! It is estimated that prophase accounts for some 85% - 95% of the total time for meiosis.

• Chromosomes condense and attach to the nuclear envelope

• Synapsis is when a pairing of homologous chromosomes occurs and a tetrad is formed. Each tetrad is composed of four chromatids. • Crossing over may occur, and chiasmata become apparent.

• Each chromosome gets attached to the spindle fiber in transition to metaphase.

Chiasma

All chromosomes are now positioned at the spindle’s equator.The orientation of the tetrads is random, with either parental homologue on a side.

This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

As there are 46 chromosomes in human cells, that means that 23 chromosomes end up on either side.

Because of the exchange during crossing over, these chromosomes are different than they were

Each chromosome is separated from its homologue, and they are moved to opposite poles of the spindle.

Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. This is the point when many chromosomal abnormalities occur, due to incomplete separation.(Nondisjunction)

•The cytoplasm of the germ cell divides.

• Each of the daughter cells is now haploid, but each chromosome still has two chromatids.

• Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Remember…each new cell has duplicates of either mom’s or dad’s chromosomes…but not both, like what happens in mitosis.

“Cytokinesis”

In what two ways does the Meiotic Cell cycle differ from the Mitotic Cell cycle?

What happens just prior to Meiosis I?

Which stage of the meiotic cell cycle is most different from the mitotic cell cycle?

What happens during synapsis, after tetrads form?

How is the chromosomal arrangement during Metaphase I different than the chromosomal arrangement during Metaphase?

At what point during the Meiotic cycle can many chromosomal abnormalities occur?

What process divides the cytoplasm at the end of Meiosis I?

Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis and meiosis I.

Microtubules have moved one member of the centriole pair (from the centrosome) to the opposite pole of each daughter cell.Microtubules attach from centriole to the chromosomes, and they are driven toward the spindle’s equator.

Chromosomes are positioned at the spindle equator, midway between the two poles.

The kinetochores of the sister chromatids point toward opposite poles. The kinetochores assemble on the centromere, and are areas responsible for joining the chromosome to the microtubule from the spindle fiber.

The attachment between the two chromatids of each chromosome breaks.

Each of the former sisters, is now a separate chromosome in its own right.

(Similar to anaphase of mitosis)

Distinct nuclei form at the opposite poles and cytokinesis occurs. At the end of meiosis II, there are four daughter cells each with one half the number of chromosomes of the original parent cell. It is really this “second division” of Meiosis II that is responsible for reducing the number of chromosomes in each daughter.

What is a kinetochore?

How do chromosomes line up at the metaphase plate during Meiosis II, as opposed to Meiosis I?

Which part of the Meiotic Cell cycle is mostly responsible for the reduction in chromosome number to ½ in each daughter cell?

In human female embryos, primary oocytes complete interphase and prophase I, where they are frozen and remain this way until the female reaches puberty. 

If a woman is fertile on average for 40 years, then 40 X 12 = 580 eggs in her lifetime.  Thus, 400,000 eggs is plenty! 

A female is born with about 2 million primary oocytes.  By the time she reaches puberty, about 400,000 are left (most of them die through the years).  This is still way more than enough!

Of the four haploid cells that form by way of meiosis and cytoplasmic divisions, one or all may develop into gametes and function in sexual reproduction.

Asymmetrical cytokinesis leads to the production of polar bodies during oogenesis.

To conserve nutrients, the majority of cytoplasm is segregated into the secondary oocyte during meiosis I , when the secondary oocyte is formed.The remaining daughter cells generated from the meiotic events contain relatively little cytoplasm and are referred to as polar bodies. Eventually, the polar bodies degenerate.

DiploidDiploid

Haploid, but duplicated

Haploid

When the haploid egg, fuses with a sperm to become fertilized, the normal complement of chromosomes is again restored.

Spermatogonia (precursors to sperm)divide by mitosis to produce more spermatogonia or spermatocytes.

With 22 pairs of autosomes and an average of two crossovers between each pair, there is huge variation among sperm.

Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete.

Then the spermatids form sperm, losing most of their cytoplasm in the process.

When do primary oocytes form in human females?

How is cytokinesis different in oogenesis than spermatogenesis?

When is the normal complement of chromosomes again achieved?

What is a precursor to sperm called?

How are additional spermatogonia produced?

Why are sperm so small compared to eggs?