Meiosis and the Formation of Eggs and Sperm
March 1, 2004


Readings: Meiosis: Ch 9: p 138 - 143, 148 - 149, Figure 9.8 and 9.9
CD-ROM:
Good animations as usual!


"You're unique. Just like everyone else..." -- Anon.

I. Review of mitosis: 1diploid cell (2n) duplicates its DNA (In S-phase), then splits that DNA up into 2 genetically identical diploid cells (2n).

Remember that is a typical diploid cell (2n) , there are two copies of each chromosome. Homologous chromosomes, or homologues, are pairs of chromosomes identical in size, shape, and (for the most part) gene sequence, that interact during meiosis. (One Homologue may carry the blue eye color gene, while the other carries the brown eye color gene, but BOTH chromosomes carry an eye color gene! More on this later...)


II. Overview of meiosis: 1diploid cell (2n) duplicates its DNA (In S-phase), then splits that DNA up into 4 genetically unique haploid cells (n).

Question: How can a male (diploid; 46 chromosomes) and a female (diploid; 46 chromosomes) produce a child with 46 chromosomes? Answer: reduce the number of chromosomes in half from 23 pairs to 23 in the eggs and sperm - then combine them to make a new individual with 23 pairs again!)


III. Meiosis reduces chromosome number from diploid (2n) to haploid (n)
1 diploid cell (
2n) cell becomes 4 haploid (n) germ cells (eggs or sperm).

During meiosis, chromosomes are replicated once in S phase (just like mitosis) into sister chromatids, but the cell divides twice.

THIS PROCESS GENERATES THE DIVERSITY OF ALL SEXUALLY REPRODUCING ORGANISMS.
Yes, this means
you too!

Do you need to memorize all the steps of meiosis? Oh, heavens NO!! However, it is VERY important for you to remember what happens in each of the two phases - the Big picture. Crossing Over is especially important! I would also like you to be able to compare ow a cell looks in Metaphase of mitosis, Metaphase of Meiosis I, and Metaphase of Meiosis II. We will work on this in class!

Meiosis I: Separate the Homologues

But wait..there's more! Meiosis II: Separate the Sister Chromatids

 

Summary: At the end of meiosis, 4 daughter cells are formed = eggs or sperm. Each contains half as many chromosomes as the parent cell (n); each cell is genetically different from its parents and from its "siblings". Fertilization of an egg by a sperm restores the chromosome number to 2n.

Alteration of Generations: All sexually reproducing organisms (animals, plants and fungi) alternate between haploid (n) and diploid (2n) states.


IV. Meiosis: What does it all mean? Formation of human gametes eggs and sperm:

1. Spermatogenesis: (testes) 2 months from start to finish. Every day, several hundred million sperm are made by meiosis!

Spermatogonia (2n) are the cells in the testes that will undergo meiosis.
Primary spermatocytes (2n)
- [Meiosis I]
23 pairs of homologues including X and Y
Spermatids (n)
- [Meiosis II]
23 chromosomes - one of which is an X or a Y chromosome
Spermatozoa (n):
'streamlined' - cell membrane, nucleus, acrosome, mitochondria, flagella

Race for the egg: Here are a few cute (and funny) short articles - read if you are interested:
Sperm like it hot
3 Feb 03 Nature
Sperm train to win 11 July 02 Nature

2. Oogenesis: (ovaries) 15 - 30 years from start to finish!

Oogonia (2n) 2 million are formed in a baby girl before birth!
Primary Oocytes (2n)
- [Meiosis I]
23 pairs of homologues including 2X chromosomes

Note: Developmental arrest in Meiosis I for 15 -30 years, until a girl goes through puberty!
Ovulation / meiosis: at the onset of puberty, FSH triggers a few primary oocytes to progress through meiosis every 28 days.

Secondary Oocyte (n) - [Meiosis II] 23 chromosomes - one of which is an X chromosome

(In the egg, there is a division into 4 cells with unequal fates)
1 ovum (n) + 3 polar bodies (n)
- the 3 polar bodies disintegrate. The
1 ovum gets all the resources (cytoplasm, mitochondria) and may get fertilized.

Why are eggs so huge, and sperm so small? Some hypotheses - read if you are interested:
Size and the single cell - 22 Nov 99 Nature


3. The birds 'n the bees: (we'll be brief here)....

During sexual reproduction, fertilization of the ovum (n, 23 chromosomes) by the sperm (n, 23 chromosomes) restores the diploid number and creates a zygote (2n, 46) that divides and grows by mitosis to form a multicellular human.

The moment of fertilization

The former zygote
~ 1 year after fertilization.....

Cute, multicellular, and diploid...thanks to meiosis followed by LOTS of mitosis!

Instructions not included.


V. Genetic variation "reshuffling the genes" comes from several sources:

Why do you look somewhat similar - but distinctly different - from your brothers and sisters, and from your parents, if you all basically have the same chromosomes? Each meiosis and fertilization is literally like a 'roll of a dice' - so no two individuals are alike!

1. Crossing Over: The chromosomes you receive from your mom's egg and your dad's sperm and NOT the same chromosomes that your mom and dad have. They are new, 'shuffled' versions of their chromosomes - and you only get ONE of them from each parental unit....

2. Independent assortment: Homologues line up or "shuffle" randomly on the metaphase plate in Meiosis I. With 23 chromosomes assorting independently, there are 2^23, or 8 million, possible assortments of chromosomes inherited for every cell!!

3. Random fertilization: The ovum has 8 million possible chromosome combinations, so does the sperm cell. 8 million x 8 million = 64 trillion possible diploid combinations in EACH AND EVERY zygote! WOW!!

In other words, you're unique...(...just like everyone else.....).


Objectives : At the end of this lecture, you should be able to

1. Compare Homologues to chromosomes and sister chromatids
2. Describe the difference between Meiosis I and Meiosis II
3. Compare how cells look in Metaphase of Mitosis, Metaphase of Meiosis I and Metaphase of Meiosis II! Make sure you know how they look similar and different!
4. What is the outcome of meiosis?
5. Explain the stages in the formation of eggs and sperm, and what happens upon fertilization.
6. What are the 3 main ways genetic variation is generated in egg and sperm cells and in formation of the zygote?