Meiosis and Formation of Eggs and Sperm
February 16, 2000


Readings: Starr text: Ch 9 cover page, 9.1 - 9.7
CD-ROM: Good animations as usual!
Warmup: Due Weds AM


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

Outline: Meiosis

I. Review of Mitosis
II. Overview of Meiosis
III. Meiosis reduces chromosome number from diploid (2n) to haploid (n)
IV. Formation of human gametes ­ eggs and sperm:
V. Genetic variation "reshuffling the genes" comes from several sources:

I. Review of mitosis:

Mitosis allows us to make more of our body's somatic cells to grow, develop, and repair injuries.

During mitosis, all the chromosomes in a cell are copied, line up at the metaphase plate, split apart at the centromere, and segregate into two new (but genetically identical) cells...



II. Overview of meiosis:

Question: How can a male (diploid; 46 chromosomes) and a female (diploid; 46 chromosomes) produce a child with 46 chromosomes?

 


We have 46 chromosomes that, if you lined them up by size and shape (karyotyped them), are actually 23 pairs of chromosomes - one was originally from "mom" and the other from "dear 'ol dad". Cells that have paired chromosomes are said to be diploid (or "2n" - 2 copies of each chromosome).

Homologous chromosomes, or homologues, are pairs of chromosomes identical in size, shape, and gene sequence, that interact during meiosis. Sex chromosomes in a cell also interact during meiosis and are considered homologues.

Example of a human karyotype: 23 pairs of homologous chromosomes (one pair is the homologous 'sex' chromosomes - in this case X and Y = a boy)

 

 

 

 

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), but the cell divides twice.

 

THIS PROCESS GENERATES THE DIVERSITY OF ALL SEXUALLY REPRODUCING ORGANISMS.

     Draw me again
 Interphase DNA replication as usual in S phase

 

 

 MEIOSIS I [Splits the homologues apart]

 

 

 Prophase I Homologues synapse (overlap) and exchange DNA - 'crossing over'

 

 

 

 Metaphase I Homologues align in pairs

 

 

 

 Anaphase I Homologues separate; sister chromatids remain attached

 

 

 

Telophase I and cytokinesis Sister Chromatids relax

 

 

 

 

   NO interphase OR DNA replication here!  
 MEIOSIS II [Splits the sister chromatids apart]

 

 

 

 Prophase II Sister chromatids condense

 

 

 

 Metaphase II Sister chromatids align

 

 

 

 Anaphase II Sister chromatids separate

 

 

 

Telophase II and cytokinesis Chromosomes relax

 

 

 

Meiosis I - Goal: Separate the homologous chromosomes from each other
 Prophase I: Homologues condense and synapse (overlap), exchanging DNA by 'crossing over'

During meiosis, but NOT mitosis, the homologous chromosomes "find" each other and pair up, lying side by side (See p 143 and 144 in your book). During this time, an important process called Crossing over occurs:

Crossing over: sections of chromosomes from homologues are swapped (or "synapse")- but only during prophase I of meiosis, when homologues are paired. In humans, 2-3 cross-over events occur per chromosome pair. The result: chromosomes after meiosis are genetically unique and have combinations of DNA derived from both parents.

 Metaphase I: Homologues align in pairs

Anaphase I: Homologues separate; sister chromatids remain attached.

 Telophase I (not shown): 2 new cells form. Each has 1 copy of each homologue, but still has duplicated sister chromatids

Meiosis II: Goal: Separate the sister chromatids. This is essentially a mitotic division

 Prophase II (not shown): Chromosomes [sister chromatids] condense

Metaphase II (not shown): Chromosomes [sister chromatids] align at the metaphase plate

Anaphase II (not shown): Chromosomes [sister chromatids] separate

Telophase II (not shown): Chromosomes [sister chromatids] separate. There are now 4 haploid (n) cells


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. Formation of human gametes ­ eggs and sperm:

1. Spermatogenesis: (testes) 2 months from start to finish (See Figure 9.8). Every day, several hundred million sperm are made!

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

 

 

 

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 2Xs

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

(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.

 

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) (awww..) that divides and grows by mitosis to form a multicellular human.

 

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

Cute, multicellular, and diploid...

Instructions not included.

 

 

 

 

 

 

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

Ok...people may say that you may have your mom's nose and your dad's hair, but what does that mean? Why do you look different (but somewhat similar) 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 223, 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.....).

 

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

1. Compare Homologues to chromosomes and chromatids
2. Describe and draw the stages of meiosis
3. Explain the outcome of meiosis
4. Explain the stages in the formation of eggs and sperm, and what happens upon fertilization
5. Quiz yourself!