Meiosis and Formation of Eggs and
Sperm
February 11, 2002
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:
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?
Answer: reduce the number of chromosomes in half from 23 pairs to 23 - then combine them to make a new individual with 23 pairs again!)
Overview: Humans 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).
Example of a human karyotype: 23 pairs of homologous chromosomes (one pair is the homologous 'sex' chromosomes - in this case X and Y = this is a baby boy)
Homologous chromosomes, or homologues, are pairs of chromosomes identical in size, shape, and (for the most part) gene sequence, that interact during meiosis. Your two chromosome "1"s are homologues, as are your two chromosome "5"s and two chromosome "8"s ,etc! (Sex chromosomes in a cell - X and Y - also interact during meiosis and are considered homologues.)
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!
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 (the two "1"s, the two"2"s, the two"3"s etc) "find" each other and pair up, lying side by side. 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 Prophase I 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 (Looks just like Prophase of mitosis, except that there is only [duplicated] copy of each chromosome - 23 in our case)
Metaphase II (not shown): Chromosomes [sister chromatids] align at the metaphase plate (Looks just like Metaphase of mitosis, except that there is only [duplicated] copy of each chromosome - 23 in our case - not 46, lining up 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. 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
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) 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:
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.....).
Learning
Objectives: At the end of this lecture, you should be
able to
|
