本期內(nèi)容是有絲分裂的基礎(chǔ)理論部分，實(shí)驗(yàn)手冊與實(shí)驗(yàn)?zāi)M請看后兩期。本部分內(nèi)容來自 University of California, Berkeley - UC Berkeley Extension, 虛擬實(shí)驗(yàn)的內(nèi)容來自 Labster. 本部分內(nèi)容均不會標(biāo)記為為原創(chuàng)，但由于是UP主購買的課程，因此不接受非授權(quán)的轉(zhuǎn)載，謝謝您的理解。

每一個(gè)生物基礎(chǔ)實(shí)驗(yàn)均會分為三部分：第一部分為實(shí)驗(yàn)的生物理論；第二部分為實(shí)驗(yàn)的指導(dǎo)手冊；第三部分為 Labster 的虛擬實(shí)驗(yàn)?zāi)M。第一部分的基本信息由 Ying Liu, Ph.D. 提供，第二部分的實(shí)驗(yàn)手冊來自 Labster, 第三部分的實(shí)驗(yàn)?zāi)M過程均由UP主操作。

Lab 7 -?Meiosis

Animal Life Cycle

- Diploid (2n) dominant (multicellular);

- Meiosis to produce gametes (haploid / n);

- Fertilization to restore diploid stage.

Sexual Reproduction

Function of Meiosis

- Meiosis keeps the number of chromosomes in a body cell constant from generation to generation;

- If gametes are produced by mitosis, egg and sperm would contain 46 chromosomes, and when they fertilize, the zygote would contain 92 chromosomes.

Meiosis

- Meiosis: cell division that creates gametes (sperms and eggs);

- Meiosis happens in testis and ovary, and produces four haploid daughter cells.

Meiosis involves two cycles of cell division:

- Meiosis I reduces the number of chromosomes by half (produces two cells each with 23 chromosomes);

- Homologous chromosomes separate into 2 daughter cells.

Meiosis II separates the two sister chromatids of each chromosome (similar to mitosis).

Spermatogenesis and Oogenesis

- Spermatogenesis produces 4 sperm;

- Oogenesis produces 1 egg, polar bodies degenerate.

Spermatogenesis

- Primary spermatocyte (2n) goes through Meiosis I and produces 2 secondary spermatocytes (1n);

- After Meiosis II, 4 spermatids (1n) are produced;

- Spermatids mature into functional sperm.

Oogenesis

- Primary oocyte (2n) goes through Meiosis I and produces a secondary oocytes (1n) and a polar body (1n);

- After Meiosis II, 1 ovum (1n) is produced;

- At birth 2,000,000 primary oocytes (2n);

- Puberty – 300,000 remain;

- Over lifespan: about 400 reach maturity ‘ovulation.’

Meiosis Promotes Genetic Variation

Independent Assortment

- Genes on different chromosomes segregate independently during the formation of eggs and sperms;

- Each of the 23 chromosomes would have a 50/50 chance of coming from maternal or paternal chromosome.

Another example:

A: eye color (A = brown, a = blue)

B: hair color (B = dark, b = blonde)

How Many Possibilities?

- Each chromosome is a separate ‘coin toss’

- The possibilities are 2^n;

- 2^23 = > 8 million combinations!

- This allows for the maximum amount of genetic variability;

- This also explains why siblings might not look much like each other!

Crossing-Over

- During prophase I, homologous chromosomes line up side by side to form synapsis;

- Crossing-over is the exchange of genetic information between non-sister chromatids.

Mitosis and Meiosis Summary

Nondisjunction

- Nondisjunction occurs when both members of a homologous pair go into the same daughter cell during meiosis I, or when sister chromatids fail to separate in meiosis II.

- Nondisjunction of chromosome 21 results in Down syndrome;

- Nondisjunction occurs in 10% of meiosis in oocytes, less frequent in human sperm;

- Nondisjunction increases with maternal age (trisomy risk 2-3% for women in their 20s, 30% in their 40s).

Results of Nondisjunction

- Monosomy: cell has only 1 copy of a chromosome (e.g. Turner syndrome, only 1 X chromosome);

- Trisomy: cell has 3 copies of a chromosome (e.g. Down syndrome, 3 copies of chromosome 21).

Assisted Reproductive Technology (ART)

Preimplantation Genetic Diagnosis (PGD)

- Embryos beyond 8-cell stage;

- One cell can be removed for karyotyping and/or DNA testing (embryo biopsy);

- Only embryo(s) with normal chromosome numbers and no genetic mutations are implanted.

本期內(nèi)容到此結(jié)束，感謝閱讀！下一期為實(shí)驗(yàn)手冊 & 下下期將進(jìn)行 Labster 實(shí)驗(yàn)！