基础生物实验 19 - 实验七理论简介：Meiosis 减数分裂
本期内容是有丝分裂的基础理论部分，实验手册与实验模拟请看后两期。本部分内容来自 University of California, Berkeley - UC Berkeley Extension, 虚拟实验的内容来自 Labster. 本部分内容均不会标记为为原创，但由于是UP主购买的课程，因此不接受非授权的转载，谢谢您的理解。
每一个生物基础实验均会分为三部分：第一部分为实验的生物理论；第二部分为实验的指导手册；第三部分为 Labster 的虚拟实验模拟。第一部分的基本信息由 Ying Liu, Ph.D. 提供，第二部分的实验手册来自 Labster, 第三部分的实验模拟过程均由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.
本期内容到此结束，感谢阅读！下一期为实验手册 & 下下期将进行 Labster 实验！
