BIMM110      LECTURE 9


Textbook: Strachan and Read, Chapters 2 & 3                    Pasternak, Chapter 2
there is also a very detailed discussion with lots of figures in the text book by Alberts, Chapter 20 (4th edition),

SLIDES B         

A. Meiosis

1. The Basics

- understanding meiosis provides a physical basis for understanding Mendel's Laws
- it is a process in which diploid precursors of the germ cell lineage are giving rise to haploid gametes;
- a description of meiosis by a geneticist emphasizes the behaviour of the chromosomes
- cytologically distinguishable stages of meiosis are:

prophase I (leptotene, zygotene, pachytene, diplotene, diakinesis), metaphase I, anaphase I, telophase I,
metaphase II, anaphase II, telophase II     [see     MEIOSIS ]

- some important terminology and structural observations:
        synapsis, synaptonemal complex
        chiasmata and crossing over, tetrads, terminalization

- recombination frequencies and linkage form the basis for the construction of genetic maps (see lecture 10)

- recombination and linkage were not anticipated by Mendel (fortunately, all the genes he studied were unlinked)

2. Meiosis, chromosome abnormalities, and genetic diseases:

a) nondisjunction
    failure of chromosomes to segregate properly in meiosis I or meiosis II  leads to monosomies and trisomies

b) recombination involving structurally abnormal chromosomes

Individuals carrying structurally abnormal chromosomes may be normal, but their gametes may carry chromosomes with deleterious alterations.
Synapsis and recombination involving structurally abnormal chromosomes leads to gametes with genetic deficiencies and duplications.

- reciprocal translocations
- pericentric and paracentric inversions
- deletions
- Robertsonian translocations

c) Further details and examples in the next lecture


B. Gametogenesis


A complete description of this process includes meiosis, but also the distinct pathways of cellular differentiation responsible for the formation of mature spermatozoa and eggs

1. spermatogenesis
- spermatogonia - primary spermatocytes - secondary spermatocytes - spermatids - spermatozoa

2. oogenesis
- ovarian follicles - oogonium - primary oocyte (arrest at dictyate stage) - secondary oocyte - polar bodies
- completion of meiosis II after fertilization


 C. Fertilization

Image from D. Kunkel

1. Major events at fertilization

- attachment of sperm and cell fusion
- completion of meiosis II and formation of second polar body
- transient existence of two pronuclei in the zygote;
- prevention of dispermy (polyspermy)
- fate of mitochondria from sperm tail

2. In Vitro Fertilization has become a major business raising significant issues of concern to bioethicists

Eggs and sperm from models, athletes, neurosurgeons, talk show hosts etc. are becoming available via e-commerce on the Internet.
An opportunity for students at major medical schools to raise $$$ for tuition by egg donation ???

D. Normal Embryogenesis

        - beyond the scope of this course, but see Embryogenesis for an interesting introduction 

E. Parthenogenesis

- development of an unfertilized egg; common in some animal species; no credible case in humans
- ovarian teratomas (tumors): originally attributed to parthenogenesis; now believed to result from failure of secondary oocyte to divide, and re-entry into mitotic cell division;

F. Hydatidiform moles

- occurrence: 1/1500 pregnancies in Caucasian women
- partial moles: fetus arrested in development; often triploid from dispermy; benign tumor
- complete moles: fertilization of enucleated egg, followed by duplication of sperm genome; karyotype: 46, XX, never 46, YY; highly malignant




Selected References

A. Historical

 - Eiben, B., Bartels, I., Bähr-Porsch, S., Borgmann, S., Gatz, G., Gellert, G., Goebel, R., Hammans, W., Hentemann, M., Osmers, R., Rauskolb, R., and Hansmann, I. (1990). Cytogenetic analysis of 750 spontaneous abortions with the direct-preparation method of chorionic villi and its implications for studying genetic causes of pregnancy wastage. Am. J. Hum. Genet. 47, 656-663.
- Martin, R.H. and Rademaker, A. (1990). The frequency of aneuploidy among individual chromosomes in 6,821 human sperm chromosome complements. Cytogenet. Cell Genet. 53, 103-107.
- Martin, R.H., Ko, E., and Rademaker, A. (1991). Distribution of aneuploidy in human gametes: comparison between human sperm and oocytes. Am. J. Med. Genet. 39, 321-331.

 B. Recent

 - Barlow, D. P. 1994. Imprinting: A gamete's point of view. Trends Genet. 10:194-199.
- Brinster, R.L. and Zimmermann, J.W. (1994). Spermatogenesis following male germ-cell transplantation. Proc. Natl. Acad. Sci. USA 91, 11298-11302.
- Carpenter, A.T.C. (1994) Chiasma function. Cell 77, 959-962
- Denis, H. and J.-C. Lacroix. 1993. The dichotomy between germ line and somatic line, and the origin of cell mortality. Trends Genet. 9:7-11.
 - Wilmut,I., Schnieke,A.E., McWhir,J., Kind, A.J., and K.H.S Campbell. 1997. Viable offspring derivedfrom fetal and adult mammalian cells. Nature 385: 810-813