Cell Division - Meiosis: Difference between revisions
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Revision as of 17:17, 14 July 2010
Introduction
- Meiosis Links: Spermatozoa Development | Oocyte Development | Cell Division - Mitosis
Some Recent Findings
Mitosis and Meiosis
Mitosis 2 Daughter cells identical to parent (diploid)
Meiosis Germ cell division (haploid)
- Reductive division
- Generates haploid gametes (egg, sperm)
- Each genetically distinct from parent
- Genetic recombination (prophase 1)
- Exchanges portions of chromosomes maternal/paternal homologous pairs
- Independent assortment of paternal chromosomes (meiosis 1)
Cell Birth - Mitosis and Meiosis 1st cell division- Meiosis
Homologous chromosomes pairing unique to meiosis
- Each chromosome duplicated and exists as attached sister chromatids before pairing occurs
- Genetic Recombination shown by chromosomes part red and part black
- chromosome pairing in meiosis involves crossing-over between homologous chromosomes
(For clarity only 1 pair of homologous chromosomes shown)
Comparison of Meiosis/Mitosis
- After DNA replication 2 nuclear (and cell) divisions required to produce haploid gametes
- Each diploid cell in meiosis produces 4 haploid cells (sperm) 1 haploid cell (egg)
- Each diploid cell mitosis produces 2 diploid cells
Meiosis Sex Differences
Female (oogenesis)
- Meiosis initiated once in a finite population of cells
- 1 gamete produced / meiosis
- Completion of meiosis delayed for months or years
- Meiosis arrested at 1st meiotic prophase and reinitiated in a smaller population of cells
- Differentiation of gamete occurs while diploid in first meiotic prophase
- All chromosomes exhibit equivalent transcription and recombination during meiotic prophase
Male (spermatogenesis)
- Meiosis initiated continuously in a mitotically dividing stem cell population
- 4 gametes produced / meiosis
- Meiosis completed in days or weeks
- Meiosis and differentiation proceed continuously without cell cycle arrest
- Differentiation of gamete occurs while haploid after meiosis ends
Sex chromosomes excluded from recombination and transcription during first meiotic prophase
Female Gametogenesis
In females, the total number of eggs ever to be produced are present in the newborn female.
- All eggs are arrested at an early stage of the first meiotic division as a primary oocyte (primordial follicle). Following purberty, during each menstrual cycle, pituitary gonadotrophin stimulates completion of meiosis 1 the day before ovulation.
- In meiosis 1, a diploid cell becomes 2 haploid (23 chromosomes) daughter cells, each chromosome has two chromatids. One cell becomes the secondary oocyte the other cell forms the first polar body.
- The secondary oocyte then commences meiosis 2 which arrests at metaphase and will not continue without fertilization.
- At fertilization meiosis 2 completes, forming a second polar body. Note that the first polar body may also undergo this process forming a third polar body.
Female Abnormalities
Meiotic non-disjunction resulting in aneuploidy, most are embryonic lethal and not seen. The potential for genetic abnormalities increase with maternal age.
- Autosomal chromosome aneuploidy
- trisomy 21 - Down syndrome
- trisomy 18 - Edwards syndrome
- trisomy 13 - Patau syndrome
- Sex chromosome aneuploidy
- monosomy X - Turner's Syndrome
- trisomy X - Triple-X syndrome
- 47 XXY - Klinefelter's Syndrome
Male Gametogenesis
In males, sperm continues to be generated throughout life from a stem cell population in the testis. Spermatozoa maturation involves two processes meiosis and spermiogenesis
The above figure compares meiosis to the female (the polar bodies have been removed and labelling updated).
Human Spermatozoa Development
- Spermatogenesis process of spermatagonia mature into spermatazoa (sperm).
- Continuously throughout life occurs in the seminiferous tubules in the male gonad- testis (plural testes).
- At puberty spermatagonia activate and proliferate (mitosis).
- about 48 days from entering meiosis until morphologically mature spermatozoa
- about 64 days to complete spermatogenesis, depending reproduction time of spermatogonia
- follicle stimulating hormone (FSH) - stimulates the spermatogenic epithelium
- luteinizing-hormone (LH) - stimulates testosterone production by Leydig cells
Links: Spermatozoa Development | MBoC - Sperm | MBoC - Highly simplified drawing of a cross-section of a seminiferous tubule in a mammalian testis | MBoC - Cytoplasmic bridges in developing sperm cells and their precursors
Puberty
- In humans at puberty, hormonal and morphological changes occur within the gonad and other systems (secondary sex characteristics).
- Within the testis the immature Sertoli cells cease to proliferate and differentiate.
- Spermatogonium proliferate and spermatogenesis begins.
- It takes about 70 days for cells to mature from the diploid spermatogonium to a primary spermatocyte.
- This maturation occurs in waves along the seminiferous tubules.
Ejeculate
- release of spermatozoa and accessory gland secretions from the male genital tract (3.5 ml)
- 200-600 million sperm, by volume less than 10 % spermatozoa
- Accessory Gland secretions - 60 % seminal vesicle, 30 % prostate and 10 % bulbourethral
Male Abnormalities
- Oligospermia - (Low Sperm Count) less than 20 million sperm after 72 hour abstinence from sex
- Azoospermia - (Absent Sperm) blockage of duct network
- Immotile Cilia Syndrome - lack of sperm motility
Abnormalities
Meiotic Nondisjunction
- Occurs when homologues fail to separate during meiotic division I or II
- Down Syndrome
- Caused by an extra copy of chromosome 21
Chromosomal Translocations
- Philadelphia chromosome
- Chronic myelogenous leukemia
- Piece of Chr9 exchanged with Chr22 Generates truncated abl
Overstimulates cell production
References
Reviews
Articles
Search Pubmed
July 2010 "meiosis" All (18851) Review (2062) Free Full Text (6212)
Search Pubmed: meiosis
Additional Images
Cite this page: Hill, M.A. (2024, April 28) Embryology Cell Division - Meiosis. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Cell_Division_-_Meiosis
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G