MEIOSIS & SEXUAL LIFE CYCLES CHAPTER 13
THE PROBLEM In mitosis, 2 diploid (2n) daughter cells are produced exactly like the parent cell. If these 2n cells united through fertilization, the result would be a 4n zygote.
THE SOLUTION MEIOSIS 2 consecutive divisions: meiosis I and meiosis II No DNA synthesis (S phase) between the two divisions The result: 4 cells, with ½ the amount of chromosomes as the starting cell. 2n ---- n. Thus, fertilization will be n + n = 2n zygote In animals: used to produce gametes egg and sperm In plants: used to produce spores, which start gametophyte generation
Quick Overview of Meiosis
Homologous chromosomes pair in process of synapsis Proteins called synaptonemal complex attaches the homologous chromosomes along their lengths this disappears at end of prophase I Pair of homologous chromosomes called a tetrad PROPHASE I
Chromatids may criss cross called chiasmata which hold homologous pairs together until anaphase I Centrioles migrate Spindle fibers form Spindle fibers connect to kinetchores on chromosomes
METAPHASE I The tetrads are moved to the center of the cell. Sister chromatids are held together by their centromeres that provide an attachment site for the spindle fibers. Separate spindle fibers, one from each pole attach to one member of each homologous pair. This allows the precise separation of the homologues during Anaphase I.
Independent assortment
ANAPHASE I Homologous pairs of chromosomes separate & move to poles of cell
TELOPHASE I & CYTOKINESIS Chromosomes continue moving until they reach the poles Each pole has a haploid chromosome (one member of pair) Cell divides into 2 daughter chromosomes Chromosomes may unwind slightly & nuclear envelope and nucleolus may reform
MEIOSIS II NO DNA REPLICATION BETWEEN I & II Prophase II - Chromosomes recoil and spindle apparatus forms Metaphase II chromosomes line at equatorial plate Anaphase II sister chromatids separate & move toward poles Telophase II nuclei form
CYTOKINESIS Oogenesis egg divides cytoplasm unequally Polar bodies receive little or no cytoplasm and are nonfunctional ; will be reabsorbed Ootid immature egg cell; receives most cytoplasm Ovum mature egg cell Spermatogenesis equal division of cytoplasm ; 4 functional spermatids Spermatids (immature) will mature into spermatozoa (mature)
Comparison of Mitosis and Meiosis
Parent cell Metaphase Metaphase I Anaphase/telophase 2n 2n Meiosis II ½ # chromosomes as parent n n n n
Meiosis & Genetic Abnormalities: Non-Disjunction Non-Disjunction: Chromosomes don t pull apart so one cell gets both sister chromatids, while the other cell doesn't get that chromosome. Non-disjunction can occur during either meiosis I or meiosis II as illustrated in the two following graphics.
Meiosis is not flawless It is estimated that from 10-20% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy failure. These chromosome abnormalities arise from errors in meiosis, usually meiosis I; occur more often (90%) during egg formation than during sperm formation; become more frequent as a woman ages. Aneuploidy - the gain or loss of whole chromosomes - is the most common chromosome abnormality. Aneuploidy is caused by nondisjunction, the failure of chromosomes to separate correctly Zygotes missing one chromosome ("monsomy") cannot develop to birth (except for females with a single X chromosome). Three of the same chromosome ("trisomy") is also lethal except for chromosomes 13, 18, and 21 Three or more X chromosomes are viable because all but one of them are inactivated
Down Syndrome Karyotype
Variety of Sexual Life Cycles In most animals, meiosis is the formation of gametes. Through syngamy (fertilization), the diploid stage is observed.
MANY PROTISTS AND SOME The only diploid stage is the zygote, it then proceeds through meiosis immediately after syngamy Thus creating a multi-cellular haploid organism. So, how are gametes produced? mitosis FUNGI
In plants: INCLUDES both a diploid stage and a haploid stage
The diploid sporophyte stage produces haploid spores by meiosis. The spores undergo mitosis, and produce a haploid plant called GAMETOPHYTE. Gametes are produced by mitosis which are fertilized to become a zygote (which is a sporophyte)