Meiosis An Introduction An organism must inherit a
- Slides: 27
Meiosis: An Introduction § An organism must inherit a single copy of every gene from its parents. § Meiosis –number of chromosomes are cut in half through the separation of homologous chromosomes in a diploid cell.
Chromosome Number § Different organisms have different numbers of chromosomes. § Humans have 46 chromosomes, 22 pairs of autosomes and one pair of sex chromosomes. § Homologous chromosomes- A matching pair (or set) of chromosmes (one from each parent) § Homo= same
Homologous chromosomes vs Sister chromatids § Homologous chromosomes are separate chromosomes § Sister chromatids are attached by the centromere.
The “Diploid” Condition § A cell that contains both sets of homologous chromosomes is said to be diploid. = 2 N § For humans, the diploid number is 46, which can be written as 2 N=46. Diploid
The “Haploid” Condition § Meiosis results in the production of haploid gametes (sex cells) which have only 23 chromosomes. § The diploid cell that entered meiosis has become 4 haploid cells (N).
Meiosis I § Interphase I Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I
Meiosis I: Prophase I § During Prophase I each chromosome pairs with its homologous chromosome forming a tetrad. Prophase I § There are 4 chromatids in a tetrad. Tetrad
Meiosis I: Synapsis §Tetrads formed during Prophase I exchange portions of their chromatids in a process called crossing over. §Increase variation in multicellular organisms. Tetrad Allele Chromatid
Meiosis I: Metaphase I How is this different from Mitosis? § During Metaphase I, the 23 homologous pairs (tetrads) line up along the equator of the cell.
Meiosis I: Anaphase I § Homologous pairs separate and move toward the poles of the cell. § The chromatids are still in tact at the centromere. Anaphase I
Meiosis: Telophase I and Cytokinesis § Nuclear membranes form and the spindle breaks down. § Cleavage results in two cells. § The two haploid (N) cells produced by meiosis I have chromosomes from each homologous pair. • The cells are different from each other.
Meiosis II The 2 haploid cells produced by meiosis I now enter a second meiotic division which results in the production of 4 genetically different haploid cells.
§Meiosis II Telophase II and Cytokinesis II Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II
Meiosis II § During Prophase II, spindle fibers reform and move chromosomes toward cell equator.
Meiosis II • During Metaphase II Chromosomes move to Midline.
Meiosis II • During Anaphase II, chromatids separate and move toward opposite poles of the cell.
Meiosis II § During Telophase II, the nuclear membrane reforms around chromosomes in each of the four new cells, and a second round of cytokinesis produces four new haploid cells
Sperm or Ovum
Gamete Development… § Spermatogenesis: four new cells from meiosis called spermatids in males develop into mature sperm cells
Gamete Development (cont. ) § Oogenesis: During cytokinesis following meiosis in females, cytoplasm is divided unequally among the four new cells. § One cell receives most of the original cell’s cytoplasm and develops into a mature egg called an ovum. The other three cells called polar bodies die off.
Gamete Development (cont. )
Sexual Reproduction § When the nuclei of the two gametes combine during fertilization, the 2 n number is restored (2 n = 46). § The fusion of two gametes produces the first cell of the new animal called a zygote.
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