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Lecture 7 : Meiosis. Meiosis and sexual life cycles:. In the sexual reproduction التكاثر الجنسي process, two gametes fuse during fertilization to produce a zygote.
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Lecture 7: Meiosis Meiosis and sexual life cycles: • In the sexual reproduction التكاثر الجنسيprocess, two gametes fuse during fertilization to produce a zygote. • Meiosis is a specialized type of cell division which reduces the chromosome number by half (reduces chromosome number from diploid to haploid). • This process occurs in animals and plants. • Meiosis occurs only in sex gonads الغدد التناسلية الجنسيةto form gametes.
Fertilization restores يُعيدthe diploid condition by combining two haploid sets of chromosomes. • Gametes, produced by meiosis, are the only haploid cells. • Gametes undergo no divisions themselves, but fuse تندمجto form a diploid zygote that divides by mitosis to produce a multicellular organism.
Meiosis and mitosis are preceded by the replication of chromosomes. • However, in meiosis, chromosomes replicate once followed by two consecutive متعاقبcell divisions, meiosis Iand meiosis II, which results in four daughter cells. • Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice. 1) The first division is known as meiosis I (a reductional division الانقسام المنصف):separates homologous chromosomes. 2) The second is known as meiosis II (an equational division :(الانقسام المتساويseparates sister chromatids.
Occurs in two steps The phases of meiotic division: 1) Meiosis I 2) Meiosis II Prophase I Anaphase I Prophase II Anaphase II Metaphase I Metaphase II Telophase I Telophase II
The phases of meiosis I • Prior to meiosis, chromosome duplication occurs during the S of the interphase stage in order to double the sister chromatid pairs. • During prophase I: • Homologous chromosomes (tetrad)الكروموسومات المتماثلة pair up by the synaptonemal complex in a process called synapsis • Each pair of homologous chromosomes is known as a bivalent (tetrad). • The pairs of chromosomes consist of four chromatids, with one chromosome coming from each parent. • Crossing over between homologous chromosomes occurs and DNA is exchanged between the bivalents in a process called homologousrecombination.
2. At metaphase I: • homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate. • In humans, you would see 23 tetrads. 3. At anaphase I: • Homologous chromosomes, not sister chromatids are separated from each other, but the sister chromatids remain attached to each other at the centromeres until anaphase II. • The processes during the second meiotic division (meiosis II) are identical to those in mitosis.
Definitions • Genes: The units that specify an organism’s inherited characteristic. • Chromatin:A DNA-protein complex which is organized into a long thin fiber. • Chromosome:The package that formed from a condensed,coiledand foldedchromatin. • Chromatids:Two sister chromatids formed from each duplicated chromosome. They contain identical copies of the chromosome’s DNA. • Centromere:The narrow region which the two sister chromatidsare connected together. • Mitosis:A type of cell division that occurs in somatic cells and generates two identical daughter cells • Meiosis:A type of cell division that occurs in the gonads, and yieldsfour non-identical daughter cells, each with half the chromosomes of the parent.
