Meiosis and Mitosis

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3.2 Meiosis Produces Gametes for Sexual Reproduction
Reproduction can be divided into two broad categories
1. In asexual reproduction, organisms reproduce without mating and produce genetically identical offspring 2. In sexual reproduction, gametes (reproductive cells) are produced; theses unite during fertilization Males and females carry distinct reproductive tissues and structures Mating requires the production of haploid gametes from both male and female The union of haploid gametes produces diploid progeny

Meiosis versus Mitosis
Many features of meiosis are similar or identical to mitosis, e.g. interphase Meiosis is distinguished from mitosis on the basis of events during meiotic M phase and the production of hour haploid gametes Meiotic interphase is followed by two division stages

Meiosis I and II
In meiosis I homologous chromosomes separate from one another, reducing the diploid number of chromosomes to the haploid number In meiosis II, sister chromatids separate from one another to produce four haploid gametes, each with one chromosome of the original diploid pair Meiosis I

Three hallmark events occur in meiosis I
1. Homologous chromosomes pairing
2. Crossing over between homologous chromosomes(generates new combinations of alleles) 3. Segregation (separation) of homologous chromosomes, which reduces chromosomes to the haploid number Stages of Meiosis I

Meiosis I is divided into prophase I, metaphase I, anaphase I, and telophase I Pairing and recombination of homologs takes place in prophase I (generating new combos of genes) Prophase I is subdivided into five stages:

1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene
5. Diakinesis
Leptotene and Zygotene
Chromosome condensation begins in leptotene stage
The meiotic spindle forms as microtubules extend out from centrosomes The nuclear envelope disintegrates during zygotene
Homologous chromosomes undergo synapsis
Synaptonemal Complex
Homologous chromosomes align and the synaptonemal complex (protein bridge) is formed between the chromosomes This tri-layer protein structure tightly binds non-sister chromatids of homologous chromosomes Nonsister chromatids belong to different members of

Pachytene
Chromosome condensation continues in pachytene
Paired homologs are called tetrads, due to the four visible chromatids Recombination nodules can be seen at intervals in the synaptonemal complex These are aggregates of enzymes and proteins needed for crossing over between homologs Diplotene

Chromosomes continue to condense in diplotene, and the synaptonemal complex begins to dissolve Homologs pull apart slightly, revealing chiasma, at locations where crossing over has occurred Cohesin protein is present between sister chromatids, to resist the pulling forces of kinetochore microtubules Diakinesis

Kinetochore microtubules move synapsed chromosome pairs toward the metaphase plate Here, homologs align side by side
Metaphase I
In metaphase I chiasmata between homologs are dissolved; this completes crossing over Homologs align on opposite sides of the metaphase plate
Kinetochore microtubules attach to both sister chromatids of one homolog; kinetochore microtubules from the opposite pole do the same for the other homolog Anaphase I
Anaphase I begins when homologs separate from one another and are pulled to opposite poles of the cell Sister chromatids are firmly attached by cohesion
Telophase I and Cytokinesis
In telophase I the nuclear membranes reform around the separated haploid sets of chromosomes Cytokinesis follows telophase I and divides the cytoplasm to create two haploid cells Meiosis I is called the reductional division because the ploidy of the daughter cells is halved compared to the original diploid parent cell Meiosis II

Meiosis II divides each haploid daughter cell into two haploid cells, by separating sister chromatids from one another The...
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