mitosis and meiosis PDF

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Summary

Cell division notes...

Description

Meiosis I -

Homologous chromosomes separate from one another, reducing the diploid number of chromosomes (2n) to the haploid number (n).

Meiosis II -

Sister chromatids separate to produce four haploid gametes, each with one chromosome of every diploid pair.

Following the completion of meiosis, each gamete contains a single nucleus holding a haploid chromosome set. The gametes of the two sexes are often dramatically different in size and morphology. Female gametes are generally much larger than male gametes. As the fertilized ovum begins mitotic division, the organelles and cytoplasmic structures provided by the maternal gamete support its early zygotic growth. MEIOSIS I Three hallmark events take place during meiosis I: 1. Homologous chromosome pairing 2. Crossing over between homologous chromosomes 3. Segregation (separation) of the homologous chromosomes that reduces chromosomes to the haploid number Meiosis I is divided into four stages: 1. Prophase I - chromosome synapsis, and recombination take place in prophase I

a. b. c. d. e.

leptotene stage zygotene stage pachytene stage diplotene stage diakinesis stage

Prophase I has 5 substages to accurately trace the interactions and recombination of homologous chromosomes 2. Metaphase I 3. Anaphase I 4. Telophase I Chromosome synapsis

Homologous chromosome pairing (the alignment of homologous chromosome pairs.

Synaptonemal complex

Protein bridge formed when synapsis initiates A trilayer protein structure that maintains synapsis by tightly binding nonsister chromatids of homologous chromosomes to one another The function of the synaptonemal complex is to properly align homologous chromosomes before their separation and then to facilitate recombination between homologous chromosomes.

Nonsister chromatids

Chromatids belonging to different members of a homologous pair of chromosomes

The paired homologs are called a tetrad in recognition of the four chromatids Within the central element of the synaptonemal complex, new structures called recombination nodules appear at intervals. Recombination nodules

Play a pivotal role in crossing over of genetic material between nonsister chromatids of homologous chromosomes

The number of recombination nodules correlates closely with the average number of crossovers. Two important observation: 1. Their appearance and location within the synaptonemal complex are coincident with the timing and location of crossing over 2. Recombination nodules seem to be present in organisms that undergo crossing over and absent in those that do not. Chiasmata

located along chromosomes where crossing over has occurred. Chiasmata mark the locations of DNA-strand exchange between nonsister chromatids of homologous chromosomes.

Cohesin protein is present between sister chromatids to resist the pulling forces of kinetochore microtubules Karyokinesis

In, this case homologous chromosomes separate from one another and are dragged to opposite poles

PROPHASE I – LEPTOTENE - Passed through interphase and chromosome is duplicated - Condensation of chromosome begins, but can’t be seen at this stage - Centrosomes begin to migrate toward opposite poles of the cell - Asters of microtubules spindles are formed from each centrosome PROPHASE I – ZYGOTENE - Chromosomes continue to condense - Homologous chromosomes enter synapsis - Synaptonemal complex forms between homologs - Centrosome migration toward opposite poles continues as microtubule polymerization progresses - Meiotic spindle forms - Nuclear envelope breakdown begins PROPHASE I – PACHYTENE - Chromosome condensation is almost complete - Synapsed homologous chromosomes are seen as bivalent structures - Crossing over occurs between nonsister chromatids of homologous chromosomes - Kinetochore microtubules attach

to kinetochore -

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Nonkinetochore and astral microtubules come from centrosome Centrosomes are nearly on opposite poles of the cells Nuclear breakdown continues Recombination nodules appear

PROPHASE I – DIPLOTENE - Crossing over is complete - Synaptonemal complex dissolves leaving chiasmata that hold nonsister chromatids together as the synaptonemal complex begins to dissolve. The dissolution allows homologs to pull apart slightly, revealing contact points between nonsister chromatids.

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Condensation progressed and 4 chromatids of tetrads are visible - Nuclear breakdown ends PROPHASE I – DIAKINESIS - Meiotic spindle is well established - Kinetochore microtubules actively move synapsed chromosome pairs towards the metaphase plate - Nuclear membrane is fully degraded - Chiasmata between homologous chromosomes are resolved process of resolving the contacts between homologs is critical to the completion of recombination between homologous chromosomes.

METAPHASE I - Tetrads are aligned along the metaphase plate - Each chromosome of homologous pair is attached to a kinetochore microtubule - Kinetochores of sister chromatids are attached to the same centrosome - Sister chromatids are joined by cohesin to prevent them separating - Chiasmata linking nonsister chromatids are broken

ANAPHASE I - Depolymerization of kinetochore microtubules begins the disjunction of homologous chromosomes, which starts moving toward opposite poles (karyokinesis) - Sister chromatids remain joined by cohesin

TELOPHASE I - Nuclear membranes re-forms around the chromosomes clustered at each pole - Each newly formed nucleus contains a haploid set of chromosomes - Chromosomes may partially decondense - Cytokinesis – divides the cytoplasmic material by dividing the nuclear contents between the cells - Cytoplasmic division may be unequal...