Mitosis and Meiosis

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Stages of mitosis and the chromosomal events that occur at each stage
-Prophase: chromosomes codense
-Prometaphase: nuclear membrane disappears, microtubles attach to chromosomes
-Metaphase: chromosomes are condensed and lined up along metaphase plate
-Anaphase: chromosomes divide and are pulled to opposite poles of the cell
-Telophase: nuclear mem. forms, chromosomes decondense
-Cytokinesis: formation of two identical daughter cells

*Two individual cells return to the G1 stage of interphase*
Mitosis: prophase
chromosomes condense; centrosomes (with duplicated centrioles) separate and migrate to opposite poles of the cell; microtubules radiating from the centrioles begin to form the mitotic spindle.
-Each chromosome now contains two strands of dsDNA (sister chromatids) which lie parallel to one another and are
connected together at one spot by the centromere
Mitosis: prometaphase
nuclear membrane disappears; chromosomes attach to
spindle microtubules at their kynetochores
Mitosis: metaphase
Chromosomes are fully condensed, line up "single file" along the 'metaphase plate', each chromosome is attached to centriole by a microtubule forming mature spindle; spindle fibers begin to contract
Mitosis: anaphase
Centromeres divide into two, spindles pull sister chromatids
toward opposite sides of the cell (centromere first) dictated by where the centrioles formed
Mitosis: telophase
two nuclear membranes form, spindle fibers disappear,
chromosomes decondense and return to interphase
Mitosis: cytokinesis
(form two identical daughter cells): occurs after nuclear division and results in a roughly equal division of the cytoplasm into two parts. This occurs by a process known as cleavage which actually begins during the anaphase stage
Stages of meiosis and the chromosomal events that occur at each stage
-Interpahse I: chromosomes replicate
-Prophase I: chromatin condenses, chiasmata form, crossing over occurs
-Metaphase I: chromosomes line up in PAIRS along the metapahse plate
-Anaphase I: homologous chromosomes pulled to opposite poles of cell
-Telophase I: nuclear membrane reforms, cytokinesis
-Interpahse II: v. brief, no replication
-Propase II: chromosomes condense, nuc mem disappears, spindle fibers
-Metaphase II: chromosomes line up
-Anaphase II: sister chromatids split and are moved to opposite ends
-Telophase II: nuc mems form, cytokinesis occurs
Meiosis I: Interpahse I
replication of chromosomal DNA occurs (similar to mitotic
interphase)
Meiosis I: Prophase I
chromatin strands condense, the homologous chromosomes pair
up, side by side, lying together in perfect alignment during a process called synapsis, this pairing of homologous chromosomes is an important part of the cell
cycle. Chiasmata (chiasma) forms, a cross-shaped structure where the
homologous chromosomes attached forming a bivalent. A bivalent indicates 2 homologous chromosomes in the unit (or a tetrad of 4 chromatids in the unit). Each chiasmata indicates a point at which the homologues exchange genetic material between non-sister chromatids, a process called "crossing over" or "recombination,"

-5 stages: leptotene, zygotene, Pachytene (recombination), diplotene (homologs separate, remain attached at chiasmata), diakinesis
chiasmata
the process of chiasmete formation and crossing over results in the exchange of genetic material between the non-sister chromatids of homologous chromosomes

-usually at least one cross over in each arm
Meiosis I: Metaphase I
complete formation of spindle and two
centromeres of each bivalent lie on the equatorial plane.Thus, the chromosomes line up in pairs along the metaphase plate
Meiosis I: Anaphase I
chiasmata disappear and the homologous chromosomes are pulled
by the spindle fibers toward opposite poles of the cell.
Meiosis I: Telophase I
it begins when the chromosomes reach opposite sides of the cell.
New nuclear membrane begins to form. Each of the two daughter cells contain the haploid number of chromosomes and each chromosome has two sister chromatids. Cytokinesis also occurs in human
Meiosis II: Interpahse II
very brief, no DNA replication occurs
*different from interpahse I and mitotic interphase
Meiosis II: Prophase II
the cell contains only the haploid number of chromosomes,
chromosomes condense and the nuclear membrane disappear, new spindle fibers are formed.
Meiosis II: Metaphase II
spindle fibers pull the chromosomes into alignment "single file" at the equatorial plane
Meiosis II: Anaphase II
centromere of sister chromatids split and each carries a single
chromatid toward a pole of the cell. Newly separated sister chromatids may not be identical due to crossing over
Meiosis II: Telopase II
it begins when the chromosomes reach opposite poles of the cell.
New nuclear membranes are formed around each group of chromosomes. And cytokinesis occurs
Differences bt Mitosis and Meiosis: Location
Mitosis: somatic cells
Meiosis: germ line cells
Differences bt Mitosis and Meiosis: Time
Mitosis: takes about an hour
Meiosis: Female begins at 3-4 months gestation, 1st division is complete at ovulation, 2nd division is complete at fertilization.
Male begins at puberty, complete in 60-65 days
Differences bt Mitosis and Meiosis: chromosome pairing
Mitosis: chromosomes do not pair
Meiosis: homologous chromosomes pair (prophase I)
Differences bt Mitosis and Meiosis: recombination
Mitosis: usually no chiasmata or crossing over
Meiosis: chiasmata and crossing over always occur
Differences bt Mitosis and Meiosis: number of cell divisions
Mitosis: 1 cell division
Meiosis: 2 cell divisions to produce 4 daugther cells
Differences bt Mitosis and Meiosis: chromosome number
Mitosis: no change in chromosome #
Meiosis:chromosome cumber reduced to one of each pair (haploid set in each daughter cell)
Differences bt Mitosis and Meiosis: gene content
Mitosis: no change in gene content
Meiosis:
correlate meiosis with Mendelian concepts of independent assortment and segregation of alleles
All daughter cells may be genetically different, due to segregation of chromosome pairs and crossing over between homologues
segregation
The 2 members of a single gene pair (alleles) are never found in the same
gamete; they always segregate and pass to different gametes. During meiosis, the two chromosomes of a homologous pair will separate
independent assortment
Members of different gene pair assort to the gametes independently of one
another. The segregation of one pair of homologs is independent of and does not affect the segregation of other pairs of homologs
syntenic genes
genes on the same chromosome
co-segregation
syntenic alleles tend to stay together through generations
recombination
occasionally co-segregation is disrupted resulting in a new combination of alleles on a chromosome
identify the haploid and diploid stages of gametogenesis
dipliod until the end of mitosis I, then hapliod through the end of meiosis, become dipliod again following fertilization