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Chapter 11- Meiosis, gametes = sex cells (egg/sperm), Haploid (n) +…
Chapter 11- Meiosis
Meiosis
- a special form of a cell division that is used only in the production of gametes
- gemetes merge durng sextual reproduction
- bc sperm and egg need to combine during fertilization, each can only carry the haploid number of chromosomes
- in human cells meiosis reduces the diploid # of 46 chromosomes to the haploid # of 23
- it does this is 2 divisions (meiosis I and II) which results in 4 daughter cells.
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Procedure of Meiosis I
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Interkinesis
Prophase I
- Homologous chromosomes pair during synapsis
- homologous chromosomes pair up and stay closely associated during the first 2 phases of meiosis I
- synapsis allows for "crossing-over" to occur
- "crossing-over" = mixing of maternal & paternal alleles
Metaphase I
- homologous chromo. align at the metaphase plate
- either the maternal or paternal chromo in a bivalent pair can be oriented toward either pole (not all same direction)
independent assortment - because the homologous can align randomly, we generate diversity in the resulting daughter cell
Anaphase I, Telophase I, InterkinesisAnaphase - homologous separate but the chromatids DO NOT separateTelophase I - does not have to complete- nuclear envelope does not always reform and cytokinesis does not always occur
Interkinesis - a short rest period
- like interphase, but NO DNA replication happens
Meiosis II
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
Prophase II
- cells have one chromosome from each homologous pair
Metaphase II
- chromosomes align at the metaphase plate
Anaphase II
- sister chromatids split and moved toward the poles
Telophase II
- spindle disappears, nuclei reform, cytokinesis takes place
What Happens?
- separation of the sister chromatids
Nondisjunction
- failure of chromosomes to separate properly, results in gain or loss of chromosomes
- errors in crossing-over may result in extra or missing parts of chromosomes
Aneuploidy - a change in the normal # of chromosomes
Monosomy (2n - 1) - when an individual has only one of a particular chromosome
Trisomy (2n + 1) - when an individual has more than two of a particular chromosome
Both are the result of nondisjunction during meiosis
Primary nondisjunction
- occurs during Meiosis I
- both members of homologous pair go into same daughter cell
- error during anaphase I
Secondary nondisjunction
- occurs during meiosis II
- sister chromatids fail to separate and both daughter chromosomes go into the same gamete
- error in anaphase II
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