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Meiosis
Meiosis – process of reduction division
in which the number of chromosomes
per cell is cut in ½ by the separation of
homologous chromosomes in a diploid
cell.
-Meiosis refers to sexual reproduction in
which genetic info from 2 parents is
combined to make genetically different
offspring.
-Meiosis occurs in sex cells (gametes).
Meiosis – has 2 divisions
(Meiosis I & Meiosis II = 4 daughter cells)
A. Chromosome Number
-In organisms that reproduce sexually, half of the
genetic material comes from the mother & half
comes from the father.
-These 2 sets of chromosomes from each parent
are homologous chromosomes – chromosomes
that each have a matching chromosome from the
opposite-sex parent.
-Cells that contain both sets of chromosomes are
diploid or 2N, meaning “two sets”. In humans 2N = 46
chromosomes in a cell.
-Cells that contain a single set of chromosomes are
haploid or N, meaning, “one set”. In humans N = 23
chromosomes.
B. The Phases of Meiosis
*Meiosis involves 2 divisions : Meiosis I & Meiosis II
-By the end of meiosis II, the diploid cell has become 4
haploid cells.
In meiosis I :
1. Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad - structure
containing 4 chromatids.
2. Homologous chromosomes begin crossing-over exchange of portions of their chromatids.
-Crossing-over creates genetic variation among organisms.
3. Homologous chromosomes separate & form 2 cells
that are ready to divide again.
Tetrads & Cross-over
Meiosis I
In Meiosis II :
1. The 2nd meiotic division starts.
2. Chromosomes line up in center of cell, separate, &
form 4 daughter cells that are genetically different
from the parent cell.
Summarization of Meiosis
Interphase I – cells undergo DNA replication, forming
duplicate chromosomes.
Meiosis I – Stage one of division - Has 4 phases :
Prophase I – Each chromosome pairs with its matching
homologous chromosome to form a tetrad.
Metaphase I – Spindle fibers attach to the chromosomes.
Anaphase I – Fibers pull the homologous chromosomes to
opposite poles of cell.
Telophase I & Cytokinesis – Nuclear membranes form.
Cell separates into 2 cells.
Meiosis II – Stage two of division –
Has 4 phases :
Prophase II – Meiosis I results in 2 haploid
(N) daughter cells, each with ½ the number
of chromosomes as the original cell.
Metaphase II – Chromosomes line up in the
center of the cell.
Anaphase II – Sister chromatids separate &
move to opposite poles.
Telophase II & Cytokinesis – Result is 4
haploid (N) daughter cells.
C. Gamete Formation
*Meiosis is also called :
Gametogenesis – the process by which gametes
develop in the gonads (production of gametes).
Gametes – haploid (N) sex cells.
Sperm – male sex cells
&
Eggs – female sex cells
Gonads – specialized organs used for reproduction. In
males they’re testes, which produce sperm & in females
they’re ovaries, which produce eggs or ova.
-Males undergo spermatogenesis – formation of sperm
in the testes. The end result is 4 good sperm cells.
-Females undergo oogenesis - formation of eggs in the
ovaries. The end result is 1 good egg & 3 polar bodies
that aren’t used.
*Fertilization restores the diploid (2N) # of chromosomes.
Gametogenesis in males & females