Transcript Mitosis, Meiosis and Heredity: Meiosis

Mitosis, Meiosis and Heredity:
Meiosis
Unit 7
What we know:
• Genes are located on chromosomes in the
cell’s nucleus
• Each organism must inherit a single copy of
every gene from both it’s parents
• When an organism produces its own gametes
(sperm/egg) the 2 sets of genes must be
separated so each gamete ends up with only 1
set.
http://www.pearsonsuccessnet.com/snpapp/iText/products/0-13-181118-5/bm/vasegreg.htm
Chromosomes
• Consider a fruit fly with 8 chromosomes
– 4 chromosomes from mom
– 4 chromosomes from dad
• These 2 sets of chromosomes are
homologous because each of the 4
chromosomes from mom match up with each of
the 4 from dad
Ploidy
• A cell that contains both sets of
homologous chromosomes is called
diploid (2 sets, 2N)
• Drosophila 2N=8
• 2 complete sets of chromosomes and 2
complete sets of genes
Ploidy
• Gametes (sperm/egg), on the other hand,
contain only a single set of chromosomes
• Thus, they have only a single set of genes
• These cells are called haploid (1 set, N)
• Drosophila N=4
Meiosis
• Meiosis is a process of reduction division
where the number of chromosomes per cell is
cut in half through the separation of
homologous chromosomes in a 2N cell.
• 2 divisions:
– Meiosis I
– Meiosis II
Meiosis I: Interphase I
• Remember the steps G1, S, G2
• Cells go through a round of replication ,
forming duplicate chromosomes
Meiosis I: Prophase I
• Chromosomes have replicated then
centrioles migrate to poles
• Each chromosome from mom pairs with
it’s corresponding chromosome from dad
– This structure is called a tetrad. Tetrads
contain 4 chromatids.
• Crossing-over occurs. This is an
exchange in alleles between
homologous chromosomes &
produces new allele combinations
Meiosis I: Metaphase I
• Spindle fibers attach to each
chromosome in a tetrad
• The tetrads line up at the metaphase
plate
Meiosis I: Anaphase I
• Spindle fibers pull the homologous
chromosomes toward the opposite end of
the cell
Meiosis I: Telophase and Cytokinesis
• Nuclear membranes form around
the now separated chromosomes
• The cytoplasm squeezes in a
pinches off into 2 cells
• The 2 cells have 4 chromatids as
in mitosis BUT since there was
crossing-over, each cell only has
half of the parent cell’s information,
this means these new cells are
haploid
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Meiosis II: Prophase II
• There’s NO chromosome replication before
prophase II.
• Centrioles migrate to opposite
poles.
Meiosis II: Metaphase II
• Chromosomes line up in the center of the cell
at the metaphase plate (like in mitosis). Spindle
fibers attach to the centromeres of each
chromatid.
Meiosis II: Anaphase II
• The spindle fibers pull the sister
chromatids apart at the centromeres.
Chromatids move toward opposite end of
the cell.
Meiosis II: Telophase II and Cytokinesis
• New nuclear membranes form
around each of the groupings of
chromatids.
• The cytoplasm draws in a pinches
off forming 2 new cells from each
of the 2 original cells.
• The final result of Meiosis is 4
haploid (N=4) daughter cells with
2 chromosomes each
http://www.pearsonsuccessnet.com/snpapp/iText/products/0-13-181118-5/bm/vaameio2.ht
Gamete Formation
• In males the haploid gametes are called sperm.
• In some plants, pollen grains contain haploid
sperm cells
Gamete Formation
• In females, normally only 1 of the 4 haploid
cells becomes an egg (has most of the
cytoplasm). The other 3 cells are called polar
bodies and do not
take part in
reproduction
ttp://www.youtube.com/watch?v=D1_-mQS_FZ0
Mitosis
vs.
Meiosis
• 1 round of cell
division
• 2 rounds of cell
division
• Results in 2
genetically identical
daughter cells
• Results in 4 different
daughter cells
• Diploid= 2N
• Somatic Cells
• Hapliod= N
• Involved in gamete
formation
*In both mitosis and meiosis the cell divides.
http://www.youtube.com/watch?v=mr0oiws9ZvM&feature=rel
ated&safety_mode=true&persist_safety_mode=1