Download Section 11-4: Meiosis

Section 11-4: Meiosis
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Where are genes located? Chromosomes
in the nucleus
Basic principles of genetics require 2
things:
1. Each organism must receive a single copy of
every gene from both its parents
2. Each organism produces gametes which
contain just one set of genes
ANIMATION - Square Dance
Chromosome Number
• Humans have 46 chromosomes
• Work in pairs (like shoes)
• Homologous chromosomes – sets of
chromosomes
– How many pairs of chromosomes do humans
have? 23
– 23 for Mom + 23 from Dad = 23 pairs
Chromosome Number
• Diploid – two sets – 2N
– Cell that contains both sets of homologous
chromosomes
– Humans – 2N = 46
• Haploid
– Cells that contain a single set of chromosomes
– gametes
– Humans – N = 23
Phases of Meiosis
• Meiosis – reduction division
– Go from diploid cells to haploid
cells
– Chromosome number is cut in
half
• Meiosis goes thru 2 rounds of
division:
– Meiosis I and Meiosis II
– Diploid cell becomes 4 haploid
cells (gametes)
• Males – sperm - Females – egg
Meiosis I
• Chromosomes (DNA) are replicated before
division
• Prophase I – like in Mitosis
• Metaphase I – chromosomes line up at
center …But in PAIRS! – called a tetrad
– Crossing-over – portions of chromatids are
exchanged
• Anaphase I – homologous pairs are
separated
• Telophase I & Cytokinesis – 2 Daughter
Cells
Meiosis II
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2 Cells produced by Meiosis I enter Meiosis II
Chromosomes are NOT replicated
Just like 2 cells going thru mitosis
Result is 4 haploid daughter cells
Comparing Mitosis and Meiosis
• Mitosis produces 2 genetically identical
diploid daughter cells
• Meiosis produces 4 genetically different
haploid daughter cells
SQUARE
DANCING
Nondisjunction
Karyotype of a normal
male
Karyotype of a Patau’s male
(notice chromosome #13 has
Questions
1. An organism has 24 chromosomes and cell
division occurs. The resulting cells have 24
chromosomes and are described as
__________
2. With regard to #1 cells divide elsewhere in the
body and resulting cells have 12 chromosomes
and are described as ______________.
3. What name(s) are given to the cells that have
12 chromosomes.