1. Section 11-4: Meiosis
Where are genes located? Chromosomes in the nucleus
Basic principles of genetics require 2 things:
Each organism must receive a single copy of every gene from both its parents
Each organism produces gametes which contain just one set of genes
ANIMATION - Square Dance

2. 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

3. Chromosome Number Diploid – two sets – 2N Haploid
Cell that contains both sets of homologous chromosomes
Humans – 2N = 46
Haploid
Cells that contain a single set of chromosomes – gametes
Humans – N = 23

4. 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

5. 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

6. Meiosis II 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

7. Comparing Mitosis and Meiosis
Mitosis produces 2 genetically identical diploid daughter cells
Meiosis produces 4 genetically different haploid daughter cells
SQUARE DANCING

9. Karyotype of a normal male
Nondisjunction
Karyotype of a Patau’s male (notice chromosome #13 has three chromosomes instead of two
Karyotype of a normal male

10. Questions
An organism has 24 chromosomes and cell division occurs. The resulting cells have 24 chromosomes and are described as __________
With regard to #1 cells divide elsewhere in the body and resulting cells have 12 chromosomes and are described as ______________.
What name(s) are given to the cells that have 12 chromosomes.