Meiosis
Homologous Chromosomes
• Homologous
chromosomes: pairs of
chromosomes with
genes for the same
traits, but they can have
different information
about those traits
• ½ the chromosomes in a cell come from Mom,
½ the chromosomes in a cell come from Dad
Humans
• Humans have 46 chromosomes or 23 pairs of
homologous chromosomes.
• Somatic (body) cells contain 46 chromosomes.
• 46 is the diploid (2n)number for humans.
- Diploid means that a cell has both
homologues for each chromosome
• Somatic cells reproduce by mitosis
The Homologues
23 pairs of chromosomes
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Reproductive Cells
• Gametes(eggs and sperm) are called haploid because
they contain one set of chromosomes.
• Eggs also called Ovum and sperm contain 23
chromosomes.
• Eggs and sperm are produced by meiosis- a process
that reduces chromosomes by half
What is Meiosis?
• A type of cell division where ONE diploid cell (46
chromosomes) become FOUR haploid cells(23
chromosomes each).
• The cells produced by meiosis are gametes (sex cells)
• One diploid cell gives rise to 4 haploid cells
46
23
23
23
23
Who does it?
• Plants
• Fungi
• Animals (including you)
• Males start meiosis at puberty.
• Females start meiosis in utero.
Where does it occur?
• In your gonads
• Females- the ovary
• Males-the testicles
Why does it occur?
• Genetic Variation
• Mixes genes from Mom and Dad to create a unique baby
• Reduce the chromosome number by half
• Ensures that each cell produced has one copy from
each homologous pair of chromosomes
• 23 in sperm + 23 in egg unite (fertilization) leads to
a 46 zygote(baby)
Meiosis creates the cells needed for
sexual reproduction
• Two haploid gametes join to form one
diploid cell.
- the new diploid cell (fertilized egg)
is called a zygote.
Quiz
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How many chromosomes does a human have?
What are somatic cells?
How do somatic cells reproduce? (process)
How do sex cells reproduce? (process)
A fertilized egg is called a ___________.
What is the difference between haploid and diploid
cells?
• Where in your body does meiosis occur?
How does it occur?
• In two stages called Meiosis I and Meiosis II.
• Each stage contains 4 distinct phases
prophase
metaphase
anaphase
teleophase
• telophase is followed by the division of cytoplasm known as
cytokinesis.
Prophase 1
• Homologous chromosomes
join to form a tetrad
-chromosomes in the tetrad
cross over and exchange
genes
• Crossing over produces new
genetic combinations
- It is another way to increase
genetic diversity
Independent Assortment
• During Metaphase 1, the tetrads
(groups of 4 homologous chromosomes)
line up down the center of the cell
- they assort independently
-Chromosomes from each
parent line up independently
of each other
Independent Assortment
Independent Assortment
 Independent assortment increases genetic diversity
- There are 23 pairs of homologous chromosomes
in humans
- independent assortment ensures that there are
223 possible combinations
223= 8,388,608 combinations
- When two gametes join to form the zygote in fertilization the
possible number of combinations is
223 X 223 = 64 trillion
Ovum Formation
• One egg is formed
• One daughter cell keeps ALL of the
cytoplasm
• Other 3 daughter cells only have
genetic material – Polar Bodies (Barr
Bodies)
Sperm Formation
• Four sperm are formed.
• Cytoplasm and Nuclear Material
Divides evenly.
• Flagella are added last to the cells.
Extra practice videos
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Meiosis stages video
Amoeba sisters mitosis vs. meiosis review
Mitosis Review
Meiosis Song 1
Meiosis crash course