Objectives
1. Model and explain how genetic information is
transferred from one generation of cells to the
next via the processes of mitosis and meiosis
2. Compare and contrast cell division by mitosis
with cell division by meiosis
3. Represent chromosomes containing specific
alleles through cell division by mitosis and
meiosis
Meiosis: sex
Last time:
Two forms of cell reproduction
MITOSIS
MEIOSIS
• Occurs in somatic (body)
cells
• Daughter cells?
• Ploidy?
• Genetically_________
parent cell
• Purpose (function):
• Occurs in reproductive
cells
• Daughter cells?
• Ploidy?
• Genetically_________
parent cell
• Purpose (function):
Modeling the steps of meiosis
Draw a cell containing 2 pairs of
unreplicated homologous
chromosomes for the individual
to the right.
She is heterozygous for tongue
rolling and earlobe shape.
Complete in your groups.
Turn in one paper per group at the end of class.
DNA replicates
G2: Cell DNA is
replicated
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Cell Division by Meiosis
Two Divisions:
1. Meiosis 1: Homologs separate
(crossing over and independent
assortment)
2. Meiosis 2: Sister chromatids separate
Prophase 1
Homologs align at
equatorial plane
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Draw your cell with homologous
pairs aligned at the equatorial plane
Metaphase 1
Homologs align at
equatorial plane
How is this
different from
metaphase of
mitosis?
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Crossing-over
Metaphase – Anaphase 1:
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Homologous pairs
align, then separate
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First Division (Meiosis 1)
Homologs sort into different daughter cells.
Draw the products of this division.
End of Meiosis 1
2 cells – each with 1 member of homologous pair.
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Are there other possibilities? What is the consequence of
separating the homologous pairs?
Mendel’s Laws
1. Law of Independent Assortment
– Each pair of homologous chromosomes
positions independently of all other pairs in
Metaphase 1
Metaphase 2:
Centromeres of sister chromatids align
at equatorial plane.
Metaphase 2:
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Second Division (Meiosis 2)
Each cell resulting from Meiosis 1 divides.
Sister chromatids separate at the
centromere and sort into different daughter
cells. Draw the products of this division.
Metaphase –
Anaphase 2:
Sister chromatids separate into
daughter cells
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Metaphase –
Anaphase 2:
Sister chromatids separate into
daughter cells
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End of Meiosis 2: Daughter cells are “gametes”
(eggs and sperm).
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Genotypes of gametes?
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What if
homologs
lined up
differently at
Meiosis 1?
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Predict the
genotypes of
gametes.
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Genotypes of gametes?
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Why are the genotypes different?
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Mendel’s Laws
1. Law of Independent Assortment
- Each pair of homologous chromosomes
positions independently of all other pairs in
Metaphase 1;
2. Law of Segregation
- Two alleles of a given gene segregate during
gamete formation, in Metaphase 2.
What is the
consequence
of Independent
Assortment?
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What would
happen if . . .?
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Why doesn’t
this work??
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Can we predict
the number of
different types
of gametes that
could be
formed?
i.e., How many
different allele
combinations
(genotypes) could
you get in the
gametes?
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