Unit 2 Lesson 2 Meiosis
Essential Question: How do cells divide
for sexual reproduction?
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Unit 2 Lesson 2 Meiosis
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How do sex cells differ from body cells?
• Before sexual reproduction can take place, each parent
produces sex cells.
• Sex cells have half of the genetic information that body
cells have.
• When genetic information from two parents combines,
the offspring will have one full set of genetic information.
Unit 2 Lesson 2 Meiosis
How do sex cells differ from body cells?
• In body cells,
chromosomes are found in
pairs of homologous
chromosomes, which
have the same structure
and size.
• Homologous
chromosomes carry the
same genes.
• However, some may be
different versions of the
genes.
Unit 2 Lesson 2 Meiosis
How do sex cells differ from body cells?
• One chromosome pair is
made up of sex
chromosomes, which
control the development
of sexual characteristics.
• In humans, the sex
chromosomes are called
X and Y chromosomes.
• Cells with a pair of every
chromosome are called
diploid.
Unit 2 Lesson 2 Meiosis
Why do organisms need sex cells?
• Most human body cells contain 46
chromosomes.
• Gametes (sex cells – sperm and egg
cells) have only half the usual
number of chromosomes.
• Gametes are haploid, meaning they
have one chromosome from each
homologous pair.
• Gametes are made in the
reproductive organs.
Unit 2 Lesson 2 Meiosis
Why do organisms need sex cells?
• When sex cells combine during sexual reproduction, the
resulting cell has 46 chromosomes: half from the egg
and half from the sperm.
• When an egg is fertilized by a sperm cell, a new diploid
cell forms.
Unit 2 Lesson 2 Meiosis
How are sex cells made?
• Mitosis produces two new cells that contain exact copies of
the chromosomes in the parent cell.
• A different kind of cell division is needed to produce sex
cells.
• Meiosis is the type of cell division that produces haploid
sex cells such as sperm or egg cells.
Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In males, the sex cells develop into sperm cells.
• In females, the sex cells become egg cells.
• In females of some species, three cells are broken
down and only one haploid cell becomes an egg.
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Unit 2 Lesson 2 Meiosis
One Step at a Time
What are the stages of meiosis?
• Meiosis has two parts: meiosis I and meiosis II.
• Recall that homologous chromosomes have the
same genes but are not exact copies of each
other.
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Unit 2 Lesson 2 Meiosis
p104
What are the stages of meiosis?
• Before meiosis I begins, each
chromosome is duplicated.
Each half is called a
chromatid.
• Chromatids are connected by
centromeres.
• During meiosis I, pairs of
homologous chromosomes
and sex chromosomes split
apart into two new cells.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In prophase I, duplicated homologous
chromosomes pair up.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In metaphase I, the homologous chromosome
pairs line up in the middle of the cell.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In anaphase I, the homologous chromosomes
separate from their partners and move to opposite
ends.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In telophase I and cytokinesis, the nuclear
membranes re-form and the cell divides into two
cells. The chromatids are still joined.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• Meiosis II involves both of the new cells that
formed during meiosis I.
• The new cells divide during meiosis II.
• Meiosis II results in four haploid sex cells.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In prophase II, the nuclear membrane breaks
apart.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In metaphase II, the chromosomes line up in the
middle of each cell.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In anaphase II, the chromatids are pulled apart
and move to opposite sides.
.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
• In telophase II and cytokinesis, the nuclear
membranes re-form and the cells divide. Each cell
is haploid.
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Unit 2 Lesson 2 Meiosis
What are the stages of meiosis?
Unit 2 Lesson 2 Meiosis
How does meiosis compare to mitosis?
Mitosis
Meiosis
Unit 2 Lesson 2 Meiosis
How does meiosis compare to mitosis?
• Only cells that will become sex cells undergo
meiosis. All other cells divide by mitosis.
• In meiosis, chromosomes are copied once, and
the nucleus divides twice.
• In mitosis, the chromosomes are copied once, and
the nucleus divides once.
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Unit 2 Lesson 2 Meiosis
How does meiosis compare to mitosis?
Meiosis produces haploid cells. Mitosis produces diploid cells.
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Unit 2 Lesson 2 Meiosis
How does meiosis compare to mitosis?
2
1
4
Haploid
2
Diploid
Sex Cells Body Cells
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Unit 2 Lesson 2 Meiosis
How does meiosis compare to mitosis?
DNA
duplication
Ends with 2
daughter
cells
Purpose for
growth and
development
5 stages
Genetically
identical
Mitosis
Ends with 4
Haploid
daughter
cells
Meiosis
Cell
division
Creates sex
cells for
reproduction
– sperm and
egg
10 stages
Crossing over
occurs
Unit 2 Lesson 2 Meiosis
Down Syndrome
• Down syndrome is a genetic
disease caused by an error
during meiosis.
• The chromatids in
chromosome 21 do not
separate, so one of the sex
cells gets an extra copy of
chromosome 21.
• Down syndrome causes a
number of health problems
and learning difficulties, but
many people with Down
syndrome have fulfilling lives.