Unit 6 Lesson 2 Meiosis
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Unit 6 Lesson 2 Meiosis
Florida Benchmarks
• SC.7.L.16.3 Compare and contrast the general
processes of sexual reproduction requiring meiosis
and asexual reproduction requiring mitosis.
• LA.6.4.2.2 The student will record information
(e.g., observations, notes, lists, charts, legends)
related to a topic, including visual aids to organize
and record information and include a list of
sources used.
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Unit 6 Lesson 2 Meiosis
Number Off!
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.
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Unit 6 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.
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Unit 6 Lesson 2 Meiosis
How do sex cells differ from body cells?
• One chromosome pair is made up of sex
chromosomes.
• Cells with homologous chromosomes are called
diploid.
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Unit 6 Lesson 2 Meiosis
Why do organisms need sex cells?
• Normal human body cells contain 46
chromosomes.
• Sex cells are also known as gametes. Gametes
have only half of the usual number of
chromosomes.
• Gametes are haploid, meaning they have one
chromosome from each homologous pair.
Gametes are made in the reproductive organs.
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Unit 6 Lesson 2 Meiosis
Why do organisms need sex cells?
• Sex cells are needed so that in sexual
reproduction offspring receive a total of 46
chromosomes.
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Unit 6 Lesson 2 Meiosis
How are sex cells made?
• Mitosis produces two new cells that contain exact
copies of the chromosomes of 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.
• When an egg is fertilized by a sperm cell, a new
diploid cell forms.
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Unit 6 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 6 Lesson 2 Meiosis
One Step at a Time
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 6 Lesson 2 Meiosis
What are the stages of meiosis?
• In prophase I, duplicated homologous
chromosomes pair up.
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Unit 6 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 6 Lesson 2 Meiosis
What are the stages of meiosis?
• In anaphase I, the homologous chromosomes
separate from their partners and move to opposite
sides.
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Unit 6 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 6 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 6 Lesson 2 Meiosis
What are the stages of meiosis?
• In males, the four sex cells develop into sperm
cells.
• In females of some species, three cells are broken
down, and only one haploid cell becomes an egg.
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Unit 6 Lesson 2 Meiosis
What are the stages of meiosis?
• In prophase II, the nuclear membrane breaks
apart.
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Unit 6 Lesson 2 Meiosis
What are the stages of meiosis?
• In metaphase II, the chromosomes line up in the
middle of the cells.
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Unit 6 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 6 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 6 Lesson 2 Meiosis
How does meiosis compare to mitosis?
• Only 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 6 Lesson 2 Meiosis
How does meiosis compare to mitosis?
• Meiosis produces haploid cells. Mitosis produces
diploid cells.
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Unit 6 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.
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