Meiosis grade 8

Unit 7 Lesson 2 Meiosis
Copyright © Houghton Mifflin Harcourt Publishing Company

Indiana Standards
• 8.3.3 Explain that genetic information is
transmitted from parents to offspring mostly by
chromosomes.

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.

• 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.

• One chromosome pair is made up of sex
chromosomes.
• Cells with homologous chromosomes are called
diploid.

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.

Why do organisms need sex cells?
• Sex cells are needed so that in sexual
reproduction offspring receive a total of 46
chromosomes.

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.

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.

One Step at a Time
• 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.

• In prophase I, duplicated homologous
chromosomes pair up.

• In metaphase I, the homologous chromosome
pairs line up in the middle of the cell.

• In anaphase I, the homologous chromosomes
separate from their partners and move to opposite
sides.

• In telophase I and cytokinesis, the nuclear
membranes re-form and the cell divides into two
cells. The chromatids are still joined.

• 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.

• 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.

• In prophase II, the nuclear membrane breaks
apart.

• In metaphase II, the chromosomes line up in the
middle of the cells.

• In anaphase II, the chromatids are pulled apart
and move to opposite sides.
.

• In telophase II and cytokinesis, the nuclear
membranes re-form and the cells divide. Each cell
is haploid.

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.

How does meiosis compare to mitosis?
• Meiosis produces haploid cells. Mitosis produces
diploid cells.

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.

Meiosis grade 8

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Meiosis grade 8