Meiosis
Sexual Reproduction
Meiosis: What is the point?
• Cell division that produces a sperm cell or
an egg cell (Gametes).
• Starts with one cell that contains two
copies of each chromosome (Diploid). One
chromosome from female parent and one
from male parent.
• Ends with 4 Haploid (only one set of
chromosomes) daughter cells. 4 cells all
genetically different.
Vocabulary
• Meiosis- process of reduction division in
which the number of chromosomes per
cell is cut in half
• Homologous- chromosomes that each
have a corresponding chromosome from
the opposite-sex parent
• Diploid – a cell that contains both sets of
homologous chromosomes
More Vocab
• Haploid- a cell that contains only a single
set of chromosomes. (single set of
genes)
• Tetrad- Structure containing 4
chromatids that form during meiosis
• Crossing over- homologous
chromosomes exchange portions of their
chromatids during meiosis
Mendel’s principles of genetics require at least
two things:
1.Each organism must inherit a single copy
of every gene from both its “parents”.
2.When an organism produces its own
gametes, those two sets of genes must be
separated from each other so that each
gamete contains just one set of genes.
Let’s Look at a Fruit Fly: Drosophila
• A fruit fly has 8 chromosomes
• 4 of the chromosomes came from the fruit
fly’s male parent and 4 came from the
female parent.
• These two sets of chromosomes are
homologous. Meaning that each of the 4
chromosomes that came from the male
parent has a corresponding chromosome
from the female parent
• A cell that contains both sets of
homologous chromosomes is said to be
diploid. (two sets)
Diploid Cell
• A diploid cell is sometimes represented
by the symbol 2N.
• What is the diploid number for the
Drosophilia fly?
• Your answer should be 8
2N = 8
Gametes (sperm & egg cells)
• The gametes of sexually reproducing
organisms, contain only a single set of
chromosomes and therefore only a single
set of genes.
• These are called Haploid cells (one set)
• Haploid is represented as N.
• What is the Haploid number for
Drosophilia?
• Your answer should be N = 4
Why are all 4 daughter cells different?
When homologous chromosomes form tetrads in
meiosis I, they exchange portions of their
chromatids in a process called crossing over.
Crossing-over produces new combinations of
alleles.
Crossing Over
 Segments of
homologous
chromosomes break
and reattach at similar
locations.
 Results in new genetic
combinations of
offspring.
 This is the main
advantage of sexual
reproduction
Why is this an advantage?
• Offspring has more variety
• If an environment undergoes frequent
changes it is better for the offspring to
have more variety so the species will not
die
Meiosis I
• Prior to Meiosis I, each chromosome is
replicated.
• The cells then begin to divide similar to
mitosis.
• In mitosis, the 4 chromosomes line up
individually at the center of the cell.
• The 2 chromatids that make up each
chromosome then separate from each
other.
The Difference
• In prophase of Meiosis I, however, each
chromosome pairs with its corresponding
homologous chromosome to form a
structure called a tetrad.
• There are 4 chromatids in a tetrad.
• This pairing of homologous chromosomes
is the key to understanding meiosis.
How are Mitosis and Meiosis the
same?
• Division of cells
• DNA replication occurs before the division
of the nucleus.
Summary
Similar in many ways to mitosis
Several differences however
Involves 2 cell divisions
Results in 4 daughter cells with
1/2 the normal genetic
information.
The 4 cells are genetically
DIFFERENT.
Gametes or cells resulting from
meiotic division will have HALF as
many chromosomes.
Overview of Meiosis
Phases of Meiosis
Meiosis II
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis
Gamete Formation
In male animals, meiosis results in four equalsized gametes called sperm.
In many female animals, only one egg results
from meiosis. The other three cells, called
polar bodies, are usually not involved in
reproduction.
• http://www.sumanasinc.com/webcontent/a
nimations/content/meiosis.html
• video
Practice questions
Q: What is the cell process that occurs only
in organisms that reproduce
sexually?___________
Q.A sperm cell of an alligator has 16
chromosomes. What is the total number of
chromosomes normally present in a
stomach cell of this alligator? _________