Review
• Mitosis is cell division that
occurs in somatic cells – all
cells of the body EXCEPT the
sex cells.
Mitosis
• In mitosis, two daughter cells
are produced that are
GENETICALLY IDENTICAL to
the parent cell
Mitosis
• Somatic cells contain all of the
chromosomes for that
particular animal.
Mitosis
• This is represented as 2n or
diploid
• In humans, there are 23 pairs
of chromosomes for a total of
46 chromosomes. (2n=46 for
humans)
Mitosis
• Somatic cells contain pairs of
chromosomes with similar
genetic information.
Mitosis
• One chromosome came from
the mother, the other from
the father.
For Example:
• Homologous chromosomes
determine eye color. The one
from mom is for brown eyes,
the one from dad is for blue
eyes.
So…
• Why do sex cells not go
through mitosis to produce
two daughter cells?
Gametes
• Sex cells, also known as
gametes, are ova (female) and
sperm (male)
So…..
• What would happen if sex
cells each had a full set of
chromosomes then egg and
sperm united?
So….
• The new individual created
would have double the
number of chromosomes.
• Not a viable being
Meiosis
• The process of meiosis
ensures that the correct
number of chromosomes is
maintained.
Meiosis
• Meiosis – The process that
forms sex cells called gametes
(ova and sperm cells)
Meiosis
• IT DOES NOT PRODUCE TWO
NEW GENETICALLY IDENTICAL
CELLS!
Meiosis
• Cells produced by meiosis
only have half (1/2) the usual
number of chromosomes that
other cells have.
Meiosis
• This is represented as n and is
said to be haploid.
• What is the haploid number
for humans?
So How Are We All Different?
• The process of meiosis also
allows for genetic variation –
we are not exact copies of our
parents.
Meiosis
• Because of meiosis, the
offspring has a combination of
genetic material from each
parent.
Cats
Cats
• Cats have 38 chromosomes.
• What is the haploid number?
• What is the diploid number?
Cats
• How many chromosomes
would a father cat’s sperm
cells have?
Cats
• How many chromosomes
would a skin cell on a cat
have?
Meiosis
• Meiosis goes through 2
complete cycles in order to
produce 4 cells that each have
½ the number of
chromosomes (haploid)
Meiosis I - Interphase
• Interphase – DNA is replicated
Meiosis I – Prophase I
• Just like in mitosis, the
chromatin shortens and
thickens into chromosomes.
In meiosis, homologous
chromosomes pair up to form
tetrads.
Meiosis I
• A tetrad is 4 chromatids.
Meiosis I
• While lined up closely
together, homologous
chromosomes may “cross
over”
Meiosis I
• Crossing over – A piece of DNA
from one homologous
chromosome changes places
with the SAME SECTION of DNA
on the other homologous
chromosome
Meiosis I
• This is what allows for genetic
variation!
Meiosis I – Prophase I
• Nuclear membrane
disintegrates and spindle
forms
Meiosis I – Metaphase I
• Homologous pairs of
chromosomes line up at the
equator
Meiosis I – Anaphase I
• Homologues (homologous
chromosomes) are pulled
apart.
• There is now a different mix
of homologues at each pole.
Meiosis I – Telophase I
• New nuclear membranes form
around each pole.
• Each of the two new cells has
one chromosome out of each
pair of chromosomes
Meiosis II – Interphase II
• Just a growth time
• THERE IS NO DUPLICATION OF
DNA!!!!
Meiosis II – Prophase II
• Nuclear membrane breaks
down and new spindle forms
Meiosis II – Metaphase II
• Chromosomes line up in the
middle
Meiosis II – Anaphase II
• Chromosomes are pulled
apart – divides the
chromosomes into
chromatids.
Meiosis II – Telophase II
• New nuclear membranes form
around all 4 new cells
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