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Warm up:
• 1) How many chromosomes do human
somatic (body) cells have?
• 2) What gametes (sex cells) do men
produce? How about women?
• 3) How many chromosomes do human sex
cells have?
It’s…….Meiosis!!!!
Chromosome Number
• Organisms have different # of chromosomes.
Humans: 46 chromosomes, 23 pairs
• Each of the 23 pairs is called a homologous
pair: a similar pair where one came from the
mother and one came from the father.
What’s this chart called??
Quick review!
• How many PAIRS of chromosomes do
humans have?
• What is a homologous pair?
Chromosome Number
•Diploid: A cell that contains both sets of
homologous chromosomes.
• The number of chromosomes in a diploid
cell is sometimes represented by the
symbol
2N.
• Almost all of the cells in your body are
diploid (have both sets of chromosomes).
Chromosome Number
• Haploid: Cells that contain only a single
set of chromosomes, and therefore only a
single set of genes.
• (Gametes are Haploid, sperm/egg)
• These cells are and are represented by the
symbol
N.
• Think: HAPloid sounds like HALF-loid
Quick Review
• A human skin cell has 46 chromosomes (or
23 pairs of homologous chromosomes).
We call this diploid / haploid??
• What is the haploid number of
chromosomes for humans?
How are haploid cells produced?
• Haploid cells are produced in a process
called meiosis.
**The purpose of meiosis is to produce
four haploid gamete cells that are
genetically different from one another
and from the original parent cell.
3 Main purposes of Meiosis
• 1) To cut the # of chromosomes in half in
a cell (46 to 23) (haploid).
• 2) To create gametes (sex cells) egg and
sperm.
• 3) Introduce genetic variation
(independent assortment, crossing over).
Phases of Meiosis
• Meiosis involves two divisions, meiosis I
and meiosis II.
• Overview of what happens:
– Meiosis I: the chromosomes replicate and the
cell divides once
– Meiosis II: each new cell divides, producing 4
cells total
– By the end of meiosis II, the diploid cell that
entered meiosis has become 4 haploid cells.
Do you remember the stages of
MITOSIS from Unit 4?
•I
•P
•M
•A
•T
•C
Meiosis I
Interphase I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
Meiosis I
• Interphase I:
– Cells undergo a round
of DNA replication,
forming duplicate
chromosomes.
• Prophase I:
Meiosis I
– Each Homologous chromosome
pairs up to form a tetrad (with 4
chromatids).
– When homologous chromosomes
form tetrads, they exchange
portions of their chromatids in a
process called *crossing over*,
producing new combinations of
alleles (genes).
– This is why you and your brothers
and sisters can look different!
Meiosis I
• Metaphase I:
– Spindle fibers attach to
the chromosomes
– *Independent
assortment*Homologous pairs can line
up on whichever side they
want! This mixes things up
and creates genetic
variation!
Meiosis I
• Anaphase I
– The fibers pull the
homologous chromosomes
toward opposite ends of the
cell.
– ***HOMOLOGOUS PAIRS
separate.
Meiosis I
• Telophase I and
Cytokinesis
– Nuclear membranes form.
– The cell separates into two
cells.
– The two cells produced by
meiosis I have
chromosomes and alleles
that are different from each
other and from the diploid
cell that entered meiosis I.
Meiosis II
• The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, chromosome replication does NOT occur.
• Prophase II: chromosomes are condensed.
• Metaphase II: Chromosomes line up in the center of cell.
• Anaphase II: ***Sister chromatids separate to opposite ends of the cell.
Telophase II
• Telophase II: The cell pinches in.
and
• Meiosis II results in four haploid (N) daughter cells.
Cytokinesis
Prophase II
Telophase I and
Cytokinesis I
Metaphase II
Anaphase II
Quick Write!
• Name all the phases of Meoisis in order
from Prophase I to Telophase/Cytokinesis
II.
Gamete Formation
In male animals, meiosis results in four equalsized gametes called sperm.
Gamete Formation
• In many female animals, only one egg
results from meiosis. The other three cells,
called polar bodies, are usually not
involved in reproduction.
Review Questions
1. What do the terms haploid and diploid mean?
2. Describe the main results of meiosis. What do
you end up with?
3. What is the result of independent assortment
and crossing over?
4. Name two differences between mitosis and
meiosis.
3 Main purposes of Meiosis
• 1) To cut the # of chromosomes in half in a
cell (46 to 23) (haploid).
• 2) To create gametes (sex cells) egg and
sperm.
• 3) Introduce genetic variation
(independent assortment, crossing over).
• *Bonus: What would happen if each of these
purposes were not met?
3 main purposes of Meosis?
• 1)
• 2)
• 3)
Why can’t your body use mitosis to
make sperm or egg cells?
• Suppose human sperm and eggs were produced
by mitosis. How many chromosomes would
each sperm or egg have?
46 chromosomes
• If a sperm of this type fertilized an egg of this
type, how many chromosomes would the
resulting zygote have?
92 chromosomes
• Obviously, if the body used mitosis to make
sperm and eggs, the resulting zygote would
have too many chromosomes to produce a
normal body.
• In humans, to produce a normal zygote, how many
chromosomes should each sperm and egg have?
23 chromosomes
 This is why meiosis is so important and
occurs in gametes.