Aim:
Why is
meiosis
important?
• Do Now: Label
each phase of cell
division (IPMATC)
So, what is the
difference between
mitosis and meiosis?
Mitosis
•
•
2 identically
diploid cells
(same as
parent)
Makes cells
used for
growth, repair,
development &
asexual
development
Meiosis
- 4 genetically
different
haploid cells
- Makes cells
for Sexual
reproduction
- Genetic info is
exchanged/rea
rranged.
Meiosis • Contains all phases of
mitosis
I
• Homologous
chromosomes separated
I
Pray
More
At
The
Church
I P M A T C
Interphase I
• This is where
the
chromosomes
are
duplicated,
replicated, or
copied.
Eukaryotic Cell Cycle
Prophase I
• Chromosomes
condense. The
nuclear envelope
breaks down.
“Crossing Over”
occurs.
• “Crossing
Over”Chromatids
exchange
genetic
material
Metaphase I
• Pairs of
homologous
chromosomes
move to the cell’s
middle.
Metaphase =
Move Middle
Anaphase I
• Homologous
chromosomes
move to the
cell’s opposite
poles.
Telophase I
• Chromosomes
gather at the
poles.
• The cytoplasm
divides.
What about cytokinesis?
• In cytokinesis, the
nuclear membrane is
reforming around each
new cell.
Cytokinesis happens after the
division of chromosomes and
cells are made.
Meiosis II
NO
INTERPHASE
and
NO
REPLICATION
of
chromosomes!!
P M A T
Prophase II
• A new spindle
forms around
the
chromosomes.
Metaphase II
• Chromosomes
line up at the
middle.
Anaphase II
• Centromeres
divide, and
chromatids move
to opposite poles.
Telophase II
• A nuclear
envelope forms
around each set
of chromosomes.
The cells divide.
What do we end up with at the
end of meiosis?
• 4 haploid
cells
Answer to Aim:
Why is meiosis important?
• Allows genetic information
from 2 parents to combine to
form offspring that are
different from both parents.
Aim: Why is genetic
variation so important?
Do Now: Copy the chart on
the side board. Fill in the
blank areas about Mitosis
and Meiosis.
• Mitosis vs Meiosis
What is a homologous
chromosome?
Chromosomes that have the same
sequence of genes, that have the
same structure, and that pair
during meiosis.
Chromatid is a single
strand of chromosomes.
Chromosomes are 2
chromatids together,
made of condensed
chromatin.
Centromere is where
the two chromatids
meet.
Chromatin are
ribbon-liked
structures of DNA
and histone
proteins.
Label the chromosome
• “Crossing
Over”Chromatids
exchange
genetic
material
(Occurs during
Prophase I in
Meiosis.)
Why is crossing over
important?
• With out it,
every one will
look the same!
Crossing Over Video
• New
combinations of
genetics
• Variety
• Crossing over is
one of the
several
processes that
lead to genetic
variation.
• The rearranging
of genetic info
leads to genetic
variation in
offspring.
Genetic Variability
• Natural
Selection
• Allows for
adaptation and
ultimate survival
of certain
organisms when
the environment
changes.
3 Ways to Promote Genetic
Variation
• 1. Crossing over
• 2. Independent
Assortment
• 3. Random Union of
sperm and egg
• Tetrad- a pair
of homologous
chromosomes.
As the tetrads form,
the chromosomes
exchange parts of
their chromatids
during the process
of crossing over.
Disjunction
Tetrads
separate equally
as a result of a
process known as
disjunction.
During what
phase does this
occur?
Anaphase
Non-disjunction
Tetrads
separate
UNEQUALLY.
• Uneven # of
chromosomes
will result in the
gametes.
If these gametes
are fertilized, the
offspring will have
either 1 too many
chromosomes or 1
too few
chromosomes.
Example of Result of nondisjunction: Down’s Syndrome
Down’s Syndrome
• The nondisjunction of the
21st chromosome.
• Occurrence:
One in every
700 American
children.
Answer to Aim:
So why is genetic variability
so important???
Allele
Different forms
of the same gene.
For example,
humans have
genes for eye
color. However,
there exists an
allele for brown
eyes and an allele
for blue eyes.
Responsible for
Dominant or
Recessive
Traits
•
You
can
have
2
of
the
• Each individual
same alleles or 2
has 2 allele’s,
different alleles.
one on each
• The dominant allele will
chromatid.
determine what trait
you have.
• Dominant Traits:
The traits that
come out in the
individual.
* Only 1 dominant
trait is needed.
Expressed as the
capital letter.
AA, Aa, aA
• Recessive Traits:
The “weaker”
trait that we do
not see unless
there are 2 of
them.
Expressed as the
lower case letter.
aa
What color eyes does
each individual have?
Activity in text page 253 on
crossing over.