Meiosis Notes
Slide 1
Meiosis- cell division that producing
reproductive cells called
gametes (or egg and sperm)
Meiosis involves replicating DNA once and
dividing the nucleus two times in steps called
Meiosis I and Meiosis II.
By dividing twice, the chromosome number is
halved in gametes.
Slide 2
- 2 parents
- offspring genetically similar to parents
(combination of parent chromosomes)
(offspring not identical)
Gamete: Sex cell
Female: egg 23
n
Gonads:
Male – Testes
(Haploid)
Female – Ovaries
Male: sperm 23
n
Zygote: Union of sperm and egg
46
2n
(Diploid)
Slide 3
Meiosis:
Process of cell division used to produce
gametes (sex cells)
Primary Sex Cell
46
2n
SC
Interphase
46
2n
DC
Secondary
Sex Cells
Sex Cells 23
n
SC
23
n
DC
P
M
A
T
23
n
DC
23
n
SC
23
n
SC
23
n
SC
I
I
I
I
P
M
A
T
II
II
II
II
Slide 4
What happens in PMAT is the same in meiosis as in mitosis
EXCEPT in…….
Prophase I
Anaphase I
Prophase I of Meiosis I
*Double chromosomes
made during interphase*
Sister chromatids pair
with their homologous
pair, forming a tetrad
Slide 5
Homologous Chromosomes
Slide 6
Humans have 23 chromosome pairs, each
chromosome in the pair contains the same kind of
traits, like eye color; they are called homologous
chromosomes. (Humans have 23 homologous
chromosomes for a total of 46 chromosomes
Having two copies of every gene can be an
advantage. For example, if the one gene from Dad
was damaged, Mom’s good gene could take over.
Slide 7
Sister Chromatid
Sister Chromatid
Slide 8
Tetrad
Maternal chromosome replicated
Paternal chromosome replicated
Slide 9
Prophase I
tetrad
Joining of homologous chromosome
at the centromere forming a tetrad
Slide 10
A Way Meiosis Makes Lots of Different Sex Cells
Crossing-Over
Slide 11
Crossing Over: - increases variation
-changes chromosome package
-(not same as either parent)
Pieces of homologous chromosomes switch locations
P
M
P
M
Bld
Brn
Bld
Brn
Blu
Brn
Blu
Brn
Fair
Drk
Drk
Fair
Slide 12
Metaphase I of Meiosis I
Homologous pairs (tetrads) line up in the center of the cell.
Anaphase I of Meiosis I
Slide 13
Spindle fibers pull the homologues (tetrad)
away from each other during disjunction.
Telophase I of Meiosis I
Slide 14
The chromosome number in these cells
divides in half; starting with an initial four (2n)
and ending with two (n) in each new cell.
Meiosis II: Similar to Mitosis
Slide 15
In Meiosis II each chromosome lines
up and sister chromatids separate
from each other.
Prophase II
Metaphase II
Anaphase II
Telophase II
http://www.biologyinmotion.com/cell_division/
Slide 16
Meiosis in males
Occurs in testes
1:4
n
n
Meiosis in female
Occurs in ovaries
2n
2n
2n
2n
n
n n
n
4 sperm
n
n
1 egg
1:1
n
n n
n
polar
bodies
3 polar bodies
Meiosis does two things -
Slide 17
1) Meiosis takes a cell with two copies of every
chromosome (diploid) and makes cells with a
single copy of every chromosome (haploid).
This is a good idea if you’re going to combine
two cells to make a new organism. This trick
is accomplished by halving chromosome
number.
In meiosis, one diploid cells produces four
haploid cells.
Slide 18
2) Meiosis scrambles the specific forms of
each gene that each sex cell (egg or sperm)
receives.
This makes for a lot of genetic diversity. This
trick is accomplished through independent
assortment and crossing-over.
Genetic diversity is important for the evolution
of populations and species.
Slide 19
Meiosis
Parent cell –
chromosome pair
What if Meiosis doesn’t happen as planned?!?!
Chromosomes
copied
1st division - pairs split
2nd division – produces
4 gamete cells with ½
the original no. of
chromosomes
Meiosis – division error
Chromosome pair
Slide 20
Slide 21
Meiosis error - fertilization
Should the gamete with the
chromosome pair be fertilized
then the offspring will not be
‘normal’.
In humans this often occurs
with the 21st pair – producing
a child with Downs Syndrome
Trisomy 21– Downs Syndrome
Slide 22
Can you see the
extra 21st
chromosome?
Is this person male
or female?
Anaphase I
Slide 23
Disjunction: Separation of tetrad at the centromere
Nondisjunction: Uneven separation of a tetrad during
disjunction
Ex. Down’s Syndrome
(nondisjunction of 21st chromosome tetrad)
Polyploidy: Nondisjunction of all tetrads
-gamete has complete extra set of
chromosomes
Animal cells – fatal
Plant cells – larger more vigorous variety
Boy or Girl? The Y Chromosome “Decides”
Slide 24
Y chromosome
X chromosome
Boy or Girl? The Y Chromosome “Decides”
Slide 25
Slide 26
How does the image in the first picture become the person
in the last picture?
http://www.pbs.org/wgbh/nova/baby/divide.html#
Slide 27
Type of cells that perform:
Uses:
Mitosis:
Meiosis:
Somatic
Gametes
Sexual
Growth, Repair,
Asexual reproduction Reproduction
1
2
Chromosome counts:
2n2n
Genetic Variation?:
None
2nn
Yes
(crossing over)
4 sperm/
1 egg
Number of cell divisions:
Number of daughter cells:
2
Slide 28
- Organism that normally contains both sexes
(ovaries and testes)
- Usually does not self fertilize
Example: earthworm
-Development of an unfertilized egg to a complete
organism
Example: male drones bees
- Fertilized eggs give rise to queen and female
workers
Slide 29
Union of the sperm and the egg forming a zygote
Occurs outside body
of female
Produces more eggs (1000+)
Occurs inside body
of female
Produces less eggs (1-12)
Occurs in water
Secretes fluid to provide
watery environment
Birds
Reptiles
Mammals
Fish
Amphibians