MEIOSIS
… making
gametes….
Meiosis is…
• Making the cells that make
babies! (egg and sperm)
• The process of nuclear
division that reduces the
number of chromosomes by
half
– Start with somatic cell
– End with gamete
What type of cells produce
egg/sperm?
Ovary and testes cells!
Why do ovary and testes cells go
through meiosis?
Why
do
meiosis?
Meiosis is “reduction division”.
During cell division, it reduces the
chromosome number in the gametes
by one half.
From diploid (2n) to haploid (n).
Each of your body cells has 2 sets of chromosomes – one
from mom, one from dad.
Any cell that has 2 complete sets of chromosomes is said to
be DIPLOID (2n)
Cells with 1 set of chromosomes (gametes) are said to be
HAPLOID (n)
When 2 cells come together in
fertilization to make a zygote,
each can only have 1 set of
chromosomes. That way, the
zygote will have 46
chromosomes. (23 + 23 = 46)
(Instead of 46 +46 = 92!)
Chromosome number
questions…
1. If a horse retina cell has 36 chromosomes,
how many does a horse sperm cell have?
2. If the n = 4 for fruit flies, how many
chromosomes does a wing cell have?
3. If the 2n number of a pine tree is 86, how
many chromosomes would be found in a
pine ovum?
Homologous pairs.
In each body cell you have 23 homologous pairs.
2 chromosomes are homologous if:
- one is from mom, one is from dad
- they have the same gene sequence on
them.
Homologous Chromosomes have:
1. the same gene
sequences.
2. The same
banding pattern
3. The same
position of the
centromere.
Karyotype is used
to determine
1. If there is an
abnormality in
number or
structure of the
chromosomes
(eg. Down’s
syndrome)
2. The gender
HOW TO MAKE A KARYOTYPE
1. Photograph a cell in metaphase.
2. Cut out the chromosomes from the picture.
3. Paste homologous pairs together according to size,
banding pattern and centromere.
4. Lay pairs out largest to smallest, sex chromosomes
at end.
Spermatogonial
cell
2n
Oogonial cell
2n
MEIOSIS
MEIOSIS
egg (n)
sperm (n)
fertilization
zygote
MITOSIS
You
Products of MITOSIS:
2 identical diploid 2n
2n
2n
(2n) cells
Products of MEIOSIS:
4 unique haploid (n) cells
2n
n
n
n
n
Meiosis Vocabulary
• Crossing Over:
– Exchange of genetic material from
homologous chromosomes during
prophase I of Meiosis
• Genetic Recombination:
– Major source of genetic variation
resulting from crossing over OR
independent assortment
MEIOSIS I
Interphase I – Period of cell growth and
preparation for division (G1, S, G2)
PROPHASE I - TETRADS APPEAR. The homologous
chromosomes (mom’s and dad’s) come together with
another homologous pair to form a tetrad
-Crossing over occurs (SYNAPSIS)
Synapsis
increases genetic
variability – and
that’s a good
thing…
Metaphase I: Tetrads (2 homologous pairs) line up on
metaphase plate and lock on to spindle fibers
Anaphase I: Spindle fibers pull tetrad apart; one
homologous chromosome goes to each side of the cell.
Independent assortment occurs
Telophase I: nuclear
membranes form around the
two new nuclei;
Cytokinesis I: cell splits in
two.
MEIOSIS II
Interphase between the two
divisions:
NO replication!!! - so nothing changes with
the chromosomes between telophase I and
prophase II
Prophase II: When chromosomes show up you will see
sister chromatids.
Metaphase II: Sister chromatids line up on equator
Anaphase II: Sister chromatids are pulled apart.
Telophase II: Nuclear membranes form.
Cytokinesis:
cytoplasm divides. Total product is 4
cells, each with only a mom or Dad’s chromatids, in
other words, half the normal chromosome number, or 4
monoploid (n) cells, or 4 gametes.
Uneven cytokinesis in females: In oogenesis, get
one big egg and 3 non-functional polar bodies:
( who die off) and 1 big functional gamete
OOGENESIS
First division: results
in one big primary
oocyte and a little
polar body.
Second division: One
secondary oocyte and
3 polar bodies. (polar
bodies die)
Spermatogenesis
Equal divisions
producing four
useable sperm
Meiosis vs. Mitosis
(Make a chart)
• Used for Sexual
Reproduction
• Produces Gametes (Egg &
Sperm)
– Have 23 chromosomes
• 2 sets of divisions
• Begins with a diploid cell
• Homologous chromosomes
pair up
• Result: 4 genetically
different HAPLOID SEX
cells
• Used for Growth and Asexual
Reproduction
• Produces body cells
– Have 46 chromosomes
– Somatic cells
•
•
•
•
One round of division
Begins with diploid cells
No crossing over
Homologous chromosomes
don’t pair up
• Results in 2 genetically identical
DIPLOID SOMATIC cells
Compared to the number of chromosomes contained in a body cell of
a parent, how many chromosomes would normally be contained in a
gamete?
F. the same number
G. twice as many
H. one-fourth as many
J. half as many
During mitosis, the chromosomes
A. are located at the cell equator during prophase
B. are located at the cell equator during telophase
C. move toward the poles of the cell during anaphase
D. move toward the poles of the cell during metaphase
COMPARISONS:
1. Compare spermatogenesis and oogenesis
2. Compare Meiosis I and Meiosis II
3. Compare haploid and diploid cells.
4. Compare a somatic (body) cell to a gamete.
http://www.cellsalive.com/meiosis.htm
Other amazing videos!
Which structures normally have a haploid number of chromosomes?
A. Seed plant spores and animal zygotes
B. Seed plant spores and animal gametes
C. Seed plant zygotes and animal zygotes
D. Seed plant gametes and animal zygotes
Non-disjunction
= Nonseparation of
chromosomes
resulting in
diploid or
empty gametes.
Conditions caused by
non-disjunction
Down’s syndrome = Trisomy 21 - 3 copies of
chromosomes 21 equalling a total of 47 chromosomes.
= 2n + 1 (chance of occurring in oogenesis increases with
maternal age)
Turner’s syndrome = Monosomy X - has only one X
chromosomes totalling only 45 chromosomes in her
body cells. (monosomy X) = 2n-1
Down’s syndrome – Trisomy 21
A simple check for the
presence of a bone in the nose
could more accurately test
unborn babies for Down's
syndrome, scientists say.
Researchers say combining
the nose test with existing
screening methods
(amniocentesis) could lead to
a five-fold reduction in the
number of miscarriages linked
to an invasive procedure used
to confirm Down's syndrome.
Turner’s syndrome X
-Caused by missing or incomplete
X chromosome
-Genes affected are involved with
growth and sexual development
-May fail to start puberty
-Shorter than normal
-Treatment = Hormone
Replacement Therapy
Klinefelter’s syndrome XXY
Normal boy who develops
some female secondary
sex character-istics at
puberty.
-slightly lower IQ,
infertile, delayed motor,
speech, maturation
-treated with testosterone
Klinefelter’s syndrome = XXY - male which has an extra X
chromosome = 2n + 1
QUIZ TIME…
1. In streptomycin fungus n = 11. What is the diploid number
for this species?
2. If a horse egg has 98 chromosomes how many does a horse
egg cell have?
3. If n = 16 for goldfish how many chromosome in a fin cell?
4. If 2n = 108 for black spruce trees then what is the haploid
number?
2n = 14 for this species. Fill in
the chromosome numbers for
each cell.