• What is the name of the process shown in
• Where does it occur?
• What is the significance of the process?
• Name the stages.
• What happens during each stage?
• What is the difference between plant and
MITOSIS & MEIOSIS
• Arrange the cards in the correct sequence
HOTS : Please explain what has
this got to do with cell division?
• What does this
picture tell us
• How is the
from the purpose
Where does meiosis
occur in a male and
a female human?
Inside testes in
a male and
ovaries in a
Meiosis in Plant
Occurs in the
• ovary (female)
Occurs in the
• anther (male)
Meiosis is the process of nuclear division
that reduces the number of chromosomes
in new cells to half the number of
chromosomes in the parent cell.
Significance of meiosis:
• Reduce chromosome number from
diploid (2n) to haploid (n).
• Promote genetic diversity (variation).
Unique phenotypes may
give a reproductive
advantage to some
Which process restores the diploid
number of chromosomes?
Features of Meiosis
Meiosis includes two rounds of division – meiosis I and
Meiosis I Meiosis II
Prophase I Prophase II
Metaphase I Metaphase II
Anaphase I Anaphase II
Telophase I Telophase II
Short interphase Haploid cells
During meiosis I,
During meiosis II,
sister chromatids separate.
[just like mitosis]
How is the HAPLOID
ONCE but divide TWICE
Meiosis I: Reduction Division
Early Prophase I
Prophase I Metaphase I Anaphase I Telophase I
Prophase I may last a long time.
about 1 week for prophase I
1 month for entire meiotic cycle
prophase I begins before birth, and
ends up to decades later during the
monthly ovarian cycle
Tetrads Form in Prophase I
(each with sister chromatids)
to form a TETRAD
Crossing over: genetic recombination
between non-sister chromatids
- sites of crossing
Crossing over produces variation in gametes
chromosomes pair up
Crossing over occurs.
Chromosomes short & thick.
Spindle fibre forms.
Nuclear membrane &
Centrioles move to opposite
poles, aster forms
The longer the chromosome, the more
chiasmata that can form
Results of Meiosis
Gametes (egg &
Four haploid cells with
one copy of each
Anaphase I is the
Meiosis II No
(or very short -
no more DNA
Meiosis II is
similar to mitosis
Genetic variation is almost infinite as a
result of meiosis
Name and describe TWO main advantages of
meiosis relative to mitosis. (4)
1) Creates new combinations of genes which
may result in the offspring being able to
survive in a changing environment.
2) Halves the number of chromosomes so that
after fertilisation, the diploid number is
• Show the sequence of Meiosis I and
Meiosis II using plasticine on manila card.
• Check your arrangement of
2d Difference between mitosis and meiosis
The importance of meiotic cell
• Ensure the diploid number of chromosomes
• Provides genetic variation
Appreciating the movement of
chromosomes during mitosis and meiosis
• Asexual reproduction (mitosis) produces
offspring that are identical to the parent
• Sexual reproduction produces genetic
• If meiosis does not occur properly, the
gametes formed will have abnormal number
of chromosomes eg Down Syndrome &
Klinefelter Syndrome (47 chromosomes) and
Turner Syndrome (45 chromosomes).
Down Syndrome: A meiotic error
Cause: Nondisjunction during either:
anaphase I anaphase II
chromosomes fail to
Down Syndrome: trisomy 21
• In humans, if both chromosome 21 homologues
go to the same pole, and the resulting egg is
fertilised, it will be trisomic for chromosome 21.
n + 1
2n + 1
The chance of having a Down syndrome child
goes up with maternal age
Egg is in center of picture
Many cumulus cells from
the ovary are seen around
the egg. This is a low
quality, oocyte from a
woman 41 years old.
Egg is irregularly shaped
A "good" egg from a
32 year old woman.
Photos from Advanced Fertility Clinic in Chicago
The diagram below represents the karyotype of
a) Why does the
in Figure 2 have
Two sister chromatids
are joined at the
Diagrams A, B and C below show the same stage
in mitosis, meiosis I and meiosis II in a plant cell.
a)Identify the stage shown giving your
Metaphase – chromosomes aligned at equator.
b) The cell in diagram A
has 20 units of DNA.
How many units of DNA
would there be in a cell
from this plant at the
Cell A has 8 chromatids =
At end of mitosis, 4
So = 10 units
2n for Cell A is 4
chromosomes = 10 units.
At end of meiosis, 2
So = 5 units
The simplified diagram below shows a nucleus
from a cell of an insect.
a) Draw diagrams to show the two nuclei which
would be formed if the cell would divide by
b) Draw diagrams to show three different
nuclei which could be formed if the
cell divided by meiosis. (Assume that
no crossing over has taken place). (3)
Any 3 from:
Genetic Variation in meiosis occurs in
1. Crossing over: prophase I
2. Random orientation of bivalents:
3. Random orientation of sister
chromatids : metaphase II
Crossing over does not create
new alleles, it simply recombines
existing ones in new ways.
1. Crossing Over
2. Random orientation of bivalents
No. of different combinations: 2n
2n = 4
n = 2
22 = 4
Suggest explanations for the following
Meiosis, unlike mitosis, is not considered to
form part of a cell cycle. (2)
When cells divide by mitosis, cytokinesis and
interphase follow and then the same phases
are repeated. However, a cell divides by
meiosis only once. There is no repetition of
Occurs in the
Men produce about
sperm per day
Occurs in the ovaries
Two divisions produce 3 polar
bodies that die and 1 egg
Polar bodies die because of
unequal division of cytoplasm
Immature egg called oocyte
Starting at puberty, one oocyte
matures into an ovum (egg) every
First polar body
Read the passage below and insert the appropriate
terms from the table below in the blank spaces
provided. Note that each term in the table may be used
once, more than once or not at all.
Fertilisation Two Crossing-over Four Forty-six
Genetic variation Haploid Homologous Chromatids Eight
Gametes Diploid Twenty-three Chromosomes Nucleus
Meiosis is an important biological process that reduces
the chromosome number by half during the formation of
…………… in animals. During the first meiotic
"reduction division," the chromosomal pairs are
partitioned so that each gamete contains one of each
chromosomal pair that is it becomes ………… .
When haploid gametes unite during ……………… the
resulting zygote is ………… having received one
chromosome of each pair from each parent.
Meiosis involves ……… successive nuclear divisions
that produce …… haploid daughter cells. The first
division (meiosis I) is the reduction division; the
second division (meiosis II) separates the
Meiosis increases ……………………… in the
population. Each diploid cell undergoing meiosis can
produce 2n different chromosomal combinations,
where n is the haploid number.