MEIOSIS
 Every living cell goes
through the process of
mitosis………
except sex cells. There is
always one exception to
biological rule!
Mitosis Reminder
Sex Cells reminder
 Called gametes.
 Animal gametes are called...
 Sperm and ovum
 Plant gametes are called...
 Pollen grain and ovule
 These four cells have half the number
chromosomes as the body cells.
 If there is a full set of chromosomes we
say it has a diploid set.
 If there are half the number of
chromosomes in the nucleus than we say it
has a haploid set.
Spot the Difference!
 Using your mitosis/meiosis sheet, identify as
many of the biological differences between
the flow diagrams.
What is Meiosis?
Watch this animation and see if you can
define what it is
Watch this one to see a basic animation
Important Bits!
 A cell that is going to produce gametes will
replicate its DNA, then go through two rounds
of division – this produces the haploid gametes.
 In the early stages of meiosis, crossing-over
happens.
 Homologous chromosomes swap pieces.
 This increases the variation seen in offspring.
 Variation is further increased by
random/independent assortment of the
chromosomes.
 The maternal and paternal chromosomes line up
independently and orient themselves randomly on the
equator before cell division.
CROSSING-OVER
RANDOM ASSORTMENT
More on Assortment
 Assortment takes place for each of the 23
pairs of human chromosomes.
 So, any single human egg receives one of two
possible chromosomes 23 times, and the total
number of different possible chromosome
combinations is over 8 million (223). And
that's just for the eggs.
 The same random assortment goes on as each
sperm cell is made.
 Thus, when a sperm fertilizes an egg, the
resulting zygote contains a combination of
genes arranged in an order that has never
occurred before and will never occur again.
Which are in
a diploid
state and
which are in a
haploid state?
We also call it
reduction
division, can you
see why?
Stages of Meiosis I
Prophase I:



Chromatin shortens and thickens, and
chromosomes appear.
The chromosomes will replicate at this stage
(although separate chromatids cannot yet be
seen).
Homologous chromosomes match up and crossingover occurs.
Metaphase I:

Random assortment occurs; homologs line up on
equator.
Anaphase I:
 Each homologue separates and moves to
opposite poles of the cell; being pulled
by the spindle fibres.
Telophase I:
 New nuclear membranes form
 Chromosomes decondense
 Cytoplasm divides (cytokinesis).
Stages of Meiosis II
Prophase II:




Nuclear membranes break down
Chromosomes shorten and thicken
Centrioles move to poles
Spindle fibres form.
Metaphase II:

Chromosomes line up along equator, not in
homologous pairs.
Anaphase II:

Each sister chromatid is separated at the
centromere, and they move to opposite poles of
the cells.
Telophase II:





Nuclear membrane reforms
Chromosomes uncoil and lengthen
Spindle disappears
Nuclear envelopes reform.
Cytoplasm divides in cytokinesis; 4 haploid
daughter cells (gametes) are formed.
Questions
 Which of the following cells would be haploid
and which diploid?








White blood cell
Male cell in pollen grain
Guard cell (in a leaf)
Root hair cell
Ovum
Sperm
Skin cell
Egg cell in ovule?
 Where in the body of the following would
you expect meiosis to be taking place
 A human male
 A human female
 A flowering plant?
 Why are organisms produced by asexual
reproduction identical to each other?
Mitosis and Meiosis
True or false?
Mitosis is necessary for
growth, repair and
replacement of tissues
True – it is the process by
which body cells divide to make
new cells.
In meiosis the number of
chromosomes stays the
same
False – in meiosis the number of
chromosomes is halved.
Meiosis takes place in
the testes
True – it also takes place in the
ovaries.
Mitosis involves two
divisions of chromosomes
False – mitosis involves one
division of chromosomes.
Mitosis results in
genetically identical cells
True
Mitosis is known as the
reduction division
False – meiosis is known as
reduction division. [Higher]