Mitosis and Meiosis
 Aims:
 Must be able to define the terms mitosis and
meiosis.
 Should be able to describe the stages of each
process.
 Could be able to compare and contrast the two
processes.
Cell Cycle
Cytokinesis
Interphase
Basic Overview
Mitosis - Basics
 After DNA replication has occurred – mitosis
(division of the nucleus) can continue…
 There are four major stages within the mitosis stage:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
 Cytokinesis occurs (division of the cytoplasm)
Mitosis - In Detail
Nuclear
Membrane
Interphase
Centrosome, which
later forms the spindle,
is also replicated.
Early Prophase
Late Prophase
Cell enters
mitosis
Nucleolus
DNA is replicated to
form 2 chromatids
DNA continues condensing into
chromosomes and the nuclear
membrane begins to dissolve
Chromosomes continue
to coil up and appear as
double-chromatids
Metaphase
The mitotic spindle is
formed to organize the
chromosomes. The spindle
consists of fibers made of
microtubules and proteins.
Anaphase
Division of the cytoplasm
(cytokinesis) is complete.
The two daughter cells are now
separate cells in their own right.
Cytokinesis
Two new nuclei form. The cell
plate forms across the midline
of the parent cell. This is where
the new cell wall will form.
Telophase
The chromosomes
segregate, pulling
the chromatids apart
Late Anaphase
Mitosis - Micrographs
1. Interphase
2. Prophase
6. Telophase
5. Late Anaphase
3. Metaphase
4. Anaphase
Activity
 Complete the Biozone pages 211 and
212.
Meiosis
Crossing over
may occur at this
stage in meiosis
First Division
(Reduction division)
 To produce
Haploid cells
(gametes).
 Only one copy
of each
homologous
pair of
autosomes
plus one sex
chromosome
Intermediate cell
Intermediate cell
Second Division
('Mitotic' division)
Gametes
(eggs or sperm)
Meiosis i
 First division of
meiosis is called a
‘reduction’ division
because it reduces
(halves) the number
of chromosomes.
 One chromosome
from each
homologous pair is
donated to each
intermediate cell.
Intermediate cell
Interphase
DNA replication
Prophase
1
Synapsis and
crossing over
Metaphase 1
Bivalents line up
on the equator
Anaphase 1
Telophase 1
Intermediate cell
NOTE: Half the full
chromosome complement
shown
Meiosis ii
Intermediate cell
 The second
division of meiosis
is called a ‘mitotic’
division, because it
is similar to mitosis.
Prophase
2
Metaphase 2
 Sister chomatids of
each chromosome
are pulled apart
and are donated to
each gamete cell.
Anaphase 2
Telophase 2
Gamete (egg or sperm)
Gamete (egg or sperm)
Crossing Over
 The swapping of sections between
homologous pairs of chromosomes.
 Occurs during Prophase I of meiosis.
 Results in genes being swapped between
maternal and paternal chromosomes.
 Gametes show greater diversity.
Crossing Over - Process
 See diagram:
 Biozone p289/90
Meiosis i - Non-Disjunction
 The meiotic spindle normally
distributes chromosomes to daughter
cells without error.
Meiosis I
 Sometimes, homologous
chromosomes or sister chromatids
fail to separate.
Meiosis II
 One gamete receives two of the same
type of chromosome and the other
gamete receives no copy.
 This mishap, called non-disjunction,
results in abnormal numbers of
chromosomes in the gametes.
 This example shows non-disjunction
in meiosis I; homologous
chromosomes fail to separate
properly at anaphase
during meiosis I.
n+1
n+1
n–1
n–1
Non-disjunction in Meiosis I
Meiosis ii – Non-Disjunction
Meiosis I
 Non-disjunction can also occur
in meiosis II, when sister
chromatids fail to separate
during anaphase of meiosis II.
Meiosis II
 In cases of non-disjunction, if
either of the aberrant gametes
unites with a normal one at
fertilization, the offspring will
have an abnormal
chromosome number
(e.g. 2N+1 or 2N–1).
 This is known as aneuploidy
and it accounts for
chromosomal defects such as
Down syndrome (trisomy 21)
and Edward syndrome (trisomy
18).
n+1
n–1
n
n
Non-disjunction in Meiosis II
Activity
 Biozone p288.
Activity
 Complete the Biozone pages 287 and
211.
Meiosis i - Non-Disjunction
 The meiotic spindle normally
distributes chromosomes to daughter
cells without error.
Meiosis I
 Sometimes, homologous
chromosomes or sister chromatids
fail to separate.
Meiosis II
 In these cases, one gamete receives
two of the same type of gamete and
the other gamete receives no copy.
 This mishap, called non-disjunction,
results in abnormal numbers of
chromosomes in the gametes.
 This example shows non-disjunction
in meiosis I; homologous
chromosomes fail to separate
properly at anaphase
during meiosis I.
n+1
n+1
n–1
n–1
Non-disjunction in Meiosis I
Meiosis ii – Non-Disjunction
Meiosis I
 Non-disjunction can also occur
in meiosis II, when sister
chromatids fail to separate
during anaphase of meiosis II.
Meiosis II
 In cases of non-disjunction, if
either of the aberrant gametes
unites with a normal one at
fertilization, the offspring will
have an abnormal
chromosome number
(e.g. 2N+1 or 2N–1).
 This is known as aneuploidy
and it accounts for
chromosomal defects such as
Down syndrome (trisomy 21)
and Edward syndrome (trisomy
18).
n+1
n–1
n
n
Non-disjunction in Meiosis II
Activity
 Complete the Biozone pages 210, 293
and 294.