REDUCTION DIVISION
MEIOSIS
The number of
chromosomes is
characteristic of
an organism (e.g
humans have 46
chromosomes).
Replication
makes the second
identical
chromatid.
44 autosomes and 2 sex
chromosomes (gonosomes)
Normal Male
Normal Female
Where does meiosis occur
WHERE WILL MEIOSIS OCCUR IN PLANTS?
Forms sex cells (n) differ from
somatic/body cells (2n)
Sperm and Ovum
REVIEW OF MITOSIS
How meiosis takes place
Two stages can be found in the process of meiotic cell division
Meiosis 1 : is a reduction division which results in
two cells being with half the number of
chromosomes (haploid)
Meiosis 2 : is a copying division which involves the two
haploid cells formed each dividing again
by mitosis, to form 4 haploid cells.
PHASES OF MEIOSIS
Note, meiosis is a continuous process and is
divided into phases for convenience.
INTERPHASE
 DNA replication takes place
 Single stranded chromosomes become
double-stranded
 Each chromosome will now consist of two
chromatids joined by a centromere
PROPHASE 1
 Centrosome splits into centrioles which move




to opposite poles
forming spindle threads.
Nuclear membrane and nucleus start to
disappear.
Chromosomes become distinct.
Crossing over takes
Crossing over takes place as
follows
 Homologous chromosomes lie next to each




other
Chromatids of homologous chromosomes
overlap
Point of overlap is called the chiasmata
A break occurs at the chiasmata
Chromatid segments (genetic material) has
now been exchanged
METAPHASE 1
 Chromosomes align at equator in a random
fashion and in homologous pairs,
 attached to the spindle threads.
RANDOM ASSORTMENT
 Chromosomes are arranged randomly at the
equator. Different combinations of maternal
and paternal chromosomes can occur.
ANAPHASE 1
 Spindle fibres contract.
 Homologous chromosomes move apart
towards the poles.
 The cytoplasm starts to divide
TELOPHASE 1
 2 new cells have been formed.
 Each new cell has half of the chromosome
complement of the original cell (it is haploid).
 The cells are not identical.
(NB Meiosis 2 is similar to
mitosis)
PROPHASE 2
 Same as prophase 1 only no crossing over.
METAPHASE 2
 Chromosomes aligned at the equator singly
ANAPHASE 2
 Spindle fibres contract
 The centromeres separate
 Two chromatids of each chromosome move
to opposite poles ( The separated chromatids
are now called chromosomes)
TELOPHASE 2
 four daughter cells (gametes, in animals),
each with a haploid set of chromosomes are
produced.
Importance/significance of
meiosis
 Production of gametes
 Halving of the chromosome number (diploid
to haploid)
 Mechanism to introduce genetic variation
(Crossing-over and the random arrangement
of chromosomes at the equator)
Homework questions
1. List similarities between meiosis and mitosis
2. Tabulate 4 differences between meiosis and
mitosis
3. Draw a labelled and annotated diagram
showing the phases of meiosis
ABNORMAL MEIOSIS
NON-DISJUNCTION
 When a chromosome/chromatid fails to
separate during anaphase I/II
ABNORMAL MEIOSIS
CONSEQUENCES OF NON-DISJUNCTION
 Some gametes will have extra chromosomes
while others will have less.
Down’s Syndrome
 Non-disjunction in chromosome 21
 Normally occurs in human egg cell
 One or more abnormal eggs formed (n = 24)
 Fertilised by normal sperm (n = 23)
 Formation of abnormal zygote (2n = 47)
Down’s syndrome
• Down's syndrome is
caused by the presence of
three copies of the 21st
chromosome.
• This chromosomal defect
is known as Trisomy-21.
• Down's syndrome almost
always results in mental
retardation, though the
severity of the retardation
varies.
Characteristics of Down syndrome
Epicanthic fold
Small, oblique eyes
Flattened, nasal bridge
Open mouth
Protruding tongue
Broad neck
Small
underdeveloped
ears set low on
head
Wide gap between
first and second toe
Incurved
finger
Single palmar
(“simian”) crease
Short broad
hands