MEIOSIS
DR. A. TARAB
DEPT. OF BIOCHEMISTRY
HKMU
• Meiosis is a special type of cell division necessary
for sexual reproduction in eukaryotes such as
animals, plants and fungi
• The number of sets of chromosomes in the cell
undergoing meiosis is reduced to half the original
number, typically from two sets (diploid) to one
set (haploid)
• The cells produced by meiosis are either gametes
(the usual case in animals) or otherwise usually
spores from which gametes are ultimately
produced (the case in land plants)
• In many organisms, including all animals and
land plants (but not some other groups such
as fungi), gametes are called sperm in male
and egg cells or ova in females
• Since meiosis has halved the number of sets
of chromosomes, when two gametes fuse
during fertilization, the number of sets of
chromosomes in the resulting zygote is
restored to the original number
• Meiotic division occurs in two stages, meiosis I
and meiosis II, dividing the cells once at each
stage
• The first stage starts with a diploid cell that has
two copies of each type of chromosome, one
from each the mother and father, called
homologous chromosomes
• All homologous chromosomes pair up and may
exchange genetic material with each other in a
process called crossing over
• Each pair then separates as two haploid cells
are formed, each with one chromosome from
every homologous pair
• In the second stage, each chromosome splits
into two, with each half, called a sister
chromatid , being separated into two new
cells, which are still haploid
• This occurs in both of the haploid cells formed
in meiosis I
Meiosis, showing chromosomal
crossover
• Therefore from each original cell, four
genetically distinct haploid cells are produced
• These cells can mature into gametes
Gametic life cycle
Stages of meiosis
• Step 1 – Prophase I
• The chromosomes condense, and the nuclear
envelop breaks down
• Homologous chromosome pair along their length
• Crossing-over occurs when portions of a
chromatid on one homologous chromosome are
broken and exchanged with the corresponding
chromatid portions of the other homologous
chromosome
• Step 2 – Metaphase I
• The pairs of homologous chromosomes are
moved by the spindle to the equator of the
cell
• The homologous chromosomes remain
together
• Step 3 – Anaphase I
• The homologous chromosome separate
• As in mitosis, the chromosomes of each pair are
pulled to opposite poles of the cell by the spindle
fibers
• But the chromatids do not separate all their
centromeres – each chromosome is still
composed of two chromatids
• The genetic material, however, has recombined
• Step 4 – Telophase I
• Individual chromosomes gather at each of the
poles
• In most organisms, the cytoplasm divides
(cytokinesis), forming two new cells
• Both cells or poles contain one chromosome
from each pair of homologous chromosomes
• Chromosomes do not replicate between
meiosis I and meiosis II
• Step 5 – Prophase II
• A new spindle forms around the
chromosomes
• Step 6 – Metaphase II
• The chromosomes line up along the equator
and are attached at their centromeres to
spindle fibers
• Step 7 – Anaphase II
• The centromeres divide, and the chromatids
(now called chromosomes) move to opposite
poles of the cell
• Step 8 – Telophase II
• A nuclear envelope forms around each set of
chromosomes
• The spindle breaks down and the cell undergoes
cytokinesis
• The result of meiosis is four haploid cells
Independent Assortment
Meiosis in male and female animals
• Meiosis of male diploid cell results in four
haploid sperm, while meiosis of a female
diploid cell results in only one functional
haploid egg cell
Differences between meiosis and
mitosis
• Meiosis
• Definition:
• A type of cellular
reproduction in which the
number of chromosomes
are reduced by half
through the separation of
homologous
chromosomes, producing
two haploid cells.
• Mitosis
• Definition:
• A process of asexual
reproduction in which the
cell divides in two
producing a replica, with
an equal number of
chromosomes in each
resulting diploid cell.
• Function:
• sexual reproduction
• Type of reproduction:
• Sexual
• Occurs in:
• Humans, animals,
plants, fungi
• Function:
• Cellular Reproduction
and general growth and
repair of the body
• Type of reproduction:
• Asexual
• Occurs in:
• All organisms
• Genetically:
• Different
• Genetically:
• Identical
• Crossing over:
• Crossing over:
• Yes, mixing of
• No, crossing over
chromosomes can occur
cannot occur
• Number of divisions:
• 2
• Number of divisions:
• 1
• Number of daughter
cells produced:
• 4 haploid cells
• Number of daughter
cells produced:
• 2 diploid cells
• Chromosome number:
• Reduced by half
• Chromosome number:
• Remains the same
• Steps:
• The steps of meiosis are
Interphase, Prophase I,
Metaphase I, Anaphase
I, Telophase I, Prophase
II, Metaphase II,
Anaphase II and
Telophase II.
• Steps:
• The steps of mitosis are
Interphase, Prophase,
Metaphase, Anaphase,
Telophase and
Cytokinesis
• Produces:
• Sex cells only: Female
egg cells or Male sperm
cells
• Produces:
• Makes everything other
than sex cells