Meiosis
• Sexual reproduction
occurs only in
eukaryotes.
• During formation of
gametes, the number
of chromosomes is
reduced by half, and
returned to the full
amount when the two
gametes fuse during
fertilization.
Let’s Define Some Terms
• Gametes: Haploid
reproductive cells
(ovum and sperm)
• “Ploidy” is a term
referring to the
number of sets of
chromosomes.
• Haploid and diploid
are terms referring to
the number of sets of
chromosomes in a
cell.
More Terms
• Diploid: Organisms are
those with two (di) sets
of chromosomes. Human
beings (except for their
gametes), most animals
and many plants are
diploid.
• We abbreviate diploid
as 2n.
More Terms
• Haploid:
organisms/cells have
only one set of
chromosomes,
abbreviated as n.
• Organisms with more
than two sets of
chromosomes are
termed polyploid.
Polyploidy
• Polyploidy occurs in some animals,
such as goldfish, salmon, and
salamanders, but is especially
common among ferns and
flowering plants, including both
wild and cultivated species.
• Wheat, for example, after
millennia of hybridization and
modification by humans, has
strains that are diploid (two sets of
chromosomes), tetraploid (four
sets of chromosomes) with the
common name of durum or
macaroni wheat, and hexaploid
(six sets of chromosomes) with the
common name of bread wheat.
Trisomy 21(Down Syndrome)
• Down syndrome is caused by
an extra chromosome 21
(trisomy 21).
• People with Down syndrome
are usually mentally retarded
and have a host of physical
defects, including heart
disorders.
• Other individuals, called Down
syndrome mosaics, have a
mixture of normal cells and
cells with three copies of
chromosome 21, resulting in a
mild form of the disorder.
Even more terms
• Chromosomes that carry the same genes are termed
homologous chromosomes.
• Genes: Specific segments of DNA that control cell
structure and function; the functional units of
inheritance.
• Alleles: Alternate forms of a gene.
Alleles
• The alleles on homologous chromosomes may
differ, as in the case of heterozygous individuals.
• Organisms (normally) receive one set of
homologous chromosomes from each parent.
• Heterozygous: Having two different alleles (one
dominant, one recessive) of a gene pair
• Homozygous: Having identical alleles for a given
gene.
Meiosis
• Meiosis: is a special
type of nuclear division,
which segregates one
copy of each
homologous
chromosome into each
new "gamete".
Compared to Mitosis
• Mitosis maintains the cell's
original ploidy level (for
example, one diploid 2n cell
producing two diploid 2n cells;
one haploid n cell producing
two haploid n cells; etc.).
• Meiosis, on the other hand,
reduces the number of sets of
chromosomes by half, so that
when gametic recombination
(fertilization) occurs the ploidy
of the parents will be
reestablished.
Compared with Mitosis
• Most cells in the human
body are produced by
mitosis.
• These are the somatic cells.
• Cells that become gametes
are referred to as germ cells
(sex cells).
Meiosis
• The vast majority of cell
divisions in the human body
are mitotic, with meiosis being
restricted to the gonads.
• Meiosis comprises two
successive nuclear
divisions with only one
round of DNA replication.
• Four stages can be
described for each nuclear
division.
Meiosis
• Interphase: Before
meiosis begins,
genetic material is
duplicated.
Meiosis I
• Prophase 1: Duplicated
chromatin condenses. Each
chromosome consists of
two, closely associated
sister chromatids.
• Crossing-over can occur
during the latter part of this
stage.
Meiosis I
• Metaphase 1:
Homologous
chromosomes
align at the
metaphase plate.
Meiosis I
• Anaphase 1:
Homologous pairs
separate with sister
chromatids
remaining together.
Meiosis I
• Telophase 1: Two
daughter cells are
formed with each
daughter containing
only one
chromosome of the
homologous pair.
Meiosis II
• Prophase 2: DNA
does not replicate.
Meiosis II
• Metaphase 2:
Chromosomes align at
the metaphase plate.
Meiosis II
• Anaphase 2:
Centromeres divide
and sister
chromatids migrate
separately to each
pole.
Meiosis II
• Telophase 2: Cell
division is complete.
Four haploid daughter
cells are obtained.
Meiosis
• One parent cell
produces four
daughter cells.
Daughter cells have
half the number of
chromosomes found
in the original parent
cell and with crossing
over, are genetically
different.
Meiosis in females
Meiosis in Males