Download document 8310413

Meiosis
• Each organism must inherit a single copy of every
gene from both its “parents”. One from the female
and one from the male.
• When an organism produces gametes, those two
sets of genes must be separated form each other so
that each gamete contains just one set of genes.
• The process that separates the two sets of genes is
called Meiosis.
Chromosome Number
• Homologous Chromosomes ≡ the chromosomes
that came from the male parent and its
corresponding chromosome from the female
parent.
Homologous Chromosomes
Chromosome Number
• Diploid ≡ A cell that contains both sets of
homologous chromosomes.
– The number of chromosomes in a diploid cell is
sometimes represented by the symbol 2N.
• Haploid ≡ Gametes of sexually reproducing
organisms contain only a single set of
chromosomes.
– The number of chromosomes in a haploid cell is
sometimes represented by the symbol N.
Phases of Meiosis
• Meiosis ≡ a process of reduction division in
which the number of chromosomes per cell is
cut in half through the separation of
homologous chromosomes in a diploid cell.
• Meiosis has 2 phases:
– Meiosis 1
– Meiosis 2
Meiosis 1
• Prior to meiosis I, each
chromosome is replicated.
• The cells begin to divide in a
way that looks similar to
mitosis.
• Tetrad ≡ this structure
forms when each
chromosome pairs with its
corresponding
homologous chromosome
Meiosis 1
• Crossing Over ≡ when while
they are in tetrads, they
exchange portions of their
chromatids.
– Crossing Over results
in the exchange of
alleles between
homologous
chromosomes and
produces new
combinations of alleles.
 The homologous
chromosomes
separate, and two
new cells are formed.
 The two cells
produced by meiosis
1 have sets of
chromosomes and
alleles that are
different from each
other and from the
diploid cell that
entered meiosis 1
Meiosis 1
Meiosis 2
• The two cells produced by meiosis 1 now enter a
second meiotic division.
• But unlike meiosis 1 – here in meiosis 2 there isn’t
any chromosome replication – only division.
• The four daughter cells produced in meiosis 2 now
contain the haploid number of chromosomes.
Mitosis vs. Meiosis
MITOSIS
MEIOSIS
Occurs in somatic cells.
Occurs in reproductive cells.
Consists of only one nuclear division.
Consists of two nuclear divisions M-I and M-II.
Cytokinesis takes place only once.
May take place only once (simultaneous type) or twice
(successive type).
Involves division of chromosomes.
Involves separation of homologous chromosomes in M-I
and division of chromosomes in M-II.
Dividing cells can be haploid or diploid.
Dividing cells are diploid.
Does not involve either pairing of homologous
chromosomes or crossing over.
Pairing of homologous chromosomes and crossing over
occur during Prophase-I.
Two daughter cells are formed.
Four daughter cells are formed.
Number of chromosomes present in the mother cell is
maintained in both the daughter cells. Therefore
it is an equational division.
Diploid number of chromosomes is reduced to haploid in
each daughter cell. Therefore it is a reduction
division.
Original characters of the chromosomes are
maintained in the daughter cells.
Chromosomal characters are altered due to "crossing
over" causing recombination of genes.
Daughter cells are similar to each other and also to
the original mother cell.
Daughter cells differ from each other as well as from the
original mother cell.
Helps in growth and body repairs.
Helps in the sexual reproduction and regulation of
chromosome number in the life cycle of sexually
reproducing organism.