Meiosis
Meiosis Forms Gametes
• Meiosis is the process by which gametes
(sex cells) are produced
• In males, gametes are
called sperm.
• In females, gametes are
called eggs.
Meiosis Reduces Chromosome #
• Meiosis reduces the number of chromosomes
in the gamete to HALF the number (n) of
chromosomes in the parent’s body cells.
• When fertilization occurs, the union of the
two gametes will produce a zygote.
• Fertilization restores the original chromosomal
number in the zygote
Haploid (n) cell
Fertilization
Zygote - Diploid (2n) cell
Haploid (n) cell
Homologous Chromosomes
• Homologous chromosomes
are two paired chromosomes,
one from each parent, which
carries genes for a specific
trait at the same location
• Meiosis cuts the chromosome
number in half by separating
homologous chromosomes
2 Phases of Meiosis
• Meiosis occurs in 2 phases:
– Meiosis I
– Meiosis II
Meiosis
• Before Meiosis begins the homologous
chromosomes replicate.
Meiosis I
• Meiosis I consists of:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase I
• During Prophase I the homologous pairs
match up, physically overlap and exchange
pieces of genetic information.
• This is called crossing over
Crossing Over
• This process occurs more often on some
chromosomes than other chromosomes and changes
the DNA sequence within each chromosome.
• This results in an endless number of different possible
genetic combinations.
• This is GENETIC RECOMBINATION!
Metaphase I
• Homologous chromosomes line up in the
middle of the cell
Anaphase I
• The homologous chromosomes separate and
are pulled to opposite sides of the cell
Completion of Meiosis I
• Meiosis I produces two daughter cells
containing HALF the number of double
stranded chromosomes.
• These are called haploid (n) cells.
Meiosis II
• Meiosis II consists of:
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis II
• Meiosis II is just like Mitosis – the sister
chromatids of each chromosome are pulled
apart.
• Meiosis II results in the formation of four cells,
each containing HALF the number of singlestranded chromosomes.
• These are called haploid (n) cells.
Meiosis Provides Genetic Variation
• Depending on how the chromosomes line up at
the equator, four gametes with four different
combinations of chromosomes can result.
• Genetic variation is also produced during crossing
over and during fertilization, when gametes
randomly combine.
– The number of different genetic combinations a
person can produce is more than 8 million!
– When fertilization occurs, 223 x 223 different genetic
combinations can occur – that is 70 trillion!
Meiosis Summary
•
•
•
•
The number of the chromosomes is halved.
Produces four daughter cells.
All daughter cells are haploid (n).
Chromosomes are shuffled in the process, so
that each daughter cell is genetically different.
• Used to create gametes (sperm and egg).
• Sometimes called reduction division.
Mitosis VS Meiosis
Mitosis VS Meiosis