Meiosis
 Meiosis is the type of cell division that
occurs in the gametes.
 Gametes are your sex cell…the sperm
and egg
 Meiosis allows for cells from two
individuals to combine without changing
the number of chromosomes within the
cell.
Mendel and Meiosis
 Gregor Mendel demonstrated that each
organism must inherit a single copy of
every gene from its parents.
 Then when organisms produce their own
gametes they must contain just one of their
sets of genes.
 This means that when meiosis occurs it
must go through a process where it starts
with two sets of genes and ends up with
one.
Chromosome Numbers
 Homologous chromosomes are two sets
of chromosomes…one set from the
father and one set from the mother.
 Homologous means the same so these
chromosomes carry the codes for the same
genes.
 When a cell has both sets of chromosomes it
is what we call DIPLOID.
Meiosis
 Process of reduction division
 Chromosome number is cut in ½ by
seperation of homologous chromosomes
in diploid cells.
Chromosome Number in body
cells vs. gametes
 Body cells (somatic):
Diploid (2n)- have 2
sets of chromosomes
Sex cells (gametes):
Haploid (N)- 1 set of
chromosomes
Haploid
 In sexually reproducing
organisms need to pass
on one set of their
chromosomes these
cells are said to be
haploid.
 A haploid cell contains
half the number of
chromosomes as a
diploid cell.
 In humans the haploid
number is 23.
Phases of Meiosis
 Meiosis is the process of reduction
division in which the number of
chromosomes per cell is cut in half
through separation of homologous
chromosomes in a diploid cell.
 Resulting cells are genetically different
Meiosis I
 Interphase I- cells undergo DNA
replication.
 Prophase I- chromosomes pair with
homologous chromosomes
 Metaphase I- spindle fibers attach to the
chromosomes and the centromere
 Anaphase I- fibers pull homologous
chromosomes toward opposite ends of the
cell.
 Telophase I and Cytokinesis- nuclear
membranes reform and the cell seperates
into two cells.
Meiosis II
 Prophase II- Meiosis I results in two haploid
N daughter cells, each with half the number
of chromosomes.
 Metaphase II- Chromosomes line up at the
center of the cell
 Anaphase II- sister chromatids separate and
move toward opposite ends of the cell.
 Telophase II and Cytokinesis- Meiosis II
results in four haploid (N) daughter cell.
Results of Meiosis
 4 haploid (N) cells: 1 sperm or egg, and
3 polar bodies
 All cells are genetically different from
one another.
Important step of Meiosis
 During PROPHASE I- crossing over
occurs
 Crossing over is the exchange of alleles
(genes) in random order that produces
completely new combinations of genes.
Genetic Recombination
 Due to the mixing of genes in meiosis a
process called genetic recombination
occurs.
 Crossing over
 Independent assortment- independent
segregation of genes during meiosis
 Fertilization- the union of the sperm and egg to
form a zygote.
 Genetic recombination is important
because it allows for the survival of species
to improve over time…favorable traits are
those that are passed on.
Mitosis Compared to Meiosis
Event
Mitosis
Meiosis
DNA replication
Occurs during interphase before
nuclear division begins
Occurs once, during the
interphase before meiosis I
begins
Number of divisions
One, including prophase,
metaphase, anaphase, and
telophase
Two each including a prophase,
metaphase, anaphase, and
telophase
Synapsis of homologous
chromosomes
Does not occur
Synapsis is unique to meiosis:
Duirng prophase I, the homolgous
chromosomes join along their
length forming tetrads (groups of
4 chromatids); synapsis is
associated with crossing over
between nonsister chromatids
Number of daughter cells and
genetic composition
Two, each diploid (2n) and
genetically identical to the parent
cell
Four, each haploid (n), containing
half as many chromosomes as the
parent cell; genetically
nonidentical to the parent cell and
to each other
Role in the animal body
Enables multicellular adult to
arise from the zygote; produces
cells for growth and tissue repair
Produces gametes; reduces
chromosome number by half and
introduces new combinations of
genes leading to genetic
variability among the gametes
Meiosis Animation