Oh! MEIOSIS
• Cell division in which one DIPLOID CELL
produces four HAPLOID CELLS called sex cells
or gametes
• Meiosis is reduction division; the cells produced
contain half the number of chromosomes as
before meiosis
• Meiosis occurs in two steps. During meiosis I, the
chromosome pairs are divided between the two
cells produced. During meiosis II, the two sister
chromatids are segregated into each daughter cell
Stages Of Meiosis!
Genes and Chromosomes
• Organisms have tens
of thousands of
genes!
• A thousand or more
are arranged on each
chromosome
• Chromosome number
of a species is not
related to the
complexity or the
organism
Chromosomes in different
Organisms:
1400
1200
1000
Human
Garden Pea
Dog
Chimpanzee
Apple
Indian Fern
800
600
400
200
0
1st Qtr
Diploid Time:
• A cell with 2 of each kind of chromosome
• 2n
• Organisms have 2 factors (or alleles) for
each trait
• One allele is located on each of the paired
chromosomes
Happy Haploids:
• A cell with one of each kind of
chromosome
• n
• Organisms produce gametes that contain
one of each kind of chromosome
• Parent organisms give one factor (or
allele) for each trait to each of their
offspring
• Together, the 2
chromosomes of each pair in
a diploid cell help determine
what the individual looks
like
• These paired chromosomes
are called H.C.
• Have genes for the same
traits arranged in the same
order
• However, because there are
different possible alleles for
the same gene, the 2
chromosomes in a
homologous pair are not
identical to each other
Homologous
Chromosomes
Genes on a
chromosome
Interphase
• Remember that
in Interphase the
cell carries out
its usual
metabolic
activities and
replicates its
chromosomes
• Just hanging out
doing its cell
thing
Prophase 1
• Chromosomes coil up and a spindle forms
• Each pair of homologous chromosomes comes
together to form a four-part structure called a
tetrad
Tetrads Unite!
• Consists of 2 homologous chromosomes, each
made up of 2 sister chromatids
• The chromatids in a tetrad pair tightly
• In fact, they pair so tightly that non-sister
chromatids from homologous chromosomes
sometimes actually exchange genetic material in
a process known as Crossing Over
Crossing Over
• Results in new
combinations of
alleles on a
chromosome
• This is why there are
so many unique folks
on this planet!
Metaphase 1
• Tetrads line up in the midline of the spindle
• Unique to meiosis
• Homologous line up together in pairs
Anaphase
• Homologous chromosomes separate and move to
opposite ends of the cell
• Centromeres don’t split like in mitosis
• Ensures that each new cell will receive only one
chromosome from each homologous pair
Telophase 1
• Spindle is broken down and the chromosomes uncoil
• Cytoplasm divides to form 2 new cells (each cell only has
half the genetic information of the original cell because it
has only one chromosome from each homologous pair
• However, another cell division is needed because each
chromosome is still doubled, containing 2 identical sister
chromatids
OH Meiosis II!
• Meiosis 2 is similar to mitosis. However, there
is no "S" phase (Chromosomes don’t replicate
• The chromatids of each chromosome are no
longer identical because of recombination
• Meiosis II separates the chromatids producing
two daughter cells each with 23 chromosomes
(haploid), and each chromosome has only one
chromatid
Prophase II
• Spindle forms in each of the 2 new cells
• Chromosomes are still made up of sister
chromatids
Metaphase II
• Chromosomes line up
at the equator or
middle
Anaphase II
• Centromeres of each
chromosome splits
• This allows the sister
chromatids to separate
and move to opposite
poles
Telophase II
•
•
•
•
Nuclei re-form
Spindles break down
Cytoplasm divides
(These are the same
events as in mitosis)
Meiosis is Finally Over!
• Four haploid cells have been formed from the
original diploid cell
• Each haploid cell contains one chromosome
from each homologous pair
• These haploid cells will become gametes,
transmitting the genes they contain to offspring!
Difference
Between
Mitosis and
Meiosis!