Download PowerPoint

Meiosis Notes
Meiosis
 Meiosis occurs in sexual reproduction when a
diploid germ cell produces four haploid
daughter cells that can mature to become
gametes (sperm or egg).
Meiosis
Goal: reduce genetic material by half
Why? n (mom) + n (dad) = 2n (offspring)
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just
right!
Fertilization
Chromosomes
Meiosis: cell division in two parts
Sister
chromatids
separate
Chromatids
separate
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid 2n
Haploid n
Haploid n
Result: one copy of each chromosome in a gamete.
Meiosis I : the reduction division
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Prophase I
• Chomatids pair and condense.
• Crossing over occurs
• Spindle forms.
• Nuclear envelope
fragments.
Metaphase I
• Chromatid pairs align along the equator of the
cell.
Anaphase I
• Chromosomes separate and move to opposite
poles.
• Sister chromatids remain attached at their
centromeres.
Telophase I
• Nuclear envelopes reassemble.
• Spindle disappears.
• Cytokinesis divides cell into two.
Meiosis II
Gene X
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with
one copy of each chromosome and thus one copy of
each gene.
Meiosis II : the equational division
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
Non-identical
haploid
daughter cells
Prophase II
• Nuclear envelope fragments.
• Spindle forms.
Metaphase II
• Chromosomes align along equator of cell.
Anaphase II
• Sister chromatids separate and move to
opposite poles.
Telophase II
• Nuclear envelope
assembles.
• Chromosomes
unravel.
• Spindle disappears.
• Cytokinesis
divides cell into
two.
Results of meiosis
• Four haploid cells
• One copy of each
chromosome
Meiosis allows the creation of unique individuals
through sexual reproduction.
Meiosis keeps the number of chromosomes
the same for each generation.
Mitosis
Meiosis
Number of divisions
1
2
Number of daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Sex cells
When
Throughout life
At sexual maturity
Growth and repair
Sexual reproduction
Genetically identical?
Chromosome #
Role
MITOSIS
MEIOSIS
Occurs in somatic (body) cells
Occurs only in gonads (sex organs:
ovary/testes)
Produces cells for repair,
maintenance, growth, asexual
reproduction
Only produces gametes (sex cells:
egg/sperm)
Results in identical diploid (2n) daughter Reduction division results in haploid (n)
cells
cells
Mistakes in Meiosis
Nondisjunction = The failure of
homologous chromosomes to
separate properly during meiosis
 Trisomy: When a gamete with extra
chromosomes is fertilized by a normal
gamete, the zygote will have an extra
chromosome
Ex. In human, if a gamete with an extra chromosome
number 21 is fertilized by a normal gamete, the resulting
zygote has 47 chromosomes instead of 46. This will
develop into a baby with Down syndrome.
 Monosomy = When a gamete that is
missing a chromosome fuses with a
normal gamete during fertilization, the
resulting zygote lacks a chromosome.
 Most often this is lethal. One non-lethal
example is Turner syndrome, in which human
females have only a single X chromosome
instead of two.
 Polyploids = organisms with more
than the usual number of
chromosomes sets
 Often results in death of the zygote. However,
polyploids often occur in plants