Download Meiosis I and II

Genetics Lecture II
Meiosis
The formation of gametes
aka. Gametogenesis
Biology Standards Covered
 2a~
students know meiosis is an early
step in sexual reproduction in which the
pairs of chromosomes separate and
segregate randomly during cell division
to produce gametes containing one
chromosome of each type
 2b~ students know only certain cells in
a multi-cellular organism undergo
meiosis
Biology Standards Covered
 2c~
students know how random
chromosome segregation explains the
probability that a particular allele will
be in a gamete
 2d~ students know new combinations
of alleles may be generated in a zygote
through the fusion of male and female
gametes (fertilization)
 2e~ students know why approximately
half of an individual’s DNA sequence
comes from each parent
Vocabulary



Homologous Chromosomes
– two chromosomes that
are similar in appearance
carrying the same genes
but may have different
alleles for those genes
Haploid – a cell having
only one “set” of
chromosomes
Diploid – a cell that
contains both “sets” of
chromosomes (one from
each parent)
 Gamete
Meiosis I and II
production happens in two
stages and produce haploid gametes
 Meiosis I
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I
 Meiosis
II
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II
Prophase I
of Meiosis I
 The
nucleus breaks down and
the chromosomes duplicate
 Homologous chromosomes
pair up and “crossing over”
happens
Diploid (2N)
Chromosome Duplication
 Chromosome
duplication
occurs during the Sphase of the Cell Cycle
 This takes the genetic
information and makes
an exact copy “bonded”
by a centromere
 The two copies of the
chromosome are now
called “sister chromatids”
Crossing Over
 This
is the process by which pairs
of homologous chromosomes
exchange alleles by allowing their
“inside sister chromatids” to
‘touch’
 This process increases genetic
variety by 50%!
 Genetic variation – differences in
genetic information within a
population allowing for
maximum survival in a rapidly
changing world
Metaphase I
of Meiosis I
• Spindle fibers arrange the
chromosomes along the
center of the cell in
homologous pairs
• This prepares them for
separation
Diploid (2N)
Anaphase I
of Meiosis I
 Spindle
fibers attached to the
centromeres of the duplicated
chromosomes “pull” the entire
chromosome to opposite ends of
the cell
 This process separates the
homologous pairs
Diploid (2N)
Telophase I
of Meiosis I
 The cell divides into two
haploid (n) cells having ½
of the original
chromosomes
 Each cell has a unique
combination of 23
duplicated chromosomes
Haploid (n)
Haploid (n)
Prophase II
of Meiosis II
 The
nuclear membrane
breaks down and the spindle
fibers reform from the
centrioles on opposite sides
of the cell
 Both cells are still haploid (n)
containing a unique ½ of the
organism’s complete
chromosomes
Haploid (n)
Haploid (n)
Metaphase II
of Meiosis II
* The duplicated chromosomes
are arranged along the middle
of the cell by the spindle
fibers
• Each chromosome still has 2
sister chromatids at this stage
• Spindle fibers are attached to
the centromere of each
duplicated chromosome
Haploid (n)
Haploid (n)
Anaphase II
of Meiosis II
 The
sister chromatids are
“pulled” apart by the spindle
fibers bringing each ½ to
opposite sides of the cell
 Because crossing over
occurred, each sister
chromatid carries “unique”
genetic information
Haploid (n)
Haploid (n)
Telophase II
of Meiosis II
 The
final stage of Meiosis II is
Telophase II
 Meiosis II results in four
haploid (n) cells that are
genetically unique!
 All four have a unique
combination of genes from
both parents
– In Males, each of the cells
becomes a unique sperm cell
Gametogenesis in Animals