Transcript Meiotic Cell Division - The FreeZone : Midwestern State

Meiotic Cell Division
Sexual Reproduction
• Sexual reproduction is the most
common way for eukaryotic organisms
to produce offspring
• Parents make gametes with half the
amount of genetic material (haploid)
• These gametes fuse with each other
during fertilization to generate a new
organism
• Simple eukaryotes Gametes
are isogamous
– They produce gametes that are morphologically similar
• Most eukaryotic species are heterogamous
– These produce gametes that are morphologically
different
• Sperm cells
– Relatively small and mobile
• Oocytes or ova
– Usually large and nonmobile
– Store large amounts of nutrients
• Microspores (Pollen)
• Macrospores (Ovules)
How Does One Make a Haploid
• Answer – meiosisGamete?
• Haploid cells are produced from diploid cells
during gametogenesis
• The chromosomes must be distributed to
reduce the chromosome number to half its
original value
• but simultaneously sorted to assure that
each chromosome (& its genes) is
represented in each gamete
Mitosis
vs
Meiosis
• Produces two diploid
daughter cells
• Produce four haploid
daughter cells
• Produces daughter
cells that ARE
genetically identical
• Produces daughter
cells that are NOT
genetically identical
Meiosis
• Meiosis begins after a cell has
progressed through G1, S, & G2
• Meiosis involves two successive
divisions
– Meiosis I and II
– Each of these is subdivided into
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
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Meiosis
• Prophase I is further subdivided into
periods known as
– Leptotena
– Zygotena
– Pachytena
– Diplotena
– Diakinesis
Periods of
A total of 4
Prophase chromatids
I
A recognition
process
Figure 3.11
Synaptonemal Complex
Periods of Prophase I
A tetrad
A physical exchange of
chromosome pieces
2
bivalents
Spindle apparatus complete;
Stages of Meiosis
I
pairs of chromatids attached
to kinetochore microtubules
• Bivalents are organized
along the metaphase plate
• Homologous pairs of sister
chromatids aligned side by
side
– A pair of sister chromatids is
linked to one of the poles
– And the homologous pair is
linked to the opposite pole
– The arrangement is random
with regards to the (blue and
red) homologues
Figure 3.13
Stages of Meiosis I
Pairs of sister chromatids
separate from each other
The centromere remains between
sister chromatids
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Meiosis
• Telophase I & cytokinesis
of meiosis I is
followed meiosis II
• Meiosis I has reduced the number of
chromosomes in the daughter cells to the ½
the diploid number
• However, each homolog is still composed of
2 recombinant sister chromatids
– The genetic content is still 2n
• Meiosis II reduces the genetic content to n
Stages of Meiosis II
1 of each type of
chromosome (n) in each
daughter cell (gamete)
Separation
of
Alleles
During
Heterozygous (Yy) cell from
a plant with yellow seeds
Meiosis
Prophase I
y yY Y
y
Y
Metaphase I
Anaphase I
Telophase I
y y
Y
Y
Meiosis II
y
y
Y
Haploid cells
Y
Separation of Alleles During
Meiosis
r r
y Y Y Heterozygous diploid
cell (YyRr) to
undergo meiosis
R R
y
y
Meiosis I
Y
y
Y
r
R
or
r
R
Meiosis II
y
R
y
Y
R
r r
y
y
R
Y
r
R
2 Ry
Y
:
2 rY
y
Y Y
r
r
R R
y
Y
r
y
r
y
Y
r
2 ry
R
:
Y
R
2 RY