02. Meiosis

Meiosis
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Meiosis is a type of cell division where sex cells or
gametes are formed.
During meiosis, the number of chromosomes is
reduced by half
Diploid – number that refers to 46 chromosomes in
somatic cells (2n)
Haploid – number that refers to 23 chromosomes in
sex cells (n)
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When two haploid sex cells join (when sperm
joins with an egg) a diploid cell is formed.
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Meiosis takes place in the testes and ovaries.
Testes produce haploid sperm cells and ovaries
produce haploid egg cells.
Stages of Meiosis
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Meiosis involves two
nuclear divisions that
produce four haploid
cells.
Interphase
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As with mitosis, meiosis begin with interphase,
however, there is no interphase between
meiosis I and meiosis II
In this stage:
Cells grow rapidly and make structural
proteins that repair damaged parts
Cells transport nutrients and eliminate wastes
Replicate DNA
Meiosis l
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Often called reduction division because the
diploid, 2n, chromosome number is reduced to
the haploid, n, chromosome number.
Meiosis l consists of Prophase l, Metaphase l,
Anaphase l, Telophase l and cytokinesis.
Prophase l
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As in mitosis, centrioles move to opposite ends of the
cell.
Centrioles assemble and spindle fibers attach to the
centromeres of chromosomes
Nuclear membrane starts to dissolve
Original chromosome and its duplicate are attached to
each other by a centromere (while attached they are
known as sister chromatids)
Homologous chromosomes (the same shape and carry
similar genetic information) come together in pairs.
Homologous chromosome pairs are known as tetrads.
tetrad
Homologous pairs -
chromosomes in a biological cell
that pair during meiosis, or nonidentical chromosomes that
contain information for the same
biological features and contain
the same genes at the same
location but possibly different
genetic information
• For example, two chromosomes
may have genes encoding eye
color, but one may code for
brown eyes, the other for blue.
• When homologous
chromosomes pair up, it is called
synapsis.
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During synapsis, pieces of
chromosomes exchange
genetic information with
its homologous pair. This
is called crossing-over.
Metaphase l
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Homologous chromosomes line up on central
line of cell and are attached to spindle fibers.
The nuclear membrane completely dissolves.
Chromosomes are very dark and attached to
spindle fibers
Anaphase l
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The homologous pairs (tetrads) are pulled
apart in a process called segregation.
Each chromosome stays double-stranded
(attached to sister chromatid)
Telophase l
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Nuclear membrane forms around each group
of chromosomes.
Each cell contains 23 replicated chromosomes
(sister chromatids)
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After telophase, cytokinesis takes place and
two distinct cells are formed. The two cells
are haploid (23 sister chromatids) and carry
different genetic information because of
crossing-over.
Meiosis ll
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Occurs at same time in haploid daughter cells.
Unlike mitosis, in meiosis there is no
replication of chromosomes before meiosis ll
(no interphase).
Prophase ll
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Nuclear membrane starts to
dissolve
Centrioles in two new cells move
to opposite poles and new
spindle fibers form.
Sister chromatids become
attached to spindle fibers.
Metaphase ll
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Sister chromatids line up at
equatorial plate
The nuclear membrane
completely dissolves.
Chromosomes are very
dark and attached to
spindle fibers
Anaphase ll
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Sister chromatids of each
chromosome separate and
move to opposite poles.
An identical set of
chromosomes move to each
pole.
Telophase ll
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Chromosomes lengthen again and intertwine
The spindle fibers dissolve
Nuclear membrane forms around the chromosomes
Cytoplasm separates (cytokinesis) , leaving 4 haploid
daughter cells (each cell has 23 single chromosomes)
The chromosome number has been reduced by half
These cells may become gametes
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Read Handout P.108 and 109