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Meiosis
Formation of Gametes
By
Diana L. Duckworth
Rustburg High School
Campbell County
Historical Perspectives
• Recall that in 1882, chromosomes were
discovered & mitosis was observed by
Walther Flemming
• 1884 – 1888, nucleus identified as center
of inheritance
• 1887 – 1892 – Weisman & others
postulated reduction in chromosome
numbers during gamete formation &
observed meiosis
Gregor Mendel’s work rediscovered
• Gregor Mendel’s work was
published in 1866 in a local
natural history publication,
languished in obscurity
• Rediscovered in 1900
– Identified factors with different
expressions that are inherited
– Law of segregation (traits
separate during reproduction)
and
– Law of independent assortment
(inheritance of one gene does
not influence another)
http://www.nlm.nih.gov/exhibition/visibleproofs/galleries/technologies/dna.html
Walter Sutton - 1902
• Three postulates for chromosomes synthesized thinking of last decade
– A) chromosomes have individuality
– B) chromosomes occur in pairs, one from
each parent
– C) pairs separate during meiosis (gamete
formation)
Review
• Somatic cells – cells that do not produce
gametes – only undergo mitosis
– Some cell lines never undergo mitosis
– Length of cell cycle varies in different
organisms & different cell types
• Sex cell lines produce gametes
– Undergo meiosis, a reduction division from
diploid to haploid condition
• Diploid cells – have two copies of each
chromosome
• Haploid cells – have one copy of each
chromosome - gametes
Preparation for Meiosis
• Chromosomes occur in pairs – called
homologous chromosomes
• All chromosomes are copied during
S-phase of cell cycle
– Each chromosome is now duplicated, so
there are four chromosomes instead of
a pair of homologous chromosomes
– Chromatids are exact copies
– Sister chromatids are joined by
centromere
– Homologous pair now consists of two
sets of sister chromatids
Meiosis I – Prophase 1
• Chromosomes condense,
become visible
• Nuclear envelope dissolves
• Crossing over occurs
between sister chromatids
End of one sister chromatid is exchanged with
end of other sister chromatid.
Note: to identfy sister chromatids, I have made individual
Chromatids a different color.
Meiosis I – Metaphase 1
• Pairs of homologous
chromosomes move
to equator of cell
• Remember, each of
the homologous
chromosomes
consists of 2 sister
chromatids
• Spindles attach to
centromeres
Meiosis I – Anaphase 1
• Homologous
chromosomes
move to opposite
poles of cell
Note that homologous chromosome
separate, not sister chromatids!
Meiosis I – Telophase 1
• Chromosomes cluster at
poles
• New nucleus forms
• Cytoplasm divides to
form two new cells each
of which has two sister
chromatids from one of
the homologous
chromosomes
• Still diploid!
Meiosis II – Prophase 2
• Cells now go into
Prophase II without
any duplication of
chromosomes!
• Nuclear envelope
dissolves & new
spindles form
around
chromosomes
Meiosis II – Metaphase 2
• Pairs of sister
chromatids line up
along the equator of
cell
Meiosis II – Anaphase 2
• Spindles attach to
centromere and
separate sister
chromatids to
opposite ends of
cell.
Meiosis II –Telophase 2
• Chromosomes
cluster at poles of
cell
• New nuclear
envelope develops
• Cell undergoes
cytokinesis
• Result: 4 haploid
gametes!
Gametes Unite in Fertilization
Haploid Sperm
Haploid Egg
Diploid Zygote
http://homepages.ius.edu/DPARTIN/
http://www.biology.iupui.edu/biocourses/n100/2k4ch9meiosisnotes.html
http://www.biology.iupui.edu/biocourses/n100/2k4ch9meiosisnotes.html
Darwin Revisited
• Recall Darwin’s problems: how does inheritance
occur & where does variability come from?
• Inheritance – genetic material is in
chromosomes; both parents contribute genetic
material to offspring through meiosis &
fertilization
• Variation – recombination during fertilization;
crossing over; independent assortment of
chromosomes contribute enormously to
variation. Number of possible gametes = 2n,
where n = number of homologous
chromosomes.
Mendel Revisited
• Mendel identified factors (genes) with
traits (expression of genes), now called
alleles.
• To explain his ratios, offspring had to get
one trait for a factor from each parent.
• Meiosis and fertilization ensure that for
any given gene, the offspring get one
allele (on one chromosome) from each
parent.