meiosis formation of gametes by diana l. duckworth rustburg high school campbell county
TRANSCRIPT
Meiosis
Formation of Gametes
ByDiana L. Duckworth
Rustburg High SchoolCampbell 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 withend of other sister chromatid.
Note: to identfy sister chromatids, I have made individualChromatids 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 chromosomeseparate, 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.