By Diana L. Duckworth Rustburg High School Campbell County 2 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
3 Gregor Mendels work rediscovered
Gregor Mendels 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/ga lleries/technologies/dna.html 4 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)
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
6 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
7 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. 8 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
9 Meiosis I Anaphase 1
Homologous chromosomes move to opposite poles of cell
Note that homologous chromosome separate not sister chromatids! 10 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
11 Meiosis II Prophase 2
Cells now go into Prophase II without any duplication of chromosomes!
Nuclear envelope dissolves new spindles form around chromosomes
12 Meiosis II Metaphase 2
Pairs of sister chromatids line up along the equator of cell
13 Meiosis II Anaphase 2
Spindles attach to centromere and separate sister chromatids to opposite ends of cell.
14 Meiosis II Telophase 2
Chromosomes cluster at poles of cell
New nuclear envelope develops
Cell undergoes cytokinesis
Result 4 haploid gametes!
15 Gametes Unite in Fertilization Haploid Sperm Haploid Egg 16 Diploid Zygote 17 http//homepages.ius.edu/DPARTIN/ 18 http//www.biology.iupui.edu/biocourses/n100/2k4ch 9meiosisnotes.html 19 http//www.biology.iupui.edu/biocourses/n100/2k4ch 9meiosisnotes.html 20 Darwin Revisited
Recall Darwins 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.
21 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.
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