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.