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)

Walter Sutton 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

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 = 2 n, 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.