Meiosis

Chapter 13

What you need to know!

• The role of meiosis and fertilization in sexually reproducing organisms

• The importance of homologous chromosomes to meiosis

• How the chromosome number is reduced from diploid to haploid through the stages of meiosis

• Three important differences between mitosis and meiosis

• The importance of crossing over, independent assortment, and random fertilization to increasing genetic variability

Meiosis

• A process by which an organism produces haploid (n) sex cells from diploid (2n) germ cells

• Meiosis is divided into meiosis 1 and meiosis 2

• Meiosis 1 = separation of homologous chromosomes

• Meiosis 2 = separation of sister chromatids

Interphase

• G1, S, G2• S Phase:• Duplication of DNA 2n 4n• Assembly of centrioles

Prophase I

• 90% of meiotic time• Condensation of chromosomes• Nuclear envolope disappears• Spindles form• Synapsis:

– Homologous chromosomes pair up as tetrads– The chiasmata are where crossing-over takes

place

Crossing Over

• Leads to the creation of novel chromosomes• Genes from homologous maternal and paternal

chromosomes mixa) Synapsis: homologous chromosomes form tetradsb) Chiasmata: homologous chromosomes overlap on multiple

pointsc) Crossing Over: identical parts break off and reattach

Crossing Over Frequency

• The recombination frequency between two genes on one chromosome is mathematically related to the distance between them

• The further apart 2 genes are the higher the probability of a crossing over event separating them

• Recombination frequencies vary between 0% and 50%

• This method is useful for establishing gene maps (location of genes on a chromosome

• Example: Recombination frequencies between genes A and B is 45% while A and C is only 20%. Map the genes.

Metaphase 1

• Homologous chromosomes line up at the equator side by side

• One spindle attaches to each chromosome• 4n = 92 chromosomes in humans

Anaphase 1

• Homologous chromosomes separate (sister chromatids remain together)

Telophase 1, Cytokinesis, and Interkinesis

Telophase 1/cytokinesis:• Two cells separate• Each contains one homologous chromosome

(haploid) with two sister chromatids• Often no nuclei form and the DNA remains

condensed 2 x (n) = 46 (no pairs)

Interkinesis:• Short interphase with little growth• Sometimes partial uncoiling of the chromosomes• Duplication of centrioles (no S phase)

Prophase 2

• Spindle fibers form• 2 kinetochore spindle fibers attach to each

chromosome

Metaphase II

• Chromosomes line up at the equator of the cell

• Spindle fibers attach to each Centromere of the sister chromatids

Anaphase 2

• Sister chromatids are pulled apart• Sister chromatids are not identical

(crossing over)

Telophase 2 and Cytokinesis

• Four new cells form• All are genetically different• All are haploid with 1n chromosomes

– Humans 23