Slides 7: Meiosis and Genetic Diversity

purposes of sexual reproduction

• genetic diversity

• differences from parence

• better survival chances in a changing environment

2 processes in sexual reproduction

gametogenesis: a diploid cell (2n) divides into two haploid gametes (n) via meiosis

fertilization: two haploid gametes (n) fuse into one diploid zygote (2n)

somatic cell

non-reproductive cell: all cells in body besides gametes, which are diploid and contain 23 pairs of chromosomes

germ line cell

somatic cells that divide to produce gametes: alternate between haploid and diploid

Meiosis 1

duplicated homologous chromosomes pair into tetrads, recombine, and are separated into two (replicated) haploid cells

Prophase 1

Chromosomes condense into Xs, pair up with homologous pairs to form tetrads, and exchange genetic material through crossing over

synaptonemal complex

several proteins hold together homologous chromosome pairs forming tetrads

chiasmata

sites of crossing over

process of crossing over

1. DNA breaks at chiasmata

2. Synaptonemal complex forms

3. small pieces of DNA switch places traveling across synaptonemal proteins

metaphase 1

tetrads line up single file (homologous pairs are double file) in the center of the cell. Their orientations are random. Microtubules attach to kinetochores on centromeres

Number of possible combinations from independent assortment

2^n, where n = the number of chromosomes in a haploid cell

anaphase 1

tetrads are broken apart and moved to the poles of the parent cell

telophase 1

nuclear membrane reforms around 2 haploid X sets of DNA, and cytokinesis divides the cell membrane

Meiosis 2

Each pair of sister chromatids separates, and two I haploid cells are formed, similar to mitosis

prophase 2

nuclear membrane breaks down, spindle apparatus forms, chromosomes move towards metaphase plate

metaphase 2

X chromosomes line up in the center of the cell, and microtubules attach to kinetochores

anaphase 2

sister chromatids separate and are pulled by microtubules to opposite ends of the cell

telophase II

nuclear membrane reforms around I haploid set of DNA, chromosomes begin to decondense, microtubules are destroyed

two elements that contribute to genetic diversity

independent assortment and crossing over

nondisjunction

mistakes in moving chromosomes during meiosis

effects of nondisjunction in meiosis 1

tetrads tail to separate properly so resulting cells are missing or have extra of some homologous pairs, two resulting cells have an extra chromosomes and two have missing chromosomes

effects of nondisjunction in meiosis 2

sister chromatids fail to separate, resulting in daughter cells with extra or missing chromosomes

trisomy and monosomy

effects of nondisjunction where somatic cells have either 3 copies of a chromosome or 1 copy, which will affect the cells ability to undergo mitosis

XX chromosome expression

only one (random) X chromosome is active and the other one is condensed into a tight ball (barbody) which cannot be expressed

nondisjunction of sex chromosomes

usually works okay because only one X chromosome is expressed in any scenario