Chapter 13 – Meiosis and Sexual Life Cycles

Heredity Heredity: the transmission of traits from one generation to the next Variation: offspring differ somewhat in appearance from parents and siblings Genes: segments of DNA that encode for proteins, the hereditary unit Locus: a genes specific location on a chromosome Asexual reproduction: produces clones example budding in hydra Sexual reproduction: results in variation

Human life cycle 23 pairs of homologous chromosomes (46) 1 pair of sex and 22 pairs of autosomes = a set Somatic cells – body cells Gametes – sex cells Sex chromosomes – X and Y Autosomes – non-sex chromosomes Gametes are haploid while all other cells are diploid Haploid (1n) = 1 set of chromosomes Diploid (2n) = 2 sets of chromosomes Fertilization (syngamy) results in a zygote

Karyotype Micrograph of chromosomes, usually arranged by size

Alternative life cycles Fungi/some algae - meiosis produces 1n cells that divide by mitosis to produce 1n adults (gametes by mitosis) Plants/some algae - Alternation of generations: 2n sporophyte, by meiosis, produces 1n spores; spore divides by mitosis to generate a 1n gametophyte; gametes then made by mitosis which then fertilize into 2n sporophyte

Meiosis Overview Preceded by chromosome replication (interphase) Followed by 2 successive cell divisions (Meiosis I & Meiosis II) Reduces the chromosome number Produces 4 haploid daughter cells Variation

Meiosis 1 overview

Meiosis 2 overview

Prophase I Occupies 90% of the time to complete meiosis Chromosomes condense Homologous chromosomes pair up, aligning gene to gene Synapsis – tetrads (4 homologous chromatids) are grouped together and held in place by a protein structure called the synaptonemal complex Each tetrad has one or more chiasmata – criss-crossed regions where crossing over occurs; holds tetrad together until Anaphase I Crossing over – non-sister chromatids exchange DNA Everything from mitosis prophase and prometaphase also occurs except the kinetochore microtubules only attach to each homologous pair, not each chromatid

Prophase I

Crossing Over

Metaphase I Tetrads line up on metaphase plate One chromosome from each homologous pair facing a pole

Metaphase I

Anaphase I Pairs of chromosomes move towards the poles Original sister chromatids remain attached

Anaphase I

Telophase I and Cytokinesis At end of telophase, each new cell has a complete set of chomosomes, but each is still attached as sister chromatids Cytokinesis occurs simultaneously with telophase I same as mitosis The cells do not enter interphase but enter a stage of interkinesis (a pause between meiosis I and meiosis II)

Telophase I and Cytokinesis

Meiosis II Same as mitosis End product is 4 haploid gametes with genetic variation

Meiosis II

Meiosis vs. Mitosis Synapsis/tetrad/ chiasmata (prop I) Homologous vs. indiv. chromosomes (meta I) Sister chromatids do not separate (ana I) Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

Origins of Genetic Variation Independent assortment: homologous pair of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II n

Origins of Genetic Variation Crossing over (prophase I): the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) Random fertilization: 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!