Meiosis & Sexual Life Cycles A prelude to Genetics….

Heredity vocab: Heredity: the transmission of traits from one generation to the next Gene- a segment of DNA on a chromosome which codes for certain traits. Chromosome: thread like structures which consist of a single long DNA molecule (highly folded & coiled), along with proteins. Contains 100s to 1000s of genes each Locus- a specific location on a chromosome which contains a gene Somatic Cell- any cell other than a sperm or egg. Humans somatic cells contain 46 chromosomes (23 pair) Sperm & egg cells also called “gametes”

Heredity vocab continued…. Homologous Chromosomes: have genes for the same type of traits at the same loci*. Each came from a different parent *with the exception of sex chromosomes Sex Chromosomes- X & y determine an individual’s sex Female- homologous set of X chromosomes Male- 1- X and 1 y chromosome Autosome- those that are not sex chromosomes Thus- humans have 22 pair of autosomes and 1 pair of sex chromosomes. Gametes are haploid (1N) Somatic cells are diploid (2N) Fertilization (syngamy) results in a zygote Meiosis: cell division to produce haploid gametes

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 Just like Mitosis, it is preceded by chromosome replication, but…. It is followed by 2 cell divisions: Meiosis I & Meiosis II 4 daughter cells 1/2 chromosome number (1N) Lots of variation!! Meiosis Animation

Meiosis vs. Mitosis Synapsis/tetrad/chiasmata (prophase I) Homologous vs. individual chromosomes (metaphase I) Sister chromatids do not separate (until anaphase II) 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 during metaphase I. Combinations possible: 2n n = haploid number of the organism.

Origins of Genetic Variation continued…. Crossing over (prophase I): • the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I Random fertilization: • 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 70+ trillion diploid combinations! Animate!