1. Meiosis & Sexual Life Cycles
A prelude to Genetics….

2. 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”

3. 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

4. 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

5. 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

6. 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.

7. 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.

8. 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!