MEIOSIS & Sexual Reproduction

Meiosis- cell division in which the daughter cells receive only half the number of chromosomes present in the parent cells Homologous chromosomes- pairs of similar chromosomes (same size, shape, & carrying alleles [genes] for the same trait) 1. Humans have 46 chromosomes (22 pairs of homologous chromosomes & one pair of sex chromosomes) 2. Autosomes- are the chromosomes other than the sex chromosomes ex) somatic or body cells

Diploid cells- contain the full # of chromosomes (2n) Haploid cells- gametes that contain half the # of chromosomes (n); no homologous pairs Gametes- specialized sex cells Female gametes = egg cells or ova (ovum is singular; egg cell) Male gamete = sperm cell Fertilization- the fusion of sperm & ova nuclei to form a zygote

Stages of Meiosis- "reduction division" in specialized cells for reproduction. IPMATPMAT It is a process which prepares sex cells.

A. Prophase I 1. Each chromosome has already replicated & consists of 2 chromatids a. Synapsis results in 4 chromatids joining together to form a tetrad 2. Each chromosome lines up relatively close with its homologous chromosomes & become attached at their centromeres. This process is called SYNAPSIS. 3.while tetrads form, CROSSING OVER occurs, which is the exchange of segments between chromatids

Metaphase I 1. Centromeres are attached by the spindle fibers 2. Tetrads line up at the equatorial plane Anaphase I 1. DISJUNCTION- separation of tetrads into homologous chromosomes NON DISJUNCTION - is the failure to separate homologous chromosomes (very bad)

Sexual Reproduction Advantages 1. Offspring are not identical to either parent 2. Variations that increases adaptability in a changing environment 3. Individuals are able to move into new environments 4. By making population more varied helps to ensure survival of the species

Sexual Reproduction in Simple Organisms- CONJUGATION (permits the recombination of hereditary material, to form new variety of organisms) A. Spirogyra- the flow of cell material between 2 cells through a conjugation tube and the formation of a zygospore (protective wall) B. Paramecium- formation of a protoplasmic bridge between 2 and the exchange of material. Don't have sexes, but different mating types positive/negative isogametes. C. Bacteria

Sexual Reproduction in Animals specialized organs that produce gametes Gonads- Ovaries- female that produce ova (egg cells) Testis- male that produce sperm cells contain both ovaries and testes Hermaphrodism -

Gametogenesis - production of gametes Oogenesis – production of ova in the ovaries Meiotic division that produces 3 polar bodies and one mature ovum production of sperm in the testes Spermatogenesis – Meiotic division that produces 4 sperm cells

Comparison of egg and sperm Egg cells contain yolk, and are larger than sperm Sperm is motile, eggs are sessile

fusion of sperm and egg nuclei to form a zygote Fertilization - 1. Diploid (2n) chromosomes restored takes place outside of body - more eggs are produced 2. External fertilization - a. Amphibians b. fish takes place inside body, less eggs are produced 3. Internal fertilization - a.Mammals b. birds

Parthenogenesis – the development of an unfertilized egg into an adult animal 1. Bees 2. Ants 3. Aphids