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)

Telophase I 1. Cytokinesis occurs forming daughter cells 2. Nuclear membrane reappears around the nuclei. 3. No further replication of chromosomes takes place, but the remainder of the division is exactly like mitosis

Prophase II [both daughter cells divide] [homologous chromosome line up on the equator] Metaphase II Anaphase II [homologous chromosomes separate] [4 different haploid cells are produced, these (n) are gametes] Telophase II

Number of daughter cells Number of cell divisions COMPARISON CHART MITOSIS vs. MEIOSIS Characteristic Mitosis Meiosis Number of daughter cells Number of cell divisions Chromosome number in daughter cells Comparing daughter cells and parent cell Type of reproduction 2 4 2 1 Same as parent One-half of parent identical different sexual asexual

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