MEIOSIS NOTES Chapter 11, section 4

Reproduction Asexual (relies on mitosis): Involves 1 parent; offspring are clones Advantages: happens quickly & produces large #s of offspring Disadvantages: no genetic variation Sexual (relies on meiosis): Involves 2 parents; offspring are different Advantages: allows for genetic variation Disadvantages: have to find mate; process takes longer; fewer offspring produced.

2 TYPES OF CELLS 1) SOMATIC CELLS 2) GAMETES

Types of Cells: Somatic Cells All cells in the body; not sex cells -Also called Body cells -Made by mitosis This scanning electron micrograph (courtesy of Dr. Marion J. Barnhart) shows the characteristic biconcave shape of red blood cells.

Types of Cells: Somatic Cells -Contain 2 copies of each chromosome -called homologous chromosomes -have diploid #, 2n EX. Humans =46

Types of Cells: Gametes *Also called sex cells *Eggs and sperm *Made by meiosis *Have haploid # (n) *EX. Humans =23

Sex Cells (sea urchin egg being fertilized)

Chromosome # Egg (haploid #)Sperm(haploid #) ZYGOTE (diploid #) Ex. Humans =46

Meiosis: produces gametes Reduction division -starts with 1 diploid cell and ends up with 4 haploid cells

Meiosis occurs in 2 steps: MEIOSIS I & MEIOSIS II

MEIOSIS I P1, M1, A1, T1) homologous chromosome pairs are divided two new daughter (diploid) cells are produced

Prophase I *Step 1—The chromosomes (DNA) coil up becoming visible and a spindle forms. The nuclear membrane and nucleolus disappear. *Step 2— Synapsis occurs Synapsis is the pairing of homologous chromosomes. A tetrad is formed. A tetrad is the pair of homologous chromosomes. **Crossing over can occur here. Crossing over is the exchange of genetic material (genetic recombination). This allows for genetic variation among organisms.

Crossing Over Adapted from: Morgan T.H., Sturtevant A.H., Muller H.J., and Bridges C.B., "The Mechansim of Mendelian Heredity", 1915.

Metaphase I Tetrads are moved by spindle fibers to the equator. **They line up in homologous pairs. In mitosis, they line up independently.

Anaphase I Homologous chromosomes separate; chromosomes of each pair move to opposite poles. **The chromatids do not split, as they do in mitosis.

Telophase I Chromosomes gather at the poles of the cell. The cytoplasm divides. **Each cell has 1 homologous pair. These chromosomes are still attached by centromeres forming two daughter cells. Another cell division is needed because each chromosome is still doubled, containing two identical sister chromatids.

MEIOSIS II (P2, M2, A2, T2) Sister chromatids are split into 4 haploid cells

Meiosis II is similar to mitosis in that the chromatids separate. There are four phases of Meiosis II: Prophase II –A new spindle forms around the chromosomes. Metaphase II-Pairs move to the equator of the cell. Anaphase II-Sister chromatids separate and move to opposite poles. Telophase II-Spindle fibers dissolve, nuclear membrane forms, and the cytoplasm divides.

Meiosis Diagram

Gamete Development in Males &Females MALES Meiosis occurs in testes at puberty and then occurs as needed 1 spermatocyte=4 sperm FEMALES Meiosis occurs in the ovaries before birth 1 oocyte=1 egg and 3 wasted cells

Sperm Formation-Spermatogenesis

Egg Formation - Oogenesis (unequal cytokinesis)

Spermatogenesis Oogenesis

Comparing Mitosis & Meiosis HOW MANY CELLS ARE MADE? Mitosis: 2 diploid cells Meiosis: 4 haploid cells

ARE RESULTING CELLS GENETICALLY THE SAME OR DIFFERENT? Mitosis: Genetically the same Meiosis: Genetically different Comparing Mitosis & Meiosis

WHAT TYPES OF CELLS RESULT IN THE PROCESS? (SOMATIC CELLS OR GAMETES) Mitosis: Somatic cells Meiosis: Gametes Comparing Mitosis & Meiosis

WHAT ARE THE ADVANTAGES OF EACH PROCESS? Mitosis: Growth and repair of organism Meiosis: Continuation of species and genetic recombination, which allows for genetic variation Comparing Mitosis & Meiosis