MEIOSIS AND SEXUAL REPRODUCTION

Meiosis Process of nuclear division that reduces the number of chromosomes in a cell by half 2 Stages Meiosis I: Homologous chromosomes are separated into separate cells Meiosis II: Chromatids are segregated into separate cells

Complete interphase with S phase (replication of DNA occurs) BEFORE MEIOSIS Complete interphase with S phase (replication of DNA occurs)

DNA condenses and becomes chromosomes. MEIOSIS 1 PROPHASE I DNA condenses and becomes chromosomes. Spindle forms, nucleolus and nucleus disappears. 3. Homologous chromosomes line up at equator.

4. Synapsis occurs: parallel alignment of sister chromotids (NOW CALLED A TETRAD) 5. Crossing over occurs Non sister chromatids break apart and exchange alleles and re-attach. Result: Recombination (NEW COMBOS OF GENES)

METAPHASE I Tetrads move to equator and remain together. ANAPHASE I Homologous chromosomes separate and one of each pair is pulled to opposite poles. **chromatids still joined by centromere**

Two daughter cells formed. TELOPHASE I Cytokinesis occurs. Two daughter cells formed. (do NOT have same genetic info due to crossing over) END RESULT Two daughter cells with haploid # (N) ** homologous chromosomes have separated, but sister chromatids remain together

MEIOSIS II 2 (2N) cells go into meiosis II MEIOSIS II 2 (2N) cells go into meiosis II. Chromatids are segregated into different cells. PROPHASE II 1. chromosomes condense. (brief interphase with NO DNA replication) nuclear membrane disappears. 3. chromosomes start to move to equator.

METAPAHSE II Chromosomes with sister chromatids joined by centromere line up at equator. ANAPHASE II Centromeres divide and each sister chromatid moves to opposite poles

** chromatids separate** TELOPHASE II 1. spindle dissolves & nuclear membrane reforms around chromosomes in each daughter cell 2. cleavage furrow or cell plate forms and cytokinesis occurs END RESULT Four cells with haploid # (N) ** chromatids separate** Meiosis animation

Why is it important for gametes to have the haploid number?

DIFFERENCES: MITOSIS AND MEIOSIS 1 diploid cell makes 2 diploid daughter cells daughter cells genetically identical to mother cell occurs in somatic cells unique features meiosis Meiosis 1 diploid cell makes 4 haploid cells resulting cells are genetically different from diploid cells and each other occurs in gametes comparison animation

HOW MEIOSIS CAUSES GENETIC VARIATION each of the 23 pairs of homologous chromosomes segregate independently crossing over adds to recombination random joining of two gametes with new combinations Outcomes: 223 x 223 = 64 trillion outcomes

GAMETOGENESIS Meiosis is primary event in formation of gametes. Spermatogenesis Oogenesis Spermatogenesis: Production of sperm in testes - starts with 1 diploid germ cell - result: 4 mature sperm cells Oogenesis: Production of ova (eggs) in ovaries - result: 1 ovum, 3 polar bodies - female is born with set number of eggs which are frozen at prophase I - puberty hormones cause eggs to complete meiosis and then pauses again - at fertilization: egg undergoes meiosis II

TYPES OF ASEXUAL REPRODUCTION BINARY FISSION FRAGMENTATION BUDDING bacteria penicillin yeast PARTHENOGENESIS REGENERATION rotifer egg starfish ANIMATIONS

Advantages of each type reproduction: Asexual Sexual - production of many offspring - quick production of different gene in short time period combinations - genetic diversity enhances evolution

Eukaryotic Sexual Life Cycles Haploid: simplest - Haploid cells occupy major portion of life cycle - Zygote is only diploid cell, undergoes meiosis as soon as formed, again become haploid Occurs in protists, fungi, algae

Eukaryotic Sexual Life Cycles Diploid: adults are diploid and inherit chromosomes from parents - adult individuals are diploid with each individual getting chromosomes from each parent (thru fertilization) - major portion of life is spent as diploid - diploid repro. cell undergoes meiosis to form haploid gametes (only haploid cell) - most animals

Eukaryotic Sexual Life Cycles 3. Alternation of generations: life cycle regularly alternates between a haploid phase and diploid phase - plants - Sporophytes: diploid phase that produces spores - Spore: haploid reproductive cell ,undergoes meiosis without fusing with another cell - Gametophyte: haploid phase produces whereby gametes produced by mitosis fuse and give rise to diploid phase Ex: moss: Cycles differ only in which phases become diploid.