Meiosis and Sexual Life Cycle Chapter 13

Heredity (inheritance) - transmission of traits from 1 generation to next. Variation - siblings differ from 1 another. Genetics - study of heredity and variation.

Parents give offspring coded information - genes. Genes - segments of DNA. 4 nucleotides found in DNA create specific sequences determine traits. DNA in sperm and egg fuse together to create variation.

Each chromosome has thousands of genes - located at specific spot - locus (loci).

Asexual reproduction - 1 parent passes all genetic information on to offspring. Example: Hydra (eukaryotic) reproduce by budding cells produced by mitosis.

Sexual reproduction produces variation - combination of genes from parents unique. Siblings resemble each other because of similar genes.

Humans, somatic (non-sex) cells - 46 chromosomes. Each chromosome distinguished by size, position of centromere, by pattern of staining with certain dyes. Karyotype - picture of 23 pairs of chromosomes with centromeres and sizes. Chromosomes homologous - they have pair that matches them.

Sex chromosomes not homologous in male (X and Y) Females homologous (2 X’s). Other 22 pairs autosomes (non-sex chromosomes) 1 chromosome of pair inherited from each parent.

Each egg and sperm (gametes) have 22 autosomes, 1 sex chromosome. Haploid - half the # of chromosomes. Cells fuse (syngamy) - fertilization. Fertilized egg (zygote) has 2 haploid sets of chromosomes with genes from maternal and paternal family lines.

When zygote has all chromosomes (46), - diploid. Humans - diploid # of chromosomes 46 (2n = 46). Gametes (develop in gonads) and are produced through meiosis - chromosome # halved. Fertilization and meiosis alternate in sexual life cycles.

Meiosis and fertilization timing varies among species. Fungi, some protists have 2 life cycles. Zygote starts off diploid, then divides to haploid. Haploid adult goes through mitosis.

Human life cycle

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Plants - alternation of generations. Includes haploid (gametophyte) and diploid (sporophyte) multicellular stages. Meiosis by sporophyte produces haploid spores - develop by mitosis into gametophyte.

Meiosis - 2 consecutive cell divisions (meiosis I, meiosis II) - result in 4 daughter cells. Each daughter cell - only ½ as many chromosomes as parent cell. Meiosis reduces chromosome # by copying chromosomes once, but dividing twice.

Meiosis I Stages are prophase, metaphase, anaphase, telophase. Interphase (preceding prophase) - chromosomes replicate to form sister chromatids. Single centrosome replicated.

Prophase I - chromosomes condense, homologous chromosomes pair up - form tetrads. Synapsis - proteins attach homologous chromosomes tightly together. Chromatids of homologous chromosomes cross (chiasmata), segments of chromosomes traded. Spindle forms from each centrosome.

Metaphase I - tetrads arranged at metaphase plate. Anaphase I - homologous chromosomes separate, pulled toward opposite poles.

Telophase I - movement of homologous chromosomes continues until haploid set at each pole. Each chromosome consists of linked sister chromatids. Cytokinesis - separates cytoplasm.

Prophase II - spindle apparatus forms, attaches to kinetochores of each sister chromatid, moves them around.

Metaphase II - sister chromatids arranged at metaphase plate. Anaphase II - centomeres of sister chromatids separate; travel toward opposite poles. Telophase II - separated sister chromatids arrive at opposite poles; nuclei form around chromatids.

Cytokinesis - cytoplasm separates; now 4 daughter cells.

Differences between mitosis and meiosis Chromosome # reduced by ½ in meiosis, not in mitosis. Mitosis produces 2 genetically identical cells; meiosis produces 4 unique ones.

Other differences 1 Crossing over: During prophase I, homologous chromosomes pair up (synapsis). At X-shaped regions (chiasmata) sections of nonsister chromatids exchanged.

2 Metaphase I homologous pairs of chromosomes, not individual chromosomes, aligned along metaphase plate. 3 Anaphase I - homologous chromosomes, (not sister chromatids) separate, carried to opposite poles of cell.

Contributions to Variation 1 Independent assortment - tetrads arrange themselves randomly on metaphase plate. Each homologous pair of chromosomes positioned independently of other pairs at metaphase I.

2 Crossing over - homologous portions of 2 nonsister chromatids trade places. Crossing over begins early in prophase I as homologous chromosomes pair up gene by gene.

3 Random fertilization - any sperm can fertilize any egg. Each egg sperm is 1 of 8 billion gene combinations - 70 trillion possibilities of combinations in zygote (doesn’t include crossing over)