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.
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.
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 • 1Crossing over: During prophase I, homologous chromosomes pair up (synapsis). • At X-shaped regions (chiasmata) sections of nonsister chromatids exchanged.
2Metaphase I homologous pairs of chromosomes, not individual chromosomes, aligned along metaphase plate. • 3Anaphase I - homologous chromosomes, (not sister chromatids) separate, carried to opposite poles of cell.
Contributions to Variation • 1Independent assortment - tetrads arrange themselves randomly on metaphase plate. • Each homologous pair of chromosomes positioned independently of other pairs at metaphase I.
2Crossing over - homologous portions of 2 nonsister chromatids trade places. • Crossing over begins early in prophase I as homologous chromosomes pair up gene by gene.
3Random 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)