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.

http://www.mssm.edu/genetics/fabry/images/figure1b.gif

• 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.

http://library.thinkquest.org/19037/media/basics/gene.GIF

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

http://www.cybered.net/library/Teaching_Resources/Biology/Evolution/Image_Gallery/Evolution-Loci.jpg

• 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.

http://www.brooklyn.cuny.edu/bc/ahp/LAD/C9/graphics/C9_homologous_2.GIF

• 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.

http://faculty.sulross.edu/ericsson/img0067b.jpg

• 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.

http://ghr.nlm.nih.gov/handbook/illustrations/mitosismeiosis

Other differences

• 1Crossing over: During prophase I, homologous chromosomes pair up (synapsis).

• At X-shaped regions (chiasmata) sections of nonsister chromatids exchanged.

http://www.synapses.co.uk/genetics/crosso1.gif

• 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)

http://www.itech.pjc.edu/fduncan/bsc1094/ap2c29ppt_files/slide0007_image008.jpg