Meiosis and Sexual Life Cycles

Transmission of Traits Based on heredity, where specific genes are passed on to offspring via chromosomes (or DNA) Gametes transmit these chromosomes (or DNA) to the next generation in plants and animals

Asexual and Sexual Reproduction Asexual reproduction will result in offspring that are exact copies of the one parent produced via mitosis The ‘child’ would be a clone of the parent Sexual reproduction allows for genetic variability because the offspring is a combination of both parents Both of these have reproduced asexually

Inheritance and Cells Normal somatic cells in humans will have 23 pairs of chromosomes or 46 chromosomes 22 pairs or 44 chromosomes will be autosomes 1 pair or 2 will be sex chromosomes Can be XX or XY (normally) We inherit one chromosome in each pair from each parent Karyotype showing the homologous pairs of chromosomes in an individual

Diploid and Haploid We will receive 23 chromosomes from our mother and 23 from our father. Counting one of those sets (or 23 chromosomes in humans) is called n or haploid. Counting both of those sets is 2n or diploid In a cell that has completed S phase, the chromosome number has not changed; however, DNA has been replicated and each chromosome has a sister chromatid

Fertilization and Growth Sperm (n) will fertilize an egg (n) creating an offspring (2n) All cells in the body will be 2n (and undergo mitosis) except for the gametes which will be produced by germ cells in the gonads (and undergo meiosis)

Variation in Life Cycles Normal animal sexual life cycle (same as seen in humans)

Variation in Life Cycles Plants and some algae undergo Alternation of Generations Diploid and haploid stages are both multicellular Diploid stage is called Sporophyte Gives rise to a haploid spore which divides via mitosis Haploid stage is called gametophyte Gives rise to a gamete which fuses with another creating a diploid zygote

Variation in Life Cycles Occurs in most fungi and some protists (including some algae) Gametes fuse, forming a diploid zygote. Meiosis occurs producing haploid cells (not gametes) which then divide via mitosis Can give rise to either unicellular descendents or multicellular haploid adult organisms Further mitosis will give rise to gametes

Meiosis Begins with one diploid cell which goes through two stages (Meiosis I and Meiosis II) to end up with four genetically different haploid cells Begins similarly to mitosis with Interphase

Prophase I Chromosomes condense, homologs pair up gene by gene in synapsis, allowing crossing over to occur Synapsis ends in mid-prophase, allowing homologs to move apart slightly; homologs are still attached at chiasmata The remainder of prophase I occurs much like prophase in mitosis

Metaphase I Homologous chromosomes line up at the metaphase plate, one chromosome in each pair is facing each pole Sister chromatids remain together

Anaphase I Proteins responsible for attachment of homologous chromosomes breakdown Homologous chromosomes are pulled to opposite sides of the cell

Telophase I & Cytokinesis Two haploid cells form, but each chromosome still has an attached sister chromatid No DNA replication occurs prior to Meiosis II

Meiosis I

Prophase II Spindle forms and prophase progresses similarly to mitosis

Metaphase II Chromosomes line up on the metaphase plate All kinetochores attach to spindle fibers

Anaphase II Proteins holding sister chromatids together breakdown Sister chromatids move to opposite sides of the cell

Telophase II & Cytokinesis Nuclei form, chromosomes decondense and cytokinesis occurs One Meiotic division produces four genetically different haploid daughter cells

Meiosis II

Meiosis vs. Mitosis

Genetic Variation Genetic variation in meiosis arises via Independent assortment of chromosomes Crossing over Random fertilization