BC Science 9: p

Sexual Reproduction  Meiosis is an important aspect of sexual reproduction.  Sexual reproduction, through the shuffling of DNA, produces genetic diversity.  Variation within the offspring may produce individuals that have an advantage over other individuals.

Role of Gametes  Normal body cells have a diploid number of chromosomes (ie. in pairs). Eg. A human sperm cell (23) fuses with an egg cell (23), fertilization takes place forming a zygote (46).  The zygote undergoes repeated mitosis to form an embryo that will develop until it becomes mature.  As sexual maturity is reached, gametes (ie. either sperm or egg cells) will begin to be produced.  Gametes are haploid (ie. half as many chromosomes).

Meiosis  Meiosis is the process of producing gametes.  Meiosis is split into Meiosis I and Meiosis II.

Meiosis I  Matching chromosome pairs (ie. homologous chromosomes) move to opposite poles of the cell.  The end result is two diploid daughter cells.

Meiosis II  Chromatids of each chromosome are pulled apart – the end result is four haploid cells.  Each cell has half the number of chromosomes (ie. haploid).  Four cells are produced that are genetically different from one another.

Crossing Over  In meiosis I, chromatids of chromosome pairs can cross over each other.  During the crossing over, DNA segments are exchanged.  This process increases genetic possibilities and produces more variation.

Independent Assortment  The pairs of chromosomes separate independently during meiosis I.  This allows for many different combinations of chromosomes in the daughter cells.  This increases genetic variation between the daughter cells that are produced.

Gamete Formation in Males and Females  In males, all four cells that result from meiosis will develop into a sperm cell.  In females, one cell gets most of the cytoplasm and becomes the egg.

Mutations During Meiosis  Chromosome mutations can occur spontaneously. Parts of chromosomes may be inverted, deleted, duplicated or moved to another spot.  Chromosome mutations can occur because of mutagens (eg. radiation). This can lead to genetic diseases or death.

Genetic Disorders  Failed separation of chromosomes in meiosis has serious consequences. A gamete may end up with no chromosomes, or have too many of a chromosome. Zygotes that result from these gametes rarely survive. If the zygote does survive it will likely have a serious genetic disorder.

Karyotypes  The chromosomes of an individual can be studied using a karyotype. Geneticists can diagnose genetic disorders or syndromes by viewing a karyotype. Eg. Down syndrome usually occurs when there is an extra twenty- first chromosome.