Chapter 10 Meiosis

Asexual Reproduction Asexual Reproduction: one parent passes a duplicate of its genetic information to its offspring, which can only be genetically identical clones of the parent Clone of the parent Example: Bacteria

Sexual Reproduction Each parent contributes one gene for each trait –Genes: unit of information for inheritable trait –passed from parent to offspring –Genes for each trait come in slightly different forms called alleles, originally produced by mutations –Meiosis shuffles the alleles during gamete formation, and fertilization produces offspring with unique combinations of alleles

Continue… The variation generate by sexual reproduction is the testing ground for natural selection and is the basis for evolutionary change

How Meiosis Halves the Chromosome Number? Meiosis begins with diploid (2n=46 in humans) germ cells can produce haploid gametes (n=23) –In 2n cells there are two chromosomes of each type called homologous chromosomes –Homologous chromosome pairing up one from each parent except sex cells –Homologous chromosomes line up (even unequally matched sex chromosome) during meiosis

Continue… Autosome: all cells except sex cells Sex Chromosomes: determine the sex of an organisms –Humans (23 rd pair) –Males XY and Female XX Meiosis produces gametes that have one of each pair of homologous chromosomes

Two Division, Not One In some ways, meiosis resembles mitosis The chromosomes are duplicated during interphase to form sister chromatids held together at the centromere Chromosomes are moved by the microtubules of the spindle fibers

Continue… Meiosis has two division (meiosis I and meiosis II) –During meiosis I, homologous chromosomes pair and cytokinesis follows Each of the daughter cells receives a haploid number of chromosomes Each chromosome is still duplicated –In meiosis II does not go through interphase –Four haploid cells are produced

Crossing Over in Prophase I Homologous chromosomes pair up (synapsis) –Nonsister chromatids exchange segments in a process called crossing over –Because alleles for the same trait can vary, new combinations of genes in each chromosome can result; this is one source of genetic variation –After crossing over, the nonsister chromaids begin to partially separate but remain attached by chiasmata Figure10.5Figure10.5

Metaphase I aligns During metaphase I, homologous chromosomes randomly line up at the spindle fibers equator During anaphase I, homologous chromosomes (still duplicated) separated into two haploid cells each of which has a random mix of maternal and paternal chromosomes

From Gametes to Offspring Plants Germ cells within plant tissues produce haploid spores by meiosis –Each spore undergoes meiosis to produce a haploid spores by meiosis –Fertilization results in a diploid sporophyte (example: Pine tree)

Gamete Formation Animals The life cycle of mulitcelled animals proceeds from meiosis to gamete formation  fertilization  growth by mitosis

Gamete Formation in Males In males, meiosis and gamete formation are called spermatogenesis –Germ cells (2n)  primary spermatocyte (2n)  Meiosis I  two secondary spermatocytes (n)  meiosis II  four spermatids (n) –Spermatids change in form; each develops a tail to become mature sperm.

Female Gamete Formation In females, meiosis and gamete formation are called oogenesis –Germ cells (2n)  primary oocyte (2n)  Meiosis I  secondary oocyte (n, and large in size) plus polar body (n and small in size)  Meiosis II  one large ovum (n), plus three polar bodies (n, small) (dissolved back in the body) –The single ovum is the only cell capable of being fertilized by a sperm, the polar bodies wither and die

Gene Shufflings at Fertilization The diploid chromosome number is restored at fertilization when two very different gamete nuclei fuse to form the zygote

Variation 1- Crossing over occurs during prophase I 2- Random alignment at metaphase I lead to millions of combinations of maternal and paternal chromosomes in each gamete 3- Of all the genetically diverse gametes produced, chance will determine which two will determine which two will meet

Meiosis and Mitosis Look at drawing