MEIOSIS, THE BASIS OF SEXUAL REPRODUCTION Why do kids look different from the parents? How are they similar to their parents? Why aren t brothers or sisters more alike?
Meiosis A process where the number of chromosomes in each cell is cut in half to produce the sex cells -- egg & sperm A Reduction - Division because the # is reduced before cells divide
Why do we need meiosis? To reduce the number of chromosomes going into the sex cells (gametes) from 46 to 23 Why reduce the chromosomes in gametes? At fertilization the male and female sex cells will provide ½ of the chromosomes each so the offspring has genes from both parents
Homologous Chromosomes maternal (mother) and paternal (father) in origin and carry info for the same traits, but not always identical info [ex. both have the trait for eye color but, one carries brown eye color, the other blue] all cells, except reproductive cells (gametes) have homologous chromosomes these chromosomes are like matching pairs from opposite sex partners
Each color is a gene Pair of homologous chromosomes Centromere Sister chromatids Homologous chromosomes: matching pairs of chromosomes Karyotype
somatic cell a normal body cell 46 chromosomes in a human sex cell or gamete or reproductive cell Only found in testes and ovaries 1/2 the amount = 23 chromosomes.
Gametes and the Life Cycle of a Sexual Organism The life cycle of a multi-cellular organism is the sequence of stages leading from the adults of one generation to the adults of the next Meiosis Haploid gametes (n = 23 Chromosomes) Egg cell Sperm cell Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46 Chromosomes) Fertilization Mitosis and development
Diploid: means having 2 bits of info for every trait in your body (one from each parent) All somatic (body) cells are diploid 46 chromosomes 2n Haploid: means having 1 bit of info or half the diploid number Sperm and Egg (gametes or reproductive cells) are haploid 23 chromosomes n
Sexual Reproduction = the process involving the joining of egg & sperm to produce offspring Fertilization = the fusion of sperm and egg (when they actually join) Zygote = fertilized egg --diploid (2n) Gametogenesis = is the making of gametes (egg/ sperm in the gonads Gonads = sex organs (testes in males, ovaries in females)
Meiotic Divisions: 1 Chromosomes 2 duplicate Homologous chromosomes separate 3 Sister chromatids separate Homologous pair of chromosomes in diploid parent cell Homologous pair of duplicated chromosomes Sister chromatids Interphase before meiosis Meiosis I Meiosis II Figure 8.14
Meiosis and genetic variation Two events happen which increase genetic variation: 1. Random segregation (independent assortment) of chromosomes (how the chromosomes line up) 2. Crossing over This is why we don t look exactly like our parents or each other
Meiosis and genetic variation Random segregation or Independent Assortment During metaphase 1 homologous chromosomes arrive at the equator, arranged randomly by the spindle fibers
Independent Assortment of Chromosomes In independent assortment every chromosome pair orients independently of the others during meiosis Possibility 1 Possibility 2 Metaphase of meiosis I Metaphase of meiosis II Gametes Combination a Combination b Combination c Combination d
Meiosis and genetic variation Random segregation Human cells have 23 chromosomes so there are 2 23 different possible ways the chromosomes can segregate! That is 8,388,608 different combinations of chromosomes
Meiosis and genetic variation Crossing over During prophase 1 homologous chromosomes come together in pairs, this is called synapsis. They form a Tetrad & they exchange genetic info
Crossing Over Crossing over Homologous chromosomes exchange genetic information (trade pieces) Genetic recombination occurs (new combinations & variations produced) Prophase I of meiosis Metaphase I Metaphase II Gametes Tetrad- 4 chromosomes Chiasma, site of crossing over Recombinant chromosomes Spindle microtubules Figure 8.18
Locus (loci) = place on a chromosome where a gene is found
There s another source of variation During fertilization any male gamete can join with any female gamete, its purely random--- chance, so there s thousands of millions of new combinations possible.
Meiosis Parent cell chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division produces 4 gamete cells with ½ the original no. of chromosomes
Meiosis mouse testes Parent cell 1 st division 2 nd division 4 gametes
Meiosis Cell division that is vital for sexual reproduction Takes place in reproductive organs Results in formation of haploid gametes The four daughter cells are not genetically identical
Gamete Formation In males, meiosis results in 4 sperm cells In females, meiosis results in 1 egg cell and three polar bodies, which are not used in reproduction.
Mitosis vs Meiosis Results in Cells are Occurs in Mitosis 2 Diploid Cells (2N) Genetically Identical Somatic (Body) Cells Meiosis 4 Haploid Cells (N) Genetically Different Sex Cells