BIO 101 1st EditionLecture 17Outline of Last LectureI. Cell divisiona. Genetic materialII. Cell Cyclea. Interphaseb. Mitosisi. Prophaseii. Prometaphaseiii. Metaphaseiv. Anaphasev. Telophasec. CytokinesisIII. Cell Division Controlsa. CheckpointsOutline of Current LectureI. Reproductiona. Asexualb. Sexuali. Meiosis1. Meiosis I2. Meiosis IIii. FertilizationII. Sources of Genetic Variationa. Independent Assortment of Chromosomesb. Crossing Overc. Random FertilizationChapter 13- Meiosis and Sexual Life Cycles2 kinds of reproduction in living things1. Asexual Reproduction- single individual is sole parent and passes all of its genetic information to offspring. Occurs basically by cell division (process we just covered in Chapter 12). Offspring are GENETICALLY IDENTICAL to the parent and to all other offspring of that parent 2. Sexual Reproduction- 2 parents give rise to offspring with a unique combination of genes. Sexually produced offspring differ genetically from their parents and from other offspringSexual Reproduction includes 2 Major Events:1. Meiosis2. FertilizationTo understand sexual reproduction, we need to look more closely at chromosomes.- Every organism has a characteristic number of chromosomesBIO 101 1st Edition- All the cells of the organism have this characteristic number of chromosomes EXCEPT for the SEX CELLS=GAMETES=EGGS AND SPERM- The gametes (egg and sperm) have exactly ½ that number- Number of chromosomes in body cells (somatic cells) is called the DIPLOID number= 2N number- Number of chromosomes in sex cells (gametes) is called the HAPLOID number= 1N number Examples: 2N (# in body cells) 1N (# in gametes)Mosquito 6 3Corn 20 10Dogs 78 39Humans 46 23- Thus there are twice as many chromosomes in DIPLOID (2N) body cells as in HAPLOID (1N) gamete cells- That is because chromosomes in DIPLOID CELLS come in PAIRS- each chromosome has a PARTNER- The pairs of chromosomes in 2N cells are called HOMOLOGS or HOMOLOGOUS PAIRSo The chromosomes in a pair are similar to one another in SIZE, SHAPE, and GENETIC INFORMATIONo One comes from the MOTHER (maternal homolog)o The other comes from the FATHER (paternal homolog)- Gametes have ½ as many chromosomes as body cells. But not just any half: **Gametes have ONE of each homologous PAIR**- Meiosis- a special type of cell division that produces haploid (1N) cells starting from special DIPLOID cells called GERMLINE CELLs. In humans, meiosis occurs in ovaries of female and in testes of maleSexual Life Cycle in HumansFertilization= fusion of haploid gametes to form a diploid zygoteMeiosis- a special kind of cell division to make gametes- Many steps in common with cell division (mitosis)- Uses much of same cell machinery- Occurs in 2 stages: MEIOSIS I separates homologous pairs MEIOSIS II separates sister chromatidsMeiosis I- first the chromosomes replicates (just as they do prior to mitosis)Prophase- replicated homologs pair up *cross over occurs between homologsMetaphase- replicated homologs line up in pairs at metaphase plate (remember that in mitosis the chromosomes line up single file- NOT in pairs)Anaphase- replicated homologs pulled to opposite ends of cellTelophase- nucleus divides, cytokinesis occursBIO 101 1st EditionAt the end of Meiosis I there are 2 cells, each with one of each homologous pair of chromosomes- it is a replicated chromosomeMeiosis II- each of these 2 cells from meiosis I divides again, this time it is like mitosis- separates sister chromatids.Ex: human germline allOverview: Cell Division vs. Gamete FormationCell Division: Mitosis Gamete Formation: MeiosisStarting cell may be either 1N or 2N Only 2N cells special germ-line cells1 division 2 cells 2 divisions 4 cellsDaughter cells have SAME number of chromosomes as the parent cellDaughter cells have ½ the number of chromosomes as the parent (ONE of eachhomologous pairDaughter cells are genetically identical Daughter cells have NEW genetic combinationsSources of Genetic Variation- Main advantage of sexual reproduction is that it provides GENETIC VARIATION= each individual is unique- Genetic variation is important because it is the raw material for natural selection (not all individuals are the same- only the best suited survive and reproduce)- Meiosis and fertilization are the primary sources of genetic variation due to sorting and recombining of chromosomes- Three major sources of variation:1. Independent Assortment of ChromosomesBIO 101 1st Edition- Chromosomes come in homologous pairs (paternal and maternal)- In Meiosis I metaphase these homologs line up at the metaphase plate IN PAIRS- Each pair aligns RANDOMLY (paternal homolog on side, maternal on the other)- Each pair orients independently of the other pairs (i.e. they don’t all line up the same- it is a RANDOM PROCESS) INDEPENDENT ASSORTMENT- Therefore, at the end of Meiosis I, when the cell divides to give 2 cells, each will have ONLY ONE of each homologous PAIR of chromosomes and there are all possible combinations of the PATERNAL and MATERNAL homologs- The number of possible combinations of paternal and maternalhomologs in the gametes depends on the number of chromosome pairs- Number of possible combinations= 2^N where N=haploid number- Examples:N # possible combinations of homologs in gametes1 2^1= 22 2^2=43 2^3=2x2x2=84 2^4=2x2x2x2=1623 2^23= 8x10^6Example- germline cell with 2N=4 chromosomes- Why is independent assortment of maternal and paternal homologs important?- Because maternal and paternal homologs may carry different alleles at many corresponding loci- Allele= alternative form of geneBIO 101 1st EditionEx: gene locus caries code for protein involved in NOSE SIZEAlleles big nose or cute little nose2. Crossing Over- - During prophase of Meiosis I, the homologous pairs of chromosomes come into gene-for-gene contact= SYNAPSIS- During synapsis, the paternal and maternal chromosomes often exchange pieces of DNA= CROSSING OVER- Results in immense genetic scrambling- Each homolog is a random mix of paternal and maternal genes 3. Random Fertilization- Human egg (female gamete) is 1 of 8 million possible combinations of paternal and maternal homologs- Will be fertilized by a sperm (male gamete) that is also 1 of 8 million possible combinations of homologs- Resulting ZYGOTE (new little baby) is 1 of 64 trillion DIPLOID