2What Cell Reproduction Accomplishes Production of new cells“Daughter” cells are identical to each other and to their “Parent”Chromosomes are duplicated and divided among the two daughtersA few genes are found in DNA in mitochondria and chloroplastsReplaces damaged or lost cellsPermits growthIncrease in the number of cellsAllows for reproductionSingle-celled organisms – simple cell divisionMulti-celled organisms – asexual or sexual
3Sexual & Asexual Reproduction Sea stars can grow new individuals from fragmented piecesGrowing a new plant from a clippingGrowth and maintenance of multicellular organismsthe lone parent and its offspring have identical genesSexual ReproductionTwo parents, each produces gametes (e.g. egg and sperm)Offspring are a combination of the two genomes
4Chromosomes most genes are located on chromosomes in the cell nucleus* #/cell depends on the speciesChromosomes aremade of chromatin, fibers composed of roughly equal amounts of DNA and protein molecules andnot visible in a cell until cell division occursDNA in a cell is packed into an elaborate, multilevel system of coiling and foldingHistones are proteins used to package DNA in eukaryotesNucleosomes consist of DNA wound around histone molecules
5Chromosome Duplication results in two copies called sister chromatids containing identical genesTwo sister chromatids are joined together tightly at a narrow “waist” called the centromereWhen the cell divides, the sister chromatids of a duplicated chromosome separate from each otherOnce separated, each chromatid isconsidered a full-fledged chromosome andidentical to the original chromosome
6Cell Cycle A cell cycle is the ordered sequence of events that extend from the time a cell is first formed from a dividing parent cellto its own division into two cellsconsists of two distinct phases:interphase andthe mitotic phaseMost of a cell cycle is spent in interphaseperforms its normal functions,doubles everything in its cytoplasm, andgrows in size.
7Mitosis Mitosis consists of four distinct phases: Figure 8.7, p 126 Prophase:Chromosome duplication and thickening;Mitotic Spindle Forms: football shaped structure of microtubules, guides the separation of two sets of daughter chromosomesMetaphase: chromosomes line up along a central pointAnaphase: sister chromatids separate and move toward opposite polesTelophase: nuclear membranes re-form; chromosomes uncoil
8Cytokinesis Generation of two daughter cells Animal cells Plant cells begins during telophasedivides the cytoplasmdifferent in plant and animal cellsAnimal cellsknown as cleavagebegins with the appearance of a cleavage furrow, an indentation at the equator of the cellPlant cellsvesicles containing cell wall material collect at the middle of the cellthey fuse, forming a membranous disk called the cell plate
9Cancer Cells: Growing Out of Control Normal plant and animal cells have a cell cycle control system that consists of specialized proteins, which send “stop” and “go-ahead” signals at certain key points during the cell cycleCancera disease of the cell cycleInvolves mutations of one or more regulatory sitesCancerous cellsCan form (solid) tumors or tumorsBenign: cells remain at their original site; grow slowlyMalignant: grow rapidly and often spread from their initial site through metastasisInterrupt or interfere with normal body functions
10Cancer Treatment May involve radiation therapy, which damages DNA and disrupts cell divisionchemotherapy, the use of drugs to disrupt cell division or target specific mutant genesSurgery to remove the tumor
11Meiosis Sexual Reproduction Homologous Chromosomes Depends on meiosis offspring contain a unique combination of genes from the parentsAll individuals of a single species have the same # and type of chromosomesHomologous Chromosomesmatching pairs of chromosomescan possess different versions (alleles) of the same genesHuman somatic cell has 46 chromosomes, 23 pairs22 pairs of autosomes and 1 pair of sex chromosomesXX or XY
12Life Cycle of Sexual Organisms Figure 8.12, page 131the sequence of stages leading from the adults of one generation to the adults of the nextFertilization results in a new zygotic cell that has one set of chromosomes from each parentOffspring are unique, and not identical to either parentMitosis and cellular differentiation allow the zygote to grow into an adult
13Meiosis Halves the Chromosome Number Figure 8.13 Page 131; Figure 8.14 pageSomatic (body) cells are diploid: contain two sets, or one pair of each of the chromosomes; undergo mitosisReproductive cells undergo meiosis produce gametes that have only one chromosome of each typeInterphaseEach chromosome in a diploid cell is duplicated; 4 copies of each chromosomeMeiosis IHomologous chromosomes separateResults in return to 2x statusMeiosis IICells divide againEach gamete 4 gametes, each with 1 copy of each chromosomeIn humans: 22 autosomes and one sex chromosome
14Genetic VariationOffspring of sexual reproduction are genetically different from their parents and one anotherIndependent Assortment of ChromosomesWhen aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chanceEvery chromosome pair orients independently of all of the others at metaphase IFor any species, the total number of chromosome combinations that can appear in the gametes due to independent assortment is 2n, where n is the haploid number.Humans: n=23; 8,388,608 different chromosome combinations possible in a gamete
15Random FertilizationA human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygoteIf each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 × 8,388,608, or more than 70 trillion different possible chromosome combinationsCrossing Overnonsister chromatids of homologous chromosomes exchange corresponding segments andgenetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs.
16When Meiosis Goes Awrymistakes can result in genetic abnormalities that range from mild to severeErrors may alter chromosome numberNondisjunctionMembers of a chromosome pair fail to separate at anaphasegametes have an incorrect number of chromosomes: 2n+1Down Syndrome: An Extra Chromosome 21
17Evolution Connection: The advantages of sex Asexual reproduction conveys an evolutionary advantage when plants aresparsely distributed and unlikely to be able to exchange pollen orsuperbly suited to a stable environment.Asexual reproduction also eliminates the need to expend energyforming gametes andcopulating with a partner.Sexual reproduction may convey an evolutionary advantage byspeeding adaptation to a changing environment orallowing a population to more easily rid itself of harmful genes