MEISOSIS & SEXUAL LIFE CYCLES

MEISOSIS & SEXUAL LIFE CYCLESCAMPBELL and REECE Chapter 13Definitions Heredity: transmission of traits from 1 generation to the nextVariation: differences between members of same speciesGenetics : scientific study of heredity & hereditary variationGenes: discrete unit of hereditary information consisting of specific nucleotide sequence in DNA or RNA (some viruses)Gametes: a haploid reproductive cell (ova, sperm)Somatic Cells: any cell in multicellular organism except ova or spermLocus: specific place along length of a chromosome where a given gene is locationInheritance of Genesgenetic program written in language of DNADNA: the polymer of 4 nucleotidesmost genes program cells to synthesize specific enzymes & proteins whose cumulative actions produces organisms inherited traitsGametesvehicle used by plants & animals to transmit genes to next generationmale + female gamete unite during fertilization

Asexual Reproductionoffspring are identical genetically to parentused by unicellular & some multicellular eukaryotic organismscells of offspring derived by mitosisaka clonesmutations can occur in asexual reproduction

Sexual Reproduction2 parents give rise to offspring that are genetically different from either parent

Life Cyclesis the generation-to-generation sequence of stages in reproductive history of an organism

Human Chromosomes46 (23 pair) (2n)22 pair autosomes1 pair sex chromosomes condense during mitosis so large enough to separate based on:sizeposition of centromerepatterns of colored banding produced by stainsKaryotype ordered display of chromosome pairs of a cell, arranged by size & shapepairs of matching chromosomes have same genes @ same loci & are called homologous chromosomes or homologs

Karyotype : Human

Not homologousdetermine individuals sex:XXXYSex ChromosomesDiploid # / Haploid #Diploid #Haploid #2 sets of chromosomes in cell: 1 set maternal source; 1 set paternal sourceHumans = 46

Gametes have 1 set of chromosomesHumans = 23Behavior of Chromosome Sets in Human Life Cyclehaploid sperm + haploid ova fertilize fusion of their 2 nucleifertilized egg called a zygote (diploid)thru mitosis this fertilized egg will produce all somatic cells in mature adultonly cells not produced by mitosis are the gametes which develop from germ cells in gonads (ovary or testes)

Meiosis creates 1n gametes so when fertilization occurs 2n offspringhuman life cycle typical of most sexually reproducing animals, plants, fungi, & protisitsVariations of Sexual Life Cyclesall sexual reproduction involves meiosis & fertilization but there is variation in timing 1. gametes are the only haploid cells humans & most other animalsVariations of Sexual Life Cycles2. Alternation of Generationsplants & some species of algaeinvolves both diploid & haploid multicellular stagesSporophyte: multicellular diploid stateSpores: haploid cells produced by sporophyte, divides by mitosis Gametophyte: haploid multicellular stage that produces gametes (1n) which will fertilize producing 2n sporophyte

Variations of Sexual Life Cycles3. Haploid Cells Predominantoccurs in most fungi & some protistshaploid gametes fuse & form diploid zygote meiosis produces haploid cells (not gametes) which then either divided by mitosis forming multicellular haploid organisms or unicellular descendants

Stages of Meiosis

Meiosis Istarts with diploid cell with duplicated chromosomes

ends with separation of homologous chromosomes in 2 daughter cells which are each haploid but with duplicated chromosomes (reductional division)

Prophase Ichromosomes condense*homologs align along their lengthspaired homologs become physically connected to each other along their lengths by a zipper-like protein structure: synaptonemal complexthis state called synapsisProphase I continuedCrossing Over: genetic rearrangement between nonsister chromosomes involves exchange of segments of DNAChiasma: exists wherever cross over has occurredspindle forms, nuclear envelope fragments, (as in prophase in mitosis)Prophase I

Metaphase Ipairs of homologous chromosomes arranged @ metaphase plateboth chromatids of 1 homolog attached to kinetochore microtubule from 1 pole; those of other homolog attached to kinetochore from other poleMetaphase I

Anaphase Iproteins that hold sister chromatids together breakdown allowing homologs to separatehomologs move toward opposite poles guided by spindlessister chromatid cohesion persists at the centromere so duplicated chrtomosomes move to opposite polesAnaphase I

Telophase I & Cytokinesis Ieach of cell has complete haploid set of duplicated chromosomescytokinesis usually forms simultaneously w/telophaseforms 2 haploid daughter cells in some species, chromosomes decondense & nuclear envelope reformsNo chromosome duplication occurs between Meiosis I & II

Meiosis IIstarts with 2 haploid cells (with duplicated chromosomes)sister chromatids separateends with 4 haploid daughter cells each genetically different than parent cellProphase IIspindle apparatus forms

Metaphase IIchromosomes positioned @ metaphase plate2 sister chromatids no longer genetically identical (due to crossing over)Kinetochores of sister chromatids attached to microtubules extending from opposite polesMetaphase II

Anaphase IIbreakdown of proteins holding the sister chromatids together @ centromere allows chromatids to separatechromatids move toward opposite poles as individual chromosomesAnaphase II

Telophase II & Cytokinesis IInuclei formchromosomes condensecytokinesis occurs4 daughter cellseach with haploid set of unduplicated chromosomes4 daughter cells are genetically distinct from each other & from parent cell

Telophase II & Cytokinesis II

Meiosis

Mitosis / Meiosis

Origins of Genetic Variation Independent Assortment of ChromosomesCrossing OverRandom FertilizationIndependent Assortmentorientation of pairs of homologous chromosomes at metaphase I is randomeach pair may orient with either its maternal or paternal homolog closer to a given pole50% chance that a particular daughter cell of meiosis I will contain the maternal chromosome of a certain homolog (50% chance paternal)Independent Assortmentbecause humans have n = 23 the # of possibilities due to independent assortment is 2 or about 8.4 million possible combinations of maternal/paternal chromosomes in any 1 gameteCrossing Overproduces recombinant chromosomes: individual chromosomes that carry genes derived from 2 different parentsin humans: average of 1 to 3 crossover events occur per chromosome pairdepends on size of chromosome & position of centromereCrossing Overhttp://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/meiosis_with_crossing_over.htmlhttp://www.hhmi.org/biointeractive/media/meiosis-lg.mov

Random Fertilizationeach sperm or egg represents 1/84,000,000 possible combinationszygote represents 1/70 trillion diploid combinations possible for these 2 parentsEvolutionary Significance of Genetic Variationability of sexual reproduction to generate genetic diversity is considered 1 of most plausible explanations for evolutionary persistence of this processgenetic diversity very important in changing environment

Stable Environments favor Asexual Reproductionmore advantageous because it ensures perpetuation of successful combinations of allelesasexual reproduction less costly to organism in terms of nrg costsAsexually Reproducing Animalsonly reproduce asexuallyraremicroscopic bdelloid rotifer~400 species living in variety of environments

Bdelloid Rotiferstheir existence does not disprove that sexual reproduction has evolutionary advantagesthere are other mechanisms the rotifers use to increase their genetic variationif environment becomes very dry they form state of suspended animation during which rotifers exchange DNA with neighbors