Chapter 13: Meiosis & Sexual Life Cycles Genetics & Inheritance Genes- hereditary units Genes- hereditary units program specific traits in the offspring.

Chapter 13: Meiosis & Chapter 13: Meiosis & Sexual Life CyclesSexual Life Cycles

Genetics & InheritanceGenetics & InheritanceGenes- hereditary unitsGenes- hereditary units

program specific traits in the program specific traits in the offspringoffspring

Most program cells to synthesize specific Most program cells to synthesize specific enzymes & proteinsenzymes & proteinsCumulative action of proteins produces Cumulative action of proteins produces traitstraits

Specific gene location on a Specific gene location on a chromosome = locuschromosome = locus

Gametes- reproductive cells which Gametes- reproductive cells which transmit genes between transmit genes between generationsgenerations

Asexual vs. Sexual Asexual vs. Sexual ReproductionReproduction

Asexual ReproductionAsexual Reproduction All offspring’s genes originate from one All offspring’s genes originate from one

parentparent New offspring produced through mitosisNew offspring produced through mitosis

Sexual ReproductionSexual Reproduction Offspring’s genes originate from two Offspring’s genes originate from two

parentsparents Unique combinations of genes can be Unique combinations of genes can be

made so that each offspring varies from made so that each offspring varies from its siblings (exception- identical twins)its siblings (exception- identical twins)

Fertilization & Meiosis Fertilization & Meiosis alternate life cyclesalternate life cycles

Life cycleLife cycle Generation-to generation sequence of stages in Generation-to generation sequence of stages in

an organism’s life from conception to an organism’s life from conception to reproductionreproduction

Example organism: HumansExample organism: Humans Somatic cell (body cell) = 46 chromosomesSomatic cell (body cell) = 46 chromosomes Homologous chromosomes = carry genes Homologous chromosomes = carry genes

controlling the same inherited trait controlling the same inherited trait 22 pairs of autosomes22 pairs of autosomes 1 pair of sex chromosomes1 pair of sex chromosomes Diploid cells = have all chromosome sets (2n)Diploid cells = have all chromosome sets (2n) Haploid cells = have one chromosome from Haploid cells = have one chromosome from

each set (n); egg or sperm cellseach set (n); egg or sperm cells

Fertilization & Meiosis Fertilization & Meiosis alternate life cyclesalternate life cycles

FertilizationFertilization Fusion of egg & sperm nuclei = zygote (2n)Fusion of egg & sperm nuclei = zygote (2n)

MitosisMitosis Used to develop zygote into sexually mature Used to develop zygote into sexually mature

adult; Forms all somatic cells from zygoteadult; Forms all somatic cells from zygote MeiosisMeiosis

Occurs in gonadsOccurs in gonads Reduces diploid number to half Reduces diploid number to half Creates sperm & eggCreates sperm & egg Maintains diploid number for organisms; when Maintains diploid number for organisms; when

fertilization occurs the diploid number is fertilization occurs the diploid number is restoredrestored

Sexual Life CyclesSexual Life Cycles Alternation of fertilization & meiosis Alternation of fertilization & meiosis

is characteristic of all sexually is characteristic of all sexually reproducing organismsreproducing organisms

Timing of fertilization & meiosis in Timing of fertilization & meiosis in the life cycle varies between the life cycle varies between different speciesdifferent species

Three types of sexual life Three types of sexual life cycles:cycles:

AnimalsAnimals Gametes are only n cellsGametes are only n cells Gametes do not divide before Gametes do not divide before

fertilizationfertilization 2n zygote divides by mitosis after 2n zygote divides by mitosis after

fertilizationfertilization

Three types of sexual life Three types of sexual life cyclescycles

Alternation of generationsAlternation of generations Plants & some algaePlants & some algae Has n & 2n multi-cellular stagesHas n & 2n multi-cellular stages 2n multi-cellular stage = sporophyte2n multi-cellular stage = sporophyte

Formed by ferilization of gametesFormed by ferilization of gametes Makes spores by meiosisMakes spores by meiosis

n multi-cellular stage = gametophyten multi-cellular stage = gametophyte Formed by mitosis of sporesFormed by mitosis of spores Makes gametes by mitosisMakes gametes by mitosis

Three types of sexual life Three types of sexual life cyclescycles

Most fungi & some protistsMost fungi & some protists Diploid zygote forms & immediately Diploid zygote forms & immediately

goes through meiosis forming haploid goes through meiosis forming haploid cellscells

Haploid cells formed during meiosis Haploid cells formed during meiosis divide by mitosis to form a multi-cellular divide by mitosis to form a multi-cellular n adult organismn adult organism

n adult forms gametes through mitosisn adult forms gametes through mitosis Only 2n cell is the zygoteOnly 2n cell is the zygote

Concept CheckConcept Check How does the karyotype of a human How does the karyotype of a human

male differ from that of a human male differ from that of a human female?female?

How does the alternation of meiosis & How does the alternation of meiosis & fertilization in life cycles maintain the fertilization in life cycles maintain the normal chromosome number for that normal chromosome number for that species?species?

What process mitosis or meiosis is more What process mitosis or meiosis is more directly involved in gamete production:directly involved in gamete production: In animals?In animals? In plants & most fungi?In plants & most fungi?

MeiosisMeiosis Has two Has two

consecutive consecutive cell divisionscell divisions

Results in 4 Results in 4 haploid haploid daughter cellsdaughter cells

Stages of Meiosis: Meiosis IStages of Meiosis: Meiosis I Prophase IProphase I

90% of meiosis spent in this phase90% of meiosis spent in this phase Chromosomes condenseChromosomes condense Crossing over may occur between Crossing over may occur between

homologous chromosomeshomologous chromosomes Synapsis occurs holding homologues Synapsis occurs holding homologues

togethertogether Tetrads formTetrads form Nucleoli disperse; nuclear envelope Nucleoli disperse; nuclear envelope

fragments; formation of spindlesfragments; formation of spindles Metaphase IMetaphase I

Tetrads arrange on metaphase plateTetrads arrange on metaphase plate

Stages of Meiosis: Meiosis IStages of Meiosis: Meiosis I

Anaphase IAnaphase I Chromosomes move toward poles (sister Chromosomes move toward poles (sister

chromatids still attached)chromatids still attached) Telophase I & cytokinesisTelophase I & cytokinesis

In some species nuclear envelope & In some species nuclear envelope & nucleoli reformnucleoli reform

Cleavage furrow (animals) or cell plate Cleavage furrow (animals) or cell plate (plants) forms(plants) forms

Stages of Meiosis: Meiosis IIStages of Meiosis: Meiosis II No replication of chromosomes No replication of chromosomes

between meiosis I & meiosis IIbetween meiosis I & meiosis II Prophase IIProphase II

Spindle apparatus formsSpindle apparatus forms Metaphase IIMetaphase II

Chromosomes position of metaphase plateChromosomes position of metaphase plate Anaphase IIAnaphase II

Sister chromatids move toward opposite Sister chromatids move toward opposite polespoles

Telophase II & CytokinesisTelophase II & Cytokinesis Nuclei form; chromosomes de-condenseNuclei form; chromosomes de-condense Cleavage furrow or cell plate forms Cleavage furrow or cell plate forms

Comparison of Mitosis & Comparison of Mitosis & MeiosisMeiosis

Meiosis reduces chromosome number Meiosis reduces chromosome number by half; mitosis conserves by half; mitosis conserves chromosome numberchromosome number

Synapsis, crossing over, & tetrad Synapsis, crossing over, & tetrad formation occur in prophase I of formation occur in prophase I of meiosismeiosis

Tetrads take position on metaphase Tetrads take position on metaphase plate in metaphase I of meiosisplate in metaphase I of meiosis

Homologues separate during anaphase Homologues separate during anaphase I of meiosis; sister chromatids remain I of meiosis; sister chromatids remain attached during meiosis Iattached during meiosis I

Concept CheckConcept Check Explain how mitosis conserves Explain how mitosis conserves

chromosome number, while meiosis chromosome number, while meiosis reduces the chromosome number by reduces the chromosome number by half.half.

Compare the chromosomes present Compare the chromosomes present in metaphase of mitosis to those in in metaphase of mitosis to those in metaphase II of meiosis. metaphase II of meiosis.

Sexual Reproduction & Sexual Reproduction & evolutionevolution

Sexual reproduction produces Sexual reproduction produces genetic variation which can genetic variation which can contribute to evolutioncontribute to evolution

Genetic variationGenetic variation MutationsMutations Independent assortmentIndependent assortment Crossing overCrossing over Random fertilizationRandom fertilization

Genetic Variation: independent Genetic Variation: independent assortmentassortment

Random orientation of homologues Random orientation of homologues during metaphase I leads to unique during metaphase I leads to unique combinations of maternal & paternal combinations of maternal & paternal chromosomes in the egg & spermchromosomes in the egg & sperm

Genetic variation: crossing Genetic variation: crossing overover

Allows genes to exchanged between Allows genes to exchanged between maternal & paternal chromosomes maternal & paternal chromosomes creating unique chromosomescreating unique chromosomes

Genetic Variation: random Genetic Variation: random fertilizationfertilization

Each egg & sperm cell is uniqueEach egg & sperm cell is unique Which sperm fertilizes which egg Which sperm fertilizes which egg

contributes to extreme variation contributes to extreme variation among offspringamong offspring

22nn = # of possible combinations per = # of possible combinations per gametegamete

Evolutionary Significance of Evolutionary Significance of Genetic VariationsGenetic Variations

Populations evolve as a result of Populations evolve as a result of differential reproductive success differential reproductive success among its variant membersamong its variant members

Individuals with variations best suited Individuals with variations best suited to the environment reproduce more to the environment reproduce more thus transmitting more of their genesthus transmitting more of their genes

Success of variations dependant on Success of variations dependant on environmentenvironment

Concept CheckConcept Check In fruit flies 2n= 8, while in honeybees In fruit flies 2n= 8, while in honeybees

2n=32. Assuming no crossing over, is 2n=32. Assuming no crossing over, is genetic variation higher among fruit genetic variation higher among fruit flies or honeybees? Explain.flies or honeybees? Explain.

When would crossing over not When would crossing over not contribute to increased genetic contribute to increased genetic variation?variation?

If a human cell had 22 autosomes & a If a human cell had 22 autosomes & a Y chromosome what kind of cell is it?Y chromosome what kind of cell is it?

What is the immediate product of What is the immediate product of meiosis in a plant?meiosis in a plant?

What feature is unique to a plant life What feature is unique to a plant life cycle versus an animal life cycle?cycle versus an animal life cycle?