A. Inasexualreproduction, one parental passes a duplicate of its genes (DNAmolecules) come itsoff­spring, which deserve to only it is in genetically identical clonesof the parent.

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B. Insexualreproduction, each parental contributes one gene because that each trait.

1. Genes foreach properties come in slightly various forms dubbed alleles, originallyproducedby mutations.

2. Meiosis shuffles thealleles during gameteformation, and also fertilization produce offspring through uniquecombinations ofalleles.

3. Thevariation generated by sex-related reproduction is the experimentation ground fornaturalselection and is the basis because that evolutionary change.

II. How Meiosis Halves theChromosome Number

A. Think“Homologues”

1. Meiosisbegins with diploid (2n = 46) germcells and produces haploid gametes (n = 23).

a. In2n cells there are two chromosomes ofeach type, referred to as homologous chromosomes.

b. Homologouschromosomes line up (even unequally matched sex chromosomes!) duringmeiosis.

2. Meiosisproduces gametes that have one the eachpair of homologous chromosomes, i.e., they room haploid.

B. 2 Divisions,Not One

1. In someways meiosis each other mitosis:

a. Thechromosomes are duplicated during interphase to form sister chromosomesheldtogether in ~ the centromere.

b. Chromosomesare relocated by the microtubules that the spindle apparatus.

2. Unlikemitosis, meiosis has actually two series of divisions—meiosis I and also II.

a. Duringmeiosis I, homologous chromosomes pair and the cytoplasm divides later.

CrossingOver inProphase I: SHUFFLING that THE genes occurs! Increasing genetic DIVERSITY -- really important!

1) Eachof the 2 daughter cells receives a haploid variety of chromosomes.

2) Eachchromosome is still duplicated.

b. Inmeiosis II, the sister chromatids of every chromosome separate; thecytoplasmdivides again, result in four haploid cells.

III. AVisual tourism of the stages of Meiosis

IV. ACloser watch at crucial Events that Meiosis I

A. Crossing end inProphase I

1. Homologouschromosomes pair up.

a. Nonsisterchromatids exchange segment in a process called crossing over.

b. Becausealleles because that the very same trait deserve to vary, brand-new combinations of genes in eachchro­mosomecan result; this is one source of genetic variation.

2. Crossingover leader to hereditary recombination.

B. Metaphase IAlignments

1. Throughout metaphase I,homologous chromosomesrandomly heat up in ~ the spindle equator.

2. Throughout anaphase I,homologous chromosomes(still duplicated) separate right into two hap­loid cells, each of whichhas a random mix that maternal and paternalchromosomes.

V. FromGametes to Offspring

A. Gamete Formationin Plants

1. Eventssuch together spore formation may occur between meiosis and gamete formation.

2. Haploidspores germinate into haploid gamete-producing bodies.

3. Gamete-producingbodies and also spore-producing bodies construct during the life cycle ofplants.

B. Gamete Formationin Animals

1. In males,meiosis and also gamete formation are called spermatogenesis.

a. Germcell (2n) ——> main spermatocyte(2n) ——> MEIOSIS ns ——> twosecondary sperma­tocytes (n) ——>MEIOSIS II ——> four spermatids (n).

b. Spermatidschange in form; each creates a tail to come to be a mature sperm.

2. Infemales, meiosis and gamete development are dubbed oogenesis.

a. Germcell (2n) ——> main oocyte (2n)——> MEIOSIS i ——> secondaryoocyte (n, and big in size) pluspolar human body (n,and tiny insize) ——> MEIOSIS II ——> one large ovum (n) plusthree polar bodies (n,small).

b. Thesingle ovum is the just cell capable of being fertilized by a sperm;the polar bodieswither and die.

C. Much more Shufflingsat Fertilization

1. Thediploid chromosome number is revitalized at fertilization once two verydifferentgamete nuclei fuse to type the zygote.

2. Variationpresent at fertilization is from 3 sources:

a. Crossing over occurs during prophase I.

b. Randomalignments in ~ metaphase I lead to millions of combine of maternalandpaternal chromosomes in every gamete.

c. Ofall the genetically diverse gametes produced, opportunity will determinewhich twowill meet.

VI. Meiosis and also Mitosis Compared

A. Mitotic celldivision to produce clones; this kind of department is common insingle-celled,asexually reproduc­ing organisms and in the growth procedure ofmulticelledforms.

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B. Meiosis occursonly in the germ cells offered in sexual reproduction; it provides rise tonovelcombinations that alleles in offspring.