Meiosis Part Meiosis Part 22

The End The End result…result…

Let’s CompareLet’s Compare

Why aren't these siblings identical?

Origins of Genetic Variation, IOrigins of Genetic Variation, I• Independent Independent

assortmentassortment::

• homologous pair of homologous pair of chromosomes chromosomes position and orient position and orient randomly randomly (metaphase I) (metaphase I)

• nonidentical sister nonidentical sister chromatids during chromatids during meiosis IImeiosis II

• Combinations Combinations possible: 2 possible: 2 n n ; with ; with n the haploid n the haploid number of the number of the organismorganism

Origins of Genetic Variation, IIOrigins of Genetic Variation, II

• Crossing overCrossing over • prophase I: prophase I:

• the reciprocal the reciprocal exchange of exchange of genetic material genetic material between nonsister between nonsister chromatids during chromatids during synapsis of meiosis synapsis of meiosis I (recombinant I (recombinant chromosomes)chromosomes)

Crossing-OverCrossing-OverA source of Genetic VariationA source of Genetic Variation

• Chromosomes line Chromosomes line up during first up during first division of Meiosisdivision of Meiosis

• Homologous Homologous chromosomes can chromosomes can cross overcross over

The new The new chromosomes are chromosomes are no longer exactly no longer exactly like the parent like the parent chromosomes..chromosomes..they are a they are a combination .combination .

Origins of Genetic Variation – IIIOrigins of Genetic Variation – IIIRandom FertilizationRandom Fertilization

• Random fertilizationRandom fertilization: :

1 sperm (1 of 8 million possible 1 sperm (1 of 8 million possible chromosome combinations)chromosome combinations)

x 1 ovum (1 of 8 million different x 1 ovum (1 of 8 million different possibilities) possibilities)

= 64 trillion diploid combinations!= 64 trillion diploid combinations!

No wonder no two are the same!

7.2 Sexual and Asexual 7.2 Sexual and Asexual ReproductionReproduction

ReproductionReproduction

•The process of producing The process of producing offspringoffspring

•Can be sexual or asexualCan be sexual or asexual

Asexual ReproductionAsexual Reproduction

• One parent passes all its genetic One parent passes all its genetic information to each of its offspringinformation to each of its offspring

• No fusion of haploid gametesNo fusion of haploid gametes• Produces many organisms in little Produces many organisms in little

timetime• ““Clone”Clone”

Basic cell division

Hydra

Red sea star

Evolution of Sexual Evolution of Sexual ReproductionReproduction

• Some protists and prokaryotes can Some protists and prokaryotes can reproduce “sexually” in times of reproduce “sexually” in times of environmental stressenvironmental stress

• May allow them to repair their DNAMay allow them to repair their DNA

• Bacterium Bacterium passing DNA passing DNA molecule to molecule to anotheranother

Sexual Life CyclesSexual Life Cycles

Life CycleLife Cycle

The entire span on the life of an The entire span on the life of an organism from one generation to organism from one generation to the nextthe next

Eukaryotes alternate between Eukaryotes alternate between haploid and diploid stages during haploid and diploid stages during

their life cyclestheir life cycles

1nHaploid

2nDiploid

Diploid life cycle

•Humans

•Adults are diploid

•Only gametes produced by meiosis are haploid

There are There are many many variations of variations of cell cyclescell cycles

sporophyte

gametophyte