““Meiosis andMeiosis and

Sexual Reproduction”Sexual Reproduction”

Chapter 7 NotesChapter 7 Notes

I. MEIOSISI. MEIOSIS

A. Formation of Haploid A. Formation of Haploid CellsCells

1. Meiosis reduces the number of chromosomes by half to form gametes.

2. During fertilization the gametes unite to form a zygote and restore the original diploid number.

3. Result of meiosis is 4 haploid gametes from 1 diploid cell.

MEIOSISMEIOSIS

Picture of a sperm cell fertilizing an egg cell Picture of a sperm cell fertilizing an egg cell

B. Meiosis and Genetic B. Meiosis and Genetic VariationVariation

1. Genetic variation is necessary for evolution to occur.

2. Three events during meiosis contribute to genetic variation:– a. independent assortment – random distribution

of homologous chromosomes during meiosis.– b. crossing-over – exchange of genetic material

between homologous chromosomes during meiosis

– c. random fertilization – random fertilization of an egg by a sperm

a. a. independent assortmentindependent assortment

b. b. crossing-overcrossing-over

c. c. random fertilizationrandom fertilization

SEGREGATION & CROSSING OVERSEGREGATION & CROSSING OVERtogether make even more together make even more

combinationscombinations

See ananimation

http://www.tokyo-med.ac.jp/genet/anm/mimov.gi

C. Meiosis and Gamete C. Meiosis and Gamete FormationFormation 1. Spermatogenesis – process by

which sperm are produced in males.

A diploid cell increases in size to become a germ cell, goes through meiosis, forms 4 haploid sperm cells.

C. Meiosis and Gamete C. Meiosis and Gamete FormationFormation

2. Oogenesis – process by which an ovum (egg cell) is produced in females.

A diploid cell increases in size to become a germ cell. Unequal division of the cytoplasm during meiosis results in a larger cell that becomes the egg and 3 smaller cells that become polar bodies and die.

II. SEXUAL II. SEXUAL REPRODUCTIONREPRODUCTION

A. Sexual and Asexual A. Sexual and Asexual ReproductionReproduction

11. Asexual – formation of offspring from 1 parent. Produces clones that are genetically identical to parents.– a. fission– b. fragmentation– c. budding

Disadvantage of asexual reproduction is lack of genetic variation. (always produces a clone)

A. Sexual and Asexual A. Sexual and Asexual ReproductionReproduction

2. Sexual – formation of offspring through union of gametes from 2 parents.Offspring are genetically different than the parents.

23 chromosomes

23 chromosomes

46 chromosomes

B. Sexual Life Cycles in B. Sexual Life Cycles in EukaryotesEukaryotes

1. Haploid life cycle – simplest of all life cycles. Haploid cell major portion of life cycle. Ex: algae and fungi

2. Diploid life cycle – adults are diploid and are major portion of life cycle. Only haploid cells are gametes. Ex: animals including humans

3. Alternation of Generations – some organisms such as plants have a life cycle that alternates between haploid and diploid phases..