MEIOSIS AND SEXUAL REPRODUCTION

• CONTINUITY – Genetic information passes from generation to generation

• GENETIC VARIATION – differences between individuals; these are the differences evolution “sorts” through

Kinds of Chromosomes

• Autosomes• Sex chromosomes• Karyotypes

Mitosis vs. Meiosis

Mitosis vs. Meiosis

• Mitosis– 1 division– daughter cells

genetically identical to parent cell

– produces 2 cells

– 2n 2n (or n n)

– produces cells for growth & repair

– no crossing over

• Meiosis– 2 divisions– daughter cells genetically different

from parent– produces 4 cells– 2n 1n– produces gametes– crossing over

THE VALUE OF MEIOSIS• Meiosis introduces genetic variation

– gametes of offspring do not have same genes as gametes from parents

• Behavior of chromosomes during meiosis and fertilization is responsible for most of the genetic variation that arises each generation during sexual reproduction

• 3 MECHANISM OF GENETIC VARIATION due to meiosis and fertilization

1. INDEPENDENT ASSORTMENT2. RANDOM FERTILIZATION3. CROSSING OVER*** mutation is also a source of genetic variation

INDEPENDENT ASSORTMENT

INDEPENDENT ASSORTMENTHOW MANY COMBINATIONS OF CHROMOSOMES ARE POSSIBLE IN A HUMAN GAMETE?

223 OR ABOUT 8 MILLION

n= the haploid number of the species

CROSSING OVER• During Prophase I– homologous pairs

swap pieces of chromosome

– nonsister chromatids intertwine and exchange genes

– crossing over

• Crossing over occurs early in prophase I • Homologous chromosomes pair up gene by

gene – synapsis; visible as group of 4 chromatids called tetrad

• Synaptonemal complex (proteins) forms between homologous chromosomes

• In crossing over, homologous portions of two nonsister chromatids trade places; physical location of cross overs visible as chiasmata– In humans, this occurs 2-3 times per

chromosome pair.• Independent assortment of these

nonidentical sister chromatids during meiosis II increases the number of genetic types of gametes that can result from meiosis.

RANDOM FERTILIZATION

• Any 2 parents will produce a zygote with over 70 trillion (223 x 223) diploid combinations

ONE MORE TIME!!!!• 3 sources of genetic variability in a sexually

reproducing organism are:1. Independent assortment of homologous

chromosomes during meiosis I and of nonidentical sister chromatids during meiosis II.

2. Crossing over between homologous chromosomes during prophase I.

3. Random fertilization of an ovum by a sperm.

• All three mechanisms reshuffle the various genes carried by individual members of a population.

• Mutations, still to be discussed, ultimately create a population’s diversity of genes – source of “new” genes

Differences in Meiosis:Males vs Females

• In males the 4 cells produced during meiosis become four sperm (spermatogenesis)

• In females only one of the 4 cells produced during meiosis becomes an egg (oogenesis)

Differences in Meiosis:Males vs. Females

• Males begin producing sperm at puberty; number and quality of sperm decline with age

• Females form all eggs during 5th month of embryonic development– oocytes remain in prophase I until ovulation begins

during puberty– meiosis I completed by ovulation;– meiosis II completed after fertilization;– at menopause ovulation ceases

**uneven division of cytoplasm produces 1 ovum and 3 polar bodies

•Meiosis in sporophyte (2n) produces spores (n)

•Spores grow into multicellular gametophyte (n)

•Gametophyte produces gametes by mitosis

•Fertilization produces zygote (2n)

•Zygote divides by mitosis to form embryo in seed

•Seed germinates and embryo grows into new sporophyte