Meiosis and Sexual Reproduction

Chapter 10

10.1 Introducing Alleles

Asexual reproduction produces genetically identical copies of a parent (mitosis)

Sexual reproduction introduces variation in the combinations of traits among offspring (meiosis)

Meiosis and Homologous Chromosomes

Meiosis A nuclear division mechanism that precedes

cytoplasmic division of immature reproductive cells in sexually-reproducing eukaryotic species

If meiosis did not occur, the chromosome number would continue to double with each new generation.

Homologous Chromosomes Same length, shape, centromere in the same location,

are in pairs (one from the mother and one from the father), separate from each other during meiosis I, have alleles for the same characteristics even though the gene expression may not be the same.

Genes and Alleles

Genes are regions in an organism’s DNA that encode information about heritable traits• In sexual reproduction, pairs of genes are

inherited on pairs of chromosomes Alleles are different or alternate forms of the

same gene• Offspring of sexual reproducers inherit new

combinations of alleles (ex. Eye color = hazel eyes (mother), blue eyes (father)

• Study Figure 10.2

Halving the Diploid Number

A diploid cell has two nonidentical copies of every chromosome (except XY sex chromosomes)• Humans have 23 pairs of homologous

chromosomes

Meiosis in germ cells halves the diploid number of chromosomes (2n) to the haploid number (n), producing haploid gametes aka sex cells (egg, sperm, spores, gametophytes)

Restoring the Diploid Number

Human gametes (eggs and sperm) have 23 chromosomes – one of each type of homologous chromosomes.

The diploid number (23 pairs or 46 chromosomes) is restored at fertilization, when two haploid gametes fuse and form a diploid zygote

Two Divisions, Not One

In meiosis, DNA is replicated once and divided twice (meiosis I and meiosis II), forming four haploid cells (four daughter cells)

In meiosis I, each duplicated homologous chromosome is separated from its partner

In meiosis II, sister chromatids are separated

Fig. 10-5a, p. 158

C Anaphase I D Telophase I

one pair of homologous chromosomes

B Metaphase I

newly forming microtubules of the spindle

breakup of nuclear envelope

centrosome with a pair of centrioles, moving to opposite sides of nucleus

A Prophase I = crossing over

plasma membrane

Meiosis I

Stepped Art

Fig. 10-5b, p. 159

F Metaphase II G Anaphase II H Telophase IIE Prophase II

There isno DNAreplicationbetween thetwo nucleardivisions.

Meiosis II

Stepped Art

Animation: Meiosis step-by-step

10.4 How Meiosis Introduces Variation in Traits

Crossovers the random sorting or alignment of

chromosomes genetic recombination of alleles fertilization in meiosis introduce novel

combinations of alleles into gametes (egg and sperm), resulting in new combinations of traits among offspring

Crossing Over in Prophase I

Crossing over• The process by which a chromosome and its

homologous partner (two of the same chromosomes; one from the mother and one from the father) exchange heritable information in corresponding segments

• Homo = same

Animation: Crossing over

Segregation of Chromosomes into Gametes

Random assortment (independent assortment) produces (8,388,608) possible combinations of homologous chromosomes

Gametes (egg and sperm; 223 ) Fertilization (fusion or combining of the egg and

sperm; (223)2 = Over 70 trillion different zygotes – the first cell that is formed after fertilization) are possible.

Animation: Random alignment

Fig. 10-8b, p. 162

Gamete Formation in Animals

Males• Meiosis of primary spermatocytes produces four

haploid spermatids Females• Meiosis of a primary oocyte forms cells of

different sizes; the secondary oocyte gets most of the cytoplasm and matures into an ovum ; other three cells (polar bodies – have no function, are dumping places for excess genetic material, are produced only during meiosis.)

• Study figure 10.10

Sperm Formation in Animals

Egg Formation in Animals

More Shufflings at Fertilization

Chance combinations of maternal and paternal chromosomes through fertilization produce a unique combination of genetic information

Fertilization• The fusion of two haploid gametes (sperm and

mature ova - egg) resulting in a diploid zygote (the first cell of a new individual with 46 chromosomes.

Fig. 10-11c, p. 164

Fig. 10-11d, p. 165

Animation: Comparing mitosis and meiosis

Animation: Generalized life cycles

Animation: Meiosis

Video: Why sex?