Post on 02-Jan-2016
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?