Last time… • We saw how cells divide by mitosis to produce genetically identical daughter cells • 2n parent cell --> 2 daughter cells, both 2n • Why Mitosis? • Growth (new cells) • Cell replacement (skin, wound healing) • Asexual reproduction- produces genetically identical offspring
In sexually reproducing organisms… • Specialized cells called gametes from 2 different individuals fuse together to form a one-celled zygote, which develops into the offspring of the next generation • If gametes were formed by mitosis, the chromosome number would double each generation Cell division (mitosis) And development fertilization + Sperm egg One-celled zygote gametes
Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms • Meiosis reduces the chromosome number by half, enabling sexual recombination to occur. • Meiosis of diploid cells produces haploid daughter cells, which may function as gametes. 2n Cell division (mitosis) And development fertilization + Sperm egg (1n) (1n) One-celled Zygote--> 2n gametes meiosis
During meioisis, chromosome number is reduced In meiosis I, homologous pairs split In meiosis II, sister chromatids split
A full complement of chromosomes is restored during fertilization. Femalegamete (1n) Malegamete (1n) Fertilization Diploid offspringcontains homologouspairs of chromosomes
Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms • Meiosis and fertilization introduce genetic variation in three ways: • 1. Crossing over between homologous chromosomes at prophase I.
Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms • Meiosis and fertilization introduce genetic variation in three ways: • 2. Independent assortment of homologous pairs at metaphase I: • Each homologous pair can orient in either of two ways at the plane of cell division. • The total number of possible outcomes = 2n (n = haploid number of chromosomes).
The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I
Hypothetical example Eye color Hair color Allele thatcontributes to browneyes (B) Allele thatcontributes to blueeyes (b) allele thatcontributes to black hair (r) allele thatcontributes to red hair (R) Maternalchromosome Paternalchromosome Maternalchromosome Paternalchromosome
During metaphase of meiosis I, tetrads can line up two different ways before the homologs separate. OR Brown eyesBlack hair (Br) Blue eyesRed hair (bR) Brown eyesRed hair (BR) Blue eyesBlack hair (br)
Bbrr bbRr Br br br bR Meiosis is a Special Type of Cell Division that Occurs in Sexually Reproducing Organisms • Meiosis and fertilization introduce genetic variation in three ways: • 3. Random chance fertilization between any one female gamete with any other male gamete. Brown eyes, black hair + OR Blue eyes, red hair +
EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING 2. Crossingover duringmeiosis I. 3. Homologs separate.(Pairing of chromosomes depends on independentassortment.) 1. Parent cellwith four chromosomes. 5. Offspring produced by selfing (only some of the possibilities shown.) 4. Gametes produced by meiosis II.
Meiosis Occurs in Different Places in the Life Cycles of Different Organisms • Diploid life cycles: diploid phase predominates; meiosis directly produces gametes. • Haploid life cycles: haploid phase predominates; meiosis occurs in the zygote. • Alternation of generations: Haploid and diploid phases alternate with one another.
MEIOSIS Haploidgametes Diploid adult MITOSIS FERTILIZATION Diploidzygote Diploid dominant
Haploid dominant MEIOSIS MITOSIS Haploid cell Diploid cell Haploid adult MITOSIS FERTILIZATION Haploid gametes
Alternation of generations MEIOSIS MITOSIS Haploid cells Diploid plant Haploid gametes Haploid plant Diploid cell MITOSIS MITOSIS FERTILIZATIION