Meiosis

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. Female gamete (1n) Male gamete (1n) Fertilization Diploid offspring contains homologous pairs of chromosomes Figure: 9.2c Caption: Meiosis reduces chromosome number by one-half. In diploid organisms, the products of meiosis are haploid.

Figure 13.7 The stages of meiotic cell division: Meiosis I

Figure 13.7 The stages of meiotic cell division: Meiosis II

Figure 13.8 A comparison of mitosis and meiosis

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.

Crossing over can lead to new combinations of alleles on a chromosome

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 that contributes to brown eyes (B) Allele that contributes to blue eyes (b) allele that contributes to black hair (r) allele that contributes to red hair (R) Figure: 9.5a Caption: (a) In this hypothetical example, genes that influence hair color and eye color in humans are located on different chromosomes. Maternal chromosome Paternal chromosome Maternal chromosome Paternal chromosome

During metaphase of meiosis I, tetrads can line up two different ways before the homologs separate. Figure: 9.5b Caption: (b) This diagram shows how gametes with different combinations of genes result from separation of homologous chromosomes during meiosis I. Brown eyes Black hair (Br) Blue eyes Red hair (bR) Brown eyes Red hair (BR) Blue eyes Black hair (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. Bbrr Brown eyes, black hair Br br + OR bbRr br Blue eyes, red hair bR +

EVEN SELF-FERTILIZATION LEADS TO GENETICALLY VARIABLE OFFSPRING Figure: 9.6 Caption: This example shows some of the possible results of self-fertilization in an organism with four chromosomes (2n = 4).   Exercise To the last line in this figure, add sketches showing the chromosome complements in additional offspring produced by selfing. 1. Parent cell with four chromosomes. 2. Crossing over during meiosis I. 3. Homologs separate. (Pairing of chromosomes depends on independent assortment.) 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.

Diploid dominant MEIOSIS Haploid gametes Diploid adult MITOSIS FERTILIZATION MEIOSIS Haploid gametes Diploid zygote Diploid dominant Figure: 9.7a Caption: (a) In animals, the gametes are the only haploid cells. Meiosis occurs in special reproductive tissues.

Haploid dominant MEIOSIS MITOSIS Haploid cell Diploid cell Figure: 9.7b Caption: (b) In many algae, the fertilized egg is the only diploid cell. When this cell undergoes meiosis, the haploid cells that are produced go on to form a multicellular adult. Haploid adult MITOSIS FERTILIZATION Haploid gametes

Alternation of generations MEIOSIS MITOSIS Haploid cells Diploid plant Haploid gametes Figure: 9.7c, upper Caption: (c) In land plants and some algae, there is a multicellular diploid stage and a multicellular haploid stage. Typically, one of these two stages is larger in size and longer in life span than the other. In ferns, the diploid stage is more prominent; in mosses the haploid stage is more prominent. Haploid plant Diploid cell MITOSIS MITOSIS FERTILIZATIION