Lecture 21: Meiosis

Lecture outline 10/25/05 Review mitosis Finish cell cycle –Mutations in checkpoint genes can lead to cancer Sexual and asexual life cycles Meiosis –Mechanics of meiosis –Independent assortment and Crossing over produce genetic variation

Mitosis produces an exact copy of the parent cell. –Used for growth and asexual reproduction. Meiosis produces reduced (haploid) gametes, which are genetically unique. –Necessary for sexual reproduction.

Figure 13.5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) The human life cycle Meiosis occurs during gamete formation Gametes are the only haploid cells

Examples of Asexual Reproduction In asexual reproduction one parent produces genetically identical offspring by mitosis. (basically just branching) Figure 13.2 Parent Bud 0.5 mm Hydra

Aspens connected underground Dandelions make asexual seeds

Siblings share 1/2 of their genes, on average Each get one chromosome from mother and one from father, but they might not both get the same chromosome from a parent.

Diploid cells have two copies of each chromosome Homologous chromosomes contain copies of the same genes One maternal, one paternal

Figure 13.5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) The human life cycle Meiosis occurs during gamete formation Gametes are the only haploid cells

MEIOSISFERTILIZATION n n n n n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Mitosis Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Figure 13.6 B Plants and some algae –Exhibit an alternation of generations –The life cycle includes both diploid and haploid multicellular stages

MEIOSIS FERTILIZATION n n n n n 2n Haploid multicellular organism Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C In most fungi, and some protists –Meiosis produces haploid cells that give rise to a haploid multicellular adult organism –The haploid adult carries out mitosis, producing cells that will become gametes

Germline vs Soma Meiosis only occurs in germ cells. Produces gametes Eggs and sperm –Human oocytes start meiosis before birth and arrest in prophase 1; then they mature one at a time at ovulation –Human spermatocytes produce sperm continuously 28 d for meiosis another 35 d to become mature sperm Germ means “seed”Soma means “body”

Mitosis vs Meiosis Homologs pair in meiosis 1 Sister chromatids separate in meiosis 2 haploid Not paired diploid

Sister chromatids Chiasmata Spindle Tetrad Tertads line up Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Meiosis 1 Figure 13.8

TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming chromosomes are still double Figure 13.8 Meiosis 2

Chromosome condensation and pairing The Synaptonemal Complex holds the homologs together Segments of this cartoon show the same Chromosome at different times in meiosis

What makes Meiosis different from Mitosis? Synapsis and crossing over –Homologous chromosomes physically connect and exchange genes Paired homologous chromosomes at metaphase 1 Separation of homologues, producing haploid gametes –At anaphase I of meiosis, homologous pairs separate –In anaphase II of meiosis, the sister chromatids separate

Meiosis produces genetic variation Random alignment of chromosomes in meiosis 1 –Independent assortment of maternal and paternal chromosomes into gametes Crossing over 2 23 = 8x10 6 possible combinations of chromssomes in a gamete Random fusion of gametes will produce about 2 23 x 2 23 =64 trillion diploid combinations! Futher shuffling of genes within chromosomes

Independent Assortment Figure Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4 Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

Crossing Over Produces recombinant chromosomes that carry genes derived from two different parents Figure Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

Crossovers are random This tetrad has 3 crossovers: 2&3 1&3 2&4

Crossovers are essential for correct alignment at metaphase 1 Spindle fibers from one pole attach to BOTH sister chromatids Spindle fibers from the other pole attach to the other homolog Chiasmata hold the pair together while “Tug of war” Aligns tetrads