Lecture 21 Meiosis Lecture outline 102505 Review mitosis

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.

The human life cycle Key Haploid gametes (n = 23) Haploid (n) Diploid (2 n) Meiosis occurs during gamete formation Ovum (n) Gametes are the only haploid cells Sperm Cell (n) FERTILIZATION MEIOSIS Ovary Testis Mitosis and development Figure 13. 5 Multicellular diploid adults (2 n = 46) Diploid zygote (2 n = 46)

Examples of Asexual Reproduction • In asexual reproduction one parent produces genetically identical offspring by mitosis. • (basically just branching) Parent Bud 0. 5 mm Figure 13. 2 Hydra

Aspens connected underground www. robertturnerphoto. com 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

The human life cycle Key Haploid gametes (n = 23) Haploid (n) Diploid (2 n) Meiosis occurs during gamete formation Ovum (n) Gametes are the only haploid cells Sperm Cell (n) FERTILIZATION MEIOSIS Ovary Testis Mitosis and development Figure 13. 5 Multicellular diploid adults (2 n = 46) Diploid zygote (2 n = 46)

• Plants and some algae – Exhibit an alternation of generations – The life cycle includes both diploid and haploid multicellular stages Haploid multicellular organism (gametophyte) n Mitosis n n n Spores Gametes MEIOSIS Diploid multicellular organism (sporophyte) Figure 13. 6 B 2 n (b) Plants and some algae FERTILIZATION 2 n Mitosis Zygote

• 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 Haploid multicellular organism Mitosis n n n Gametes MEIOSIS FERTILIZATION 2 n Figure 13. 6 C n Zygote (c) Most fungi and some protists

Germ means “seed” Soma means “body” 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

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

Meiosis 1 MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Sister chromatids Chiasmata Spindle Tetrad Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange Figure 13. 8 segments; 2 n = 6 in this example Homologous chromosomes separate Pairs of homologous Tertads line up chromosomes split up

Meiosis 2 MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II Cleavage furrow chromosomes are still double Figure 13. 8 METAPHASE II ANAPHASE II Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming

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 Futher shuffling of genes within chromosomes 223 = 8 x 106 possible combinations of chromssomes in a gamete Random fusion of gametes will produce about 223 x 223 =64 trillion diploid combinations!

Independent Assortment Key • Each pair of chromosomes Maternal set of sorts its maternal and chromosomes Possibility 1 Paternal set of paternal homologues into chromosomes daughter cells independently of the other pairs Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Figure 13. 10 Combination 1 Combination 2 Combination 3 Combination 4

Crossing Over Prophase I of meiosis • Produces recombinant chromosomes that carry genes derived from two different parents Metaphase I Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase II Daughter cells Figure 13. 11 Recombinant chromosomes

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

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