Chapter 10 Meiosis and Sexual Reproduction BIOLOGY Mader Chapter 10 Meiosis and Sexual Reproduction

Types of Reproduction Parent Bud 0.5 mm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 13.4 Describing chromosomes Key Maternal set of chromosomes (n = 3) Paternal set of 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 13.5 The human life cycle At fertilization, a sperm fuses with an egg, forming a diploid zygote 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) Repeated mitotic divisions lead to the development of a mature adult The adult makes haploid gametes by meiosis All of these processes make up the sexual life cycle of organisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chromosomes are matched in homologous pairs MEIOSIS Chromosomes are matched in homologous pairs Somatic cells of each species contain a specific number of chromosomes Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Centromere Sister chromatids Figure 8.12 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Gametes have a single set of chromosomes Cells with two sets of chrom. are said to be diploid (2n = 46 for humans) Gametes are haploid, with only one set of chromosomes (n = 23 for humans) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Homologous chromosomes carry different versions of genes The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene (called alleles) at corresponding loci (i.e. gene for eye color might say “blue” on dad, but “brown” on mom) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Figure 8.17A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Biology, 9th ed,Sylvia Mader Chapter 10 Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Four haploid daughter cells centrioles nucleolus sister chromatids synapsis centromere chromosome duplication 2n = 4 2n = 4 n = 2 n = 2 MEIOSIS I Homologous pairs synapse and then separate. MEIOSIS II Sister chromatids separate, becoming daughter chromosomes. Meiosis & Sexual Reproduction

Overview of meiosis: reduces chromosome number Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II Overview of meiosis: reduces chromosome number Meiosis, like mitosis, is preceded by chromosome duplication (during interphase) However, in meiosis the cell divides twice to form four daughter cells, each of which is haploid (n = 23). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Meiosis Men vs. Women Spermatogenesis: Oogenesis Men Testes 4 cells produced 145 rounds 6 days Oogenesis Women Ovaries Uneven separation results in 4 cells, one called egg Born = 1-2 million oocytes (prophase I) Puberty = 400,000

The Meiotic Division of an Animal Cell Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tetrads line up Metaphase plate 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells, each of which are haploid (n = 23). Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Meiosis - 2 divisions....WHY? In the first division, meiosis I, homologous chromosomes are paired. (While they are paired, they can cross over and exchange genetic information) The homologous pairs are then separated, and two daughter cells are produced, which at this point are haploid (n = 23). But because each chromosome has double the genetic info (2 sister chromatids), another division is necessary. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 13.8 The Meiotic Division of an Animal Cell TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Genetic Variation: Independent Assortment Biology, 9th ed,Sylvia Mader Chapter 10 Genetic Variation: Independent Assortment Independent assortment: When homologues align at the metaphase plate: They separate in a random manner The maternal or paternal homologue may be oriented toward either pole of mother cell Causes random mixing of blocks of alleles into gametes Meiosis & Sexual Reproduction

The independent assortment of homologous chromosomes in meiosis Key Maternal set of chromosomes Paternal set of Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Independent Assortment Biology, 9th ed,Sylvia Mader Chapter 10 Independent Assortment Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Meiosis & Sexual Reproduction

What leads to variability/diversity? Why are we not all identical? Independent assortment Crossing over Random fertilization Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Processes allowing for increased GENETIC DIVERSITY Random Fertilization - Each chromosome of a homologous pair comes from a different parent. (Which sperm will fertilize the egg? Or Which 2 people will produce offspring together?) Each chromosome thus differs at many points from the other member of the pair (different alleles) Crossing over Independent assortment Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Genetic Variation: Significance Biology, 9th ed,Sylvia Mader Chapter 10 Genetic Variation: Significance Asexual reproduction produces genetically identical clones Sexual reproduction cause novel genetic recombinations Asexual reproduction is advantageous when environment is stable However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous Offspring adapt to that environment Meiosis & Sexual Reproduction

Random Fertilization

Genetic Variation: Fertilization Biology, 9th ed,Sylvia Mader Chapter 10 Genetic Variation: Fertilization When gametes fuse at fertilization: Chromosomes donated by the parents are combined In humans, (223)2 = 70,368,744,000,000 chromosomally different zygotes are possible If crossing-over occurs only once (423)2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible Meiosis & Sexual Reproduction

Figure 13.10 The independent assortment of homologous chromosomes in meiosis Key Maternal set of chromosomes Paternal set of Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Crossing over further increases genetic variability

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis vs. Mitosis Meiosis Requires two nuclear divisions Chromosomes synapse and cross over Centromeres survive Anaphase I Halves chromosome number Produces four daughter nuclei Produces daughter cells genetically different from parent and each other Used only for sexual reproduction Mitosis Requires one nuclear division Chromosomes do not synapse nor cross over Centromeres dissolve in mitotic anaphase Preserves chromosome number Produces two daughter nuclei Produces daughter cells genetically identical to parent and to each other Used for asexual reproduction and growth Meiosis & Sexual Reproduction

PARENT CELL (before chromosome replication) Site of crossing over MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE PROPHASE I Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication 2n = 4 Chromosomes align at the metaphase plate Tetrads align at the Metaphase plate METAPHASE METAPHASE I ANAPHASE I TELOPHASE I ANAPHASE TELOPHASE Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Haploid n = 2 Daughter cells of meiosis I 2n = 4 2n = 4 No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cells of mitosis Haploid n = 2 n n n n Daughter cells of meiosis II Figure 8.15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Changes in Chromosome Number Euploid is the correct number of chromosomes in a species. Aneuploid is change in the chromosome number Results from nondisjunction Monosomy - only one of a particular type of chromosome, Trisomy - three of a particular type of chromosome

Changes in Chromosome pair of homologous chromosomes pair of Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of homologous chromosomes pair of homologous chromosomes Meiosis I nondisjunction normal nondisjunction Meiosis II normal Fertilization Zygote 2n 2n 2n + 1 2n - 1 2n + 1 2n + 1 2n - 1 2n - 1 a. b.

Trisomy Trisome 21 Occurs when an individual has three of a particular type of chromosome The most common autosomal trisomy seen among humans Also called Down syndrome Recognized by these characteristics: short stature eyelid fold flat face stubby finger wide gap between first and second toes

a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers Trisomy 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. extra chromosome 21 a. b. a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers

Changes in Sex Chromosome Result from inheriting too many or too few X or Y chromosomes Nondisjunction during oogenesis or spermatogenesis Turner syndrome (XO) Female with single X chromosome Short, with broad chest and widely spaced nipples Can be of normal intelligence and function with hormone therapy Klinefelter syndrome (XXY) – a male Male with underdeveloped testes and prostate; some breast overdevelopment Long arms and legs; large hands Near normal intelligence unless XXXY, XXXXY, etc. No matter how many X chromosomes, presence of Y renders individual male

Changes in Chromosome Structure Changes in chromosome structure include: Deletions One or both ends of a chromosome breaks off Two simultaneous breaks lead to loss of an internal segment Duplications Presence of a chromosomal segment more than once in the same chromosome Translocations A segment from one chromosome moves to a non-homologous chromosome Follows breakage of two nonhomologous chromosomes and improper re-assembly

Changes in Chromosome Structure Duplication A segment of a chromosome is repeated in the same chromosome Inversion Occurs as a result of two breaks in a chromosome The internal segment is reversed before re-insertion Genes occur in reverse order in inverted segment

Types of Chromosomal Mutation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a a a b b b b c c c c + a d d d d e e e e f d f f e g g g f g a. Deletion b. Duplication a a a a b b l b b l c m c c m d d n d n d c e o e o e e f p f p f f g q q g g g h r r h c. Inversion d. Translocation

Types of Chromosomal Mutation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a a b b + h deletion lost c c d d e e f f g g h a. b. b: Courtesy The Williams Syndrome Association

Types of Chromosomal Mutation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a s a s b t b t c c u u d translocation v d v e w e w x x f f g y y g z h z h a. b. b: American Journal of Human Genetics by N. B. Spinner. Copyright 1994 by Elsevier Science & Technology Journals. Reproduced with permission of Elsevier Science & Technology Journals in the format Textbook via Copyright Clearance Center