Meiosis and Sexual Reproduction Chapter 10 Meiosis and Sexual Reproduction

10.1 Halving the Chromosome Number Biology, 9th ed,Sylvia Mader Chapter 10 10.1 Halving the Chromosome Number Meiosis & Sexual Reproduction Meiosis Special type of cell division Used only for sexual reproduction Halves the chromosome number prior to fertilization Parents are diploid (2n) Meiosis produces haploid (n) gametes Gametes fuse in fertilization to form a diploid (2n) zygote The zygote becomes the next diploid (2n) generation

Halving the Chromosome Number Biology, 9th ed,Sylvia Mader Chapter 10 Halving the Chromosome Number Meiosis & Sexual Reproduction In diploid body cells, chromosomes occur in pairs Humans have 23 different types of chromosomes Diploid (2n) cells have two chromosomes of each type Chromosomes of the same type are said to be homologous chromosomes (homologues) They have the same length Their centromeres are positioned in the same place One came from the father (the paternal homolog) the other from the mother (the maternal homolog) When stained, they show similar banding patterns

Homologous Chromosomes Biology, 9th ed,Sylvia Mader Chapter 10 Homologous Chromosomes Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Sister chromatids nonsister chromatids duplication duplication kinetochore centromere chromosome homologous pair chromosome paternal chromosome maternal chromosome b. a: © L. Willatt/Photo Researchers, Inc.

Halving the Chromosome Number Biology, 9th ed,Sylvia Mader Chapter 10 Halving the Chromosome Number Meiosis & Sexual Reproduction Homologous chromosomes have genes controlling the same trait at the same position Each gene occurs in duplicate Many genes exist in several variant forms in a large population Homologous copies of a gene may encode identical or different genetic information The variants that exist for a gene are called alleles An individual may have: Identical alleles for a specific gene on both homologs (homozygous for the trait), or A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)

Halving the Chromosome Number Biology, 9th ed,Sylvia Mader Chapter 10 Slide #6 Halving the Chromosome Number Meiosis & Sexual Reproduction Overview of Meiosis Meiosis I Chromosomes are replicated prior to meiosis I Each chromosome consists of two identical sister chromatids Homologous chromosomes pair up – synapsis Homologous pairs align themselves against each other side by side at the metaphase plate The two members of a homologous pair separate Each daughter cell receives one duplicated chromosome from each pair Meiosis II DNA is not replicated between meiosis I and meiosis II Sister chromatids separate and move to opposite poles The four daughter cells contain one daughter chromosome from each pair Each daughter chromosome consists of a single chromatid The daughter cells are haploid 6

Overview of Meiosis Meiosis & Sexual Reproduction 7 Biology, 9th ed,Sylvia Mader Overview of Meiosis Chapter 10 Slide #7 Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First division Second division Four haploid daughter cells centrioles nucleolus sister chromatids centromere synapsis chromosome duplication 2n = 4 2n = 4 n = 2 n = 2 MEIOSISI Homologous pairs synapse and then separate. MEIOSISII Sister chromatids separate, becoming daughter chromosomes. 7

Biology, 9th ed,Sylvia Mader Chapter 10 10.2 Genetic Variation Meiosis & Sexual Reproduction Meiosis brings about genetic variation in two key ways: Crossing-over between homologous chromosomes, and Independent assortment of homologous chromosomes Crossing Over: Exchange of genetic material between non-sister chromatids during meiosis I At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues Then homologues separate and are distributed to different daughter cells

Crossing Over Occurs During Meiosis I Biology, 9th ed,Sylvia Mader Chapter 10 Crossing Over Occurs During Meiosis I Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. nucleoprotein lattice sister chromatids of a chromosome sister chromatids of its homologue A A a a A a B B b b B b chiasmata of nonsister chromatids 1 and 3 c C C c C c D D d d D d 1 2 3 4 1 2 3 4 1 2 3 4 Bivalent forms Crossing-over has occurred Daughter chromosomes a. b. c. d. a: Courtesy Dr. D. Von Wettstein

Biology, 9th ed,Sylvia Mader Chapter 10 Genetic Variation Meiosis & Sexual Reproduction Independent assortment When homologous chromosome pairs 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

Independent Assortment Biology, 9th ed,Sylvia Mader Chapter 10 Independent Assortment Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Combination 7 Combination 3 Combination 5 Combination 1 Combination 2 Combination 4 Combination 6 Combination 8

Biology, 9th ed,Sylvia Mader Chapter 10 Genetic Variation Meiosis & Sexual Reproduction Fertilization – union of male and female gametes 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 Crossing-over may occur several times in each chromosome

Biology, 9th ed,Sylvia Mader Chapter 10 Genetic Variation Meiosis & Sexual Reproduction Significance of genetic variation: Asexual reproduction produces genetically identical clones Sexual reproduction causes genetic recombinations among members of a population Asexual reproduction is advantageous when the environment is stable However, if the environment changes, genetic variability introduced by sexual reproduction may be advantageous

Biology, 9th ed,Sylvia Mader Chapter 10 10.3 The Phases of Meiosis Meiosis & Sexual Reproduction Meiosis I: Prophase I A spindle forms The nuclear envelope fragments The nucleolus disappears Each chromosome is duplicated (consists of two identical sister chromatids) Homologous chromosomes pair up and physically align themselves against each other side by side (synapsis) Synapsed homologs are referred to as a bivalent (two homologues) or a tetrad (four chromatids) Metaphase I Homologous pairs are arranged at the metaphase plate Bivalents are aligned independently of one another

Biology, 9th ed,Sylvia Mader Chapter 10 The Phases of Meiosis Meiosis & Sexual Reproduction Meiosis I Anaphase I Homologous chromosomes of each bivalent separate from one another Homologues move towards opposite poles Sister chromatids do not separate Each is still a duplicated chromosome with two chromatids Telophase I Daughter cells have one duplicated chromosome (n) from each homologous pair

Biology, 9th ed,Sylvia Mader Chapter 10 The Phases of Meiosis Meiosis & Sexual Reproduction Interkinesis Two haploid (n) daughter cells, each with one duplicated chromosome of each type Interkinesis is similar to mitotic interphase except It is usually shorter DNA replication does not occur

Biology, 9th ed,Sylvia Mader The Phases of Meiosis Chapter 10 Meiosis & Sexual Reproduction Meiosis II (mitosis of two haploid cells) Prophase II – Chromosomes condense Metaphase II – Chromosomes align at metaphase plate Anaphase II Centromere dissolves Sister chromatids separate and become daughter chromosomes Telophase II and cytokinesis Four haploid (n) cells all genetically unique

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Meiosis I in Plant and Animal Cells

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis & Sexual Reproduction Meiosis I in Plant and Animal Cells

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis II in Plant and Animal Cells Meiosis & Sexual Reproduction

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis II in Plant and Animal Cells Meiosis & Sexual Reproduction

10.4 Meiosis Compared to Mitosis Biology, 9th ed,Sylvia Mader Chapter 10 10.4 Meiosis Compared to Mitosis Meiosis & Sexual Reproduction 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 Compared to Mitosis Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis Compared to Mitosis Meiosis & Sexual Reproduction

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis Compared to Mitosis Meiosis & Sexual Reproduction

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis Compared to Mitosis Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2n = 4 Prophase I Synapsis and crossing-over occur. MetaphaseI Homologous pairs align independently at the metaphase plate. Anaphase I Homologous chromosomes separate and move toward the poles. MEIOSISI 2n = 4 Prophase Metaphase Chromosomes align at the metaphase plate. Anaphase Sister chromatids separate and become daughter chromosomes. MITOSIS

Biology, 9th ed,Sylvia Mader Chapter 10 Meiosis Compared to Mitosis Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Telophase I Daughter cells are forming and will go on to divide again. n = 2 Daughter cells Sister chromatids separate and become daughter chromosomes. Four haploid daughter cells. Their nuclei are genetically different from the parent cell. n = 2 n = 2 MEIOSIS I cont'd MEIOSIS II Daughter cells Telophase Daughter cells are forming. Two diploid daughter cells. Their nuclei are genetically identical to the parent cell. MITOSIS cont'd

Biology, 9th ed,Sylvia Mader Chapter 10 The Cycle of Life Meiosis & Sexual Reproduction In animals: “Individuals” are diploid and produce haploid gametes The only haploid part of the life cycle is the gametes The products of meiosis are always gametes Meiosis occurs only during gametogenesis Production of sperm Spermatogenesis All four cells become sperm Production of eggs Oogenesis One of the four nuclei receives the majority of the cytoplasm Becomes the egg or ovum Others wither away as polar bodies

Biology, 9th ed,Sylvia Mader Chapter 10 The Cycle of Life Meiosis & Sexual Reproduction Human Life Cycle: Sperm and egg are produced by meiosis A sperm and egg fuse at fertilization Results in a zygote The one-celled stage of an individual of the next generation Undergoes mitosis Results in a multicellular embryo that gradually takes on features determined when the zygote was formed All growth occurs as mitotic division As a result of mitosis, each somatic cell in the body Has same number of chromosomes as zygote Has the same genetic makeup, which was determined when the zygote was formed

Meiosis & Sexual Reproduction Biology, 9th ed,Sylvia Mader Life Cycle of Humans Chapter 10 Slide #29 Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MITOSIS 2n 2n 2n MITOSIS 2n zygote 2n = 46 diploid (2n) FERTILIZATION MEIOSIS haploid (n) n = 23 n n egg sperm 29

Biology, 9th ed,Sylvia Mader Spermatogenesis and Oogenesis in Mammals Chapter 10 Meiosis & Sexual Reproduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SPERMATOGENESIS OOGENESIS primary spermatocyte primary oocyte 2 n 2n Meiosis I Meiosis I first polarbody secondary spermatocytes n secondary oocyte n Meiosis II n Meiosis II spermatids Meiosis II is completed after entry of sperm (fertilization) n second polarbody n Metamorphosis and maturation Fertilization egg n sperm Sperm nucleus n n fusion of sperm nucleus and agg nucleus zygote 2n

10.6 Changes in Chromosome Number and Structure Euploidy is the correct number of chromosomes in a species. Aneuploidy is a 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

Nondisjunction 32 pair of homologous chromosomes pair of homologous Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of homologous chromosomes pair of homologous chromosomes nondisjunction Meiosis I normal Meiosis II normal nondisjunction Fertilization Zygote 2n + 1 2n + 1 2n - 1 2n - 1 2n 2n 2n + 1 2n - 1 a. b. 32

Changes in Chromosome Number and Structure Trisomy occurs when an individual has three of a particular type of chromosome The most common autosomal trisomy seen among humans is Trisomy 21 Also called Down syndrome Recognized by these characteristics: short stature eyelid fold flat face stubby fingers 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 Chromosome Number and Structure Changes in sex chromosome number: Results from inheriting too many or too few X or Y chromosomes Nondisjunction during oogenesis or spermatogenesis Turner syndrome (XO) Female with a single X chromosome Short, with broad chest and widely spaced nipples Can be of normal intelligence and function with hormone therapy

Changes in Chromosome Number and Structure Changes in sex chromosome number: Klinefelter syndrome (XXY) 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 are present, the presence of a chromosome Y renders the individual male

Changes in Sex Chromosome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Turner syndrome missing chromosome X b. Klinefelter syndrome extra chromosome X a(top): Courtesy UNC Medical Illustration and Photograph; b(top): Courtesy Stefan D. Schwarz, http://klinefeltersyndrome.org; a, b(bottom): © CNRI/SPL/Photo Researchers, Inc

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

Changes in Chromosome Number and Structure Changes in chromosome structure include: 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 the 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