Chp. 16 Notes - Exam 3 PDF

Chapter 16 Transmission of Genetic Information from Parents to Offspring II: Epigenetics, Linkage, and Extranuclear Inheritance 16.1 Overview of Epigenetics Section 16.1 Learning Outcomes...

Chapter 16 Transmission of Genetic Information from Parents to Offspring II: Epigenetics, Linkage, and Extranuclear Inheritance 16.1 Overview of Epigenetics Section 16.1 Learning Outcomes 1. Define epigenetics and epigenetic inheritance 2. Outline the types of molecular changes that underlie epigenetic effects on gene expression 16.1 Overview of Epigenetics Female honeybees are of two types: queen bees and worker bees The differences between queens and workers are not due to differences in alleles, rather the difference in development is due to epigenetic modifications related to differences in their diet Although many genes follow Mendelian inheritance patterns, Loading… some genes follow other patterns 16.1 Overview of Epigenetics 16.1 Overview of Epigenetics Epigenetics is the study of mechanisms that lead to changes in gene expression that can be passed from cell to cell that are reversible that do not involve a change in the sequence of DNA Loading… Some epigenetic changes repress transcription while others activate transcription Some epigenetic changes are relatively permanent during an individual’s life while others may be reversible Epigenetic inheritance occurs in multicellular species that reproduce via gametes; epigenetic changes are passed from parent to offspring 16.1 Overview of Epigenetics Common types of molecular changes that underlie epigenetic effects on gene expression are DNA methylation, chromatin remodeling, covalent histone modification, and localization of histone variants 16.2 Epigenetics: Genomic Imprinting Section 16.2 Learning Outcomes 1. Predict the outcome of crosses for imprinted genes 2. Explain the molecular basis of genomic imprinting 16.2 Epigenetics: Genomic Imprinting Genomic imprinting refers to a segment of DNA being “marked”; the mark is retained and recognized throughout the life of the organism A gene can be marked by females during egg formation or by males during sperm production, but not both The marking process involves epigenetic modifications and affects whether or not the gene is expressed The offspring expresses either the maternal allele or the paternal allele, but not both Imprinted genes do not follow a Mendelian pattern of inheritance Several hundred to over a thousand human genes are thought to be imprinted 16.2 Epigenetics: Genomic Imprinting DNA Methylation Affects the Transcription of an Imprinted Gene DNA methylation is the marking process that occurs during the imprinting of certain genes, including Igf2 For most genes, methylation silences gene expression; for some it may enhance gene expression Each time a somatic cell divides, the methylation state is retained The methylation state can be altered when individuals make gametes The reactions that add or remove methyl groups are catalyzed by enzymes 16.2 Epigenetics: Genomic Imprinting For Imprinted Genes, the Gene from Only One Parent Is Expressed One of the first imprinted genes to be identified was Igf2, which encodes a growth hormone called insulin-like growth factor 2 The Igf2- mutation blocks the function of the Igf2 hormone 16.2 Epigenetics: Genomic Imprinting For Imprinted Genes, the Gene from Only One Parent Is Expressed If female parent is homozygous for the mutant allele all normal If male parent is homozygous for mutant allele all dwarf offspring The normal size and dwarf offspring all have the same genotype (Igf2 Igf2-) but have different phenotypes Igf2 is imprinted such that only the paternal allele is expressed Loading… 16.3 Epigenetics: X-Chromosome Inactivation Section 16.3 Learning Outcomes 1. Explain how X- chromosome inactivation may affect the phenotype of female mammals 2. Describe the process of X- chromosome inactivation at the cellular level 16.3 Epigenetics: X-Chromosome Inactivation Female mammals carry two X chromosomes (XX) whereas males carry one X chromosome (XY) During embryonic development in female mammals, one of the X chromosomes undergoes an epigenetic change called X- chromosome inactivation (XCI) The inactivated X chromosome (Barr body) becomes highly compacted, which silences the genes that it carries 16.3 Epigenetics: X-chromosome Inactivation In Female Mammals, One X Chromosome Is Inactivated in Each Somatic Cell The patchy pattern of calico cats is due to permanent inactivation of one X chromosome in each skin cell Coat color is determined by an X-linked gene Orange allele, XO Black allele, XB Heterozygous XOXB female will be calico 16.3 Epigenetics: X-chromosome Inactivation In Female Mammals, One X Chromosome Is Inactivated in Each Somatic Cell In early development, one of the two X chromosomes is randomly inactivated in each somatic cell, including the cells that give rise to the hair-producing skin cells The pattern of XCI is maintained during subsequent cell divisions Female mammals can be described as mosaics because they are composed of two types of cells 16.3 Epigenetics: X-chromosome Inactivation In Female Mammals, One X Chromosome Is Inactivated in Each Somatic Cell It is proposed that XCI achieves dosage compensation, equalization of levels of expression of X-linked genes in male and female cells Cells of humans and other mammals can “count X chromosomes” and allow only one X chromosome to be active Extra X chromosomes are converted to Barr bodies 16.3 Epigenetics: X-chromosome Inactivation The X Chromosome Has an X Inactivation Center That Controls Compaction into a Barr Body A short region of the X chromosome called the X inactivation center (Xic) plays a critical role in XCI The expression of a specific gene, Xist (X inactive specific transcript) is required for the compaction of the X chromosome into a Barr body The Xist gene product is a long RNA molecule that does not encode a protein; instead the Xist RNA coats one of the two X chromosomes during the process of XCI After coating, proteins associate with the Xist RNA and cause epigenetic changes that promote the compaction of the chromosome This chromosome is maintained as a Barr body for an individual’s life 16.4 Epigenetics: Effects of Environmental Agents Section 16.4 Learning Outcomes 1. Explain how chemicals in the diet may affect an individual's phenotype 2. List examples of chemicals that cause epigenetic changes that may contribute to cancer 16.4 Epigenetics: Effects of Environmental Agents Chemicals in an Individual’s Diet May Cause Epigenetic Changes That Affect Phenotype Studies of the Agouti gene in mice have demonstrated how chemicals in the diet can promote epigenetic changes This gene encodes the Agouti signaling peptide that controls the deposition of yellow pigment in developing hairs Several mutations have been identified; Avy is a gain of function mutation due to the insertion of a new promoter next to the normal promoter Overexpression of Agouti causes mice to be yellow, however there is wide variation in phenotype of mice carrying the Avy allele 16.4 Epigenetics: Effects of Environmental Agents Chemicals in an Individual’s Diet May Cause Epigenetic Changes That Affect Phenotype The new promoter in Avy mice is very sensitive to epigenetic changes; coat colors correlate with the degree of methylation at the new promoter Nutrients known to inhibit DNA methylation affect coat color in Avy mice; offspring of females fed a supplemental diet tended to have darker coats 16.4 Epigenetics: Effects of Environmental Agents Environmental Agents May Cause Epigenetic Changes That Are Associated with Human Diseases Like Cancer Researchers have identified many examples in which epigenetic changes are associated with a particular disease (Alzheimer, diabetes, multiple sclerosis, asthma, cardiovascular diseases) The role of epigenetics in cancer has been most studied; several environmental factors have been associated with specific cancers 16.5 Extranuclear Inheritance: Organelle Genomes Section 16.5 Learning Outcomes 1. Describe the general features of mitochondrial and chloroplast genomes 2. Explain why chloroplast and mitochondrial genes usually exhibit maternal inheritance 3. List human diseases associated with mutations in mitochondrial genes 16.5 Extranuclear Inheritance: Organelle Genomes The transmission of genes located outside the nucleus is called extranuclear inheritance Mitochondria and chloroplasts contain their own genome The endosymbiosis theory describes the origin of these semiautonomous organelles 16.5 Extranuclear Inheritance: Organelle Genomes Chloroplast and Mitochondrial Genomes Are Small but Contain Genes That Encode Important Proteins Mitochondrial and chloroplast genomes are composed of a single, circular DNA molecule Typically, the mammalian mitochondrial genome has 37 genes 24 encode tRNAs and rRNA needed for translation inside the mitochondrion 13 encode proteins for oxidative phosphorylation Chloroplast genomes of flowering plants contain 100-200 genes Many encode proteins vital to photosynthesis 16.5 Extranuclear Inheritance: Organelle Genomes Chloroplast Genomes Are Often Maternally Inherited Leaf pigment in the four-o’clock plant does not obey Mendel’s law of segregation Leaf pigmentation of offspring depends solely on the pigmentation of the maternal plant; this phenomenon is called maternal inheritance The white phenotype is caused by a mutation in a gene within the chloroplast genome that prevents the synthesis of most chlorophyll The maternal inheritance pattern occurs because the chloroplasts are only transmitted through the cytoplasm of the egg 16.5 Extranuclear Inheritance: Organelle Genomes Chloroplast Genomes Are Often Maternally Inherited Chloroplasts develop from proplastids An egg cell contains several proplastids so an offspring from a variegated maternal plant can be green, white or variegated In seed-bearing plants, maternal inheritance of chloroplasts is the most common transmission pattern Some species exhibit biparental inheritance (both pollen and egg contribute chloroplasts) and others exhibit paternal inheritance (only pollen contributes chloroplasts) 16.5 Extranuclear Inheritance: Organelle Genomes Mitochondrial Genomes Are Maternally Inherited in Humans and Most Other Species Maternal inheritance is the most common pattern of mitochondrial transmission in eukaryotic species Some species do exhibit biparental or paternal inheritance Mutations in human mitochondrial genes can cause a variety of rare diseases; usually affect organs and cells that require high levels of ATP 16.6 Linkage of Genes on the Same Chromosome Section 16.6 Learning Outcomes 1. Describe how linkage violates the law of independent assortment 2. Explain how experimental crosses can demonstrate linkage 16.6 Linkage of Genes on the Same Chromosome When two genes are close together on the same chromosome, they tend to be transmitted as a unit, a phenomenon called linkage Loading… 16.6 Linkage of Genes on the Same Chromosome Linkage and Crossing Over Produce Nonrecombinant and Recombinant Types In the P generation, b+b+ c+c+ (grey body, straight wings) were crossed with bb cc (black body, curved wings) As expected, all F1 had gray bodies and straight wings (dominant traits) F1 were mated with a fly that was homozygous recessive for both traits in a test cross 16.6 Linkage of Genes on the Same Chromosome Linkage and Crossing Over Produce Nonrecombinant and Recombinant Types If the genes for body color and wing shape are on different chromosomes and assort independently, the F2 offspring will have four possible phenotypes in a 1:1:1:1 ratio Instead, the two most abundant phenotypes are those with the combinations of characteristics from the P generation: gray bodies with straight wings or black bodies and curved wings