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17 Genome Sequencing, Molecular Biology, and Medicine

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  • 17 Genome Sequencing, Molecular Biology, and Medicine 17.1 How Do Defective Proteins Lead to Diseases?17.2 What Kinds of DNA Changes Lead to Diseases?17.3 How Does Genetic Screening Detect Diseases?17.4 What Is Cancer?17.5 How Are Genetic Diseases Treated?17.6 What Have We Learned from the Human Genome Project?

  • 17.1 How Do Defective Proteins Lead to Diseases?Genetic mutations are often expressed as proteins that differ from wild-type.Genetic diseases can result from abnormalities in enzymes, receptor proteins, transport proteins, structural proteins, etc.

  • 17.1 How Do Defective Proteins Lead to Diseases?Phenylketonuria (PKU) was traced to its molecular phenotype in the 1950s.Results from an abnormal enzyme phenylalanine hydroxylasenormally catalyzes conversion of dietary phenylalanine to tyrosine.The abnormal enzyme has tryptophan instead of arginine in position 408.

  • Figure 17.1 One Gene, One Enzyme

  • 17.1 How Do Defective Proteins Lead to Diseases?People with PKU have light skin and hair color.Melaninpigment in dark skin and hair, is made from tyrosine, which people with PKU can not synthesize.

  • 17.1 How Do Defective Proteins Lead to Diseases?Polymorphism in proteins does not always mean disease.There can be numerous normal alleles of a gene which produce normally functioning proteins.

  • 17.1 How Do Defective Proteins Lead to Diseases?The first human disease known to be caused by an abnormal protein was sickle-cell disease.The abnormal allele produces abnormal hemoglobin that results in sickle-shaped blood cells.The sickle-shaped cells block blood flow in capillaries.

  • 17.1 How Do Defective Proteins Lead to Diseases?Hemoglobinprotein with quaternary structure; 2 and 2 chains.In sickle-cell disease, one of 146 amino acids in the -globin chain is different: glutamic acid (negatively charged) is replaced by valine (neutral).Changes shape of the hemoglobin and causes anemia.

  • 17.1How Do Defective Proteins Lead to Diseases?Variation in hemoglobin has been well documented.There are many amino acid substitutions; many have no effect on the protein function.

  • Figure 17.2 Hemoglobin Polymorphism

  • 17.1 How Do Defective Proteins Lead to Diseases?Some diseases result from altered membrane receptors or transport proteins.Familial hypercholesterolemia (FH)excess cholesterol can accumulate on artery walls and block them, causing heart attacks and strokes.

  • 17.1 How Do Defective Proteins Lead to Diseases?People with FH are unable to transport cholesterol to the liver and other cells that use it.Cholesterol travels as a lipoprotein (LDL). LDL binds to a receptor on a liver cell, and is taken up by endocytosis.In FH, the receptor protein is nonfunctional.

  • Figure 17.3 Genetic Diseases of Membrane Proteins (A)

  • 17.1How Do Defective Proteins Lead to Diseases?In cystic fibrosis, thick dry mucous lines surfaces such as the respiratory tract and prevents passage of air, and prevents cilia from functioning.This is caused by a nonfunctional chloride transporter protein. Normally, this ion channel releases Cl outside the cell. Water leaves cell by osmosis, providing moist surfaces.

  • Figure 17.3 Genetic Diseases of Membrane Proteins (B)

  • 17.1 How Do Defective Proteins Lead to Diseases?Duchenne muscular dystrophy and hemophilia are diseases caused by altered structural proteins.In Duchenne muscular dystrophy, there is no functional dystrophin, which normally connects actin fibers of muscle cells to the extracellular matrix. Without it, muscle cells are structurally disorganized, and stop working.

  • 17.1 How Do Defective Proteins Lead to Diseases?Hemophilia is caused by the absence of a blood clotting protein.Normally, clotting proteins are always present in the blood.

  • 17.1 How Do Defective Proteins Lead to Diseases?Transmissible spongiform encephalopathies (TSEs) are degenerative diseases in which holes develop in the brainincludes mad cow disease.Results from errors in conformation of proteins.

  • Figure 17.4 Mad Cow Disease in Britain

  • 17.1How Do Defective Proteins Lead to Diseases?TSEs can be transferred by eating animals that had the disease.Kurua TSE is found in a tribe in New Guinea that practiced ritual cannibalism.The infectious agent is a prionproteinaceous infective particle.

  • 17.1 How Do Defective Proteins Lead to Diseases?Normal brain cell membranes have a protein called PrPc.In TSE infected tissue, the protein has a different shape, called PrPsc. This protein piles up as fibers and causes cell death.The abnormal PrPsc causes the normal protein to change conformation.

  • Figure 17.5 Prion Diseases are Disorders of Protein Conformation

  • 17.1 How Do Defective Proteins Lead to Diseases?Most human diseases are multifactorialcaused by interactions of many genes and proteins and the environment.Alleles that cause genetic diseases may be inherited in a dominant or recessive pattern, and may be carried on autosomes or sex chromosomes.Some diseases result from extensive chromosomal abnormalities.

  • 17.1 How Do Defective Proteins Lead to Diseases?PKU, sickle-cell disease, and cystic fibrosis are autosomal recessive.If both parents are carriers (heterozygotes with normal phenotypes), every time a child is conceived there is a one in four chance that it will have the disease.

  • 17.1 How Do Defective Proteins Lead to Diseases?Familial hypercholesterolemia is caused by an autosomal dominant allele.Presence of only one mutant allele is enough to cause the disease.

  • 17.1 How Do Defective Proteins Lead to Diseases?Hemophilia is X-linked recessive.A son that inherits the allele from the mother will have the disease, because there is no allele on the Y chromosome.All rare X-linked diseases are much more common in men than women.

  • 17.1 How Do Defective Proteins Lead to Diseases?Chromosomal abnormalities include the gain or loss of chromosomes (aneuploidy), deletions, and translocations.Some are inherited, some result from meiotic events.Fragile-X syndrome is a constriction at the tip of the X chromosome. Causes mental retardation in some people.

  • Figure 17.6 A Fragile-X Chromosome at Metaphase

  • 17.2 What Kinds of DNA Changes Lead to Diseases?Some disease-causing mutations are determined when the abnormal protein phenotype is known, the gene can be cloned.In other cases, the defective protein is unknown until the gene is isolated.

  • 17.2 What Kinds of DNA Changes Lead to Diseases?For sickle-cell disease, mRNA was isolated from immature red blood cells, a cDNA copy was made and used to probe a human gene library.Then gene sequencing was used to compare normal and sickle-cell genes.

  • 17.2 What Kinds of DNA Changes Lead to Diseases?Duchenne muscular dystrophy was thought to be X-linked, but the abnormal protein nor the gene could be identified.A small chromosome deletion was discovered in the X chromosome.Comparing with normal chromosomes allowed the gene to be isolated.

  • Figure 17.7 Two Strategies for Isolating Human Genes

  • 17.2 What Kinds of DNA Changes Lead to Diseases?When no abnormal protein or chromosome deletion can be identified, positional cloning is used.Genetic markers can be positioned anywhere on the DNA. They must be polymorphic (more than one allele).

  • 17.2 What Kinds of DNA Changes Lead to Diseases?RFLPs (Restriction Fragment Length Polymorphisms)If there is a mutation in a restriction site, it will not be cut by a restriction enzyme, resulting in a larger fragment. These can be seen in gel electrophoresis.An RFLP band pattern is inherited in Mendelian fashion.

  • Figure 17.8 RFLP Mapping (Part 1)

  • Figure 17.8 RFLP Mapping (Part 2)

  • 17.2 What Kinds of DNA Changes Lead to Diseases?SNPs (single nucleotide polymorphisms) are widespread in eukaryotic genomes.SNPs can be detected by direct sequence comparisons or chemical methods such as mass spectrometry.

  • 17.2 What Kinds of DNA Changes Lead to Diseases?Genetic markers such as RFLPs and SNPs can be used to find genes if the genes are polymorphic too.The gene and the marker must always be inherited togetherpedigrees are constructed to determine this.

  • 17.2 What Kinds of DNA Changes Lead to Diseases?To isolate a gene, the neighborhood around an RFLP might be screened with other restriction enzymes.When a relatively short sequence of DNA is identified as a candidate for the gene, it can be cut in fragments and tested with probes made from mRNA from affected cells.

  • 17.2 What Kinds of DNA Changes Lead to Diseases?DNA sequencing has shown that mutations occur most often in certain base pairshot spots for mutation.Often where cytosine has been methylated to 5-methylcytosineUnmethylated cytosine can lose its amino group to form uracilthis error is detected and repaired.

  • 17.2 What Kinds of DNA Changes Lead to Diseases?When 5-methylcytosine loses its amino group, it forms thymine, which is ignored by DNA repair mechanism.Mismatch repair recognizes the mistake, GT instead of GC, but cannot tell which member of the pair was incorrect.

  • Figure 17.9 5-Methylcytosine in DNA Is a Hot Spot for Mutations (Part 1)

  • Figure 17.9 5-Methylcytosine in DNA Is a Hot Spot for Mutations (Part 2)

  • 17.2 What Kinds of DNA Changes Lead to Diseases?Larger mutations can involve many base pairs.In Duchenne muscular dystrophy, the deletion may be small, covering only part of the gene for dystrophin, or the entire gene may be deleted.Other mutations involve millions of base pairs.

  • 17.2 What Kin

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