Biochemistry and Genetics: Pretest Self-Assessment and Review

8320 Wilson_hyf_k.qxd 11/14/01 10:35 AM Page 1 High-Yield Facts in Biochemistry and Genetics HORMONAL CONTROL OF METABOLISM Metabolism is precisely regulated by hormones controlling the level of blood fuels and their delivery to tissues. The primary control hormones of metabolism are insulin and glucagon. Epinephrine has effects similar to those of glucagon, except that glucagon has a greater effect on the liver while epinephrine has a greater effect on muscle. Blood levels of glucose, amino acids, fatty acids, and ketone bodies are maintained by variations in the [insulin]/[glucagon] ratio. When blood sugar is high, the ratio increases and insulin signals the fed state, promoting anabolic activities. The ratio decreases as glucagon is released to direct catabolic activities when blood glucose falls between meals, during fasting, and during starvation. Epi- nephrine or norepinephrine is released during exercise to promote catabo- lism of glucose and fat that supports muscular activity. Under normal conditions, the very precise interplay between insulin and glucagon main- tains homeostatic blood fuel levels at about: glucose, 4.5 mM; fatty acids, 0.5 mM; amino acids, 4.5 mM; ketone bodies, 0.02 mM. Blood levels of ketone bodies and fatty acids rise during fasting or during starvation, with blood glucose levels being maintained. However, during uncontrolled juvenile diabetes, blood glucose levels rise greatly. The lack of insulin in this disease otherwise mimics starvation. The activity of various pathways during different metabolic states is summarized in the following table. 1 Copyright 2002 The McGraw-Hill Companies. Click Here for Terms of Use. 8320 Wilson_hyf_k.qxd 11/14/01 10:35 AM Page 2 2 Biochemistry and Genetics ACTIVITY OF METABOLIC PATHWAYS Pathway Fed Fasted Diabetes Glycogen synthesis + ? ? Glycolysis (liver) + ? ? Triacylglyceride synthesis + ? ? Fatty acid synthesis + ? ? Protein synthesis + ? ? Cholesterol synthesis + ? ? Glycogenolysis ? + + Gluconeogenesis (liver) ? + + Lipolysis ? + + Fatty acid oxidation ? + + Protein breakdown ? +/? +/? Ketogenesis (liver) ? + + Ketone body utilization ? + + (non-hepatic tissues) KEY FACTS ABOUT INHERITANCE • Human gametes have 23 chromosomes (haploid chromosome number n = 23), while most somatic cells have 46 chromosomes (diploid chro- mosome number 2n=46). • Genes occupy sites on chromosomes (loci) and occur in alternative forms (alleles). • Mendelian diseases exhibit autosomal dominant, autosomal recessive, or X-linked inheritance, while multifactorial diseases (e.g., cleft palate, dia- betes mellitus, schizophrenia, hypertension) are determined by multiple genes plus the environment. • Characteristics of autosomal dominant diseases include a vertical pedi- gree pattern, affliction of both males and females, variable expressivity (variable severity among affected individuals), frequent new mutations, and a 50% recurrence risk for offspring of affected individuals (see pedi- gree A on chart). Corollary: germ-line mosaicism may produce affected siblings with autosomal dominant disease when neither parent is affected. • Characteristics of autosomal recessive diseases include a horizontal pedi- gree pattern, affliction of males and females, frequent consanguinity 8320 Wilson_hyf_k.qxd 11/14/01 10:35 AM Page 3 High-Yield Facts 3 (inbreeding), frequent carriers (heterozygotes without manifestations of disease), and a 25% recurrence risk for carrier parents (see pedigree B on chart). Corollary:normal siblings of individuals with autosomal recessive disease have a 2/3 chance of being carriers. • Characteristics of X-linked recessive diseases include an oblique pedi- gree pattern, affliction of males only, frequent female carriers, and a 25% recurrence risk for carrier females (see pedigree C on chart). Corollary: Haldane’s law predicts a 2/3 chance that the mother of an affected male with X-linked recessive disease is a carrier (and a 1/3 chance the affected male represents a new mutation). • Ethnic correlations with Mendelian disorders include higher frequencies of cystic fibrosis in whites, sickle cell anemia in blacks, ?-thalassemia in Italians and Greeks, ?-thalassemia in Asians, and Tay-Sachs disease in Jews. • Advanced maternal age is associated with higher risks for chromosomal disorders (e.g., Down’s syndrome, trisomy 13), while advanced paternal age is associated with higher risks for new mutations (e.g., those pro- ducing achondroplasia or Marfan’s syndrome). • The Hardy-Weinberg law predicts allele frequencies in an idealized pop- ulation according to the formula p2+2pq +q2=1. Applied to cystic fibro- sis, the law predicts that homozygotes (q2) have a frequency of 1 in 1600, predicting that carriers (2pq) have a frequency of 1 in 20. • A karyotype is an ordered arrangement of chromosomes that is described by cytogenetic notation. A karyotype can be obtained from dividing cells (blood leukocytes, bone marrow, fibroblasts, amniocytes), but not from frozen or formalin-fixed cells. • Cytogenetic notation includes the chromosome number (usually 46), description of the sex chromosomes (usually XX or XY), and indication of missing, extra, or rearranged chromosomes. Examples include 47,XY,+21 (male with Down’s syndrome); 47,XX,+13 (female with tri- somy 13); 45,X (female with monosomy X or Turner’s syndrome); 46,XX,del(5p) (female with deletion of the chromosome 5 short arm). • DNA diagnosis examines specific regions of genes for altered nucleo- tide sequences or deletions that affect gene expression and function; techniques include Southern blotting, gene amplification with the polymerase chain reaction (PCR), and mutant allele detection by 8320 Wilson_hyf_k.qxd 11/14/01 10:35 AM Page 4 4 Biochemistry and Genetics Pedigree symbols and pedigree patterns. 8320 Wilson_hyf_k.qxd 11/14/01 10:35 AM Page 5 High-Yield Facts 5 hybridization with allele-specific oligonucleotides (ASOs). Chromosome microdeletions encompass several genes and are detected by fluorescent in situ hybridization (FISH). • Non-Mendelian inheritance mechanisms include mitochondrial inheri- tance (exhibiting maternal transmission), expansion of triplet repeats (exhibiting anticipation in pedigrees as in the fragile X syndrome), and genomic imprinting (exhibiting different phenotypes according to maternal or paternal origin of the aberrant genes). • Prenatal diagnosis can include fetal ultrasound, maternal serum studies, or sampling of cells from the fetoplacental unit by chorionic villus sam- pling [CVS at 8 to 10 weeks, amniocentesis at 12 to 18 weeks, or percu- taneous umbilical sampling (PUBS) from 16 weeks to term]. 8320 Wilson_hyf_k.qxd 11/14/01 10:35 AM Page 6 6 Biochemistry and Genetics GENETICALLY BASED BIOCHEMICAL DISEASES Disease and Incidence* Defect Symptoms Glycolysis-based Deficient glycolytic Hemolytic anemia hemolytic anemias enzymes Glucose-6-P Deficient enzyme of Hemolytic anemia with dehydrogenase pentose phosphate antimalarial drugs deficiency shunt (up to 1 in 3) Glycogen storage diseases Deficient glycogen Glycogen accumulation (one type XLR) catabolism Type 1a von Gierke’s Glucose-6-phosphatase Large liver, disease (1 in 100,000) deficiency hypoglycemia Type II Pompe’s disease Lysomal ?-glucosidase Short PR interval on (1 in 100,000) deficiency ECG, fatal Type III Cori’s disease Debranching enzyme Large liver, mild deficiency myopathy Type V McArdle’s disease (1 in 100,000) Lipid storage diseases Deficiencies of Sphingolipid storage sphingolipid neurodegeneration metabolism Tay-Sachs disease Hexosaminidase A Cherry red spot, [1 in 4,000 (Jews); deficiency neurodegeneration 1 in 100,000] Krabbe’s disease Galactosylceramide Neurodegeneration, (1 in 100,000) ?-galactosidase demyelination deficiency Niemann-Pick disease Sphingomyelinase Organomegaly, type A (1 in 50,000) deficiency neurodegeneration Gaucher’s disease type I Glucosylceramide Organomegaly, [1 in 1,000 (Jews); ?-glucosidase fractures 1 in 100,000] deficiency Fabry’s disease (XLR) ?-galactosidase Angiokeratoma, nerve (1 in 40,000) deficiency pains Lipid transport Abnormality in plasma Fatty serum, diseases (most AD) lipoprotein receptors atherosclerosis or enzymes 8320 Wilson_hyf_k.qxd 11/14/01 10:35 AM Page 7 High-Yield Facts 7 GENETICALLY BASED BIOCHEMICAL DISEASES (CONT.) Disease and Incidence* Defect Symptoms Familial Defective apo-B100 Xanthomas, hypercholesterolemia LDL receptors hypercholesterolemia type IIa [1 in 500 (AD)] Familial Increased synthesis or Hypertriglyceridemia, hypertriglyceridemia decreased catabolism atherosclerosis type IV [1 in 50,000 of VLDLs (AD)] Ion channel diseases Sodium transport High sweat chloride, (cystic fibrosis) deficiency lung disease Nucleotide catabolism PRPP synthetase Gouty arthritis diseases (some XLR) abnormalities Lesch-Nyhan syndrome Deficient HGPRT Self-mutilation [(XLR) 1 in 100,000] DNA repair diseases Exonuclease deficiency Skin cancer (xeroderma pigmentosum) Hereditary DNA mismatch repair Colon cancer nonpolyposis defects colorectal cancer [(AD) 1 in 50,000] RNA-processing diseases Imbalance of ?- or ?- Anemia [thalassemias (1 in hemoglobin chains 50,000; higher in Mediterraneans (?) or Asians (?)] Porphyrias Heme biosynthesis Abdominal pain, [(one form AD) enzyme defects psychosis, 1 in 1 million] skin rash Amino acid metabolism Phenylalanine Mousy odor, pale skin, diseases hydroxylase deficiency blond hair [Phenylketonuria (1 in 12,000) Maple syrup urine Branched-chain amino Seizures, acidosis disease acid dehydrogenase (1 in 100,000)] deficiency *All diseases are autosomal recessive unless otherwise indicated. AD, autosomal dominant; XLR, X-linked recessive; PRPP, 5-phosphoribosyl-1-pyrophosphate; HGPRT, hypoxanthine-guanine phosphoribosyl- transferase. 8320 Wilson_01_k.qxd 11/14/01 10:36 AM Page 9 Storage and Expression of Genetic Information Copyright 2002 The McGraw-Hill Companies. Click Here for Terms of Use. 8320 Wilson_01_k.qxd 11/14/01 10:36 AM Page 11 DNA Structure, Replication, and Repair Questions DIRECTIONS: Each item below contains a question or incomplete statement followed by suggested responses. Select the one bestresponse to each question. 1. Patients with Hurler’s syndrome (252800) are known to have mutations at the L-iduronidase locus. The diagnosis of Hurler’s syndrome is most effi- ciently made by analyzing a patient’s DNA for a. A region of DNA that does not encode RNA b. Alternative forms of the L-iduronidase gene c. The entire set of genes in one leukocyte d. A nucleotide substitution in the L-iduronidase gene e. The position of the L-iduronidase gene on a chromosome 2. Which of the following statements regarding a double-helical molecule of DNA is true? a. All hydroxyl groups of pentoses are involved in linkages b. Bases are perpendicular to the axis c. Each strand is identical d. Each strand has parallel, 5?to 3?direction e. Each strand replicates itself 3. A sample of human DNA is subjected to increasing temperature until the major fraction exhibits optical density changes due to disruption of its helix (melting or denaturation). A smaller fraction is atypical in that it requires a much higher temperature for melting. This smaller, atypical frac- tion of DNA must contain a higher content of a. Adenine plus cytosine b. Cytosine plus guanine c. Adenine plus thymine d. Cytosine plus thymine e. Adenine plus guanine 11 Copyright 2002 The McGraw-Hill Companies. Click Here for Terms of Use.