DNA Structure ,  Replication, and Repair  

DIRECTIONS:   Each item below contains a question or incomplete statement followed by suggested responses. Select the one best response to  each question

۱-              Patients with Hurler’s syndrome (252800) are known to have mutations at the L-iduronidase locus. The diagnosis of Hurler’s syndrome is most efficiently 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

۲-                      ؟ 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

۳-                    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 fraction 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

                                                                                                                                                                                                                 A polymorphism is best defined as                                                                                          - ۴ 

                a.  Cosegregation of alleles

                b.  One phenotype, multiple genotypes c.  Nonrandom allele association

d.  One locus, multiple abnormal alleles

                e.  One locus, multiple normal alleles

۵-        The process that occurs at the 5 position of cytidine and often correlates with gene inactivation is

a.  Gene conversion

b.  Sister chromatid exchange

                c.  Pseudogene

d.  Gene rearrangement

                e.  DNA methylation

جوابها: 

1-              The answer is b.  (Murray, pp 412-434. Scriver, pp 3-45. Sack, pp 3-29. Wilson, pp 99-121.) The most efficient DNA diagnosis would involve analysis of the two L-iduronidase genes in an individual, looking for any change in DNA sequence (nucleotide substitutions, deletions, duplications) that would alter function of the L-iduronidase protein. Alternative forms of a gene are called alleles, and an allele that has changed during transmission from parent to child may be called a mutant allele. Many human characteristics are encoded by genes, each occupying a particular address or locus on a chromosome. Genes consist of DNA segments that encode RNA along with flanking DNA sequences that regulate gene expression (see the figures below). Within the coding regions are DNA segments that are transcribed and then translated into protein (exons), and those that are transcribed but removed by RNA splicing (introns). If a DNA mutation produces a disease through  its  altered  protein,  as  with  L-iduronidase  gene  mutations  in Hurler’s syndrome, the mutant gene (strictly, the mutant allele) may be called abnormal. Human autosomes (chromosomes 1 through 22) and X chromosomes in females have two homologous loci in each individual, harboring two identical (homozygous) or different (heterozygous) alleles. Males have only one X chromosome and often only one allele per sex chromosome locus because the Y has minimal coding material. The complete set of genetic material (all the genes and loci) in each biological species is called the genome.  

             ۲-                          The answer is b.  (Murray, pp 412-434. Scriver, pp 3-45. Sack, pp 3-29. Wilson, pp 99-121.) In the classic double-helical model of DNA proposed by Watson and Crick, the purine (adenosine and guanine) and pyrimidine (cytosine and thymine) bases (see the figure above) attached to the sugar backbone are perpendicular to the axis and parallel to each other. They are paired (A to G or T to C) and held together by hydrogen bonds. The DNA strands (nucleotide polymers) are joined by linkages between the 3′-hydroxyl of each pentose  (deoxyribose) and the  5′-phosphate of its deoxyribose neighbor. Each strand composing the double helix is different and antiparallel. The 3′ end of one strand is opposite the 5′ end of its complement and vice versa (see the figure above). It is this complementary nature of DNA that allows the strands to be templates for one another during DNA replication

                ۳-               The answer is b.  (Murray, pp 412-434. Scriver, pp 3-45. Sack, pp 3-29. Wilson, pp 99-121.) The melting temperature Tm  of duplex DNA is the temperature at which half the base pairs are denatured. Adenine-thymine (A-T) base pairs have two hydrogen bonds, in contrast to cytosineguanine (C-G) base pairs, which have three hydrogen bonds. Duplex DNA molecules rich in A-T base pairs have a much lower Tm than those rich in C-G base pairs. As DNA is heated, fractions with a higher A-T content melt or denature before those with a higher C-G content. Most mammals, including humans, have satellite DNA fractions that are highly repetitive and  clustered  in  particular  chromosome  regions.  Satellite  DNAs  are named for their altered density (satellite band) on centrifugation, caused by higher G-C content. Their function is unknown

                ۴-            The answer is e.  (Murray, pp 412-434. Scriver, pp 3-45. Sack, pp 3-29. Wilson, pp 99-121.) Polymorphic loci have multiple alleles because of DNA sequence variation, including one or more with frequencies greater than 1%. This higher frequency and benign connotation differentiate polymorphic loci from those that harbor multiple disease-causing alleles. The DNA sequence changes may alter restriction sites [producing restriction fragment length polymorphisms (RFLPs)], change the numbers of repeated segments [producing variable numbers of tandem repeats (VNTRs)], or alter the genetic code (producing variant proteins, or protein polymorphisms). Polymorphisms may cosegregate (be inherited together) with disease alleles, allowing diagnosis by linkage analysis or estimates of risk

through allele association (a.k.a. linkage disequilibrium, as with certain HLA alleles and diabetes mellitus). Different mutant alleles may cause indistinguishable phenotypes (allelic heterogeneity), as may mutations at pG dinucleotides that often cluster in at the upstream promoter regions of genes  (CpG islands). While these are generally correlated with gene inactivation, there are many exceptions. Double crossovers at meiosis can substitute a normal allele for a mutant allele (conversion), and reverse transcriptases can copy intronless mRNA into complementary DNAs (cDNAs) that integrate into the  genome  as  pseudogenes.  Immunoglobulin  genes  undergo  gene rearrangement to unite variable, joining, and constant regions for expression of a unique antibody. Unequal crossing over between sister chromatids is thought to be an important mechanism for variation in copy number within gene clusters