Single-base substitution

There is a normal variation of DNA sequence just as is true of more obvious aspects of human structute. Variations of DNA sequence, polymorphisms, occur approximately once in evety 500 nucleotides, or about 10 times per genome. There are without doubt deletions and insertions of DNA as well as single-base substitutions. In healthy people, these alterations obviously occur in noncoding regions of DNA or at sites that cause no change in function of the encoded protein. This heritable polymorphism of DNA structure can be associated with certain diseases within a large kindred and can be used to search for the specific gene involved, as is illustrated below. It can also be used in a variety of applications in forensic medicine. 

M14. Matsuura, S., Igarashi, M Tanizawa, Y., Yamada, M., Kishi, F Kajii, T Fujii, H., Miwa, S Sakurai, M., and Nakazawa, A., Human adenylate kinase deficiency associated with hemolytic anemia. A single base substitution affecting solubility and catalytic activity of cytosolic adenylate kinase. J. Biol. Chem. 264, 10148-10155 (1989). 

Myers, R.M., Lumelsky, N., Lerman, L.S. and Maniatis, T. (1985c) Detection of single base substitutions in total genomic DNA. Nature 313, 495-498. 

Sheffield, V.C., Beck, J.S., Kwitek, A.E., Sandstrom, D.W. and Stone, E.M. (1993) The sensitivity of single strand conformation polymorphism analysis for the detection of single base substitutions. Genomics 16, 325-332. 

Single-base substitutions, which affect the amino acid sequence of proteins and lead to altered protein function, are the most frequent type of polymorphisms associated with many disease phenotypes as well as with variation in drag response... 

Molecular analysis of the fibrillin gene in these family members identified the mutation associated with Marfan syndrome as a single base substitution at the 3 end of exon 2. The result of this substitution was a deletion of the entire exon 2 firom the mRNA, and consequently 41 amino acids from the fibrillin protein. The mutation was most likely a... 

Point mutations or single base substitutions are classified as transitions or transversions. 

A. Single-base substitutions may occur as a result of DNA damage, chemical mutagenesis, or unrepaired errors in replication. 

Transitions and transversions that may arise from single-base substitutions have been reviewed in Chapter 11. 

This may be caused by single-base substitutions, insertions, or deletions. 

M4. Myers, R. M., Larin, Z., etal. Detection of single base substitutions by ribonuclease cleavage at mismatches in RNA DNA duplexes. Science 230(4731), 1242—1246 (1985). 

Myers RM, Fischer SG, Lerman LS, Maniatis T (1985) Nearly all single base substitutions in DNA fragments joined to a GC-clamp can be detected by denaturing gradient gel electrophoresis. Nucleic Acids Res 31 3131-3145... 

When a mutation introduces a termination codon in the interior of a gene, translation is prematurely halted and the incomplete polypeptide is usually inactive. These are called nonsense mutations. The gene can be restored to normal function if a second mutation either (1) converts the misplaced termination codon to a codon specifying an amino acid or (2) suppresses the effects of the termination codon. Such restorative mutations are called nonsense suppressors they generally involve mutations in tRNA genes to produce altered (suppressor) tRNAs that can recognize the termination codon and insert an amino acid at that position. Most known suppressor tRNAs have single base substitutions in their anticodons. 

Genetic diseases have always been with us, but it was not until 1949 that the first disease, sickle cell anemia (Box 7-B), was understood at the molecular level. A single base substitution in DNA and the resultant single amino acid substitution in hemoglobin... 

A single-base substitution causing no change in the amino-acid sequence a silent mutation... 

The hypercoagulable states can be inherited or acquired. One inherited hypercoagulable state is antithrombin (AT) deficiency. AT deficiency is an autosomal dominant condition. The type I antithrombin deficiency is characterized by reduced synthesis of normal protease inhibitor molecules. The antigenic and functional activities of antithrombin are reduced due to small deletions or insertions or single base substitutions. Type II antithrombin deficiency is due to defects within the protease inhibitor. While the immunologic activity levels are normal, the plasma levels of antithrombin are reduced. About 42% of afflicted individuals develop clinical manifestations spontaneously. Manifestations related to pregnancy, parturition, oral contraceptives, or trauma occur in 58% of AT deficient individuals (190). 

White MB, Carvalho M, Derse D, O Brien SJ, Dean M. Detecting single base substitutions as heteroduplex polymorphisms. Genomics 1992 12 301-306. 

Several rapid and sensitive methods have been developed to detect mutations in cDNA and genomic DNA. These include ribonuclease (RNase) protection analysis, denaturing gradient gel electrophoresis, and single-strand conformation polymorphism analysis. These methods can be used in conjunction with the PCR technique to detect small deletions or insertions and single base substitutions. 

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