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Genetic Mutations and Analysis Methods

This text describes the characteristics of recessive and dominant mutations, temperature-sensitive mutations, complementation analysis, cDNA and genomic DNA libraries, gene expression systems, DNA polymorphisms, expression analysis, dominant negative alleles, and RNA interference.

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Genetic Mutations and Analysis Methods

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  1. Chap. 5 Problem 1 Recessive mutations must be present in two copies (homozygous) in diploid organisms to show a phenotype (Fig. 5.2). These mutations show a loss-of-function phenotype. Lethal recessive mutations can be maintained in diploid organisms in heterozygous form. In contrast, dominant mutations must be present in only one copy (heterozygous) in diploid organisms to show a phenotype. Dominant mutations often cause a gain-of-function phenotype and can be lethal in heterozygotes.

  2. Chap. 5 Problem 2 A temperature-sensitive (ts) mutation is a mutation that causes a phenotype only under the condition of high temperature (is conditional). ts mutations often are caused by amino acid substitutions that make a protein incorrectly folded and therefore inactive at high (nonpermissive), but not low (permissive) temperatures. ts mutations are useful for analysis of the functions of essential genes in haploid organisms (Fig. 5.6a). It is not possible to create a viable cell that lacks an essential gene.

  3. Chap. 5 Problem 3 Complementation analysis can be used to determine if two recessive mutations reside in the same or in different genes. The wild type phenotype will be observed in a heterozygous diploid organism if the mutations reside in different genes (Fig. 5.7). The mutant phenotype will be observed in a heterozygous diploid organism if the mutations reside in the same gene since neither allele is functional. Dominant mutations cannot be analyzed by complementation analysis as the mutant phenotype will be observed in the presence of the wild type allele.

  4. Chap. 5 Problem 7 A cDNA library is a collection of cloned DNA fragments corresponding to all mRNAs transcribed in a certain tissue or organism. The DNA fragments are derived by reverse transcription of mRNA. A genomic DNA library is a collection of cloned DNA fragments representing all of the DNA of an organism. This includes both protein-coding and non-protein-coding segments of DNA. To clone a gene expressed only in neurons, you could start with either of the genomic DNA libraries, or you could use the cDNA library produced from neurons. You should not use the skin cell cDNA library, as the cDNA for the neuronal gene will not be present in the library.

  5. Chap. 5 Problem 10 To express a foreign gene, the plasmid must have a promoter that will drive expression of the gene (Fig. 5.31). One common method by which expressed proteins are purified is via the attachment of an amino acid sequence such as a polyhistidine sequence (His-tag) that serves as a tag for affinity purification. Mammalian cell expression systems offer the advantage that post-translational modifications such as glycosylation can occur. Glycosylation reactions do not occur in bacteria.

  6. Chap. 5 Problem 13 A number of DNA polymorphisms exist in DNA. SNPs are single nucleotide changes between individuals. SSRs are repeating one-, two-, or three-base sequence duplications that vary in length between individuals. Both of these polymorphisms can be linked to a disease gene and used to map its location in the genome. The tighter the linkage of a marker to a disease gene the closer its location to gene.

  7. Chap. 5 Problem 15 In expression analysis, the expression of a candidate disease gene is is compared in tissues from normal and affected individuals by Northern blotting. Northern blotting allows a comparison of both the level and length of transcripts from a gene, which may be changed in the tissue from the diseased individual. If the disease is caused by a point mutation that changes an amino acid in the encoded protein, then expression analysis may be inconclusive. In this case, DNA sequencing will be required to identify the SNP that causes the disease (Fig. 5.38).

  8. Chap. 5 Problem 17 Dominant negative alleles inactivate the function of a wild type allele in the heterozygous state (Fig. 5.44). Thus both copies of the gene of interest need not be modified to observe a phenotype. In RNA interference, gene expression is inactivated by selective destruction of the mRNA transcribed from that gene. To accomplish this, a short double-stranded RNA is synthesized in vitro (Fig. 5.45 a & b) and then introduced into cells. The base pairing of the introduced RNA with the target mRNA leads to nuclease destruction of the mRNA and silencing of the gene. Alternatively, the RNA can be synthesized in vivo.

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