recap

15.3 recap

Large DNA molecules can be cut into smaller pieces by restriction digestion and then sorted by gel electrophoresis. PCR is used to amplify sequences of interest from complex samples. These techniques are used in DNA fingerprinting to analyze DNA polymorphisms for the purpose of identifying individuals. Genes involved in disease can be identified by first detecting the abnormal DNA sequence and then the protein that the wild-type allele encodes. Scientists hope to be able to identify all species using DNA analysis.

learning outcomes

You should be able to:

  • Explain how DNA sequencing can be used to identify mutations.

  • Explain how a mutation can be identified as a cause for disease.

Question 1

Cystic fibrosis is an autosomal recessive disease in which thick mucus is produced in the lungs and airways. The gene responsible for this disease encodes a protein composed of 1,480 amino acids. In most patients with cystic fibrosis, the protein has 1,479 amino acids: a phenylalanine is missing at position 508. A baby is born with cystic fibrosis. He has an older brother who is not affected. How would you test the DNA of the older brother to determine whether he is a carrier for cystic fibrosis?

Direct DNA sequencing of the cystic fibrosis gene could be done. A person who is a carrier will test positive for both the normal and the mutant alleles.

Question 2

How can a gene mutation that causes a disease be mapped and detected before its protein product is known?

Mapping a disease-causing mutation can be done by linkage analysis. A polymorphic DNA marker such as an STR can be linked to the occurrence of a disease in many patients. This means that the marker must lie on the chromosome near the mutant disease-causing gene. DNA sequencing can then isolate the gene involved. From the gene sequence or genetic technology, the protein encoded by the gene can then be isolated and its function described. So genotype precedes phenotype.

The determination of the precise molecular phenotypes and genotypes of various human genetic diseases has made it possible to diagnose these diseases even before symptoms appear. Let’s take a detailed look at some of these genetic screening techniques.