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Modern Genetic Analysis Chapter 3. Gene Function
Summary

Information transfer in cells is by transcription of DNA into RNA and translation of RNA into protein. The functions of DNA and RNA are based on two main principles: base complementarity between single strands and attachment of various DNA and RNA binding proteins. Most genes code for proteins, and their transcripts are mRNA or pre-mRNA; however, a minority code for functional RNAs such as tRNAs and rRNAs, and these RNAs are never translated. RNA is a single-stranded molecule that has a nucleotide sequence complementary to the DNA strand from which it was transcribed. In any one gene, only one DNA strand acts as the transcription template. This strand is in 3 5 orientation, and synthesis of the RNA transcript begins at the transcript's 5 end, proceeding in a 5 3 direction.

Eukaryotic mRNA molecules are capped at the 5 end, and a poly(A) tail is added to the 3 end. Many eukaryotic genes have introns interrupting the coding sequence; these are excised from the transcript before translation.

Protein function is ultimately determined by its primary structure (its amino acid sequence), which is determined by nucleotide sequence. The nucleotide sequence in mRNA is translated three nucleotides (a codon) at a time in the 5 3 direction. Each codon stands for a specific amino acid. Each amino acid is joined to a specific tRNA, which aligns with the proper mRNA codon on the ribosome. Changes in nucleotide sequence in DNA result in altered amino acid sequence, and generally this leads to loss of protein function.

A defective protein-coding allele is recessive if, when paired with a normal allele in a heterozygote, the normal allele can provide adequate protein for the cell's needs (haplo-sufficiency). If this is not the case (haplo-insufficiency), the defective allele is dominant.

© 1999 by W. H. Freeman and Company. All rights reserved.