Chapter 17: Gene Expression

Overview of Gene Expression

• Gene expression is the process by which DNA directs protein synthesis, consisting of two stages: transcription and translation
• Proteins are the link between genotype and phenotype
• RNA is the bridge between DNA and protein synthesis, having a similar structure to DNA but with ribose instead of deoxyribose and uracil instead of thymine

Transcription

• During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand
• The process produces a messenger RNA (mRNA) molecule
• Transcription of a protein-coding gene produces a complementary RNA strand

Translation

• Translation is the process of building a polypeptide, consisting of three stages: initiation, elongation, and termination
• All three stages require protein "factors" that aid in the translation process
• Initiation involves the binding of mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
• The small ribosomal subunit binds with mRNA and a special initiator tRNA, which carries methionine
• Elongation involves the addition of amino acids one by one to the growing chain, requiring energy expenditure and codon recognition

RNA Splicing

• Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides called introns, which lie between coding regions (exons)
• RNA splicing removes introns and joins exons to create an mRNA molecule with a continuous coding sequence
• The signal for RNA splicing is a short nucleotide sequence at each end of an intron

Transfer RNA (tRNA)

• tRNA molecules are translators, translating a particular mRNA codon into a particular amino acid
• Each tRNA molecule bears a specific amino acid at one end and a nucleotide triplet called an anticodon at the other end, which base-pairs with a complementary codon on mRNA
• tRNA molecules are transcribed from DNA templates, reading a nucleic acid word (the mRNA codon) and interpreting it as a protein word (the amino acid)