Chapter 17: Gene Expression: From Gene to Protein

Introduction to the Central Dogma of Molecular Biology

The central dogma of molecular biology describes the flow of genetic information within a biological system. It explains how genetic information stored in DNA is used to synthesize proteins, which determine an organism's traits. This process involves two main stages: transcription and translation.

The Relationship Between Genes and Proteins

• Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein.

• Proteins are the link between genotype (genetic makeup) and phenotype (observable traits).

• A single change in DNA (mutation) can result in a dramatic change in phenotype, as seen in genetic disorders like sickle cell anemia or albinism in animals.

Experimental Discovery: Genes Specify Proteins

The relationship between genes and proteins was elucidated through experiments using model organisms such as the bread mold Neurospora crassa. These experiments demonstrated that specific genes are responsible for the production of specific enzymes, which catalyze steps in metabolic pathways.

Beadle and Tatum's Experiment

• Wild-type Neurospora can grow on minimal medium, which contains only basic nutrients.

• Mutants generated by X-ray exposure failed to grow on minimal medium, indicating a loss of function in a metabolic pathway.

• By supplementing the medium with specific amino acids or vitamins, researchers identified which step in the pathway was disrupted.

Classification of Mutants

Mutants were classified based on their ability to grow when supplemented with pathway intermediates. This led to the formulation of the "one gene–one enzyme" hypothesis, later refined to "one gene–one polypeptide" as not all proteins are enzymes and many proteins are composed of multiple polypeptides.

The Flow of Genetic Information

• The information content of genes is encoded in the specific sequence of nucleotides in DNA.

• Gene expression involves two main stages:

  • Transcription: Synthesis of RNA from a DNA template.

  • Translation: Synthesis of a polypeptide (protein) from an mRNA template.

Gene Expression in Prokaryotes vs. Eukaryotes

• In prokaryotes, transcription and translation occur in the cytoplasm and can happen simultaneously due to the absence of a nuclear envelope.

• In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm. Eukaryotic mRNA undergoes additional processing (e.g., splicing, capping, polyadenylation) before translation.

Refinements of the Central Dogma

• Not all RNA molecules code for proteins. Major types of RNA include:

  • mRNA (messenger RNA): Encodes proteins.

  • tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

  • rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

• Some genes code for functional RNAs (tRNA, rRNA) that are not translated into proteins.

Reverse Transcription and Viruses

• Some viruses (e.g., retroviruses like HIV) use reverse transcriptase to synthesize DNA from an RNA template, integrating viral DNA into the host genome.

• This process is called reverse transcription and is an exception to the standard flow of genetic information.

The Genetic Code

• The genetic code is the set of rules by which nucleotide sequences in mRNA are translated into amino acid sequences in proteins.

• It is a triplet code: three nucleotides (a codon) specify one amino acid.

• There are 64 possible codons (43), but only 20 amino acids, making the code degenerate (more than one codon can specify the same amino acid).

• The code is unambiguous: each codon specifies only one amino acid.

• Three codons (UAA, UAG, UGA) are stop codons that signal the end of translation.

Experimental Evidence for the Triplet Code

• Mutagenesis experiments using insertions and deletions (indels) demonstrated that the genetic code is read in non-overlapping triplets.

• Single or double indels disrupt the reading frame (frameshift mutation), while triple indels may restore the reading frame, supporting the triplet nature of the code.

Reading the Genetic Code Table

• To determine the amino acid specified by a codon, use the first base (left), second base (top), and third base (right) to locate the correct box in the genetic code table.

• For example, the codon GAA codes for glutamic acid (Glu, E).

Evolution of the Genetic Code

• The genetic code is nearly universal among all organisms, providing strong evidence for a common evolutionary origin.

• This universality allows genes from one organism to be expressed in another, a principle used in biotechnology (e.g., expressing a jellyfish gene in bacteria to produce fluorescence).

Summary Table: Key Terms and Concepts