Gene Expression: From Gene to Protein – The Central Dogma and Genetic Code

Gene Expression: From Gene to Protein

Overview of Gene Expression

Gene expression is the process by which information encoded in a gene is used to direct the synthesis of a protein, ultimately determining an organism's traits. This process involves two main stages: transcription and translation.

• Transcription: The synthesis of RNA from a DNA template.

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

• Central Dogma: The flow of genetic information is typically described as DNA → RNA → Protein.

Example: The production of hemoglobin in red blood cells is controlled by the expression of specific genes.

One Gene–One Polypeptide Hypothesis

Early experiments by Beadle and Tatum established that genes act by coding for enzymes, each of which affects a single step in a metabolic pathway. This led to the "one gene–one enzyme" hypothesis, later refined to "one gene–one polypeptide" as not all proteins are enzymes and some proteins consist of multiple polypeptides.

• Mutational Analysis: Mutants unable to synthesize certain nutrients helped map gene function to specific enzymes.

• Example: Neurospora crassa mutants lacking enzymes for arginine synthesis.

The Molecular Basis of Genes and Proteins

Structure of DNA and RNA

Genes are composed of DNA, a polymer of nucleotides. Each nucleotide contains a sugar, a phosphate group, and a nitrogenous base (A, T, C, G for DNA; A, U, C, G for RNA).

• DNA: Double-stranded helix; bases are adenine (A), thymine (T), cytosine (C), guanine (G).

• RNA: Usually single-stranded; uracil (U) replaces thymine.

Additional info: The sequence of bases in DNA determines the sequence of amino acids in proteins via the genetic code.

Transcription: DNA to RNA

Transcription is the process by which a segment of DNA is copied into RNA by the enzyme RNA polymerase.

• Initiation: RNA polymerase binds to the promoter region of a gene.

• Elongation: RNA polymerase synthesizes a complementary RNA strand.

• Termination: Transcription ends when RNA polymerase reaches a terminator sequence.

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA}$

Translation: RNA to Protein

Translation is the process by which the sequence of an mRNA molecule is decoded to build a polypeptide chain (protein).

• Codons: Triplets of nucleotides in mRNA that specify amino acids.

• tRNA: Transfer RNA molecules bring amino acids to the ribosome.

• Ribosome: The molecular machine that assembles proteins.

Equation:

$\text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

The Genetic Code

Codons and the Triplet Code

The genetic code consists of three-nucleotide sequences called codons, each of which specifies a particular amino acid or a stop signal.

• Start Codon: AUG (methionine) – signals the start of translation.

• Stop Codons: UAA, UAG, UGA – signal the end of translation.

• Redundancy: Multiple codons can code for the same amino acid.

Example: The codon UUU codes for phenylalanine; AUG codes for methionine and also serves as the start signal.

Codon	Amino Acid	Function
AUG	Methionine	Start
UUU	Phenylalanine	Amino Acid
UAA	None	Stop
UAG	None	Stop
UGA	None	Stop

Additional info: The genetic code is nearly universal among all organisms, highlighting evolutionary conservation.

Regulation and Processing of Genetic Information

RNA Processing in Eukaryotes

In eukaryotic cells, the initial RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.

• 5' Cap: Added to the beginning of the RNA transcript.

• Poly-A Tail: Added to the end of the RNA transcript.

• Splicing: Removal of non-coding regions (introns) and joining of coding regions (exons).

Central Dogma and Its Modifications

The Central Dogma describes the flow of genetic information from DNA to RNA to protein. However, exceptions exist, such as reverse transcription in retroviruses (RNA → DNA).

• Reverse Transcriptase: Enzyme used by retroviruses to synthesize DNA from RNA.

• Additional info: Some genes code for functional RNAs (e.g., rRNA, tRNA) rather than proteins.

Evolutionary Significance of the Genetic Code

Universality of the Genetic Code

The genetic code is nearly universal, shared by all living organisms, indicating a common evolutionary origin.

• Application: Genetic engineering uses the universality of the code to transfer genes between species.

• Example: Inserting a jellyfish gene into a firefly to produce bioluminescence.

Additional info: These notes expand on the original material by providing definitions, examples, and context for key concepts in gene expression and the genetic code, suitable for exam preparation in General Biology.