From Genes to Proteins: Decoding Genetic Information

Overview of Genetic Information Flow

The process of gene expression involves the conversion of genetic information stored in nucleic acids (DNA and RNA) into functional proteins. This process is fundamental to all living organisms and is divided into two main stages: transcription and translation.

• Genetic material: Composed of nucleic acids (DNA and RNA).

• Proteins: Polymers made of amino acids, responsible for most cellular functions.

• Key Question: How is information translated from the language of nucleotides (in RNA) to the language of amino acids (in proteins)?

The Genetic Code

The Triplet Nature of the Genetic Code

The genetic code is the set of rules by which the information encoded in nucleic acids is translated into proteins. Francis Crick and Sydney Brenner (1961) demonstrated that the genetic code is read in consecutive, non-overlapping groups of three nucleotides, called codons, each specifying a single amino acid.

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid.

• Triplet code: Each codon consists of three nucleotides.

• Non-overlapping: Codons are read one after another, without overlap.

• Example: The codon UUU codes for the amino acid phenylalanine (Phe).

Discovery and Deciphering of the Genetic Code

• Marshall Nirenberg (1961) identified the first codon (UUU) for phenylalanine.

• The complete genetic code was determined by Nirenberg and Gobind Khorana by the mid-1960s.

• Nirenberg and Khorana received the Nobel Prize in 1968 for this work.

Structure of the Genetic Code Table

The genetic code table shows which codons correspond to which amino acids. It is nearly universal among organisms.

• Start codon: AUG (codes for methionine, Met)

• Stop codons: UAA, UAG, UGA (signal termination of translation)

• Degeneracy: Most amino acids are specified by more than one codon.

Transcription and Translation

Transcription

Transcription is the process by which a DNA template is used to synthesize messenger RNA (mRNA). The mRNA carries the genetic information from the DNA in the nucleus to the ribosome, where translation occurs.

• Template strand: The DNA strand used to synthesize mRNA.

• mRNA: A single-stranded RNA molecule complementary to the DNA template.

Translation

Translation is the process by which the sequence of codons in mRNA is used to assemble a corresponding sequence of amino acids, forming a polypeptide (protein).

• Codon: Three-nucleotide sequence in mRNA specifying an amino acid.

• Anticodon: Three-nucleotide sequence in tRNA complementary to the mRNA codon.

Transfer RNAs (tRNAs) and Ribosomes

Role of tRNAs

Transfer RNAs (tRNAs) are adaptor molecules that match specific amino acids to their corresponding codons in mRNA during translation.

• Each tRNA has an anticodon that base-pairs with a codon in mRNA.

• Aminoacyl-tRNA synthetases are enzymes that attach the correct amino acid to its corresponding tRNA.

Wobble Effect and Degeneracy

The wobble effect refers to the flexibility in base-pairing between the third base of the codon and the corresponding base of the anticodon. This allows some tRNAs to recognize multiple codons, contributing to the degeneracy of the genetic code.

• Codons with different bases at the third position often specify the same amino acid.

• This flexibility reduces the number of tRNAs needed.

Role of Ribosomes

Ribosomes are the cellular machinery responsible for protein synthesis. They bring together mRNA, tRNAs, and the growing polypeptide chain.

• Ribosomes have binding sites for mRNA and tRNAs.

• They catalyze the formation of peptide bonds between amino acids.

Stages of Translation

Initiation

Translation initiation involves the assembly of the ribosome on the mRNA at the start codon (AUG).

• Small ribosomal subunit binds to mRNA.

• Initiator tRNA binds to the start codon.

• Large ribosomal subunit joins to complete the initiation complex.

Elongation

During elongation, amino acids are added one by one to the growing polypeptide chain.

• Each new amino acid is brought by a tRNA whose anticodon matches the next codon in the mRNA.

• Peptide bonds are formed between the carboxyl group of one amino acid and the amino group of the next.

• This is a dehydration synthesis reaction:

Termination

Translation ends when a stop codon is reached. Release factors promote the release of the newly synthesized polypeptide and the disassembly of the ribosome.

• Stop codons: UAA, UAG, UGA.

• Release factors recognize stop codons and trigger hydrolysis of the bond between the polypeptide and tRNA.

Polyribosomes

Multiple ribosomes can simultaneously translate a single mRNA molecule, forming a structure called a polyribosome or polysome. This increases the efficiency of protein synthesis.

Coupling of Transcription and Translation in Prokaryotes

In prokaryotes, transcription and translation are coupled, meaning translation can begin on an mRNA while it is still being synthesized. This is possible because both processes occur in the same cellular compartment (the cytoplasm).

• Allows rapid response to environmental changes.

• Not possible in eukaryotes, where transcription occurs in the nucleus and translation in the cytoplasm.