From DNA to Protein: Transcription and Translation

Posted on Sep 5, 2024 in Biology

Transcription: mRNA Synthesis from DNA

Gene to Protein

Genes, located within specific DNA regions, hold the instructions for building specific proteins. Protein synthesis happens outside the nucleus, on ribosomes. To bridge this gap, a molecule is needed to carry the DNA code and ensure accurate protein production.

Transcription Process

  1. Initiation: RNA polymerase unwinds the DNA segment containing the target gene.
  2. Elongation: RNA polymerase adds RNA nucleotides from the nucleus to the DNA template strand, following base-pairing rules (A with U, G with C). It also forms covalent bonds between the mRNA nucleotides.
  3. Termination: Once the gene is transcribed, the mRNA strand detaches and exits the nucleus through a nuclear pore. It then travels to the ribosomes for protein synthesis.

Note: The mRNA strand uses uracil (U) instead of thymine (T). The DNA strand matching the mRNA sequence is the sense (coding) strand, while the other is the antisense (template) strand.

Translation: Polypeptide Synthesis on Ribosomes

Decoding mRNA

Translation is the process of creating polypeptides with specific amino acid sequences based on the mRNA’s base sequence, which is determined by the gene. This process occurs on ribosomes in the cytoplasm or on the rough endoplasmic reticulum (ER).

Ribosomes: The Protein Factories

Ribosomes, composed of a large and a small subunit, are made of rRNA and protein. They serve as the sites for polypeptide synthesis.

The Genetic Code: Linking mRNA to Amino Acids

mRNA’s Role

Messenger RNA (mRNA) carries the genetic information from a specific gene to the ribosomes to synthesize the correct polypeptide. Each mRNA molecule is specific to a single polypeptide.

Codons: Triplets of Bases

The mRNA strand consists of three-base units called codons. Each codon specifies a particular amino acid. The protein’s amino acid sequence is determined by the mRNA sequence, which is a complementary copy of the gene in the DNA.

Decoding the Code

The 64 possible triplet codons can code for one of the 20 amino acids. Three codons (UAA, UAG, and UGA) do not code for amino acids and act as stop signals for translation. The AUG codon codes for methionine and serves as the start codon, initiating translation. The genetic code is considered “degenerate” because multiple codons can code for the same amino acid.

Translation: mRNA-tRNA Interaction

Decoding mRNA with tRNA

Translation involves the assembly of amino acids into proteins (polypeptides). mRNA carries codons (3 base pairs) that dictate the polypeptide’s amino acid sequence. tRNA molecules have anticodons that bind to their complementary mRNA codons, carrying the corresponding amino acid.

Ribosomes and Polypeptide Synthesis

rRNA provides binding sites for mRNA and tRNA and catalyzes the formation of the polypeptide chain. After transcription, the mRNA exits the nucleus through a nuclear pore and binds to a ribosome in the cytoplasm or on the rough ER.

Initiation and Elongation

  1. The mRNA binds to the ribosome’s small subunit, with its first two codons within the ribosome’s binding sites. The first codon, AUG (the start codon), codes for methionine.
  2. The corresponding tRNA carrying methionine binds to the mRNA, initiating the polypeptide chain.
  3. A second tRNA binds to the mRNA at the second binding site, carrying the amino acid specified by the second codon.
  4. The two amino acids are joined by a condensation reaction.

Application: Human Insulin Production in Bacteria

Universality of the Genetic Code

A gene determines the production of a specific polypeptide in an organism. Due to the universality of the genetic code, transferring a gene from one species to another does not alter the amino acid sequence of the resulting polypeptide.

Producing Human Insulin

Animal insulin has been used to treat diabetes, but some individuals develop allergic reactions. Since 1982, human insulin has been produced using gene transfer techniques with E. coli bacteria.

Gene Transfer Process

  1. mRNA Extraction: mRNA coding for insulin is extracted from pancreatic cells.
  2. cDNA Synthesis: Reverse transcriptase is used to create a complementary DNA (cDNA) strand from the mRNA.
  3. Plasmid Preparation: Bacterial plasmids (small circular DNA molecules) are cut with a restriction enzyme, creating “sticky ends” for cDNA attachment.
  4. cDNA Insertion: DNA ligase seals the cDNA into the plasmid.
  5. Transformation: The plasmid carrying the insulin gene is inserted into plasmid-free E. coli bacteria (host cells).
  6. Fermentation: The insulin-producing bacteria reproduce rapidly during fermentation.
  7. Insulin Extraction and Purification: Insulin is extracted from the bacteria and purified for use by diabetics.