Genetic Message Expression and Protein Synthesis

Posted on Nov 10, 2024 in Biology

Expression of the Genetic Message

The expression of the genetic message is the process by which the information encoded in DNA is used to synthesize specific proteins. These proteins, including enzymes, are responsible for the structural and functional characteristics of an organism.

Codons

Each codon, a sequence of three nucleotides, captures a specific amino acid. AUG is the initiation triplet (methionine), meaning all proteins begin with it. UAA, UAG, and UGA are termination codons, signaling the end of protein synthesis.

Central Dogma of Molecular Biology

The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

  • Transcription: DNA is copied into a messenger molecule of RNA.
  • Translation: Ribosomes use the information in mRNA to synthesize a protein.

RNA: The Genetic Messenger

RNA plays an indispensable role as an intermediary in the two stages of genetic message expression. There are three main types of RNA:

Messenger RNA (mRNA)

  • A copy of a portion of DNA, carrying the information used by ribosomes to join amino acids in the correct order to form a specific protein.
  • Very short life span.
  • Monocistronic (used only once).
  • Represents 3-5% of total cellular RNA.

Ribosomal RNA (rRNA)

  • Part of the ribosomes, also called structural RNA.
  • Participates in the process of joining amino acids to synthesize proteins.
  • Represents 80-85% of total cellular RNA.

Transfer RNA (tRNA)

  • Transports amino acids to the ribosomes.
  • Each tRNA molecule carries a specific amino acid.
  • Represents 10% of cellular RNA.
  • Forms a cloverleaf structure with four arms, three with loops at the ends, and the other end containing the 3′ and 5′ ends.

Transcription: RNA Synthesis

Transcription, the synthesis of RNA, occurs inside the nucleus and requires:

  • A DNA strand that acts as a template.
  • Enzymes, including RNA polymerase.
  • Ribonucleotide triphosphates (ATP, GTP, CTP, UTP).

Transcription Process

The transcription process can be divided into three stages:

Initiation

  • Begins when RNA polymerase recognizes a promoter site on the DNA, a short sequence of nitrogenous bases that signals the start of transcription.
  • Helicase unwinds the DNA double helix to expose the DNA bases.
  • Ribonucleotides begin to pair with the exposed DNA bases.

Elongation

  • RNA polymerase reads the DNA template strand in the 3′ to 5′ direction and synthesizes RNA in the 5′ to 3′ direction.
  • The synthesized RNA strand is complementary to the DNA template strand.
  • Complementarity between DNA and RNA bases: G-C, A-U, T-A, C-G.

Termination

  • RNA polymerase recognizes a termination signal on the DNA, indicating the end of transcription.
  • The DNA double helix rewinds, and the RNA polymerase separates from the DNA and the newly synthesized RNA transcript.
  • A sequence of adenine ribonucleotides (poly-A tail) is added to the 3′ end of the RNA transcript.
  • A cap is added to the 5′ end, which will be important for translation.

Translation: Protein Synthesis

Translation, the synthesis of proteins, occurs in the cytoplasm and requires:

  • Ribosomes.
  • Messenger RNA (mRNA).
  • Amino acids.
  • Transfer RNA (tRNA).
  • Enzymes.
  • Energy (ATP).

Ribosomes

Ribosomes are cytoplasmic organelles composed of two subunits:

  • Small subunit: binds to mRNA.
  • Large subunit: binds to tRNA and catalyzes peptide bond formation.

Translation Process

Initiation

  • The mRNA initiator codon (AUG) binds to the small ribosomal subunit.
  • The first aminoacyl-tRNA, carrying methionine and with the anticodon UAC, binds to the start codon.
  • The large ribosomal subunit joins the complex.
  • The ribosome has two tRNA binding sites: the P site (peptidyl site) and the A site (aminoacyl site).

Elongation

  • A new aminoacyl-tRNA enters the A site, carrying the amino acid specified by the next codon on the mRNA.
  • A peptide bond is formed between the amino acid in the P site and the amino acid in the A site.
  • The ribosome moves along the mRNA by one codon, shifting the tRNA in the P site to the E site (exit site) and the tRNA in the A site to the P site.
  • The process repeats, adding amino acids to the growing polypeptide chain.

Termination

  • When the ribosome reaches a stop codon (UAA, UAG, or UGA), a release factor binds to the A site.
  • The polypeptide chain is released from the tRNA in the P site.
  • The ribosomal subunits dissociate from the mRNA.