Transcription, mRNA Processing, and Translation in Protein Synthesis

Overview of Gene Expression and Central Dogma

  • Information flows sequentially within cellular systems from the DNA gene through messenger RNA (mRNA) during transcription, and subsequently from mRNA to a polypeptide during translation.

  • In eukaryotic cells, genetic information is first transcribed into an initial precursor molecule called pre-mRNA.

  • Pre-mRNA must undergo mandatory enzymatic processing within the cell nucleus to become functional mRNA before it can pass through nuclear pores into the cytoplasm.

Overview of cellular transcription, mRNA processing, nuclear export, and translation

Transcription Process and Mechanism

  • Definition and Core Function:

    • In biological systems, transcription transfers genetic information from a DNA base sequence to the complementary base sequence of an mRNA molecule.

    • While the physical format changes from double-stranded DNA to single-stranded RNA, the exact informational content is conserved.

    • Once synthesized, the mRNA molecule detaches from the DNA strand, exits the nucleus through a nuclear pore, and travels to the protein synthesis machinery located at the ribosome.

  • Preparatory Steps and Transcription Factors:

    • Transcription cannot initiate until gene-activating chemical signals called transcription factors stimulate histones at the gene transcription site to loosen.

    • Transcription factors bind to the promoter, a specialized DNA sequence containing the exact start point of the gene to be transcribed.

    • The promoter determines which of the two unwound DNA strands serves as the template strand.

    • The uncoiled DNA strand not used as a template is called the coding strand because its nucleotide sequence matches the sequence of the mRNA being constructed (with uracil, U, in mRNA substituting for thymine, T, in DNA).

    • RNA polymerase is the primary enzyme that oversees and catalyzes the synthesis of mRNA.

  • Phases of Transcription:

    • 1. Initiation: RNA polymerase binds to the promoter region, pries apart the two complementary DNA strands, and initiates mRNA synthesis at the start point within the promoter region.

    • 2. Elongation: RNA polymerase moves along the template strand, joining complementary RNA nucleotides one by one to elongate the mRNA transcript. As it advances, it unwinds the DNA double helix ahead of the transcription bubble and rewinds the DNA helix behind it.

    • The DNA-RNA Hybrid: At any given moment during elongation, a small segment of approximately 1616 to 1818 base pairs of DNA remains unwound, and the newly synthesized RNA strand stays hydrogen-bonded to the DNA template strand. This small overlapping region is called the DNA-RNA hybrid.

    • 3. Termination: mRNA synthesis finishes when RNA polymerase reaches a specialized nucleotide sequence termed the termination signal. At this point, RNA polymerase and the completed mRNA transcript are released from the DNA.

Detailed stages of transcription including initiation, elongation, termination, and the DNA-RNA hybrid region

Post-Transcriptional mRNA Processing

  • Pre-mRNA and Introns:

    • Because DNA is transcribed sequentially, the raw mRNA initially produced is termed pre-mRNA and contains non-coding region sequences called introns.

    • Before the newly formed RNA can function as a messenger for protein synthesis, sections corresponding to introns must be removed.

  • Spliceosome Function:

    • Large molecular complexes composed of RNA and proteins, called spliceosomes, snip out the non-coding introns.

    • Spliceosomes then splice together the remaining coding sections, called exons, producing a mature, functional mRNA molecule.

  • Cytosolic Export:

    • Once splicing is complete, functional mRNA exits the nucleus through nuclear pores, enters the cytosol, and engages in translation.

The Genetic Code

  • Language Translation in Protein Synthesis:

    • Translation converts genetic information from the language of nucleic acids (base sequences) into the language of proteins (amino acid sequences).

    • The rules by which the base sequence of a gene is translated into an amino acid sequence are known as the genetic code.

  • Codons and DNA Triplets:

    • For each base triplet (three-base sequence) on DNA, the corresponding three-base sequence synthesized on mRNA is called a codon.

    • Since there are 44 distinct RNA (and DNA) nucleotides, there are 43=644^3 = 64 possible codon combinations.

  • Codon Distribution and Redundancy:

    • Out of the 6464 possible codons, 33 codons serve as "stop signs" (stop codons) that signal the termination of polypeptide synthesis.

    • The remaining 6161 codons code directly for specific amino acids.

    • Because there are only approximately 2020 standard amino acids, many amino acids are specified by more than one codon.

    • This redundancy in the genetic code provides protection against transcription and translation errors (mutations).

Structure and Function of tRNA and Ribosomes

  • tRNA Structure and Dual Binding Function:

    • Transfer RNA (tRNA) is shaped like a handheld drill and performs a dual-binding role by connecting to both an amino acid and an mRNA codon.

    • An amino acid from the cytoplasmic pool attaches to one end of the tRNA molecule.

    • At the opposing end of tRNA is the anticodon, a unique three-base sequence that forms complementary hydrogen bonds with an mRNA codon.

    • Example: If an mRNA codon is AUA (which codes for isoleucine, Ile), the tRNA molecule carrying isoleucine will possess the complementary anticodon UAU.

Structure of tRNA showing the attached amino acid isoleucine and the UAU anticodon

  • Aminoacyl-tRNA Formation:

    • There are approximately 4545 distinct types of tRNA molecules in a cell, each capable of binding a specific amino acid.

    • The attachment of an amino acid to its corresponding tRNA is controlled by the enzyme aminoacyl-tRNA synthetase and activated by ATP energy.

    • Once loaded with its amino acid cargo, the molecule is termed an aminoacyl-tRNA and diffuses to the ribosome.

  • Ribosomal Binding Sites:

    • The ribosome acts as a firm coordinate mechanism, holding mRNA and tRNA close together to facilitate accurate codon-anticodon pairing.

    • Each ribosome contains one binding site for mRNA and three active binding sites for tRNA:

    • A (aminoacyl) site: Receives the incoming charged aminoacyl-tRNA.

    • P (peptidyl) site: Holds the tRNA that carries the growing polypeptide chain.

    • E (exit) site: Holds the uncharged outgoing tRNA immediately before it exits the ribosome.

Sequence of Events in Translation

  • General Properties:

    • Translation occurs in the cytosol and proceeds through three continuous phases: Initiation, Elongation, and Termination.

    • Each phase requires energy input in the form of ATP (or GTP), along with specific protein factors and enzymes.

  • Phase 1: Initiation:

    • A small ribosomal subunit binds to a special initiator tRNA carrying methionine.

    • The small subunit-tRNA complex then binds to the mRNA molecule to be decoded.

    • The small ribosomal subunit scans along the mRNA transcript until it reaches the start codon, which is the first AUG triplet.

    • The initiator tRNA's UAC anticodon recognizes and base-pairs with the AUG start codon.

    • A large ribosomal subunit then joins the small subunit to form a fully functional ribosome.

    • At the end of initiation, mRNA is anchored in the groove between ribosomal subunits, the initiator tRNA sits in the P site, and the A site is vacant to accept the next incoming aminoacyl-tRNA.

  • Phase 2: Elongation:

    • Elongation proceeds as a cyclic three-step process in which the ribosome moves along the mRNA transcript in one direction, adding one amino acid at a time to the polypeptide chain:

    • 2a. Codon Recognition: An incoming aminoacyl-tRNA binds to its complementary codon in the ribosomal A site.

    • 2b. Peptide Bond Formation: An enzymatic component in the large ribosomal subunit catalyzes peptide bond formation between the amino acid of the tRNA in the P site and the amino acid of the tRNA in the A site, transferring the growing chain to the A-site tRNA.

    • 2c. Translocation: The ribosome translocates (shifts) forward by one codon along the mRNA strand. This movement shifts the tRNA in the A site into the P site. Simultaneously, the uncharged tRNA in the P site shifts to the E site, where it is released back into the cytosol to be recharged with another amino acid by aminoacyl-tRNA synthetase.

  • Polyribosomes:

    • As a ribosome advances along the mRNA track, the initial segment of mRNA clears the ribosome and can bind to additional ribosomes in sequence.

    • A multiple ribosome-mRNA assembly reading the same mRNA message simultaneously is called a polyribosome (or polysome).

    • Polyribosomes allow efficient, simultaneous synthesis of multiple identical copies of a protein from a single mRNA molecule.