Transcription: rewriting the code
The first stage of biosynthesis consists of the synthesis of an immature messenger RNA molecule on a DNA template. The main catalyst is the enzyme RNA polymerase. In eukaryotic organisms, functions are divided: polymerase I synthesizes ribosomal RNAs, polymerase II synthesizes messenger RNAs, and polymerase III synthesizes transfer RNAs.
The process includes three sequential stages:
- Initiation. The enzyme recognizes the starting site on the DNA — the promoter — and firmly binds to it.
- Elongation. The DNA double helix unwinds locally. RNA polymerase moves in the 5' → 3' direction and builds the RNA chain, selecting nucleotides strictly according to the principle of complementarity.
- Termination. Synthesis continues until the enzyme reaches a special stop signal. After that, the newly synthesized chain separates.
RNA processing in eukaryotes
Unlike bacteria, in nuclear organisms, transcription takes place in the nucleus, and the resulting RNA is immature. It requires maturation — processing. It includes three key modifications:
- Capping. 7-Methylguanosine is added to the 5'-end of the molecule. This "cap" protects the RNA from degradation and later helps the ribosome properly initiate translation.
- Polyadenylation. A long poly-A "tail" is formed at the 3'-end, providing structural stability to the molecule.
- Splicing. Since eukaryotic genes are interrupted, special complexes (spliceosomes, which include small nuclear RNAs) cut out non-coding fragments (introns) and ligate the informative ones (exons).
Alternative splicing is of paramount importance: by combining exons in different ways, the organism can synthesize several different proteins based on the same gene. Mature mRNA exits into the cytoplasm through nuclear pores.
Translation: polypeptide assembly
The final stage of translating the nucleotide text into an amino acid sequence takes place on ribosomes. This requires messenger RNA, transfer RNAs (carrying amino acids and having anticodons), aminoacyl-tRNA synthetases, and energy sources (ATP and GTP).
Stages of translation:
- Initiation. The small ribosomal subunit binds to mRNA. The start codon AUG enters the P-site (peptidyl). The initiator tRNA brings the first amino acid — methionine, after which the large ribosomal subunit attaches.
- Elongation. New transfer RNAs with amino acids arrive at the vacant A-site (aminoacyl). Peptide bonds are formed between neighboring amino acids. The ribosome takes a step, and the cycle repeats many times.
- Termination. Synthesis halts when one of the stop codons (UAA, UAG, or UGA) enters the A-site. The complex disassembles, releasing the finished protein.
Regulation of gene expression
Mechanisms for controlling gene activity depend on the type of cell organization.
In prokaryotes, regulation is often based on the operon model — a group of genes controlled by a single promoter and operator.
- Lac operon (induction): in the absence of lactose, a special repressor protein blocks the operator, and transcription does not occur. The appearance of lactose (inducer) binds the repressor, clearing the path for RNA polymerase.
- Trp operon (repression): excess of the end product (tryptophan) activates the repressor itself, turning off further enzyme synthesis.
In eukaryotes, control is multi-level: from chromatin condensation and enhancer influence to processing. A huge role is played by epigenetics — heritable changes in expression without altering the DNA code. Such mechanisms include DNA methylation (usually suppresses genes), histone modification (e.g., acetylation activates reading), and RNA interference.
Briefly: answers to topical questions
What enzymes facilitate DNA replication before cell division?
The leading enzymes ensuring the elongation of the polynucleotide chain during replication are DNA polymerases.
- Eukaryotic DNA polymerases — there are 7 known types of enzymes ($\alpha, \beta, \gamma, \delta, \epsilon, \zeta, \eta$) that localize and function in the nucleus, mitochondria, and plastids.
- Prokaryotic DNA polymerases (E. coli) — there are 5 known types of polymerases (I, II, III, IV, V) that are responsible for DNA replication and repair in various combinations.
This complex process occurs via a semi-conservative model and requires the participation of a whole complex of specific enzymes and proteins.
How does amino acid activation occur before attaching to tRNA?
The activation of amino acids is carried out by specialized enzymes — aminoacyl-tRNA synthetases, which catalyze the reaction of binding an amino acid to a transfer RNA.
- Aminoacyl-tRNA synthetases — amino acid activation enzymes that provide the recognition stage (recognition of the amino acid by the enzyme) at the pre-translational stage.
For example, to involve leucine in protein biosynthesis, an activation reaction of leucine with tRNA-Leu is necessary. This process is catalyzed by the aminoacyl-tRNA synthetase specific for leucine. After the formation of the amino acid-tRNA complex, the molecule is incorporated into the growing polypeptide chain at the translation elongation stage.
What is the exact mechanism of spliceosome and small nuclear RNA action during intron excision?
The exact mechanism of intron removal is provided by the spliceosome, whose catalytic activity is due to RNA components (ribozymes).
Stages of intron excision:
- Initiation — small nuclear RNA (snRNA) interacts with the 5'-end of the intron. Specific sequences (splicing sites: AGGU and GAGG) are recognized by the principle of complementarity.
- Complex assembly — snRNPs form a spliceosome, fix the primary transcript, and form an intron loop.
- Lariat formation — cleavage of the 3',5'-phosphodiester bond is catalyzed. The 5'-end of the intron is cut and attaches to a site within the intron, forming a lariat-like loop structure.
- Removal and ligation — the 3'-end of the intron is cut, the intron is completely excised as a lariat, the spliceosome disassembles, and the exon ends are joined (ligated).
To avoid confusing the ribosome centers during translation, focus on the letters: the Amino acid arrives at the A-site, while the growing Peptide is held in the P-site.
What else to study on this topic
- Alternative splicing and protein diversity
- Structure and functions of transfer RNAs
- Mechanism of spliceosome and small nuclear RNA action
- Tryptophan operon regulation model
- Histone modifications: acetylation and methylation
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