Proteins are versatile macromolecules that perform structural and functional roles in living organisms — enzymes, hormones, transporters, receptors and structural components. The sequence of amino acids in a protein is encoded in DNA and is produced in the cell through the process called protein synthesis. This process has two main stages: transcription (DNA → RNA) and translation (RNA → protein). This flow of information is summarized by the Central Dogma of Molecular Biology.
The central dogma describes the directional flow of genetic information: DNA → RNA → Protein. DNA stores information, RNA conveys the information, and proteins perform cellular functions. For example, the gene for hemoglobin is transcribed into mRNA and translated to produce the hemoglobin protein that carries oxygen in red blood cells.
The genetic code determines how nucleotide triplets (codons) in mRNA map to amino acids. Key features:
- Triplet nature: Each codon is three bases long (e.g., UUU = Phenylalanine).
- Degeneracy: Multiple codons can code for the same amino acid (e.g., GGU/GGC/GGA/GGG → Glycine).
- Start codon: AUG (codes for methionine; initiation point).
- Stop codons: UAA, UAG, UGA (terminate translation).
- Universality: Nearly universal across organisms.
- Non-overlapping: Codons are read sequentially without overlap.
Definition: Transcription is the process of copying a DNA segment into RNA using RNA polymerase.
- mRNA (messenger RNA): carries coding information for proteins.
- tRNA (transfer RNA): carries specific amino acids and has anticodons to pair with mRNA codons.
- rRNA (ribosomal RNA): forms the structural and catalytic core of ribosomes.
- snRNA, miRNA, siRNA (eukaryotes): regulatory functions.
- Initiation: RNA polymerase binds to the promoter (e.g., TATA box in eukaryotes). DNA unwinds locally. In prokaryotes, sigma factor helps promoter recognition.
- Elongation: RNA polymerase synthesizes an RNA strand in the 5' → 3' direction using the DNA template (read 3' → 5'). Base pairing uses U (uracil) instead of T (thymine).
- Termination: RNA polymerase stops at terminator sequences. In prokaryotes termination may be rho-dependent or rho-independent; in eukaryotes specific signals and processing cause release.
- 5' capping: Addition of a 7-methylguanosine cap to the 5' end — protects mRNA and assists ribosome binding.
- Polyadenylation: Addition of a poly-A tail (50–250 A residues) at the 3' end — increases stability and aids export from nucleus.
- Splicing: Removal of introns and joining of exons by the spliceosome. Alternative splicing can produce multiple proteins from one gene.
Definition: Translation decodes mRNA into a polypeptide sequence using ribosomes, tRNAs, and various factors.
- mRNA: template that provides codons.
- tRNA: adaptor molecules with anticodon and attached amino acid (charged by aminoacyl-tRNA synthetases).
- Ribosomes: sites of protein synthesis (70S in prokaryotes: 30S + 50S; 80S in eukaryotes: 40S + 60S). Three functional sites — A (aminoacyl), P (peptidyl), and E (exit).
- Translation factors: initiation, elongation and release factors that regulate and assist the process.
- Initiation: Small ribosomal subunit binds to mRNA near start codon (AUG). Initiator tRNA carrying methionine (Met or fMet in bacteria) occupies the P site. Large subunit joins to form the complete ribosome.
- Elongation: Charged tRNA enters the A site; peptide bond forms between the amino acid at the A site and the growing polypeptide at the P site (peptidyl transferase activity). Ribosome translocates, moving tRNAs from A → P → E and freeing the A site for the next charged tRNA.
- Termination: When a stop codon (UAA, UAG, UGA) enters the A site, release factors promote hydrolysis of the bond between the polypeptide and the tRNA, releasing the finished polypeptide. Ribosomal subunits dissociate.
Newly synthesized polypeptides frequently require processing to become fully functional:
- Folding: Correct 3D structure achieved with the help of molecular chaperones.
- Proteolytic cleavage: Removal of signal peptides or propeptides (e.g., many hormones are synthesized as precursors).
- Chemical modifications: phosphorylation, glycosylation, methylation, acetylation — affect activity, stability and localization.
- Targeting: Signal sequences direct proteins to organelles (ER, mitochondria, nucleus) or secretion pathways.
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Location | Transcription and translation are coupled in the cytoplasm (no nucleus). | Transcription occurs in nucleus; mRNA processed and exported; translation occurs in cytoplasm. |
| mRNA processing | Generally absent (no capping or poly-A tail). | Capping, polyadenylation and splicing (introns removed). |
| Ribosomes | 70S (30S + 50S) | 80S (40S + 60S) |
| Initiator tRNA | Formyl-methionine (fMet) | Methionine (Met) |
| Coupling | Transcription and translation are simultaneous. | Separated in space and time. |
Regulation occurs at multiple levels to ensure appropriate protein production:
- Transcriptional control: promoters, transcription factors, chromatin remodeling (prominent in eukaryotes).
- Post-transcriptional control: mRNA stability, alternative splicing, miRNA-mediated regulation.
- Translational control: initiation factors, ribosome availability, upstream open reading frames (uORFs).
- Post-translational control: modifications and targeted degradation (e.g., ubiquitin–proteasome pathway).
- Operons: In prokaryotes, operons (e.g., lac operon) coordinate expression of functionally related genes.
- Produces enzymes for metabolism and structural proteins for cell architecture.
- Generates signaling molecules (hormones, receptors) and immune effectors (antibodies).
- Errors in protein synthesis may cause disease (e.g., sickle cell anemia arises from a mutation affecting protein structure).
- Major target for antibiotics: many antibiotics (tetracyclines, aminoglycosides, macrolides) selectively inhibit bacterial protein synthesis.
