Protein synthesis is a multi-step process that involves both transcription and translation. Here are the key steps involved:
1. Transcription
- Initiation: The process starts in the nucleus, where the DNA unwinds and unzips to expose the gene that encodes the desired protein.
- RNA Polymerase Binding: An enzyme called RNA polymerase binds to the promoter region of the gene.
- Elongation: RNA polymerase moves along the DNA template strand, synthesizing a complementary strand of messenger RNA (mRNA) by adding ribonucleotides that are complementary to the DNA template.
- Termination: When RNA polymerase reaches a termination signal in the DNA, it stops transcribing, and the newly formed mRNA strand is released.
- Post-Transcriptional Modifications: The mRNA undergoes capping (addition of a 5' cap) and polyadenylation (addition of a poly-A tail) and splicing (removing introns and joining exons together) before leaving the nucleus.
2. Translation
- Initiation: The processed mRNA exits the nucleus and enters the cytoplasm, where it binds to a ribosome. The ribosome scans the mRNA for the start codon (AUG).
- tRNA Binding: Transfer RNA (tRNA) molecules, each carrying a specific amino acid, bind to the corresponding codons on the mRNA. The first tRNA fits into the ribosome's P site, while the next one enters the A site.
- Peptide Bond Formation: The ribosome facilitates the formation of peptide bonds between the amino acids, creating a growing polypeptide chain.
- Translocation: The ribosome moves along the mRNA, shifting the tRNA from the A site to the P site, and the next codon enters the A site for further tRNA binding.
- Termination: This process continues until the ribosome reaches a stop codon (UAA, UAG, or UGA) on the mRNA, which does not code for an amino acid. Release factors bind to the stop codon, causing the newly synthesized polypeptide to be released from the ribosome.
3. Post-Translational Modifications
- After translation, the polypeptide chain may undergo folding and additional modifications (such as phosphorylation, glycosylation, or cleavage), enabling it to become a functional protein.
These steps collectively enable the cell to accurately synthesize proteins based on the genetic instructions encoded in DNA.