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Protein synthesis is a fundamental process that occurs in cells, where proteins are synthesized from the genetic information encoded in DNA. This process involves two main steps: transcription and translation.
Transcription is the first step in protein synthesis and takes place in the nucleus of eukaryotic cells or the cytoplasm of prokaryotic cells. It begins with the unwinding and unzipping of the DNA double helix by the enzyme RNA polymerase. The RNA polymerase then reads the DNA template strand and synthesizes a complementary RNA molecule called messenger RNA (mRNA). This process is guided by the base pairing rules, where adenine (A) pairs with uracil (U) instead of thymine (T).
Once the mRNA molecule is synthesized, it undergoes a process called RNA processing in eukaryotic cells. During this process, the non-coding regions called introns are removed, and the remaining coding regions called exons are spliced together to form a mature mRNA molecule. In prokaryotic cells, this processing step is not required.
The next step in protein synthesis is translation, which occurs in the cytoplasm. Translation involves the conversion of the mRNA sequence into a specific amino acid sequence, which forms the primary structure of a protein. This process takes place on ribosomes, which are composed of ribosomal RNA (rRNA) and proteins.
Translation begins with the binding of the mRNA to the small subunit of the ribosome. The ribosome then scans the mRNA until it reaches the start codon, which is usually AUG. At this point, a specific initiator tRNA molecule carrying the amino acid methionine binds to the start codon.
Once the initiation complex is formed, the large subunit of the ribosome joins, and the process of elongation begins. During elongation, the ribosome moves along the mRNA molecule in a 5' to 3' direction, reading the codons and bringing in the corresponding aminoacyl-tRNA molecules. Each aminoacyl-tRNA carries a specific amino acid that matches the codon on the mRNA.
As the ribosome moves along the mRNA, it catalyzes the formation of peptide bonds between the amino acids, creating a growing polypeptide chain. This process continues until a stop codon is reached, which signals the termination of translation. The newly synthesized polypeptide is then released from the ribosome.
After translation, the polypeptide may undergo further modifications, such as folding into its three-dimensional structure or undergoing post-translational modifications like phosphorylation or glycosylation. These modifications are crucial for the protein to acquire its functional form.
In summary, protein synthesis is a complex process that involves transcription, where the genetic information in DNA is transcribed into mRNA, and translation, where the mRNA is translated into a specific amino acid sequence to form a protein. This process is essential for the functioning and survival of cells.