Protein Synthesis: From DNA to Functional Proteins
Introduction to Protein Synthesis
Protein synthesis is the process by which cells use the information stored in DNA to create proteins. This process is crucial for the function and maintenance of all living organisms. In this detailed overview, we will discuss how proteins are made using information from DNA in both prokaryotic and eukaryotic cells. We will cover the steps of transcription, RNA processing, translation, and post-translational modifications.Transcription
Location
In eukaryotic cells, transcription occurs in the nucleus. Prokaryotic cells, on the other hand, do not have a nucleus and perform transcription in the cytoplasm.
Process
1. Initiation
The process begins with the binding of RNA polymerase, an enzyme, to a specific region of the DNA called the promoter. Promoters signal the start of a gene and are recognized by RNA polymerase to initiate transcription.
2. Elongation
During elongation, RNA polymerase unwinds the double-stranded DNA and synthesizes a messenger RNA (mRNA) strand using one of the DNA strands as a template. The mRNA synthesized is complementary to the DNA template: adenine (A) pairs with uracil (U) in RNA, and cytosine (C) pairs with guanine (G).
3. Termination
The transcription process terminates when RNA polymerase reaches a termination signal in the DNA sequence. When this happens, RNA polymerase detaches from the DNA, and the newly formed mRNA is released.
RNA Processing
Only applicable in eukaryotes:
Location
RNA processing primarily occurs in the nucleus before the RNA molecules can be translated.
Process
1. Capping
A 5' cap is added to the beginning of the mRNA for stability and recognition by cellular machinery.
2. Polyadenylation
A poly-A tail is added to the 3' end of the mRNA, which enhances stability and aids in the export of the mRNA from the nucleus to the cytoplasm.
3. Splicing
Introns, which are non-coding regions of the RNA, are removed, and exons, which are coding regions, are joined together to form a mature mRNA molecule.
Translation
Location
Translation occurs in the cytoplasm on ribosomes.
Process
1. Initiation
The mature mRNA leaves the nucleus and binds to a ribosome in the cytoplasm. The ribosome scans the mRNA until it finds the start codon, usually AUG, which signals the beginning of protein synthesis.
2. Elongation
Transfer RNA (tRNA) molecules, each carrying a specific amino acid, bring the amino acids to the ribosome. The tRNA anticodons are complementary to the mRNA codons, ensuring the correct amino acid is added to the growing polypeptide chain. The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, elongating the polypeptide chain.
3. Termination
Translation continues until a stop codon (UAA, UAG, or UGA) is reached. These codons do not correspond to any tRNA, signaling the end of translation. The ribosome releases the completed polypeptide chain into the cytoplasm.
Post-Translational Modifications
After translation, many proteins undergo additional modifications to achieve their final functional state and activity. These modifications can include phosphorylation, glycosylation, and other chemical modifications.
Summary
In conclusion, proteins are synthesized based on information encoded in DNA through the processes of:
Transcription: DNA is transcribed into mRNA. Translation: mRNA is used to assemble amino acids into proteins.This intricate and finely tuned process is essential for cellular function and the expression of genetic information.