Introduction
DNA store the genetic information, this information has to be converted into “product” to perform the cellular function. The process is called “the central dogma”, DNA is first transcribed into messenger RNA (mRNA), then the protein is synthesized according to the information on mRNA. This way the information is passed from DNA to protein and the proteins are the “executive” molecules in cells.
RNA
RNA contains 4 bases: A, U, G and C, they can pair with bases from DNA (T, A, C and G, in that order). There are 4 classes of RNA: mRNA, transfer RNA (tRNA), ribosome RNA (rRNA) and small RNAs including snRNA, miRNA and ncRNA. The mRNA molecules are the “messenger” from DNA to protein. The rRNA and tRNA both function in protein synthesis.
Transcription
Transcription is the process in wich DNA is converted into a complementary RNA, catalyzed by RNA polymerase. Transcription is initiated when RNA polymerase complex assembles at promoter. RNA polymerase catalyzes the elongation of the RNA while the DNA template is unwound and rewound. Transcription complex responds to specific termination signals and disassembles, that is the termination of transcription.
RNA processing
RNA processing includes: 5’ capping for RNA stabilization and ribosome binding; splicing for removing intron sequence and 3’ polyadenylation for protecting mRNA from 3’ exonuclease, extending the half life of mRNA.
Genetic Code
From mRNA to protein, the genetic code is read in a continuous fashion, there is no no comma, they are non-overlapping, unambiguous, almost universal with few exceptions. Many codes may code for same amino acid (degeneration). There is special start codon (AUG) and three stop codons (UAA, UAG and UGA). In the third position of the codon, it is more likely the nucleotide is different but it still codes for same amino acid (wobble).
Translation
Translation requires tRNAs, which bring in amino acid and line them up according to the genetic code in mRNA. Translation include three steps: Initiation: the ribosome subunit binds to 5’ end of mRNA. Elongation: an incoming aminoacyl-tRNA binds to codon at A-site, peptide bond is formed between new amino acid and growing chain. The peptide moves one codon position and gets ready for next one. Stop codons is not recognized by any tRNA.
This leads to disassembly of ribosomes and release of polypeptide.
