DNA transcription

The upper most strand is the (coding strand) DNA base sequence (triplet) of the gene codes for synthesis of a particular polypeptide chain. The second strand is the mRNA base sequence (template strand used for copying) codon of the transcribed mRNA. The process for going from the upper stand to the second strand is called Transcription and involves an enzyme called polymerase. The polymerase attaches to the promoter region (start codon) and reads the nucleotide base sequence until it gets to a termination signal (stop codon) and then it jumps off because it has the whole recipe copied. The polymerase uses complimentary base pairing to attach RNA nucleotides to the second strand with the exception that it attaches Uracil in place of Thymine. The row of “guitars” represent the transfer RNA (tRNA) which transfers a specific active amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. Each tan ball in the diagram represents an amino acid. The multi-color “piano keys’ represent the four nucleotide bases A, G, T, and C. In the (first) upper stand the A, G, T, and C bases are grouped into a sequence of three, called a triplet. Each triplet specifies a particular amino acid. The second strand contains RNA which differs from DNA because it is single stranded and it has the sugar ribose instead of deoxyribose, and the base uracil (U) instead of thymine (T). Unlike DNA, the messenger RNA (mRNA) leaves the nucleus through the nuclear pore into the cytosol and attaches to a ribosome to undergo translation (tRNA). The difference between mRNA and tRNA is that the mRNA codon is the complimentary base pairing of the DNA triplet, whereas tRNA is an anticodon, a three-base sequence opposite complementary base pairing of the mRNA codon. The anticodon is identical to the template DNA sequence except for the substitution of uracil (U) for thymine (T). In the translation step of protein synthesis (going from second strand to the “guitars”) the language of nucleic acids (base sequence) is translated into the language of proteins (amino acid sequence). The mRNA leaves the nucleus through the pore into the cytosol where it attaches to a ribosom. Specialized enzymes activate and assemble a large and small ribosomal subunit. Using the rules referred to as the genetic code, the ribosome reads the codons and matches them with the corresponding anticodon. Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme. The ribosom has three active sites A, P and E. Translation starts when the incoming aminoacyl-tRNA recognizes the complementary codon calling for it at the A site on the ribosome. It forms a hydrogen bond with the codon via its anticodon. The ribosome travels along the mRNA and each codon is read in sequence and a new amino acid is added to the protein chain, then the tRNA in the A site is moved to the P site. Once the amino acid is released from the P site, tRNA then leaves through the E site released to reenter the cytoplasm. The polypeptide is released when the stop codon is read.