Polypeptide Synthesis Essay

873 Words4 Pages
Polypeptide Synthesis The whole process from DNA to amino acid sequence involves:- • The making of a temporary complementary copy of a gene called pre-messenger RNA. This is called transcription. • The removal of ‘junk’ base sequences called ‘introns’ from the pre-mRNA to form messenger RNA (mRNA) • The translation of the base sequence in mRNA into an amino acid sequence (polypeptide) using transfer RNAs (tRNAs) and ribosomes. Transcription & Splicing DNA never leaves the nucleus, but proteins are synthesised in the cytoplasm, so a copy of each gene is made to carry the “message” from the nucleus to the cytoplasm. This copy is called mRNA, and its production is called transcription and splicing. 1. The start of each gene on DNA is marked by a special sequence of bases called the promoter. 2. The enzyme, DNA helicase breaks hydrogen bonds between base-pairs causing the strands of DNA to separate and the molecule to effectively unwind. The RNA molecule is built up from the four nucleotides (A, C, G and U) in the nucleoplasm. The RNA nucleotides are attached to the bases on the DNA by the enzyme RNA polymerase by complementary base pairing, just as in DNA replication. However, only one strand of RNA is made. The DNA strand that is copied is called the template strand. The other strand is a complementary copy, called the non-template strand. 3. The new nucleotides are joined to each other by strong covalent phosphodiester bonds. 4. Only about 8-12 base pairs remain attached at a time, since the mRNA molecule peels off from the DNA as it is made. DNA-helicase enzyme rewinds the DNA. 5. At the end of the gene the transcription stops at a ‘stop codon’, so the pre-mRNA molecule detaches and is then modified into the final mRNA. 6. The mRNA diffuses out of the nucleus through a nuclear pore into the cytoplasm. There, it

More about Polypeptide Synthesis Essay

Open Document