Describe the production of protein molecules through transcription and translation.
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For a gene to be expressed, i.e., translated into RNA, that portion of the DNA has to be uncoiled and freed of the protective proteins. An enzyme, called DNA polymerase, reads the DNA code (the sequence of bases on one of the two strands of the DNA molecule) and builds a single-stranded chain of the RNA molecule. Again, where there is a G in DNA, there will be C in the RNA and vice versa. Instead of thymine, RNA has uracil (U). Wherever in the DNA strand there is an A, there will be a U in the RNA, and wherever there is a T on the DNA molecule, there will be an A in the RNA.
Once the whole gene (100s to 10,000s of bases in a row) is transcribed, the RNA molecule detaches. The RNA (called messenger RNA or mRNA) may be further modified by addition of more A bases at its tail, by addition of other small molecules to some of the nucleotides and by excision of some portions (introns) out of the chain. The removal of introns (the non-coding regions) and putting together the remaining segments - exons - into a single chain again, is called RNA splicing. RNA splicing allows for one gene to code for multiple related kinds of proteins, as alternative patterns of splicing may be controlled by various factors in the cell.
Unlike DNA, the mRNA molecule is capable of exiting the nucleus through the pores in the nuclear membrane. It enters the endoplasmatic reticulum and attaches itself to one of the membranes in the rough ER.
Three types of RNA are involved in the translation process: mRNA which carries the code for the gene, rRNA which aids in the formation of the ribosome, and tRNA which brings individual amino-acids to the ribosome. Translation is controlled by various enzymes that recognize specific nucleotide sequences.
The genetic code (nucleotide sequence of a gene) translates into a polypeptide (amino-acid sequence of a protein) in a 3-to-1 fashion. Three nuclotides in a row code for one amino-acid. There are a total of 20 amino-acids used to build all proteins in our bodies. Some amino-acids are coded by a single triplet code, or codon. Other amino-acids may be coded by several different RNA sequences. There is also a START sequence (coding for fMet) and a STOP sequence that does not code for any amino-acid. The genetic code is (almost) universal. Except for a few microorganisms, all of life uses the same genetic code.
When the ribosome is assembled around a molecule of mRNA, the translation begins with the reading of the first triplet. Small tRNA molecules bring in the individual amino-acids and attach them to the mRNA, as well as to each other, forming a chain of amino-acids. When a stop signal is reached, the entire complex disassociates. The ribosome, the mRNA, the tRNAs and the enzymes are then either degraded or re-used for another translational event.