The third difference between RNA and DNA is that there are three different types of RNA, mRNA (messenger RNA), tRNA (transfer RNA) and rRNA (ribosome RNA). mRNA carries the genetic code (instructions how to assemble the protein) from the DNA in the nucleus to ribosomes in the cytoplasm. tRNA picks up and transfers amino acids from cytoplasm to the mRNA on the ribosomes and is shaped similar to a cloverleaf. rRNA forms a structural part of ribosome that helps join the amino acids together form a protein.
DNA is a giant twisted molecule made up of many nucleotides linked together resembling a double helix. It provides a base triplet, a sequence of three bases on one of the strands of DNA, that code for one amino acid. The sequence of base triplets on DNA molecules determines the order of the amino acids on the protein chain.
In the first phase of transcription, the first process of protein synthesis that occurs in the nucleolus, a portion of a DNA molecule unwinds and serves as a template. Free nucleotides floating in the nucleoplasm pair up with their complimentary bases on the DNA strand. (Except that uracil replaces thymine). The nucleotides form sugar-phosphate bonds with each other and become an mRNA strand but they do not form bonds with the DNA strand. The sequence of three exposed bases on mRNA, that are complimentary to the base triplet on the DNA, are known as codons. Once the mRNA strand is complete it moves from the DNA in the nucleus, through the nuclearpore into the cytoplasm where it drapes itself over the ribosomes with their codons exposed.
Floating in the cytoplasm are tRNA molecules which job is to pick up specific amino acids and transport them to where the mRNA is draped. This is done by means of the aminoacyl attachment site (the site at which the amino acid is attached to the tRNA molecule). Each tRNA molecule, by means of their anticodons (a sequence of three exposed free bases complimentary to that of the codons on mRNA) will temporarily attach to its specific complementary codon on the mRNA. (For example, anitcodon G A U will attach to the codon C U A). This allows the amino acids to be arranged in the correct order according to the sequence of the codons on the mRNA strand. Peptide bonds form between the adjacent amino acids after the tRNA attaches to the mRNA. (Bonds in a protein chain are called polypeptide bonds).
The ribosomes (rRNA's) move along the mRNA strand reading codon after codon and assist the linking of amino acids to each other. Eventually, with this process, long chains of amino acids are linked to form a protein. As the ribosomes move along the mRNA they will eventually reach a nonsense codon (codons on mRNA that do not code for any amino acids. Their function is to indicate the start and end of a protein chain) on mRNA, the protein chain is then completed. The proteins then break away from the tRNA’s and moves of into the cytoplasm and carry out their tasks. The tRNA molecules also separate from the mRNA strand and move away to pick up another specific amino acid. The mRNA strand can then either be used again to create more proteins or be broken down into their separate nucleotides.
Protein Synthesis is the process whereby DNA codes for the production of essential proteins. This process can be divided into two parts, transcription and translation. Transcription is the production of mRNA from a DNA template. mRNA carries the information from the nucleus to the cytoplasm, which is where protein synthesis occurs. Before the synthesis of a protein begins, amino acids are attached to the tRNA molecule, which transport and attach them to the mRNA with the aid of rRNA, this is known as translation. Peptide bonds then form between the amino acids. Once the chain is completed the polypeptide chain separates from the tRNA and move in to the cytoplasm and in this way a protein is formed.
