There are three types of RNA that are involved in protein synthesis. These are mRNA, rRNA and tRNA. All three types are synthesized directly on DNA and the amount of RNA in each cell is directly related to the amount of protein synthesis.
mRNA ( Messenger RNA): 3-5 % of the total RNA of the cell is mRNA. It is a single stranded molecule formed on a single strand of DNA by a process called transcription. The base sequence of mRNA is a complementary copy of the DNA strand being copied and varies in length according to the length of the polypeptide chain for which it codes. It enters the cytoplasm where is associates with the ribosomes and acts as a template for protein synthesis. Most mRNA exists in the cell for a short time.
rRNA (Ribosomal RNA): It makes up approximately 80% of the total RNA of the cell. It is synthesized by genes present on the DNA of several chromosomes. It is a large, complex molecule made up of both double and single helices. The base sequence of rRNA is similar in all organisms. It is found in the cytoplasm where it is associated with protein molecules which together form the cell organelles known as ribosomes.
tRNA ( Transfer RNA): It transfers amino acids present in the cytoplasm to the ribosome and acts as an intermediate molecule between the triplet code of mRNA and the amino acid sequence of the polypeptide chain. It is a small molecule ( about 80 nucleotides ) comprising a single strand. It is manufactured by DNA and makes upto 10-15% of the cell’s RNA and all types are basically similar. It forms a clover-leaf shape, with one end of the chain ending in a cytosine-cytosine-adenine sequence. It is at this point that an amino acid attaches itself. There are at least 20 types of tRNA, each carrying a different amino acid. At an intermediate point along the chain is an important sequence of three bases, called the anticodon. These line up alongside the appropriate codon on the mRNA during protein synthesis.
Since DNA is a code for the production of protein molecules, it is quite clear that the sequence of bases in the DNA is a code for the sequence of amino acids in protein molecules. This relationship between bases and amino acids is known as the genetic code. There are 20 common amino acids used to make proteins and that the bases in the DNA must code for. Only a code composed of three bases could incorporate all 20 amino acids into the structure of protein molecules; the code is therefore a triplet code and is called a codon. The code is universal; the same triplet code for the same amino acids in all organisms, it is degenerate; a given amino acid may be coded for by more than one codon, and it is non-overlapping which means that each triplet is read separately. For example, CUGAGCUAG is read as CUG-AGC-UAG and not CUG-UGA-GAG-AGC, where each triplet overlaps the previous one.
Protein synthesis involves four main stages:
- Synthesis of amino acids: in plants, the formations of amino acids occur in the mitochondria and chloroplasts from nitrates absorbed from the soil. Animals usually obtain their amino acids from the food they ingest, although they have some capacity to synthesize their own non-essential amino acids. The remaining ones – essential amino acids – must be provided in the diet.
- Transcription: This is the making of mRNA from DNA where a length of DNA is copied into a mRNA molecule. Transcription is basically the mechanism by which the base sequence of a section of DNA representing a gene is converted into the complementary base sequence of mRNA. A specific region of the DNA molecule, called a cistron, unwinds. This unwinding is the result of hydrogen bonds between base pairs in the DNA double helix being broken. This exposes the bases along each strand. Each base along one strand attracts its complementary RNA nucleotide, i.e. a free guanine base on the DNA will attract an RNA nucleotide with a cytosine base. However, uracil, and not thymine, is attracted to adenine, since RNA is being formed. The enzyme RNA polymerase moves along the DNA adding one complementary RNA nucleotide at a time to the newly unwound portion of DNA. The region of base pairing between the DNA and the RNA is only around 12 base pairs at any one time as the DNA helix reforms behind the RNA polymerase. The DNA thus acts as a template against which mRNA is constructed. When sufficient number of mRNA molecules have been formed from the gene, the RNA polymerase molecule leaves the DNA and the two strands ‘zip up’ again, reforming the double helix. Being too large to diffuse across the nuclear membrane, the mRNA leaves instead through the nuclear pores. In the cytoplasm, it is attracted to the ribosomes. Along the mRNA is a sequence of triplet codes which have been determined by the DNA and each triplet, as mentioned before, is called a codon.
- Amino acid activation: This is the process by which amino acids combine with the tRNA using energy from ATP. Each type of tRNA binds with a specific amino acid, which means there must be at least 20 types of tRNA. Each type differs, among other things, in the composition of a triplet of bases called the anticodon. All tRNA molecules have a free end in common, which terminates in the triplet CCA. It is to this free end that the individual amino acids become attached. The tRNA molecules with attached amino acids then move towards the ribosomes.
- Translation: This is the mechanism by which the sequence of bases in a mRNA molecule is converted into a sequence of amino acids in a polypeptide chain. It takes place on the ribosomes. Several ribosomes may become attached to a molecule of mRNA and the whole structure is known as a polyribosome or polysome. The advantage of such an arrangement is that it allows several polypeptides to be synthesized at the same time. Each ribosome is composed of a small and a large subunit. The complementary anticodon of a tRNA-amino acid complex is attracted to the first codon on the mRNA. The second codon likewise attracts its complementary anticodon. The ribosome acts as a framework holding in position, the mRNA, tRNa and the associated enzymes controlling the process, until a peptide bond forms between the adjacent amino acids. Once the new amino acid has been added to growing polypeptide chain the ribosome moves one codon along the mRNA. Once each amino acid is linked, the tRNA which carried it to the mRNA is released back into the cytoplasm. It is again free to combine with its specific amino acid. This sequence of the ribosome reading and translating the mRNA code continues until it comes to a codon signaling stop. These terminating/nonsense codons are UAA, UAG and UGA. At this point, the polypeptide chain, now with its primary structure as determined by the DNA, leaves the ribosome and the translation is complete. The polypeptide so formed must now be assembled into proteins. This may involve the spiraling of the polypeptide to give a secondary structure, its folding to give a tertiary structure and its combination with other polypeptides and/or prosthetic groups to give a quaternary structure.
Hence, simple/complex proteins are manufactured by the above processes and are used to perform a variety of essential functions in the human body.
