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DNA and Protein Synthesis
DNA is the molecule which controls the synthesis of proteins. Proteins are used for growth and repair and also as enzymes, in which form they catalyse all other cellular activities.
Thus DNA is able to exert a controlling influence over the whole cell and ultimately, the whole organism. The segments of DNA which hold the key to this control are the genes.
Beadle and Tatum performed experimental work using the haploid mould Neurospora. Being haploid, this organism expresses all genes present, regardless of dominance.
Beadle and Tatum demonstrated that irradiated moulds developed mutations which affected their ability to synthesise amino acids. One mutation affected only one amino acid synthesis (a critical enzyme was absent) and was also passed to subsequent generations.
Their work led to the principle of one gene = one enzyme.
Later this principle was modified to one gene = one polypeptide.
It has since has become one cistron = one polypeptide.
A cistron is the shortest length of DNA that can code for a whole polypeptide.
Protein synthesis relies on the effective communication of the coded information held in the genes to the sites of protein manufacture, the ribosomes in the cytoplasm.
Since DNA is part of larger structures (chromosomes), which are unable to move from the nucleus, intermediate messenger molecules are needed. These are messenger RNA molecules.
To begin with, the DNA duplex unzips to expose the base sequence on the coding strand. RNA nucleotides then move in and align themselves according to the rules of base pairing (A-U and G-C) with U replacing T in the RNA molecule.
The RNA is assembled using the enzyme RNA polymerase. This process is called transcription.
The DNA template strand is read from the 3' to the 5' end and the mRNA is made from the 5' to the 3' end. During transcription only the coding parts of the DNA are copied (the exons). Non coding parts or introns are ignored.
The completed mRNA molecule detaches from the DNA template and exits the nucleus via the nuclear pores, moving into the cytoplasm.
The mRNA is now ready for translation, which is organised by the ribosomes, which now attach themselves to the mRNA.
If more than one ribosome attaches then a polysome (polyribosome) is formed, with the appearance of beads on a string. Each ribosome controls the formation of one polypeptide. The mRNA is read as triplets of bases called codons.
The ribosome attaches to the mRNA by its small subunit. Magnesium ions are involved in the attachment process.
The larger subunit of the ribosome can accommodate two codons of the mRNA. One is held in the P (peptidyl) site and the other in the A (aminoacyl) site. Each codon triplet then attracts a complementary triplet or anticodon.
Each anticodon forms part of one transfer RNA (tRNA) molecule. Each tRNA carries one specific amino acid in the cytoplasm. The anticodon and codon bind together temporarily by means of hydrogen bonds.
This causes two amino acids to be held next to one another long enough for the formation of a peptide bond between them.
The first amino acid in any polypeptide is usually methionine. The codon for this is AUG, which has come to be known as the initiation codon as a result. The formation of the peptide bond is catalysed by the enzyme peptidyl transferase, which is an integral part of the large ribosome subunit.
Once the peptide bond has formed, the first tRNA detaches and travels into the cytoplasm to pick up another amino acid.
The ribosome shifts along the mRNA by exactly one codon, so that the second codon now occupies the P site and the third the A site.
This movement is a process called translocation.
A third tRNA now moves to the correct position and a second peptide bond forms. This process is then repeated until the polypeptide is complete.
The ribosome moves along the mRNA from the 5' towards the 3' end. The completion of this process of translation is signalled by nonsense or stop codons. These do not correspond with any tRNA, but signal the ribosome to detach from the mRNA.
The polypeptide is then ready for modification into a specific protein. A stop codon may be UAA, UAG or UGA.
Watson and Crick originally proposed the triplet theory on the grounds that this is the minimum number which could give at least one unique combination for each amino acid. There are in fact 64 possible codons, so each amino acid has more than one to code for it. The genetic code is thus described as being degenerate.
Evidence for the genetic code being in the form of triplets included the work of Crick (1962). He showed that if single bases were removed from the DNA of T4 bacteriophages, then frame shifts were caused in the translation to polypeptides.
The code was broken by the work of Nirenberg. He synthesised mRNA with known base sequences and observed the resulting amino acid sequences. For example, mRNA made only of uracil (polyuradylic acid or poly-U) gives polypeptides made only of phenylalanine. Thus the codon UUU corresponds with the amino acid phenylalanine.
The genetic code is also universal (the same in all organisms) and non overlapping (triplets are adjacent).