In the early years of the twentieth century, researchers began to realize a connection between genes and proteins. This groundbreaking realization of George Beadle and Edward Tatum (Campbell, 2005) came years before the knowledge needed to understand the nuts and bolts of the process known as protein synthesis. The structure of the DNA molecule had not yet been discovered and the genetic code had not been determined. It was not until these two prerequisite Nobel Prize winning tidbits of information were in place that the mechanisms of protein synthesis were elucidated.
DNA, deoxyribonucleic acid, is the genetic material found within all cell types. It contains four nitrogenous bases: adenine, guanine, cytosine, and thymine. It is these bases that make up the genetic code. DNA, specifically genes within the DNA, codes for specific proteins as suspected by Beadle and Tatum. Each gene contains a sequence of nitrogen bases that when deciphered codes for a specific sequence of amino acids, the building blocks of protein. It was Marshall Nirenberg who first worked out the genetic code. (Campbell, 2005) He realized that the genetic code was a triplet code and that a sequence of three nitrogen bases, called a codon, codes for a specific amino acid. Today, there are 64 codons in the genetic code, 61 of which code for the 20 amino acids used in protein synthesis and 3 that act as "stop" signals.
Many years and much scientific work later, it is now known that several steps are required to move from the information contained within DNA to the outward expression of a gene. The two main processes required are transcription and translation. Transcription involves the copying of information in DNA into a second type of nucleic acid, ribonucleic acid or RNA. The RNA formed by transcription acts as a blueprint used during the construction of the protein and is called messenger RNA (mRNA). Translation involves the building of a protein by the assembly of the amino acid building blocks. This process involves the mRNA copied from the DNA and two other types of RNA: ribosomal RNA (rRNA) and transfer RNA (tRNA). Ribosomes are the cellular construction site of proteins and are composed primarily of rRNA. The tRNA molecules act as construction workers physically carrying amino acids to the ribosome for protein construction.
It is important to note that there are significant differences between protein synthesis in prokaryotic and eukaryotic cells due to the structural differences of these two cell types. Since prokaryotic cells lack a nuclear membrane, the processes of both transcription and translation occur within the cytoplasm and occur simultaneously. However, in a eukaryote, the presence of the nuclear membrane separates transcription from translation spatially and temporally, in space and time, respectively. Another important difference is that the product of transcription in eukaryotes is called a primary transcript or pre-mRNA and must undergo processing before it moves out of the nucleus and to the ribosome for translation.
What follows is a description of transcription, RNA processing, and translation in the eukaryote. Transcription involves three main steps: initiation, elongation, and termination. The initiation process involves the formation of a complex of molecules at a specific location along a DNA strand. This location is known as the promoter and is recognizable due to the presence of a specific nitrogen base sequence called the "TATA" box. The sequence is TATAAAA. An enzyme called RNA polymerase joins to the promoter with the aid of several specialized proteins called transcription factors. Once joined this complex is called the transcription initiation complex and can now act as a molecular machine that will build the RNA in the elongation step. Elongation progresses as RNA polymerase reads the DNA and assembles complimentary RNA nucleotides. This involves nitrogen base pairing. In DNA, adenine bonds to thymine (A-T) while guanine bonds to cytosine (G-C). The base pairing rules are similar for RNA except that RNA nucleotides do not contain thymine. Instead, a nitrogen base called uracil is present. From the following DNA template, TAC CCG AAT GTG ACT, the RNA formed would have the sequence: AUG GGC UUA CAC UGA. The process of base pairing would continue until a termination signal is reached in the DNA. An excellent animation of this process can be seen at http://vcell.ndsu.nodak.edu/animations/transcription/index.htm
RNA processing occurs in eukaryotes because of the presence of what are known as split genes. For reasons still under scientific scrutiny, the genes are segmented into exons, coding regions, and introns, noncoding regions. In order to create mRNA, the introns must be removed and the exons must be spliced together. This process of RNA splicing is carried out by another specialized molecular machine called a spliceosome. RNA splicing occurs in the nucleus and is followed by modification of the ends of the mRNA. At the leading end a guanine cap is added while a polyadenine tail is placed at the other end. These end modifications are believed to aid in transport of the mRNA out of the nucleus and help to protect the mRNA from degradation in the cytoplasm. An excellent animation of this process can be seen at http://vcell.ndsu.nodak.edu/animations/mrnaprocessing/index.htm
Finally, the process of translation and protein synthesis can begin. Translation also involves initiation, elongation, and termination. Initiation involves the formation of a translation initiation complex that includes the ribosome, the mRNA, the initiator tRNA molecule that recognizes the "start" codon AUG, and some specialized proteins called initiation factors. Once formed the process of elongation may begin. Elongation involves tRNA molecules entering the ribosome and dropping off the amino acid building blocks in the appropriate sequence to build a functional protein. This is done by a special type of nitrogen base pairing. Codons of the mRNA are "read" by a special sequence of three nitrogen bases found in the tRNA molecules called anticodons. A codon-anticodon match signals where to drop a specific amino acid. For example, the codon UUU in mRNA codes for the amino acid phenylalanine. The tRNA carrying phenylalanine would therefore have the anticodon sequence AAA, which is complimentary to UUU. The ribosome moderates this interaction between mRNA and tRNA and moves along the mRNA one codon at a time. As the elongation progresses, a polypeptide chain or protein is being assembled. Eventually, a "stop" codon is read and termination of protein synthesis occurs and translation is complete. An excellent animation of this process can be seen at http://vcell.ndsu.nodak.edu/animations/translation/index.htm
The protein created through translation undergo post-translational modifications and folding to take on its functional conformation. The processes of transcription, RNA processing, and translation have successfully created the outward expression of a gene, the protein. The protein can then carry out its function whether it be a structural protein, a transport protein, or an enzyme. The road to gene expression is complete.
Campbell, Neil and Reece, Jane. Biology, 7th edition. Benjamin Cummings, San Francisco, 2005.