The activity of any living cell, and by extension life itself, depends on protein synthesis and the transcription of deoxyribonucleic acid, or DNA.
If proteins are the machinery of cellular function, then DNA strands are the machine assembly lines. These lines are responsible for accurately and efficiently transcribing protein messengers, structures, and enzymes.
DNA transcription begins in the nucleus of a cell when an enzyme called RNA (ribonucleic acid) polymerase binds to the DNA strand. Sequences within the DNA direct the polymerase to specific start and endpoints within the genetic code.
Next, proteins unwind the DNA strand in a process called elongation. This allows the RNA polymerase to copy a single-stranded messenger RNA (or mRNA) from one strand of the double-stranded DNA.
Finally, RNA polymerase transcribes along the DNA sequence until it reaches a signal telling it to stop, known as a terminator sequence. When the RNA polymerase detects this sequence, it releases the completed mRNA polymer and separates from the DNA.
After an mRNA copy is transcribed from the original DNA strand, protein synthesis continues through a process called translation. Two cellular components — transfer RNA (tRNA) and ribosomes — are necessary to convert the single strand of nucleotides into functioning proteins.
The entire process of translation takes place in the cytoplasm of the cell, outside the nucleus. First, the mRNA segment to be translated must leave the nucleus. After entering the surrounding cytoplasm, its genetic code sequence is modified several times before being translated. Some sections of the code, called introns, do not get translated into proteins and are removed. The mRNA is also capped at one end by a series of adenine bases, referred to as a poly-A tail, and at the other end by a guanosine-triphosphate cap. Once these modifications are in place and the introns are removed, the mRNA sequence is ready for the next step in the protein synthesis process: translation.
To begin the translation process, mRNA attaches to a ribosome. This piece of cellular machinery acts as a matchmaker, bringing together the mRNA and tRNA molecules. This will string together a chain of protein-forming amino acids.
Next, tRNA attaches to the nucleotide sequence of mRNA and begins to chain together a corresponding string of amino acids. The 3-leaf-clover structure of tRNA is essential to this process. At the stem of the clover is an amino acid attachment site. The leaf opposite the stem holds a section called the anticodon. The anticodons bind to a specific sequence of nucleotides on the mRNA called codons. These two sections allow for the correct amino acid to be inserted into the chain as determined by the nucleotide code in the mRNA.
Once the chain of amino acids is formed, it is released into the cell. The product of protein synthesis is then modified within the cell for a specific purpose, such as the coding for hair or eye color.