Proteins are synthesized according to instructions carried in the genetic material, DNA (deoxyribonucleic acid). Protein synthesis can be divided into three phases: transcription, translation, and post-translation modification. The first two phases rely on the nucleic acids, DNA and RNA (ribonucleic acid), and on associated enzymes. Enzymes are proteins that speed up chemical reactions in living matter. The final phase involves other enzymes, but no nucleic acids. The finer details of protein synthesis differ between cells that have no nucleus (prokaryotes) and cells that do have a nucleus (eukaryotes). However, the overall process is basically the same.
Proteins are a very common type of molecule in living matter. One thing that all proteins have in common is that they are composed of chains of smaller molecules, called amino acids. A protein's ability to function properly is very closely related to having the correct sequence of amino acids. In some cases, a single misplaced amino acid can make a protein useless.
The first phase of protein synthesis is transcription of DNA. Like proteins, DNA is made up of a chain of smaller molecules. These smaller molecules are called nucleotides. There are four types of nucleotides in DNA, each containing one of four bases--adenine, guanine, cytosine, or thymine. DNA is a double-stranded, spiral-shaped molecule, often referred to as a double helix. When the two strands of DNA are lined up, their nucleotide bases face one another like the teeth of a zipper. The strands are joined by hydrogen bonds between the bases. Pairing between bases follows a strict pattern. Adenine will only pair with thymine, and vice-versa; cytosine will only pair with guanine, and vice-versa. Although there are only four types of nucleotides, the order in which they appear along the DNA strand can convey a lot of information.
However, the information flow from DNA to protein is not direct. One strand of the double helix--the coding strand or template strand--serves as a master copy of the genetic code. The cell uses RNA to construct a working copy from that strand. RNA is chemically similar to DNA. Key differences are that it is single-stranded, and uracil is used as one of the four bases rather than thymine.
Transcription begins when the enzyme, RNA polymerase, binds to the coding strand of DNA just before the start of a gene. A gene is the stretch of DNA that codes for a particular protein or polypeptide. (Polypeptides are composed of amino acids, but are smaller than proteins.) The RNA polymerase binds to DNA with the help of other proteins called transcription factors. Transcription factors can help a cell control the level of gene expression, i.e., the amount of protein synthesized.
Once RNA polymerase binds to the DNA, it begins to construct a strand of RNA. The two strands of DNA separate and, for a short while, the coding strand is paired with RNA as a matching strand. The base pairing between the DNA strand and the RNA strand follows the same rules as base pairing between two strands of DNA. The key exception is that uracil is used in place of thymine.
Once the DNA has been transcribed, the mRNA (messenger RNA) detaches from the DNA. It may be modified before moving on to the next phase of protein synthesis. Modification of mRNA is only used by eukaryotes. The mRNA is made up of coding and noncoding portions. Coding portions are called exons; noncoding portions are known as introns. Modification stabilizes the mRNA and snips out introns. Why introns exist is unknown. They may represent changes that have been added over the course of time, or they may be remnants of old genes that are no longer needed. In any case, they are removed before the mRNA is released from the nucleus into the interior of the cell.
The next phase of protein synthesis is translation of the code, which relies on two other types of RNA: ribosomal RNA (rRNA) and transfer RNA (tRNA). Although there are three types of RNA, they are all composed of adenine, cytosine, guanine, and uracil. The differences between the RNA types are based more on their function and their associated proteins, rather than the nucleic acid itself.
The rRNA is a component of a cellular structure called a ribosome. Ribosomes serve as sites for translation. To start translation, a ribosome attaches itself to a strand of mRNA. The ribosome serves to hold the mRNA in place and direct the speed of translation. The transcribed code carried on the mRNA is read three nucleotides at a time. There are 64 possible mRNA triplets, or codons. Sixty-one of them code for a specific amino acid; the remaining three serve as stop signals to end translation. Most amino acids have more than one associated codon.
As the ribosome gradually moves along the length of the mRNA, tRNA transports amino acids to the growing protein molecule. The tRNA contains a nucleotide sequence called the anticodon. The anticodon contained in the tRNA determines which amino acid it will carry. When the anticodon pairs up with a corresponding mRNA codon, it releases the amino acid to the growing protein chain. When the ribosome reaches one of the three stop codons, it detaches from the mRNA and a new protein molecule is released. Once released, the new protein may or may not undergo modification.
This is the complete article, containing 879 words
(approx. 3 pages at 300 words per page).
