Protein synthesis is the process by which cells convert amino acids into long chain polymers called proteins. Proteins are molecules that have a variety of functions in cells such as providing structure, storing energy, providing movement, transporting other substances, catalyzing biological reactions, and protecting against disease. Proteins make up more than 50% of a cell's dry weight. Protein synthesis is programmed by DNA. During this process DNA is converted to RNA which is then translated into a protein by the ribosomes.
The theories that laid the foundation for modern understanding of protein synthesis began in 1909 with Archibald Garrod. He was the first to suggest that genes were chemically expressed through enzymes that catalyze specific chemical reactions in the cell. He even theorized that an inherited disease reflected a person's inability to make a particular enzyme. Unfortunately, his ideas about inheritance were ahead of their time, and it took several decades before they were supported by further research.
This research came in the 1930s when George Beadle and Edward Tatum established the relationship between genes and enzymes. They discovered that in certain bread mold species, there were mutants that required extra nutrients to grow on a plate. These different mutants were thought to be deficient of certain metabolic enzymes. Through their research, Beadle and Tatum established the one gene-one enzyme hypothesis, which states that the function of a gene is to dictate the production of a specific enzyme. This idea was later refined when it was found that genes also dictate the production of proteins.
Genes are the genetic material that dictate the production of proteins and enzymes. The genes are located in the nucleus of the cell and are composed of DNA. The DNA is composed of nucleotides which are molecules made up of a sugar component, a phosphate group and a cyclic base. There are four different nucleotidebases in which all the information for making proteins is stored. A gene is typically hundreds or thousands of nucleotides long. Unlike DNA, proteins are made up of amino acids instead of nucleotides. To get from DNA to protein requires two steps, transcription and translation.
The first step in protein synthesis involves the transcription of DNA into messenger RNA (mRNA). While DNA and RNA are similar, there are subtle differences. For example the sugar component of DNA has one less hydroxyl group than ribose, the sugar component of RNA. Also, RNA uses the nucleotide uracil instead of thymine.
During the process of transcription an enzyme known as RNA polymerase separates an area of the double strands of DNA. It first binds to a region of DNA known as a promoter region. It then begins to transcribe, or copy, the DNA into RNA at an initiation site. Only one strand is used as a template for making the mRNA. This occurs by the base pairing of the nucleotides. The polymerase than moves along the DNA and elongates the RNA molecule. As the polymerase travels along the DNA it both unwinds, catalyzes copying and rewinds the DNA. When the polymerase comes upon a certain sequence of DNA, called the termination sequence, it stops transcription and releases the RNA. The resulting RNA contains an exact copy of the gene. This RNA is further processed by RNA splicing to remove introns. The result is mRNA which is transported out of the nucleus into the cytoplasm where it can be translated into a protein.
Translation is the actual production of the protein or polypeptide. It is the process by which the mRNA nucleotide code is translated into amino acids. It turns out that different combinations of three nucleotides code for the 20 amino acids. For example, the base triplet GCA is translated into the amino acid alanine. Thus, the genetic code is in the form of triplet codons. Since it takes three nucleotides to code for 1 amino acid, each gene must have three times as many nucleotides as amino acids that make up the protein. For example, a strand of mRNA that has 300 nucleotides would code for a protein that is 100 amino acids long.
The process of translation involves cellular organelles called ribosomes. The ribosome are made up two subunits which are composed of proteins and ribosomal RNA. During translation the small subunit of the ribosome attaches to the mRNA at the initiation site. This is a sequence of three nucleotides which is the same in every gene. The large subunit then converges on the structure carrying with it another type of RNA called transfer RNA tRNA). The tRNA has a reading portion which recognizes three nucleotides. It interacts with an enzyme called aminoacyl-tRNA synthetase which attaches specific amino acids to the tRNA.
After initiation, the amino acids are combined to create a growing polypeptide chain. The ribosome travels along the mRNA in blocks of three nucleotides, or codons. The tRNA reads the codon and new amino acids are added one by one. When the ribosome gets to a certain codon, called the stop codon, it ceases translation. The polypeptide chain is then modified by other enzymes in the cytoplasm. These posttranslational modifications can include the attachment of sugars or lipids, the removal of certain amonio acids, or the joining of one polypeptide chain with another to make a multiple unit protein.
Cracking the genetic code was done during the 1960s. The first experiment was done by Marshall Nirenberg. He created an artificial RNA molecule which contained only uracill. He put this in a test tube with ribosomes and the other materials needed to synthesize proteins. The result was a protein that contained only phenylalanine. This suggested that a codon of UUU specified the amino acid phenyalanine. After this experiment, a series of similar ones were run until the entire genetic code was determined. Now when scientists get the nucleotide sequence of a gene, they know exactly the amino acid sequence of the protein it codes for. The genetic code is nearly universal. It is shared by organisms as diverse as bacteria to humans. This is important because it has allowed the incorporation of human genes in bacteria to produce human proteins.
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