The process of Protein Synthesis involves many parts of the cell. Unlike other similar productions, this process is very complex and precise and therefore must be done in proper sequence to work effectively. The slightest error during this process could cause the action to experience difficulty or even fail. For example, in the production of starch, glucose molecules are combined to be stored and eventually utilized as usable chemical energy. The cell can break down the starch with little difficulty as if each molecule was identical, even though there is a wide variety of molecules. This is a different case in Protein Synthesis. In Protein Synthesis, there are twenty different amino acids and if one is out of place than is will effect the specificity of the protein. In a healthy person, the protein hemoglobin can be found in red blood cells, hemoglobin is helps with the transfer of respiratory gases from the blood to the tissues of the body. With an illness called sickle-cell anemia, the red blood cells are changed from a round, disk shape to a floppy looking sickle shape. These cells therefore cannot pass through small blood vessels due to their divergent shape. The actual cause of this mutation is a gene disorder, where the sixth codon of the protein glutamaric acid is changed with valine. This small change in the genetic code can cause severe defects in the effected such as blood clots, severe disorders and even death. All this can result from a misinterpretation in one codon in a chain of hundreds! Protein synthesis acts in this way, that is if there is only the most minuscule mistake it can have monstrous effects.
Protein synthesis first begins in a gene. A gene is a section of chromosome compound of deoxyribonucleic acid or DNA. Each DNA strand is composed of phosphate, the five-carbon sugar deoxyribose and nitrogenous bases or nucleotides. There are four types of nitrogenous bases in DNA. They are (A)denine, (G)uanine, (T)hymine, (C)ytosine and they must be paired very specifically. Only Adenine with Thymine (A-T) and Guanine with Cytosine (G-C).
To form a polynucleotide DNA, many nucleotides are linked together with 3`-5` phosphodiester linkages. In a complete molecule of DNA two of these polynucleotide strands are linked together by nitrogenous bases at 90 degrees to the sugar-phosphate “spine” (FIG. 1). The nitrogenous bases are held together with weak hydrogen bonds. One polynitrogenous chain runs in a 3′-5′ direction, the 3′ being the top hydroxyl and the 5′ being the bottom phosphate attached to the carbon five of the sugar. The other string runs the opposite. The two strands of the structure cannot be identical but they are complimentary. There is no restrictions on the placement and sequence of the nucleotides, which becomes important in storage of information.
Genetic information would be rendered useless if the stored information did not have a way of reaching the desired focal area. Since protein synthesis occurs in the cytoplasm and the DNA must remain in the nucleus, a way of transporting the code is essential. This comes in the form of messenger ribonucleic acid or m-RNA. Since the information on the DNA must stay the same on the m-RNA, the two have to be very similar. There are three major differences between RNA and DNA. RNA is only a single strand. The five carbon sugar of RNA is ribose opposed to deoxyribose and in RNA the pyrimidine uracil (U) replaces DNA’s pyrimidine thymine (T). Since RNA is produced from DNA, the nucleotides of RNA can hold the same information as the nucleotides of DNA because the code for amino acids is centered around the RNA structure.
The process in which m-RNA is synthesized is called transcription. This process is similar to DNA replication in the way that for transcription to occur, the double helix DNA must be unwound as in DNA replication (FIG 2). The major difference between transcription and replication is that in transcription only one of the strands is used as a template and only one m-RNA strand is produced. Transcription can be broken up into three parts in order to be understood. These steps are: i)initiation, ii)elongation and iii)termination. Initiation of transcription is how the transcription begins. The enzyme responsible for m-RNA synthesis is called RNA polymerase 2. The RNA polymerase knows where to begin transcription because it is coded into the DNA.
Elongation of transcription represents how the process happens. This occurs the same way as DNA replication, with the nucleotides being added one at a time in the 5′-3′ direction as the m-RNA strand uses the DNA strand as a template. Notice that uracil replaces thymine.
Termination of transcription represents how the process stops. Transcription is stopped by certain sequences coded into the DNA template. These sequences are called terminators. At the terminator sequence, RNA polymerase 2 stops or pauses, causing the transcription to be completed and the m-RNA to be released.
DNA can replicate prior to mitotic division. This process is called semiconservative, meaning that each daughter duplex contains one parental and a complimentary replicated chain. For DNA to replicate, it must first be unwound. This is done by an enzyme called helicase; using ATP as an energy source. The helicase helps this in process by breaking the weak hydrogen bonds between nitrogenous bases. While unwinding, the strands can become tangled and knotted. This problem is solved by an enzyme called gyrase which can make transient breaks in the strand relieving tension and then rejoins the ends. DNA replication occurs in a partially unwound are where some of the duplex region is still present, known as the replication fork. For DNA synthesis, all four nucleotides must be present. The existing DNA strands serve as templates which dictate the nucleotide sequence of the new strand. Growth of the new chain only occurs in the 5′-3′ direction.
DNA has the capacity to determine the sequences of specific proteins. The proteins are composed of amino acids; of which there are twenty types. Since there are only four types of nucleotides to “blueprint”, DNA uses combinations of three nucleotides to form codons (FIG. 3). Each gene has its own amount and series of codons, depending on the protein. There are sixty-four codons each having its own meaning. The only codon that has a double meaning is AUG. This codon symbolizes the amino acid metheonine and also signals where the polypeptide synthesis should start.
Translation is the process where the amino acid sequence is derived from m-RNA. To understand translation, one must first understand transfer RNA, t-RNA (FIG. 4). The function of t-RNA is to serve as a transporter for amino acids and an intermediate between m-RNA codons and their corresponding amino acids. Transfer RNA have anticodons which make them correspond to the codons of m-RNA. These t-RNA, that is with the help of an enzyme called aminoacyl t-RNA synthetase, carry the proper amino acids to the proper position in the m-RNA chain. When an amino acid is bonded to a t-RNA molecule, ATP supplies the energy. When an amino acid is bonded to another amino acid by a peptide bond, the ATP supplies the energy. The final component of the translation process is the ribosome. Ribosome’s are a cellular organelle that causes the t-RNA, the m-RNA and the amino acid sequence to come together and form a polypeptide chain. Ribosome’s are composed of two unequal sub-units. Each sub-unit contains ribosomal RNA and ribosomal protein. Ribosome’s are attached to the m-RNA, read the codons, make sure that the proper t-RNA is in place and then bonds the amino acids together by peptide bonds (FIG. 5). There are three m-RNA codons that cause translation termination. There are not any t-RNA’s that correspond to these codons. Instead, they are recognized by proteins as release factors. These release factors cause the release of the polypeptide chain from its t-RNA and the ribosome. Then the polypeptide chain “folds” back up into its original structure. With the release of the chain, the ribosome leaves the m-RNA. The ribosomal sub-units are then ready to repeat the process for another m-RNA. See FIG 6 for complete description.
Mutations can occur either in body cells or reproductive (germinal) cells. Only diseases of germinal cells can be passed through generations. Mutations can alter a single gene point ( point mutations) or can effect and change the structure of many chromosomes ( chromosomal mutations). Mutations are not always bad because they can cause adaptation and variation in people.
Point Mutations and Base Pair Mutations
The most common type of mutation involves a change in only a single base pair. This change only effects a single codon of the gene. There are three types of base pair mutations: silent, missense, and chain termination. Silent mutations involves the repositioning of the third codon. This does not effect the amino acid sequence. Missense mutation is where one codon is altered to code for a different amino acid (sickle cell anemia). Chain termination mutations involve the codon being changes to a stop codon. This causes the protein synthesis to remain incomplete and lose most of the biological activity.
This is the addition or deletion of one or more base pair but not multiples of three. This causes the ribosome to read the codon incorrectly causing and entirely different amino acid sequence. Mutagens are agents that increase the frequency of mutations. X-rays or other radiation are causes of mutagens.