The transfer of genetic information from one generation to the next one of all organisms is made possible by means of a nucleic acid. The latter has two types, which are as follows: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). When cells in the body are divided, DNA is being copied and passed to the next generation, containing instructions for cellular activities. On the other hand, RNA is involved in the synthesis of proteins. Information is passed through DNA to RNA (Bailey 2012). Thus, these types of a nucleic acid carry genetic information and make it possible to be passed.
Codon and the Relation between the Sequence of Bases and the Protein Produced
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The genetic code is the sequence of nucleotides in DNA or RNA that determines a specific amino acid sequence in the synthesis of proteins. It is the biochemical basis of heredity. It also relates to DNA, which stores genetic information composed of nucleotide triplets or codons. A codon is a set of three adjacent nucleotides, which also specifies a particular amino acid in protein or stops protein synthesis. It is also referred to as triplet (Codon - definition 2008). The code is said to be non-overlapping, because triplets are read in order. It is also degenerated, because each amino acid can be specified by more than one codon, being also almost universal, since almost all organisms have exactly the same genetic code (Genetic Code n.d).
Proteins play important roles in every organism. They are the end products of all information pathways. There are many types of proteins, which play various functions inside an organism. Some of these functions are catalysis, structure, movement, transport, protection, storage, regulation, and hormones (Bettelheim, Brown & March 2004). Proteins are produced or synthesized according to the cell’s needs. They are transported to specific locations in the cell to perform specific functions. Proteins are programmed to die, when they are not needed, in order not to disturb or intervene in reactions inside the cell, which they may affect (Nelson, Lehninger & Cox 2008).
As it has been stated above, proteins perform specific functions, being polymers of amino acids. They are produced, synthesized, and function in a particular way. DNA (deoxyribonucleic acid) is the one that dictates how and what should a protein do. It defines every individual’s “fingerprint”. Every DNA is unique and the same in every inch of human body, from nails to hair. The DNA synthesized by cells in the head or hair is the same as the one synthesized by cells in toes or nails. However, there is the question: why are different proteins produced when in fact they came from the same DNA sequence?
DNA is made up of two strands and it is a polymer of nucleotides. Each nucleotide is composed of a base (adenine, guanine, cytosin, or thymin), sugar (deoxy-ribose for DNA), and a phosphate group. Double-stranded DNA is hold together by a hydrogen bond between base pairs: three hydrogen bonds between adenine and thymine, and two between cytosine and guanine.
As far as RNA is concerned, adenine is paired to uracil, and cytosine is paired to guanine. During translation, mRNA, which is the product of transcription, is turned into a protein by translating its nucleotides. Each triplet nucleotide codes for a specific amino acid is called a codon. As it has been mentioned earlier, a protein is a polymer of amino acids. Thus, it is possible to say that the sequence of nucleotides is responsible for how a target protein will function, since the former sequence defines the sequence of amino acids in mRNA being translated.
Effect of a Mutation in a Single Base on a Protein
There are certain instances, when bases of DNA are altered or changed. Such process is called a mutation. A gene mutation is a permanent change in the DNA sequence. It occurs in two ways. It can be acquired during lifetime or inherited from a parent (Toland 2011). This alteration of base sequences may be due to internal mistakes, chemical reactions, or may be caused by agents that make a mutation occur. Changes are introduced by environmental agents, such as sunlight, cigarette smoke and radiation, which damage DNA. These agents that favor a mutation are called mutagens (Toland 2011).
As it has been stated above, a gene is made up of bases that make up a protein. Changes that occur in these bases alter the gene’s meaning, changing the protein that is made.
Mutations of DNA have different types. The two of these are a frameshift and point mutation. The former results from the deletion or addition of bases and a change in the reading frame, which consists of three bases corresponding to one amino acid. This kind of mutation shifts the grouping of these bases. The resulting protein is nonfunctional (Genetics Home Reference 2012). On the other hand, a point mutation is a simple change in one base of the gene sequence. It is frequently a result of mistakes made during DNA replications.
The two types of point mutations are transition and transversion. The former mutation occurs when” a pyrimidine base, such as thymine or cytosine, is substituted by another pyrimidine base, or when a purine base, adenine or guanine, is substituted by another one” (Britannica Online Encyclopedia 2012). In contrast, transversion mutations occur when “a purine base is substituted by a pyrimidine base or vice versa” (Britannica Online Encyclopedia 2012). It is further classified into a silent, missense, and nonsense mutation. The first one is a result, when a base is changed and the produced triplet nucleotide codes for the same amino acid. For example, although the third base in the TCT codon for serine has been changed to any other three bases, serine will still be encoded. A missense mutation happens when a base pair is changed, and the resulting codon codes for a different amino acid. Lastly, a nonsense mutation will take place, when a base is changed, and the resulting triplet nucleotide will code for a stop codon (UAG, UGA, and UAA) (Mutations 2012).
Conditions and Symptoms Resulting from Mutations
A mutation can be harmful or beneficial depending on the protein produced after it. Sometimes, a change in just one base can have fatal effects on an organism. However, a point mutation can sometimes have no effect at all, because in some cases, when a base is changed, the resulting triplet nucleotide will still code the same amino acid. Therefore, the function of the protein will not be altered. For example, changing U in the third position of the sequence GGU to C will still result in amino acid glycine.
However, one condition that results from this error is sickle-cell disease. Sickle-cell anemia is a disorder of the blood. “It is usually caused by inherited abnormal hemoglobin, which leads to distorted red blood cells” (Mutations 2012). Sickle-cell disease is an example of a missense mutation. The replacement of adenine by thymine in the seventeenth nucleotide of the gene by the beta chain of hemoglobin changes the codon GAG or glutamic acid to GTG or valine. Thus, the sixth amino acid in the chain becomes valine instead of glutamic acid (Mutations 2012).
“Common symptoms of the disease are a direct result of abnormally shaped, sickled red blood cells blocking the flow of blood that circulates through the tissues of the body” (Sickle Cell Anemia 1996). The major symptoms of sickle-cell anemia are fatigue, anemia, pain crises, swelling and the inflammation of hands or feet, bacterial infections, lung and heart injury, as well as eye damage (Sickle Cell Anemia 1996).
Cystic fibrosis is a disease inherited in families. It causes thick and sticky mucus, which builds up in the lungs, digestive tract and other areas of the body. It is also considered as a life-threatening disorder (National Center for Biotechnology Information 2012). Cystic fibrosis is caused by a nonsense mutation. Persons, who suffer from this disease, have mutations in the gene encoding of the cystic fibrosis trans-membrane conductance regulator protein on both alleles of chromosome. It alters the structure, function, or production of a cyclic adenosine-5’-monophoosphate-dependent trans-membrane chloride channel protein, which is critical for normal functioning of multiple organs (Genetic of Cystic Fibrosis 2012).
Symptoms of this disorder are delayed growth, a failure to gain weight, no bowel movements, and salty-tasting skin in newborns. It can cause infertility in men, and lungs-related problems and bowel functions.
Indeed, a single mutation of a base may be an insertion or deletion. If it is in one of the bases, it can cause no change at all in the protein produced, if an organism is fortunate enough. On the other hand, a base mutation may have severe effects on organisms, since most mutations in proteins being translated are dysfunctional, or in worst cases, defective. Thus, as this paper has been proven, a base mutation highly affects the protein, which is being synthesized.