This book presents the basics of what the genetic code is, so that the reader can have a basis of understanding of molecular biology. The backbone of the book is the central dogma of molecular biology, which is the idea that information flows from DNA to RNA to protein. It not only defines and discusses what the genetic code is, but discusses the rules of the genetic code and the type of mutations that can occur in the system.
The basic building blocks of both DNA and mRNA are adenine, thymine, guanine, and cytosine. These are defined as nitrogen bases and are usually labeled A, T, G, and C. These bases code for twenty specific amino acids. The twenty amino acids that can be created are as follows: alanine, arginine, asparagine, aspartate, cysteine, glutamine, glutamate, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, praline, serine, threonine, tryptophan, tyrosine, and valine. A nucleotide is defined as a linked bunch of molecules, composed of a phosphate group, nitrogenous bases, and a pentose sugar. The nucleotides code for these amino acids in groups of three, giving 64 possible combinations. The groups of threes are called codons. There are actually three codons that don't code for amino acids. These codons are called stop codons and they signal signal translation termination. These codons are UAA, UAG, and UGA.
The genetic code is said to be degenerate. This means that the code doesn't code to its capacity. Basically, the twenty known amino acids that can be created by the genetic code can be made by more than one type of nucleotide sequence. For instance, CUU, CUC, CUA, and CUG all code for the amino acid leucine. Another example is that CGU, CGC, CGA, and CGG all code for arginine.
According to the book, there are three main rules for the genetic code. The first rule is that the sequence of nitrogenous bases must follow the direction of translation. mRNA is translated in the 5' to 3' direction, so the codon sequences have to occur in the same orientation. This ensures that they will be properly translated. The first base of a codon must be located at the 5'-most end of the codon. For instance, the following three bases code for the amino acid cysteine: CGU. Two codons code for this amino acid, 5'-UGU-3' and 5'-UGC-3'. 5'-UGC-3' matches the codon CGU if you read it backwards.
The second rule is that one nucleotide can be used per reading frame. In other words, one nucleotide can only be part of one codon. For example, the code AATT could be read only as AAT or ATT, but not at the same time.
The last rule deals with start and stop codons. Basically, once you begin reading a codon from a specific nucleotide, one must continue reading it by threes until the end. The most common start codon is AUG. UAA, UAG, or UGA are the stop codons. So, the implication of this rule is that any sequence can be read in three different ways, depending on which nucleotide is put first. The three different ways of reading can yield three different amino acids.
Mutations are errors in codons caused by changes in nucleotide bases. Depending on the type of mutation, the error can cause no change or devastating change in protein created. These changes can result in positive phenotypic changes in the living organism, but usually are deleterious. The first type of mutation discussed is the base substitution. This is when one base is substituted for another. There are three main base substitutions: silent mutations, missense mutations, and nonsense mutations. Silent mutations do not change the amino acid created, due to the degeneracy of the genetic code. For example, if UGU is changed to UGC, the corresponding amino acid will still be cysteine. A missense mutation results in a substitution that changes the actual amino acid that is created by the codon. A nonsense mutation is a substitution that actually transforms the codon into a stop codon. This is generally considered to be the worst sort of base substitution mutation because it can really mess up the formation of a protein.
The next category of mutations is the insertion and deletion mutations. The main mutation that correlates to the insertion or deletion of a nucleotide in a codon is the frameshift mutation. This mutation is very profound in its effects, due to it changing every codon in a genetic sequence. It messes up the three codon structure which constitutes code, thus altering the protein that is created by the code.
The last category of mutations are the suppressor mutations. Supressor mutations change the result of an entirely different mutation. There are two types of these: extragenic and intragenic mutations. Extragenic mutations occur outside the genetic code, but has an effect on the amino acid sequence that is translated from the genetic code. Basically, one mutation can negate another mutation, due to the affects of a tRNA mutation. An intragenic mutation comes from within the genetic code. An example of this would be if an insertion of a particular nucleotide was negated by a frameshift deletion of this nucleotide. This book does a thorough job of educating the reader in the basic of the genetic code. It not only defines what the genetic code is, what rules govern it, and what mutations can occur in it, but it discusses how the genetic code is an indicator of evolution. Since all life shares the same four nitrogenous bases, which make up the codons that code for the proteins that make all living bodies, evolution from common descent seems plausible.
The basic building blocks of both DNA and mRNA are adenine, thymine, guanine, and cytosine. These are defined as nitrogen bases and are usually labeled A, T, G, and C. These bases code for twenty specific amino acids. The twenty amino acids that can be created are as follows: alanine, arginine, asparagine, aspartate, cysteine, glutamine, glutamate, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, praline, serine, threonine, tryptophan, tyrosine, and valine. A nucleotide is defined as a linked bunch of molecules, composed of a phosphate group, nitrogenous bases, and a pentose sugar. The nucleotides code for these amino acids in groups of three, giving 64 possible combinations. The groups of threes are called codons. There are actually three codons that don't code for amino acids. These codons are called stop codons and they signal signal translation termination. These codons are UAA, UAG, and UGA.
The genetic code is said to be degenerate. This means that the code doesn't code to its capacity. Basically, the twenty known amino acids that can be created by the genetic code can be made by more than one type of nucleotide sequence. For instance, CUU, CUC, CUA, and CUG all code for the amino acid leucine. Another example is that CGU, CGC, CGA, and CGG all code for arginine.
According to the book, there are three main rules for the genetic code. The first rule is that the sequence of nitrogenous bases must follow the direction of translation. mRNA is translated in the 5' to 3' direction, so the codon sequences have to occur in the same orientation. This ensures that they will be properly translated. The first base of a codon must be located at the 5'-most end of the codon. For instance, the following three bases code for the amino acid cysteine: CGU. Two codons code for this amino acid, 5'-UGU-3' and 5'-UGC-3'. 5'-UGC-3' matches the codon CGU if you read it backwards.
The second rule is that one nucleotide can be used per reading frame. In other words, one nucleotide can only be part of one codon. For example, the code AATT could be read only as AAT or ATT, but not at the same time.
The last rule deals with start and stop codons. Basically, once you begin reading a codon from a specific nucleotide, one must continue reading it by threes until the end. The most common start codon is AUG. UAA, UAG, or UGA are the stop codons. So, the implication of this rule is that any sequence can be read in three different ways, depending on which nucleotide is put first. The three different ways of reading can yield three different amino acids.
Mutations are errors in codons caused by changes in nucleotide bases. Depending on the type of mutation, the error can cause no change or devastating change in protein created. These changes can result in positive phenotypic changes in the living organism, but usually are deleterious. The first type of mutation discussed is the base substitution. This is when one base is substituted for another. There are three main base substitutions: silent mutations, missense mutations, and nonsense mutations. Silent mutations do not change the amino acid created, due to the degeneracy of the genetic code. For example, if UGU is changed to UGC, the corresponding amino acid will still be cysteine. A missense mutation results in a substitution that changes the actual amino acid that is created by the codon. A nonsense mutation is a substitution that actually transforms the codon into a stop codon. This is generally considered to be the worst sort of base substitution mutation because it can really mess up the formation of a protein.
The next category of mutations is the insertion and deletion mutations. The main mutation that correlates to the insertion or deletion of a nucleotide in a codon is the frameshift mutation. This mutation is very profound in its effects, due to it changing every codon in a genetic sequence. It messes up the three codon structure which constitutes code, thus altering the protein that is created by the code.
The last category of mutations are the suppressor mutations. Supressor mutations change the result of an entirely different mutation. There are two types of these: extragenic and intragenic mutations. Extragenic mutations occur outside the genetic code, but has an effect on the amino acid sequence that is translated from the genetic code. Basically, one mutation can negate another mutation, due to the affects of a tRNA mutation. An intragenic mutation comes from within the genetic code. An example of this would be if an insertion of a particular nucleotide was negated by a frameshift deletion of this nucleotide. This book does a thorough job of educating the reader in the basic of the genetic code. It not only defines what the genetic code is, what rules govern it, and what mutations can occur in it, but it discusses how the genetic code is an indicator of evolution. Since all life shares the same four nitrogenous bases, which make up the codons that code for the proteins that make all living bodies, evolution from common descent seems plausible.
| Written by Chris Bielke Independent Affiliate GeneWize LifeSciences Direct: 928-261-8247 Article Source: http://EzineArticles.com/?expert=Chris_Bielke |
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