Submit Cancel. February 01, Goodbye! November 16, Divide and conquer: coaxing cheaters to battle August 10, The origins of kin discrimination—telling June 22, The Evolution of Suicidal Sex. May 25, Shape Matters for Gene Expression.
April 20, What Drove the Great Dying? April 13, The pan-genome of Emiliania huxleyi April 06, Behavioral Transplants March 30, Drug resistance evolves in inbred parasites. March 23, What did the earliest skulls on land look like? December 01, Crocodilians Hunt With Tools!
November 24, What Makes a Cat? October 20, What Do Whales Taste? September 29, Island Biogeography in the Era of Humans. September 22, From chimps to chickens: how a little DNA can m July 07, The give-and-take between mothers and their off The portion of the genome that codes for a protein or an RNA is referred to as a gene. Those genes that code for proteins are composed of tri-nucleotide units called codons , each coding for a single amino acid.
Each nucleotide sub-unit consists of a phosphate , deoxyribose sugar and one of the 4 nitrogenous nucleotide bases. The purine bases adenine A and guanine G are larger and consist of two aromatic rings.
The pyrimidine bases cytosine C and thymine T are smaller and consist of only one aromatic ring. In the double-helix configuration, two strands of DNA are joined to each other by hydrogen bonds in an arrangement known as base pairing.
These bonds almost always form between an adenine base on one strand and a thymine on the other strand and between a cytosine base on one strand and a guanine base on the other. This means that the number of A and T residues will be the same in a given double helix as will the number of G and C residues. This in turn is translated on the ribosome into an amino acid chain or polypeptide. The process of translation requires transfer RNAs specific for individual amino acids with the amino acids covalently attached to them, guanosine triphosphate as an energy source, and a number of translation factors.
Individual tRNAs are charged with specific amino acids by enzymes known as aminoacyl tRNA synthetases which have high specificity for both their cognate amino acids and tRNAs. The high specificity of these enzymes is a major reason why the fidelity of protein translation is maintained.
In reality, all 64 codons of the standard genetic code are assigned for either amino acids or stop signals during translation. This RNA sequence will be translated into an amino acid sequence, three amino acids long. The standard genetic code is shown in the following tables. Table 1 shows what amino acid each of the 64 codons specifies. Table 2 shows what codons specify each of the 20 standard amino acids involved in translation.
These are called forward and reverse codon tables, respectively. Note that a codon is defined by the initial nucleotide from which translation starts.
Partial codons have been ignored in this example. Every sequence can thus be read in three reading frames , each of which will produce a different amino acid sequence in the given example, Gly-Lys-Pro, Gly-Asp, or Glu-Thr, respectively.
With double-stranded DNA there are six possible reading frames , three in the forward orientation on one strand and three reverse on the opposite strand. The actual frame a protein sequence is translated in is defined by a start codon , usually the first AUG codon in the mRNA sequence. However, as a more complex mutation, it is less likely to occur by chance. In practice, this mutation might take six months to develop in someone taking AZT monotherapy.
These mutations are like fruit machines: getting one bell is easier than getting a row of three cherries to show. You need to play more often and for longer to get the difficult combinations. It is included to show two things.
This can easily be tested by calculating the expected frequencies of amino acids and comparing to observed. The codons and observed frequencies of particular amino acids are given in the table.
The expected frequency of a particular codon can then be calculated by multiplying the frequencies of each DNA base comprising the codon. The expected frequency of the amino acid can then be calculated by adding the frequencies of each codon that codes for that amino acid.
Since 3 of the 64 codons are nonsense or stop codons, this frequency for each amino acid is multiplied by a correction factor of 1. By plotting the expected frequency against the observed frequency, we can see if some amino acids are occurring more or less often than expected by chance.
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