Genetic code mRNA sequences, Table

The sequence of the bases contains coded information for the synthesis of proteins. These sequences are transcribed into an RNA copy of the sequence messenger RNA (mRNA). The mRNA is translated in the cytoplasm. The DNA also encodes structural RNAs, with functions in transcription of the DNA, processing of the transcripts and translation of the transcripts. The genetic code shown in Table 8.2.1 is simple, but efficient. At each nucleotide position, there are only four possibilities A,... 

Table 1.5. Genetic code. Each sequence of three bases in the mRNA determines which amino acid is used in the polypeptide (refer to Table 1.1 for amino acid abbreviations).

The anticodon region consists of seven nucleotides, and it recognizes the three-letter codon in mRNA (Figure 38-2). The sequence read from the 3 to 5 direction in that anticodon loop consists of a variable base-modified purine-XYZ-pyrimidine-pyrimidine-5h Note that this direction of reading the anticodon is 3 " to 5 whereas the genetic code in Table 38—1 is read 5 to 3 since the codon and the anticodon loop of the mRNA and tRNA molecules, respectively, are antipar-allel in their complementarity just like all other inter-molecular interactions between nucleic acid strands. 

To transcribe information from DNA to mRNA, one strand of the DNA is used as a template. This is called the anticoding, or template, strand and the sequence of mRNA is complementary to that of the template DNA strand (Fig. A2.8) (i.e., C->G, G->C, T->A, and A U note that T is replaced by U in mRNA). The other DNA strand, which has the same base sequence as the mRNA, is called the coding, or sense, strand. There are 64 (4 x 4 x 4) possible triplet codes of the four bases 61 are used for coding amino acids and three for termination signals. As there are 20 amino acids for the 61 codes, some triplets code for the same amino acid. A table of the genetic code is presented in Exhibit A2.2. 

Most genetic code tables designate the codons for amino adds as mRNA sequences (Figure 1-4-1). Important features of the genetic code include ... 

Mutations are often classified according to the effect they have on the structure of the gene s protein product. This change in protein structure can be predicted using the genetic code table in conjunction with the base sequence of DNA or mRNA. A variety of such mutations is listed in Table 1-4-1. Point mutations and frameshifts are illustrated in more detail in Figure 1-4-2. 

Production of proteins from mRNA requires translation of the base sequence into an amino acid sequence. The collection of base sequences (codons) that correspond to each amino acid and to signals for termination of translation is the genetic code. The code consists of 64 triplets of bases (Table 25-2). The codons are written with the 5 terminus... 

As noted above, the genetic code used by cells is a triplet code, with every three-nucleotide sequence, or codon, being read from a specified starting point In the mRNA. Of the 64 possible codons in the genetic code, 61 specify individual amino acids and three are stop codons. Table 4-1 shows that most amino acids are encoded by more than one codon. Only two—methionine and tryptophan—have a single... 

Obviously, each codon present within mRNA must correspond to a specific amino acid. Nirenberg found that trinucleotides of known base sequence could bind to ribosomes and induce the binding of specific aminoacyl-tRNAs (i.e., tRNAs with amino acids covalently attached). As a result of these and the earlier experiments, the relationship between all 64 codons and the amino acids they specify (the entire genetic code) was determined by the mid-1960s (Table 15.1). 

Proteins are linear polymers of amino acids. The sequence of a protein s constituent amino acids determines its biochemical function. The mRNA sequence is read in groups of three, called codons. Because there are four bases in DNA or RNA, there are 64 (4 ) codons. Only 20 amino acids are specified by translation, so there is more than one codon per amino acid. In other words, the genetic code is redundant. The code also contains punctuation marks. Three codons, UAG, UAA, and UGA, specify stop signals (like the periods in a sentence). One amino acid, methionine, coded by AUG, is used to initiate each protein (like a capital letter at the beginning of a sentence). Just as a letter that starts a sentence can also appear in an uncapitalized form inside the sentence, so methionine also appears internally in proteins. See Table 4-1. 

One of the main tasks of the DNA is to initiate the synthesis of proteins as and when they are needed. Proteins are synthesised in the ribosomes of the cell cytoplasm. DNA, however, is found in the cell nucleus. So how is the information contained in the DNA passed out of the cell nucleus and into the cytoplasm First, the DNA helix unfolds, and, in a process called transcription, a complementary strand of RNA is synthesised along a crucial part of one of the single DNA strands. This is the messenger RNA (mRNA) which leaves the cell nucleus and is transported into the manufacturing centres for proteins, the ribosomes. In the ribosome, transfer RNA (tRNA) delivers the amino acids required for polypeptide synthesis. The sequence of each group of three bases on the mRNA determines which amino acid is next in the peptide sequence. For example, the sequence AGC in the mRNA specifies the incorporation of the amino acid serine. This process is referred to as translation (Fig. 1.27). The genetic code, i.e. which sequence of bases in the DNA strand refers to which amino acid is given in Table 1.5. 

The triplets of nucleotides (the codons) on mRNA are the genetic code (see Table 25.2). The code must be in the form of three bases, not one or two, because there are 20 different amino acids used in protein synthesis but there are only four different bases in mRNA. If only two bases were used, there would be only 4, or 16, possible combinations, a number too small to accommodate all of the possible amino acids. However, with a three-base code, 4, or 64, different sequences are possible. This is far more than... 

The genetic code can be expressed in mRNA codons (as we have shown in Table 25.2) or in DNA codons. We have chosen to show the mRNA codons because these are the codons that are actually read during the synthesis of polypeptides (the process called translation that we discuss next). However, each mRNA molecule (Section 25.5A) acquires its sequence of nucleotides by transcription from the corresponding gene of DNA. In transcription, RNA polymerase (along with other transcription factors) opens the DNA double helix and begins the process. 

The genetic code is based on triplet codons composed of three bases. Each triplet corresponds to a specific amino acid (Table 61.1). The mRNA codon AUG signals the start of a polypeptide reading frame in which the triplet codons determine the sequence of amino acids. The mRNA stop codons are UAA, UGA, UAG. 

Table 61.2 The genetic code. By convention, the genetic code is based on the sequence of bases, read from 5 to 3 in mRNA. For example, CUA specifies the amino acid leucine. Note that in DNA, the corresponding coding strand is CTA (in RNA U replaces T). The complementary template strand of DNA is GAT. This corresponds to the base pairing in the DNA heUx (Fig. 60.1), i.e. and A=T.

A protein is biosynthesized from its N-terminal end to its C-terminal end by a process that reads the bases along the mRNA strand in the 5 3 direction. The amino acid that is to be incorporated into a protein is specified by a three-base sequence called a codon. The bases are read consecutively and are never skipped. The three-base sequences and the amino acid that each sequence codes for are known as the genetic code (Table 26.2). A codon is written with the 5 -nucleotide on the left. For example, the codon UCA on mRNA codes for the amino acid serine, whereas CAG codes for glutamine. 

The information contained in the base sequence of the mRNA template is interpreted in sequences of three bases called codons each codon represents one amino acid. Therefore, the unit of information is the codon. Since there are four major bases in mRNA, 4 (i.e. 64) different codons are possible. The 64 triplets constitute the genetic code (Table 17.1). All codons have been assigned to amino acids or punctuation signals. Three triplets (UAA, UAG and UGA) are not complemented by anticodons on tRNAs and serve to signal that the polypeptide chain has been completed. Of the other 61 triplets which have complementary tRNAs, two (AUG and GUG) have additional roles in the initiation of protein synthesis. Since there are only 20 amino acids, most amino acids are specified by more than one codon, i.e. the code is degenerate. The genetic code applies to prokaryotes and eukaryotic nuclear and chloroplast mRNAs but not to... 

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