What does each codon code for

Following this discovery, Nirenberg, Philip Leder, and Har Gobind Khorana eventually identified the rest of the genetic code and fully described which codons corresponded to which amino acids. Reading the genetic code. Redundancy in the genetic code means that most amino acids are specified by more than one mRNA codon. Methionine is specified by the codon AUG, which is also known as the start codon.

Consequently, methionine is the first amino acid to dock in the ribosome during the synthesis of proteins. Tryptophan is unique because it is the only amino acid specified by a single codon. The remaining 19 amino acids are specified by between two and six codons each. Figure 2 shows the 64 codon combinations and the amino acids or stop signals they specify. Figure 2: The amino acids specified by each mRNA codon. Multiple codons can code for the same amino acid. Figure Detail.

What role do ribosomes play in translation? As previously mentioned, ribosomes are the specialized cellular structures in which translation takes place. This means that ribosomes are the sites at which the genetic code is actually read by a cell. Figure 3: A tRNA molecule combines an anticodon sequence with an amino acid. These nucleotides represent the anticodon sequence. The nucleotides are composed of a ribose sugar, which is represented by grey cylinders, attached to a nucleotide base, which is represented by a colored, vertical rectangle extending down from the ribose sugar.

The color of the rectangle represents the chemical identity of the base: here, the anticodon sequence is composed of a yellow, green, and orange nucleotide. At the top of the T-shaped molecule, an orange sphere, representing an amino acid, is attached to the amino acid attachment site at one end of the red tube. During translation, ribosomes move along an mRNA strand, and with the help of proteins called initiation factors, elongation factors, and release factors, they assemble the sequence of amino acids indicated by the mRNA, thereby forming a protein.

In order for this assembly to occur, however, the ribosomes must be surrounded by small but critical molecules called transfer RNA tRNA. Each tRNA molecule consists of two distinct ends, one of which binds to a specific amino acid, and the other which binds to a specific codon in the mRNA sequence because it carries a series of nucleotides called an anticodon Figure 3. In this way, tRNA functions as an adapter between the genetic message and the protein product.

The exact role of tRNA is explained in more depth in the following sections. What are the steps in translation? Like transcription, translation can also be broken into three distinct phases: initiation, elongation, and termination. All three phases of translation involve the ribosome, which directs the translation process. Multiple ribosomes can translate a single mRNA molecule at the same time, but all of these ribosomes must begin at the first codon and move along the mRNA strand one codon at a time until reaching the stop codon.

This group of ribosomes, also known as a polysome , allows for the simultaneous production of multiple strings of amino acids, called polypeptides , from one mRNA. When released, these polypeptides may be complete or, as is often the case, they may require further processing to become mature proteins. Figure 5: To complete the initiation phase, the tRNA molecule that carries methionine recognizes the start codon and binds to it.

The bases are represented by blue, orange, yellow, or green vertical rectangles that protrude from the backbone in an upward direction. Inside the large subunit, the three leftmost terminal nucleotides of the mRNA strand are bound to three anticodon nucleotides in a tRNA molecule. An orange sphere, representing an amino acid, is attached to one tRNA terminus at the top of the molecule.

The ribosome is depicted as a translucent complex bound to fifteen nucleotides at the leftmost terminus of the mRNA strand. The tRNA at left has two amino acids attached at its topmost terminus, or amino acid binding site. The adjacent tRNA at right has a single amino acid attached at its amino acid binding site. A third tRNA molecule is leaving the binding site after having connected its amino acid to the growing peptide chain. There are five additional tRNA molecules with anticodons and amino acids ready to bind to the mRNA sequence to continue to grow the peptide chain.

Figure 7: Each successive tRNA leaves behind an amino acid that links in sequence. The resulting chain of amino acids emerges from the top of the ribosome.

The ribosome is depicted as a translucent complex bound to eighteen nucleotides in the middle of the mRNA strand. The tRNA at left has five amino acids attached at its amino acid binding site, forming a chain.

Two additional tRNA molecules, each with a single amino acid attached to the amino acid binding site, are approaching the ribosome from the cytoplasm. Figure 8: The polypeptide elongates as the process of tRNA docking and amino acid attachment is repeated.

The ribosome is depicted as a translucent complex bound to many nucleotides at the rightmost terminus of the mRNA strand. A chain of 19 amino acids is attached to the amino acid binding site at the top of the tRNA molecule. The chain is long enough that it extends beyond the upper border of the ribosome and into the cytoplasm. Alternating copolymers: e. UC n programs the incorporation of Ser and Leu. But in combination with other data, e.

The genetic code. By compiling observations from experiments such as those outlined in the previous section, the coding capacity of each group of 3 nucleotides was determined. This is referred to as the genetic code. It is summarized in Table 3. This tells us how the cell translates from the "language" of nucleic acids polymers of nucleotides to that of proteins polymers of amino acids.

Knowledege of the genetic code allows one to predict the amino acid sequence of any sequenced gene. The complete genome sequences of several organisms have revealed genes coding for many previously unknown proteins. A major current task is trying to assign activities and functions to these newly discovered proteins. The Genetic Code. Position in Codon. Of the total of 64 codons, 61 encode amino acids and 3 specify termination of translation.

The degeneracy of the genetic code refers to the fact that most amino acids are specified by more than one codon. The degeneracy is found primarily the third position. Consequently, single nucleotide substitutions at the third position may not lead to a change in the amino acid encoded. These are called silent or synonymous nucleotide substitutions. They do not alter the encoded protein. This is discussed in more detail below.

The pattern of degeneracy allows one to organize the codons into " families " and " pairs ". In 9 groups of codons, the nucleotides at the first two positions are sufficient to specify a unique amino acid, and any nucleotide abbreviated N at the third position encodes that same amino acid. These comprise 9 codon "families". An example is ACN encoding threonine. There are 13 codon "pairs", in which the nucleotides at the first two positions are sufficient to specify two amino acids.

A purine R nucleotide at the third position specifies one amino acid, whereas a pyrimidine Y nucleotide at the third position specifies the other amino acid. The UAR codons specifying termination of translation were counted as a codon pair. The codons for leucine and arginine, with both a codon family and a codon pair, provide the few examples of degeneracy in the first position of the codon. Degeneracy at the second position of the codon is not observed for codons encoding amino acids.

Chemically similar amino acids often have similar codons. Hydrophobic amino acids are often encoded by codons with U in the 2nd position, and all codons with U at the 2nd position encode hydrophobic amino acids. The major codon specifying initiation of translation is AUG. Using data from the genes identified by the complete genome sequence of E.

AUG is used for genes. GUG is used for genes. UUG is used for genes. AUU is used for 1 gene. CUG may be used for 1 gene. Regardless of which codon is used for initiation, the first amino acid incorporated during translation is f-Met in bacteria.

Of these three codons, UAA is used most frequently in E. UAG is used much less frequently. UAA is used for genes. UGA is used for genes. UAG is used for genes. The genetic code is almost universal. In the rare exceptions to this rule, the differences from the genetic code are fairly small.

Differential codon usage. Various species have different patterns of codon usage. The pattern of codon usage may be a predictor of the level of expression of the gene. In general, more highly expressed genes tend to use codons that are frequently used in genes in the rest of the genome. This has been quantitated as a "codon adaptation index". Thus in analyzing complete genomes, a previously unknown gene whose codon usage profile matches the preferred codon usage for the organism would score high on the codon adaptation index, and one would propose that it is a highly expressed gene.

Likewise, one with a low score on the index may encode a low abundance protein. The observation of a gene with a pattern of codon usage that differs substantially from that of the rest of the genome indicates that this gene may have entered the genome by horizontal transfer from a different species.

The preferred codon usage is a useful consideration in "reverse genetics". If you know even a partial amino acid sequence for a protein and want to isolate the gene for it, the family of mRNA sequences that can encode this amino acid sequence can be determined easily.

Because of the degeneracy in the code, this family of sequences can be very large. Since one will likely use these sequences as hybridization probes or as PCR primers, the larger the family of possible sequences is, the more likely that one can get hybridization to a target sequence that differs from the desired one. Thus one wants to limit the number of possible sequences, and by referring to a table of codon preferences assuming they are known for the organism of interest , then one can use the preferred codons rather than all possible codons.

DNA and RNA molecules are written in a language of four nucleotides; meanwhile, the language of proteins includes 20 amino acids. Codons provide the key that allows these two languages to be translated into each other. Each codon corresponds to a single amino acid or stop signal , and the full set of codons is called the genetic code.

The genetic code includes 64 possible permutations, or combinations, of three-letter nucleotide sequences that can be made from the four nucleotides. Of the 64 codons, 61 represent amino acids, and three are stop signals.