The genetic code defines each amino acid in a protein, or polypeptide, in terms of a specific sequence of three nucleotide called codons, in the DNA. The genetic code is therefore the key to converting the information contained in genes int proteins.
The genetic code defines each amino acid in a protein, or polypeptide, in terms of a specific sequence of three nucleotides, called codons, in the deoxyribonucleic acid (DNA). Therefore, the genetic code is called a triplet code.
The four different nucleotides in DNA can form sixty-four different triplet codons. Because there are only twenty amino acids found in proteins, some amino acids are encoded by more than one codon. Therefore, the genetic code is said to be redundant, or degenerate.
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Three of the triplet codons do not encode any amino acids. These are stop codons, which identify the end of the message (similar to the period at the end of a sentence) encoded in genes. The genetic code is nearly universal; that is, specific codons code for the same amino acids in nearly all organisms.
However, a few exceptions have been found, primarily in mitochondrial DNA (mtDNA), but also in a few protozoa and a single-celled algae, such as Acetabularia. (DNA is found in three places in eukaryotic cells: the nucleus, plastids, and mitochondria).
Role of RNA
There are two distinct steps in the conversion of DNA sequences into protein sequences: transcription and translation. Transcription is the process by which the nucleotide sequence of a gene (in the DNA of a chromosome) is used to make a complementary copy of RNA (ribonucleic acid). DNA is a double-stranded molecule, and genes are arranged along DNA on each strand.
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| transcription and translation DNA |
Wherever there is a gene on one strand, called the coding strand, the other strand opposite the gene contains a nucleotide sequence that is complementary. The opposite strand is called the non-coding strand. When transcription of a gene occurs, it is the coding strand that is transcribed into a complementary stand of RNA.
The transcribed RNA is complementary in the sense that for each adenine (A) in the DNA, a uracil (U) is incorporated into the RNA. Likewise, for each uracil, guanine (G), and cytosine (C) in the DNA, an adenine, cytosine, and guanine are incorporated into the RNA, respectively. Thus, for the codon AGT in the DNA, it becomes UCA in the RNA.
In prokaryotes (bacteria), the RNA resulting from transcription is called messenger RNA(mRNA), and it is immediately ready for the next step, translation. In the subsequent translation process, the mRNA is translated into a protein sequence by a ribosome.
Ribosomes are macromolecular assemblies composed of various proteins and a second type of RNA, ribosomal RNA (rRNA). Ribosomes bind to mRNA molecules near the 5′ end and scan along the nucleotides until they reach an initiation or start codon, which indicates where translation should begin and establishes the reading frame.
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The start codon is always AUG and is the first translated codon in all mRNAs. Translation ends when a stop codon is reached. Most organisms have three stop codons (UAA, UGA, and UAG), and each gene ends with one of these.
As top codon does not code for an amino acid but simply identifies the end of the gene. Because nucleotide bases are read three at a time along a continuous chain of nucleotides, shifting the reading frame by inserting or deleting a single nucleotide within a gene can dramatically alter the amino acid sequence of the protein it can produce.
Although ribosomes are essential for translating mRNAs, they are not directly responsible for interpreting the codons. Transfer RNA(tRNA)molecules are single-stranded RNA molecules that exhibit extensive intramolecular base-pairing such that the tRNA has a two-dimensional structure with a stem and three loops resembling a three-leaf clover.
The middle loop contains a region, composed of three nucleotides, called the anticodon, which is complementary to a specific codon. The tRNA molecules directly decode the mRNA sequence or translate it into a correct amino acid sequence by their ability to bind to the right codon.
Each tRNA carries a specific amino acid to the ribosome where protein synthesis occurs. The binding of amino acids to tRNAs occurs at a place on the tRNA called the amino acid attachment site.Amino acids are added to tRNAs by special enzymes called aminoacyl-tRNA synthetases.
Each tRNA has a special region on the stem below the amino acid attachment site whose nucleotide sequence determines which amino acid needs to be attached to the tRNA. This code is often called the second genetic code, and geneticists have discovered that if the nucleotide region is changed in this region, the wrong amino acid gets attached.
There are at least one type of tRNA for each of the twenty amino acids. It is the pairing of the codons of the mRNA molecules and anticodons of tRNA molecules that determines the order of the amino acid sequence in the polypeptide chain.
The third base of the anticodon does not always properly recognize the third base of the mRNA codon. The third base is called the wobble position because nonstandard base pairing can occur there.
This phenomenon, together with the degeneracy of the genetic code means that cells do not have to have sixty-one different tRNA types (one for each codon that specifies an amino acid). For example, only two tRNAs are needed to recognize four different codons for the amino acid glycine. Consequently, most organisms have about forty-five different tRNA types.
Polypeptide Sorting
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| Polypeptide Sorting |
Protein sorting relies upon the presence of special signal sequences at one end of the protein molecule. These signal sequences direct proteins to various sites. For example, some proteins synthesized on ribosomes in the cytoplasm are targeted to organelles, such as the mitochondria or chloroplasts.
Other proteins, such as those found in the plant cell wall, are targeted to the cytoplasmic membrane for transport out of the cell to the cell wall. The signal sequences are frequently removed once the protein has reached its intended destination.
