Which of the following does NOT constitute a stop codon? The three stop codons are UAG, UAA, and UGA. These signal that the translation process should terminate at that point.
Other nucleic acids sequences can also function as stop signals. For example, the sequence of amino acids glutamine will signal a termination of translation if it is the last amino acid in a protein chain. This mechanism can prevent synthesis of potentially toxic proteins that might otherwise occur if translation continued until all of the code letters in the DNA sequence were used up.
Some genes contain signals for stopping translation prematurely. These signals are known as terminators. Terminators are nucleotide sequences located near the end of genes whose function is to allow RNA polymerase to exit the gene before the start of the next open reading frame. Thus, the ribosomes do not have time to bind up the gene after they have translated the last amino acid. Instead, the ribosomes drop off at the terminator, allowing more genes to be read from the DNA strand.
The genetic information contained in DNA consists of four different chemicals called bases. Each base can be either purine or pyrimidine.
The stop codons are UAA, UAG, and UGA, which mark the end of translation. The 64 codon combinations and the amino acids or stop signals they define are depicted in Figure 2. Figure 2: The amino acids that each mRNA codon specifies. The same amino acid can be coded for by many codons. For example, alanine can be encoded for by AGG, AGA, GGC, and GGA.
Alanine, arginine, asparagine, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine can all be encoded for by single codons. In fact, there are four different codes used for these amino acids: one code that uses three codons (arginine, glycine, and proline), one code that uses two codons (lysine and methionine), and one code that uses one codon (all other amino acids).
In addition to marking the end of an open reading frame, a stop codon can also function as a polyA signal. The presence of a stop codon just prior to the 3' end of an RNA molecule indicates that the RNA is mature and can be further processed or acted upon by cellular enzymes.
The genetic code has three STOP codons: UAG, UAA, and UGA. During translation, these codons signify the end of the polypeptide chain. Because they do not code for an amino acid, these codons are sometimes known as "nonsense codons" or "termination codons." Translation of RNA into protein is a critical step in gene expression. The accuracy of this process is important because any errors will be passed on to the protein product, which can have serious effects on cell function and development. Multiple mechanisms exist in cells to prevent erroneous translations. When these mechanisms fail, then nonsense mutations may occur. These are mutations that cause a codon sequence to read as a termination signal rather than as an amino acid residue.
Nonsense mutations can also arise during DNA replication if the DNA polymerase fails to read all the nucleotides in a template strand. If this happens often enough, you can end up with genes that are entirely made of nonsense mutations. Such genes are called null mutants because there is no functional protein produced from them. Nonsense mutations can also arise during RNA editing processes. Here, inappropriate nucleotides are inserted or deleted from the DNA of some organisms. When this occurs near a stop codon, it can change its meaning. For example, when a cytidine is inserted next to a UGA codon, it can transform this signal into an tryptophan instead of a stop.
Three of these 64 mRNA codons (UAA, UAG, and UGA) are stop codons in the normal genetic code. Crick termed it for the little "play" or wobble that occurs at the third codon position. This is different from a 4-base amino acid sequence, such as TAG, which cannot be encoded by a single triplet of DNA. Rather, each triplet of DNA codes for one amino acid; the location of the triplet within the gene determines how long it stays on the ribosome. Thus, there is no strict definition of a "codon."
All other codons are considered to have some degree of wobble. For example, GUG can form five different amino acids through RNA editing or chemical modification. The extent of wobble at each codon varies between genes and organisms. In general, the more highly expressed genes tend to use less stable wobble bases during translation.
Amino acids with non-wobbling codons include Phe, Ile, Leu, Lys, Met, Trp, and Tyr. These amino acids are called "fixated" because they do not change position within the protein chain. They are usually encoded by a single nucleotide within a gene. For example, the amino acid sequence of hemoglobin contains only Phe, Tyr, and Trp residues.
Codons that have three pauses (or stops) in them are known as triplet codes. There are four triplets that give rise to five different amino acids: AAA, AAG, UUU, UUC, and UCU. These are called stop codons because they signal the end of a protein-coding sequence.
The number of stop codons in the genetic code is significant because it determines how many proteins can be made from a gene. Genes contain information for making proteins, which can be interpreted by cells when DNA is copied during replication. This interpretation process occurs only if each nucleotide base in the gene is read as its corresponding amino acid; otherwise the base would not be translated into an amino acid and would be lost during replication. Therefore, the number of possible proteins encoded by a gene depends on the number of stop codons along with the number of coding bases (letters of the genetic alphabet).
All genes in all organisms contain the same number of letters, but some of these letters are used more often than others. For example, there are far more URs than As or Gs in any given gene.