The relationship between an mRNA codon and its corresponding amino acid is called the genetic code. The three-nucleotide code means that there is a total of 64 possible combinations 4 3 , with four different nucleotides possible at each of the three different positions within the codon.
This number is greater than the number of amino acids and a given amino acid is encoded by more than one codon Figure 1. This redundancy in the genetic code is called degeneracy. Typically, whereas the first two positions in a codon are important for determining which amino acid will be incorporated into a growing polypeptide, the third position, called the wobble position , is less critical.
In some cases, if the nucleotide in the third position is changed, the same amino acid is still incorporated. Whereas 61 of the 64 possible triplets code for amino acids, three of the 64 codons do not code for an amino acid; they terminate protein synthesis, releasing the polypeptide from the translation machinery.
These are called stop codons or nonsense codons. Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also typically serves as the start codon to initiate translation.
Each set of three nucleotides following this start codon is a codon in the mRNA message. The genetic code is nearly universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all extant life on earth shares a common origin. However, unusual amino acids such as selenocysteine and pyrrolysine have been observed in archaea and bacteria.
In the case of selenocysteine, the codon used is UGA normally a stop codon. Pyrrolysine uses a different stop codon, UAG. Figure 1. This figure shows the genetic code for translating each nucleotide triplet in mRNA into an amino acid or a termination signal in a nascent protein.
The first letter of a codon is shown vertically on the left, the second letter of a codon is shown horizontally across the top, and the third letter of a codon is shown vertically on the right. In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation. The composition of each component varies across taxa; for instance, ribosomes may consist of different numbers of ribosomal RNAs rRNAs and polypeptides depending on the organism.
However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. A ribosome is a complex macromolecule composed of catalytic rRNAs called ribozymes and structural rRNAs , as well as many distinct polypeptides.
Prokaryotes have 70S ribosomes, whereas eukaryotes have 80S ribosomes in the cytoplasm and rough endoplasmic reticulum, and 70S ribosomes in mitochondria and chloroplasts. Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassociate during the initiation of translation.
The small subunit is responsible for binding the mRNA template, whereas the large subunit binds tRNAs discussed in the next subsection. The complete structure containing an mRNA with multiple associated ribosomes is called a polyribosome or polysome. In both bacteria and archaea , before transcriptional termination occurs, each protein-encoding transcript is already being used to begin synthesis of numerous copies of the encoded polypeptide s because the processes of transcription and translation can occur concurrently, forming polyribosomes Figure 2.
This allows a prokaryotic cell to respond to an environmental signal requiring new proteins very quickly. In contrast, in eukaryotic cells, simultaneous transcription and translation is not possible. Although polyribosomes also form in eukaryotes, they cannot do so until RNA synthesis is complete and the RNA molecule has been modified and transported out of the nucleus.
Figure 2. In prokaryotes, multiple RNA polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently translate the mRNA transcripts into polypeptides. In this way, a specific protein can rapidly reach a high concentration in the bacterial cell. Bacterial species typically have between 60 and 90 types. Serving as adaptors, each tRNA type binds to a specific codon on the mRNA template and adds the corresponding amino acid to the polypeptide chain.
As the adaptor molecules of translation, it is surprising that tRNAs can fit so much specificity into such a small package. Mature tRNAs take on a three-dimensional structure when complementary bases exposed in the single-stranded RNA molecule hydrogen bond with each other Figure 3.
The anticodon is a three-nucleotide sequence that bonds with an mRNA codon through complementary base pairing. At least one type of aminoacyl tRNA synthetase exists for each of the 20 amino acids. Figure 3. Email address: Your name:. Example Question 1 : Translation. Which of the following is a true statement regarding translation in eukaryotes? Possible Answers: All translation begins in the cytoplasm on free ribosomes.
All translation begins on bound ribosomes attaches to the rough endoplasmic reticulum. Correct answer: All translation begins in the cytoplasm on free ribosomes. Explanation : Translation is a process by which polypeptides are synthesized from a mRNA transcript, which was previously synthesized from the process of transcription. Report an Error.
Example Question 2 : Translation. It was translated on a cytoplasmic ribosome II. Possible Answers: III only. Correct answer: II only. Explanation : Proteins undergo translation with the help of ribosomes, which can be found in either cytoplasm or on the rough endoplasmic reticulum rough ER.
Example Question 3 : Translation. Possible Answers: different. Correct answer: the same. Explanation : Translation begins when a start codon is recognized in the mRNA molecule.
Which of the following are the same regarding prokaryotic and eukaryotic translation? Location of translation of prokaryotic proteins and eukaryotic membrane proteins II. The start codon III. Possible Answers: I and II. Explanation : Like transcription, there are slight differences between prokaryotic and eukaryotic translation. Example Question 5 : Translation. Possible Answers: Phenylalanine. Correct answer: Methionine.
Explanation : Every protein begins with methionine, therefore, this will be found in all proteins upon completion of translation. Example Question 6 : Translation. During translation, AUG corresponds to which amino acid? Possible Answers: Serine. Explanation : 5' AUG 3' is the start codon for polypeptide synthesis and corresponds to the amino acid methionine.
Example Question 7 : Translation. Which of the following is false about the genetic code? Possible Answers: Almost all organisms except mitochondria use basically the same genetic code. Codons are traditionally written with the 5' terminal on the left. Some nucleotide triplets are never used in translation. Each codon signifies either an amino acid, or a translation stop. Correct answer: Some nucleotide triplets are never used in translation. Explanation : All nucleotide triplets can theoretically occur in translation.
Example Question 8 : Translation. Which of the following is false about ribosomal binding sites? The mRNA is shifted three nucleotides' length through the ribosome for each amino acid added. The set of three binding sites is labelled A, P, and E. Example Question 9 : Translation. Which of the following amino acids has only one possible codon that codes for it? Possible Answers: Methionine. Explanation : Among the amino acids, there are two which only have one codon that code for them: tryptophan UGG , and methionine.
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. In the cytoplasm, the peptide chain has folded in on itself several times to form three compact rows of amino acids.
Eventually, after elongation has proceeded for some time, the ribosome comes to a stop codon, which signals the end of the genetic message.
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