Protein Synthesis HOME
1) DNA & Protein
2) Overview of Protein Synthesis & RNA
Success Criteria & Vocabulary
Click this drop-down menu to see the Success Criteria.
I can explain the process of translation and the role of enzymes during this process.
I can use a codon table to match codons to amino acids.
I can compare and contrast transcription and translation.
I can discuss why translation is important for normal cell function.
Click this drop-down menu to see the list of Vocabulary.
translation: Process that uses mRNA as a template to make a specific polypeptide chain. Second stage of protein synthesis.
mRNA: The type of RNA that carries instructions from DNA in the nucleus to the ribosome in the cytoplasm during protein synthesis.
tRNA: The type of RNA that carries amino acids to the ribosome during protein synthesis.
ribosome: Organelle located in the cytoplasm, responsible for using mRNA as the template to make a polypeptide chain.
uracil: Nitrogen base that pairs with adenine in RNA.
codon: Three consecutive bases on an mRNA strand.
anticodon: Three consecutive bases on a tRNA strand.
start codon: Codon that codes for the amino acids methionine (Met), and initiates translation.
stop codon: Codon that does not code for any amino acid, but stops translation and releases the polypeptide chain from the ribosome.
codon table: Graphically organises all 64 codons according to their corresponding amino acid.
Methionine (Met): Amino acid that is always at the beginning of every polypeptide chain.
Self-Directed Learning Tasks
Protein Folding Simulation
Task 3: Complete Education Perfect:
Task called 'Translation'.
Translation comes after Transcription
After transcription, there is an mRNA copy of the DNA. The mRNA leaves the nucleus and goes out to the cytoplasm, where it joins up with a ribosome, and begins the process of translation.
The process of translation uses the code sequence on mRNA to make a polypeptide (string of amino acids). It's called translation because it basically translated the information in nucleotides into a string of amino acids.
The ribosome (below) is an organelle made of two subunits—one large and one small. (The ribosome is made up of proteins and a type of RNA called ribosomal RNA (rRNA).) mRNA binds to the ribosome to form a complex.
The ribosome reads mRNA bases (from 5’ to 3’) in a code of 3 bases at a time called codons. Ribosomes move along the mRNA from the start codon until the stop codons is reached. The tRNAs bind to the complementary nucleotides on the mRNA. (Remember that the mRNA is a copy of the information from the DNA.)
The three unpaired bases on the tRNA are known as an anticodon, and have a corresponding amino acid that attaches to it. They are complementary to a codon on the mRNA. Each codon on the mRNA is read by the ribosome and matched to the complementary anticodon (unpaired three base sequence on the tRNA). tRNA carries the amino acid to the ribosome and drops it off.
There is a different kind of tRNA for each amino acid. The specific amino acid attached to the tRNA is then added (peptide bond forms) to the polypeptide chain being made. Once the polypeptide chain is released from the ribosome, it folds into a 3D structure, becoming a functional protein. This protein can be used for cellular functions (e.g. an enzyme).
How do the amino acids know how to line up in the correct order?
Answer: transfer RNA (tRNA)!
Translation is the last major step required to create a functional protein. The process of translation itself can be broken down into several steps.
Immediately Before Translation.
First, the mRNA leaves the nucleus through the nuclear pore and travels to a ribosome in the cytoplasm. The ribosome moves along the strand of mRNA, reading the sequence of bases 1 codon at a time. The process of translation begins at the start codon, AUG.
It is very important that the ribosome begins translation at the start codon. If it starts translation in the wrong place, then a non-functional protein may be produced!
Step 1: Initiation at the START Codon (AUG)
The start codon AUG codes for an amino acid called methionine. The tRNA with the complementary sequence of bases on its anticodon, UAC, will bind with methionine and bring this amino acid to the ribosome. The process of translation will then begin.
From the start codon, the ribosome continues to move along the strand of mRNA, ‘reading’ each of the codons on the mRNA strand one at a time. The tRNA molecules bring amino acids from the cytoplasm that match the codons on the mRNA to the ribosome. The anticodon on the tRNA molecule binds to the complementary codon on the mRNA strand.
The codon on the mRNA strand and the anticodon on the tRNA will match, and methionine will detach from the tRNA to begin the formation of a polypeptide chain. Methionine will ALWAYS be the first amino acid in the polypeptide chain.
Step 2: Elongation of the Polypeptide Chain
Once the codon and anticodon bind, the amino acid detaches from the adjacent tRNA molecule in the sequence, and forms a bond with the new amino acid that has been brought to the ribosome. You can think of the polypeptide chain being formed as “jumping” from one tRNA molecule to the next, as they enter the ribosome.
Once the amino acid detaches from the tRNA molecule, the ribosome shifts along the mRNA strand to “read” the next codon in the sequence. The now-empty tRNA molecule leaves the ribosome as another tRNA molecule enters. This means that every time a ribosome moves along a codon, one amino acid is added to the polypeptide chain. Some polypeptide chains can be made up of thousands of amino acids!
Step 3: Termination at the STOP Codon (UAA, UAG, UGA)
Translation continues until a stop codon is reached. There are three different stop codons, each with a different sequence of nitrogen bases. These are UAA, UAG, and UGA.
Once one of these stop codons is reached, translation stops and the polypeptide chain is released into the cytoplasm, where it can twist and fold to form a functional protein. Translation is complete! This protein is then ready to carry out important cellular functions!
There are no amino acids or tRNA molecules associated with a stop codon. In contrast with a start codon, this means no additional amino acids are added to the polypeptide chain when a stop codon is reached.
After translation, you have both an mRNA and a protein. The mRNA can be used over and over to make more copies of protein.
But what happens next to the protein after translation?
The protein or polypeptide chain goes through protein folding to become a 3-dimensional structure with a specific function!
Proteins can't do their jobs in the cell until they fold into the correct shapes. Different proteins have different shapes, depending on their function in the cell.
But how do proteins "know how to" fold into the correct shape? Remember those 20 different amino acids that make up your proteins? Some of them are hydrophilic (interact with water), and some are hydrophobic (don't interact with water). These properties are important in determining how a protein folds.
How to use a Codon Table
There are more possible Codons than possible Amino Acids
As bases are always read in groups of three, there are 64 possible combinations of the four bases (4 x 4 x 4 =4³ = 64), and therefore 64 different codons. However, there are only 20 amino acids. This means there are more codons for one amino acid.
AUG is a start codon that signals the start site of translation.
UAA, UAG, and UGA are 3 stop codons that signal the end point of translation, and have no amino acids associated with them.
The codons and their corresponding amino acids are shown in the codon table below. Codon tables can look complicated, but we can use them to figure out which amino acid a particular codon corresponds to.
Using a Codon Table to determine the Amino Acid.
How do we know which amino acid a codon is coding for? We can use an mRNA codon table like the one shown above.
Example #1: Let’s begin with the codon UGG as an example.
We start at the left hand side of the table “First letter”. Here there are four rows labelled U, C, A, and G. Since U is the first letter of our codon, the corresponding amino acid must be in the first row (U).
Next we look at the four columns at the top of the table “Second letter”. The second letter of our codon is G, which is the fourth column across. This means the amino acid could be CYS or TRP.
Finally, we look at the right hand side of the table “Third letter”. The last letter of our codon is G, which means the corresponding amino acid must be in the fourth line of the first row.
The amino acid coded for UGG is TRP (tryptophan - you don’t need to know the proper name of the amino acid).
Example #2: Let’s use the codon UUC as an example.
We start at the left hand side of the table “First letter”. Since U is the first letter of our codon, the corresponding amino acid must be in the first row (U).
Next, we look at the four columns at the top of the table “Second letter”. The second letter of our codon is U, which is the first column across. This means the amino acid could be PHE or LEU.
Finally we look at the right hand side of the table “Third letter”. The last letter of our codon is C, which means the corresponding amino acid must be in the second line of the first row.
The amino acid coded for UUC is PHE (phenylalanine)!
Comparing Transcription and Translation
There are many similarities between the processes of transcription and translation. Both transcription and translation:
Have sequences of bases that control the start points and end points
Both processes are controlled by enzymes.
These two stages of protein synthesis occur in different locations. Transcription occurs in the nucleus, while translation occurs in the cytoplasm with the help of ribosomes. This also means that transcription and translation involve different molecules. Transcription involves the use of DNA and nucleotides in the nucleus to produce mRNA; while translation uses ribosomes, tRNA and amino acids in the cytoplasm to “read” the mRNA and produce proteins.
These two processes both use a template to create their required product, however the templates are different. In transcription, the template strand of DNA is used to create mRNA, while in translation the mRNA is used as a template to create a protein.