Anticodons are sequences of nucleotides that are complementary to codons. They are found in tRNAs, and allow the tRNAs to bring the correct amino acid in line with an mRNA during protein production.
During protein production, amino acids are bound together into a string, much like beads on a necklace. It’s important that the correct amino acids be used in the correct places, because amino acids have different properties. Putting the wrong one in a spot can render a protein useless, or even dangerous to the cell.
This graphic shows a growing protein chain. Towards the bottom left, you can see tRNAs carrying amino acids entering the ribosome complex. If all goes well, only the tRNAs with the correct anticodons will bind successfully to the exposed mRNA, so only the correct amino acids will be added:
tRNAs are responsible for bringing the correct amino acids to be added to the protein, according to the mRNA’s instructions. Their anticodons, which pair-bond with codons on mRNA, allow them to perform this function.
Function of Anticodons
The function of anticodons is to bring together the correct amino acids to create a protein, based on the instructions carried in mRNA.
Each tRNA carries one amino acid, and has one anticodon. When the anticodon successfully pairs up with an mRNA codon, the cellular machinery knows that the correct amino acid is in place to be added to the growing protein.
Anticodons are necessary to complete the process of turning the information stored in DNA into functional proteins that a cell can use to carry out its life functions.
How Anticodons Work
When genetic information is to be turned into a protein, the sequence of events goes like this:
- Genetic information in the cell’s genome is transcribed into mobile pieces of RNA using base-pairing rules. Each nucleotide has only one other nucleotide which pairs up with it.
By pairing the correct RNA nucleotide with each DNA nucleotide, RNA polymerase creates a strand of RNA that contains all the correct information to make the protein.
This “messenger RNA,” or “mRNA,” then travels to a ribosome, the site of protein production.
- At the ribosome, the rules of base-pairing are again used to ensure a correct transfer of information. Each three-nucleotide “codon” in the mRNA is matched with an “anticodon” containing the complementary bases.
The “transfer RNAs” or “tRNAs” that string proteins together each have one anticodon that corresponds to one mRNA codon, and one amino acid attached.
When the correct tRNA finds the mRNA, its amino acid is added to the growing protein chain.
Enzymes catalyze the bonding of amino acids together as tRNA anticodons bind to the correct mRNA codon.
When the tRNA’s amino acid has been added to the protein chain, the tRNA leaves to pick up a new amino acid to bring to a new mRNA.
Interestingly, this means that the tRNA anticodon has the RNA version of the same nucleotide sequence of the original gene.
Remember – the gene was transcribed using complementary nucleotides to make RNA, which then had to bond with complementary tRNA codons.
RNA Base Pairing Rules
Each RNA nucleotide can only hydrogen bond to one other nucleotide. It is by bonding the correct nucleotides together that DNA and RNA successfully transfer and use information.
The four bases of RNA are Adenine, Cytosine, Guanine, and Uracil. These bases are often referred to by just their first letter, to make it easier to show sequences of many bases. Base pairing rules for RNA are:
A – U
C – G
G – C
U – A
Put more simply, in RNA, A nucleotides always bond with U nucleotides, and C nucleotides always bond with G nucleotides.
Differences Between RNA and DNA
Of note, in DNA, the “Uracil” base is a slightly different base called “Thymine.” In DNA, A and T pair. RNA Adenine will also pair with DNA’s Thymine, and DNA Adenine will pair with RNA’s Uracil.
The difference between Uracil and Thymine is that Thymine has an extra methyl group, which makes it more stable than Uracil.
It is thought that DNA uses Thymine instead of Uracil because, as the cell’s “master blueprints,” information stored in DNA must remain stable over a long period of time. RNAs are only copies of DNA made for specific purposes, and are used by the cell for only a short period of time before being discarded.
Examples of Anticodons
Let’s look at some examples of DNA base triplets, mRNA codons, and tRNA codons to see if you can fill in the missing information using base pairing rules.
You might find it useful to use a pencil and paper to allow you to transcribe each nucleotide’s complement instead of doing it in your head.
1. mRNA codon: GCU
What is the tRNA anticodon that will bind to this mRNA codon?
Answer to Question #1
CGA. The codon GCU codes for the amino acid alanine, so the tRNA with the corresponding anticodon will be carrying that amino acid.
2. mRNA codon: ACA
What is the corresponding tRNA anticodon?
Answer to Question #2
UCU. The codon CGA codes for the amino acid cysteine, so a tRNA with anticodon UCU will be carrying cysteine.
3. DNA base triplet: CTT
What is the mRNA codon that will be transcribed from this DNA triplet?
Answer to Question #3
GAA. This mRNA codon codes for the amino acid glutamate.
4. Based on the information in the answers to the question above, what is one anticodon for a tRNA that carries glutamate?
Answer to Question #4
CUU. This anticodon that is complementary to an mRNA codon for glutamate.
Related Biology Terms
- Amino Acid – The building blocks of protein. Different amino acids have different properties, which allow cells to build proteins to serve many different functions by stringing the right combinations of amino acids together
- Codon – A three-nucleotide sequence in an mRNA molecule that codes for a particular amino acid. Most amino acids have more than one codon that codes for them, although methionine only has one.
- DNA – The substance used to store the permanent operating instructions of a cell. Information stored in DNA is stable, and can be copied to make new blueprints for daughter cells using nucleotide base pairing rules.
1. Which of the following is NOT true of anticodons?
A. They are found on tRNAs.
B. They are complementary to codons.
C. They have the RNA equivalent of the same nucleotide sequence as the original DNA instructions for the amino acid.
D. They have the same nucleotide sequence as codons.
Answer to Question #1
D is correct. Anticodons are complementary to codons, NOT the same as them.
2. Which of the following sequences is complementary to: GCUCGU
Answer to Question #2
C is correct. It’s helpful to transcribe the sequence letter by letter before answering multiple choice questions like these.
3. Which of the following is something that would NOT be coded for by a codon?
D. Stop protein production
Answer to Question #3
B is correct. tRNAs do not carry sugars. Sugars may be added to proteins later to form important substances like glycoproteins, but that is done during a later step of protein processing.
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