The Genetic Code – MCAT Biology | MedSchoolCoach

The Genetic Code

MCAT Biology - Chapter 2 - Section 1.4 - Gene Expression - The Central Dogma
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Sample MCAT Question - Genetic Code

Which of the following is a stop codon for amino acid translation?

a) UAA

b) UUG

c) AUG

d) UGU

A is correct. UAA. Ribosomes translate mRNA three nucleotides at a time. These nucleotide triplets are called codons, and depending on the particular codon, different amino acids will be added, or translation will stop. There are, in fact, three codons that will not code for specific amino acids and will effectively stop translation. These codons are known as stop codons. They are UAA, UGA, and UAG. A handy mnemonic to remember these three stop codons is that they stand for U Are Annoying, U Go Away, and U Are Gone.

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The Genetic Code

Both DNA and RNA are made from four different nucleotides. DNA is made from nucleotides containing either cytosine (G), guanine (G), adenine (A), or thymine (T), while RNA is made from nucleotides containing either cytosine (C), guanine (G), adenine (A), or uracil (U). (When DNA is transcribed to RNA, U is substituted for T.) 

 

These C’s, G’s, A’s, T’s, and U’s are the information-bearing portion of DNA and RNA, and their sequence determines the sequence of amino acids in the proteins synthesized during translation. The specific way in which sequences of nucleotides in DNA and RNA dictate the sequence of amino acids in proteins is known as the genetic code. It turns out that individual nucleotide triplets, known as codons, each code one of the 20 amino acids. These codons are “read” by the ribosome (the organelle responsible for translation) and are matched to the correct amino acid, which is then added to the growing protein chain. This process of matching certain codons with certain amino acids is the subject of this MCAT content post. 

Codons Pair with Anticodons, Which Specify Amino Acids

In eukaryotes, translation occurs in the cytosol or cell membrane of the endoplasmic reticulum. The ribosome is the central organelle responsible for translating mRNA into proteins, and it works by translating mRNA three nucleotides at a time. These nucleotide triplets, or codons, will code for particular amino acids and will be complementary to what is called the anticodon (Figure 1). 

 

The anticodon is located on a transfer-RNA (tRNA) molecule. The tRNA molecule, with its anticodon, will base pair with a complementary codon located on the mRNA molecule. In other words, depending on what the codon on the mRNA molecule is, the appropriate tRNA and anticodon will be recruited. Each tRNA molecule also carries an amino acid, so depending on the mRNA codon, a particular amino acid will be added to the protein. An mRNA codon can code for a stop signal, as well, in which case translation will be terminated.

Codons Pair with Anticodons, Which Specify Amino Acids - MCAT Biology
Figure 1. Codons Pair with Anticodons

Redundancy in the Genetic Code

We’ve said that each codon contains three nucleotides. Since there are four nucleotides total, this means that there are 4 x 4 x 4 or 64 possible codons. However, there are only 20 possible amino acids. This means that there must be some redundancy in the genetic code, meaning that multiple codons code for the same amino acid. Figure 2 shows the 64 possible codon combinations, and the different amino acids that the combinations code for. Apart from tryptophan (Trp) and methionine (Met), every amino acid has multiple codons that code for it. 

The Genetic Code: Codons Specify Amino Acids - MCAT Biology
Figure 2. The Genetic Code

Start & Stop Codons

Two codons are important to know for the MCAT exam, the start codon, and the stop codon. The start codons begin translation, and the stop codons terminate it. There is only one start codon, AUG, which codes for methionine. In terms of stop codons, there are three: UAA, UGA, UAG. A helpful mnemonic for memorizing the stop codons is “U Are Annoying, U Go Away, and U Are Gone.” Unlike the start codons, the stop codons do not code an amino acid. Of the 64 codons, three are stop codons, meaning only 61 codons code for an amino acid.

Wobble Pairing Explains Genetic Code Redundancy

The basis for the redundancy of the genetic code is a concept known as wobble pairing. The wobble hypothesis was first posited by Francis Crick, who, along with James Watson, discovered the DNA double-helix structure. What wobble pairing refers to is the flexibility of the third base pair in each codon. Recall that when a codon binds to an anticodon, both are complementary to each other. The third base pair in a codon, however, can bind to more than one base pair in an anticodon. Essentially, wobble pairing is pairing between nucleotides that do not follow the standard Watson-Crick base pairing of C to G and A to U.

 

There are four different types of wobble pairings: guanine-uracil (G-U), hypoxanthine-uracil (I-U), hypoxanthine-adenine (I-A), and hypoxanthine-cytosine (I-C). Figure 3 shows two different codons that both code for the amino acid leucine. The same tRNA is used both times. However, the one on the left is binding using traditional Watson-Crick base pairing, and the one on the right is binding using wobble pairing.

Wobble Pairing Explains Genetic Code Redundancy - MCAT Biology
Figure 3. Wobble Pairing Explains Genetic Code Redundancy

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