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Question Video: Calculating the Number of Dipeptides That Can Be Formed from 20 Amino Acids Biology

There are 20 standard amino acids that make up the proteins found in all living organisms. From these, how many different dipeptides can be made?

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Video Transcript

There are 20 standard amino acids that make up the proteins found in all living organisms. From these, how many different dipeptides can be made?

Proteins are biological macromolecules composed of amino acid monomers. They can be enzymes, structural molecules, signaling molecules, or antibodies, just to name a few of their many forms and functions.

An amino acid is built around a central carbon atom called the 𝛼-carbon. You may remember that carbon atoms are able to form four chemical bonds. The 𝛼-carbon in an amino acid is bonded with an amino group, a carboxyl group, an atom of hydrogen, and a molecular structure referred to as an R group, or side chain. The 𝛼-carbon, amino group, carboxyl group, and hydrogen are virtually the same for every amino acid. But the R group will vary, giving every amino acid its specific chemical properties. The R group can make an amino acid polar, nonpolar, aromatic, acidic, or basic. Amino acids form bonds called peptide bonds. A chain of multiple amino acids bonded together is called a polypeptide, which we can remember because the prefix poly- means many.

Now let’s take a look at how a peptide bond is formed. Amino acids form peptide bonds during a chemical reaction called a condensation reaction. During a condensation reaction, the carbon in the carboxyl group of one amino acid bonds with the nitrogen in the amino group of another amino acid. And a water molecule is released in this process.

The question is asking us about dipeptides. You likely already know that the prefix di- means two, so a dipeptide contains two amino acids, just like the diagram. We need to determine how many different dipeptides we can make with the 20 standard amino acids. A dipeptide can be made from any two amino acids, even two molecules of the same amino acid. Therefore, there are 20 possibilities for what the first amino acid could be. There are also 20 possibilities for what the second amino acid could be. So, if we multiply 20 by 20, we can calculate the number of possible combinations of dipeptides.

Therefore, the number of different dipeptides that can be made out of 20 amino acids is 400.

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