CSIR NET JRF Amino Acid notes: Answer


1. How many amino acids are involved in making proteins in humans?

The answer is 20. Although there are many more variations found in nature, only 20 are found within the human genome (list of all the bases found in DNA). These are called standard proteinogenic amino acids.

One option was 64, which is the theoretical number of amino acids that the 3 bases per amino acid codes for: There are 4 different bases in DNA, A, T, G and C, and they code for amino acids in triplets. 4 to the power of 3 is 64, but the code is redundant in many places, i.e. most amino acids are coded for by two or more codes. For example: the amino acid lysine is coded for by the triplets AAA and AAG.

2. The code Asp denotes which of these amino acids?

The correct answer was Aspartic acid

Aspartic acid is one of two acidic amino acids along with glutamic acid. The carboxylate anions (-COO[neg] instead of -COOH on the carboxylate group, where [neg] is a negative charge) of these amino acids are also active as neurotransmitters.

3. Amino acids join up into a large chain (polymer) to create what biological molecule(s)?

The answer is proteins. Proteins are long linear chains of amino acids which are joined together by peptide bonds. They are synthesized in the ribosome, which can be attached to the smooth endoplasmic reticulum (smooth ER) or floating free in the cytoplasm.

There are an estimated 100 billion different proteins; the highest number of different proteins within any one organism is an estimate of 60000!

4. Okay, a question about the nature of sidechains. There are four main groups of amino acids: acidic, basic, polar and...?

The correct answer was non polar

Other accepted answers: non-polar nonpolar

There are 9 non-polar amino acids in total: (in alphabetical order) alanine, glycine, isoleucine, leucine, methionine, phenylalanine, proline, tryptophan and valine. You didn't expect me to give you the codes for them now, did you?

5. Which of these is the one-letter code for glutamine?

The correct answer was Q

Oddly enough, Q is the correct answer.

Incorrect answers: G codes for glycine, S codes for serine, and O codes for hydroxyproline, which is not one of the 20 standard proteinogenic amino acids; it is very uncommon, and only differs from proline by an -OH group (a hydroxyl group) as evident by its name.

6. F codes for which amino acid?

The answer is phenylalanine. The clue here is the F sound at the beginning of the word.

Phenylalanine is an essential amino acid (i.e. must be contained in the diet), and is a white powdery solid found naturally in mammalian breast milk. People with the genetic disorder phenylketonuria (PKU) are unable to metabolize phenylalanine and so must be careful about the amounts they ingest in the diet.

7. What is the three-letter code for the amino acid alanine?

The correct answer was Ala

The correct answer is Ala. This is one of the more easy ones to remember; it is simply the first three letters of the full name!

8. Which of these three-letter codes is correct regarding the amino acid isoleucine?

Your Answer: Ile

Ile is the only real code. The other three I just made up.

9. Which amino acid is coded for by the letter H?

The correct answer was Histidine

Histidine is considered an essential amino acid in children, therefore they must obtain it from their diet. However, humans start to produce histidine when they are a few years old and no longer need to acquire it in the diet as adults, therefore it is classed as a non-essential amino acid.

10. The amino acid lysine has the one-letter code L.

It seems strange, but lysine actually has a code letter of K. L codes for the amino acid leucine.

Lysine is an essential amino acid, found in foods such as meat, eggs, pulses, and also some types of fish including cod.

11. The amino acid coded for by the letter E is basic.

The correct answer was f

E actually stands for glutamic acid, which, as the name suggests, is acidic. Mentioned earlier, the anion of glutamic acid, glutamate, is a neurotransmitter, and is actually the most abundant excitatory neurotransmitter in the whole mammalian nervous system!

12. Which amino acid is coded for by the letter W?

The correct answer was Tryptophan

The answer is tryptophan.

Tryptophan is another essential amino acid, which is found in foods such as chocolate, oats, bananas, red meat, and poultry. It contains a functional group called an indole, which is naturally occurent in human feces and has a strong fecal smell. Strangely enough, at low concentrations, indole appararently has a flowery odor!

13. Which amino acid is coded for by the letter Y?

The correct answer was Tyrosine

This one was a bit harder. Some of the one letter codes appear to have nothing in common with the full names. The only clue here is the Y in tYrosine, although other amino acids also have a Y in their names.

Tyrosine is a non-essential amino acid (it can be synthesized in the body) and is found in milk and cheese and other high protein foods. It can be synthesized from phenylalanine, another amino acid.

14. Which amino acid is coded for by the three-letter code Met?

The correct answer was Methionine

Methionine is the only proteinogenic amino acid to contain sulfur, except for cysteine. It is coded by a single codon (base triplet) of DNA, which is AUG, which is also a start codon for protein synthesis, therefore methionine is always the first amino acid in a synthesized protein strand, although it is commonly removed afterwards in a method called post-translational modification.

15. Last question; a bit of a bonus. An unspecified or unknown amino acid is given the one-letter code X, but what is the three-letter code?

The correct answer was Xaa

The answer is Xaa. As far as I know, there's no reason for this code, but there may well be. Unk is also sometimes used, but more rarely.