Properties of Genetic Code Mutation

Properties of Genetic Code

We often call the genetic code a degenerate codon. Four choices of nucleotides (A, U, G, C). 3 combinations of nucleotides are used to form a codon (4C3 or 64 codons are possible). There are 3 Stop codons. In total, 61 codons code for 20 amino acids. Selenocysteine is not included as it is coded by a stop codon.

The following are the properties of the genetic code:

1. Degeneracy

More than one codon can code for a single amino acid, which is explained by the Wobble phenomenon. Two amino acids that don't follow degeneracy are Methionine and tryptophan.

To learn more about the Wobble Phenomenon and the Examples of degeneracy, read the notes and watch the videos by signing up on the prepladder app.

2. Unambiguous

One amino acid will code for only one codon.

3. Non-overlapping

Codons should not be overlapped with each other.

4. Not punctuated

There is no punctuation mark after Third nucleotide. Hypothetically, if there is a loss of one nucleotide, then the amino acid coded alone will be defective. Then the nucleotides will be combined with one another (the entire frame of reading will be shifted).

This might lead to frameshift mutations.

5. Universal

There is a Unique genetic code for every cell with few exceptions. At one point in time, genetics were very puzzling because they understood only G. The genetic composition of all the cells is identical, but the expression of genes is different in every cell. UGA is a stop codon, but it codes for tryptophan in mitochondrial DNA. Usually, AGG and AGA codes for arginine but in mitochondrial DNA, they act as a stop codon.

Types of Mutation

• Change in the sequence of amino acids.
• These are classified at three levels.
  • Effects on nucleotide sequence
  • Effect on Amino acid sequence
  • Effect of function of a protein

Effects on Nucleotide Sequence

Following are the mutations that happen by the effect of the nucleotide sequence

Point Mutation

• One nucleotide is substituted by other
• Depending on the nucleotide substitution, it is divided into.
  • Transition
  • Transversion

Frameshift Mutation

• When there is a loss or gain of nucleotides, the entire frame of reading will be shifted
• It has two types.
  • Deletion
  • Insertion

Transition

• Purine is replaced with a purine.
• Pyrimidine is replaced with pyrimidine.

Transversion:

• Purine is replaced with pyrimidine.
• Pyrimidine is replaced with purine.

Deletion

•  Loss of nucleotide

Insertion

• Gain of nucleotide

Effect on Amino Acid Sequence

Following are the mutations that happen by the effect of the amino acid sequence.

Effect of Function of a Protein

Following are the mutations that happen by the effect of the function of protein

Acceptable mutation

There is a change in the nucleotide sequence. That results in alternating the amino acid sequence. However, there is no change in the function of the proteins. All hemoglobin variants are accidentally found during electrophoresis or chromatographic separation. HbA1C is a glycated hemoglobin (to assess the long-term glycemic control of a person). When chromatography or electrophoresis is done, you will observe the difference in the movement of hemoglobin.

Examples: Hb Milwaukee, Bristol, and Sydney

• The 67 amino acid of the beta-globin chain is usually valine.
• If it is replaced with aspartic acid, it is called Bristol.
• Hb Milwaukee: Valine is replaced with glutamic acid. 
• Sydney: Valine is replaced with alanine.
• But the oxygen-carrying capacity is normal.

Partially acceptable mutation

There is a change in the nucleotide sequence. That results in alternating the amino acid sequence. Hence, the function of the proteins is altered. But it is not immediately life-threatening

Example: Sickle cell anemia-Hbs (Mutation it fits point mutation, transversion, missense mutation)

• Mutation of beta globin gene (6th codon is GAG that codes for polar glutamic acid)
• It is replaced with GTG (non-polar amino acid valine)
• Glutamic acid is replaced with valine.
• The 6th codon present in the inner region of hemoglobin, is released after dehydration and deoxygenation.
• If it is polar, it doesn't cause any damage.
• If it is nonpolar, it exhibits a sticky patch with the membrane (due to hydrophobic interaction).
• It results, in the accumulation of globin chains, it loses its deformability.
• Finally, it causes a sickling crisis as the membrane is pinched off.

Unacceptable mutation

It is immediately life threatening 

Example: Congenital methemoglobinemia- HbM

• Fe2+ is replaced with Fe3+ (present in the center of the tetrapyrrole ring)
• Fe2+ is covalently attached to one amino acid (histidine F8) of the globin chain.
• In this condition, histidine F8 is replaced with a tyrosine has a phenol (hydrophilic) group. 
• It accepts the electrons from Fe2+
• As a result, it becomes Fe3+ (cannot carry oxygen)

Also Read: Hartnup Disease and Glycine Metabolic Effects

Frequently Asked Questions

Question: No of possible codons 

1. 64 
2.  61 
3.  20 
4. 31

Answer: 64

Question: No.of codons which code for an amino acid 

1. 64 
2. 61 
3. 20 
4. 31

Answer: 61

Question: Codon consists of: 

1. 3 base pairs. 
2. 2 base pairs 
3. 5 base pairs 
4. 3 nucleotides

Answer: 3 nucleotides

Question: Stop codon: 

1. UAG 
2. UCA 
3. UAC 
4. AUG

Answer: UAG

Question: The amino acid which does not follow degeneracy of codon is, 

1. Glycine 
2. Glutamine
3.  Tryptophan 
4. Tyrosine

Answers: Tryptophan

Questions: The wobble phenomenon explains which of the following, 

1.  Degeneracy 
2. Unambiguity 
3. Ambiguity 
4. Punctuation

Answer: Degenerac

Questions: Transition mutation of GATCCT is

1. GGTCCT 
2. GTTCCT 
3. GAACCT 
4. GATACT

Answer: GGTCCT

Question: A 45-year-old male is detected to have fasting and postprandial hyperglycemia in the diagnostic range of diabetes. To understand his long term glycemic control, he is asked to estimate his HbA1C Chromatogram detects an abnormal Hemoglobin peak, corresponding to Hb Bristol. His oxygen-carrying capacity is normal. Hb Bristol is an example of 

1. Silent Mutation 
2. Acceptable mutation 
3. Partially acceptable mutation 
4. Nonsense mutation

Answer: Acceptable mutation

Question: How many choices of nucleotides do we have to make a codon?

Answer: 4 choices of nucleotides (A, U, G, C)

Question: Which property of the genetic code explains the Wobble phenomenon?

Answer: Degeneracy

Question: What are the amino acids that do not follow degeneracy?

Answer: Methionine and Tryptophan

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