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11.2.1. Gene or point mutation

Gene or point mutation

The gene mutations are those that alter the sequence of nucleotides of a single gene, so also called pointThey can be produced by:

  • Base pair substitution: For example, instead of a thymine nucleotide there is a cytosine nucleotide.
  • Loss of nucleotides.
  • Insertion of new nucleotides.
  • Nucleotide inversion.
  • Translocation of complementary nucleotide pairs.

Distintos tipos de mutaciones génicas

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Substitution of bases

They are produced when one nitrogen base changes in one position for another (it is the nitrogenous bases that distinguish the nucleotides of a chain). They account for about 20% of gene mutations. They can be of two types:

  • Transitions When a puric base is substituted for another puric one (adenine for guanine and vice versa) or a pyrimidine for another pyrimidine (cytosine for thymine and vice versa).

By Sara (Own work) [CC BY-SA 3.0], via Wikimedia Commons

  • Transversions. When a puric base is substituted for another pyrimidine, or vice versa.

    These mutations only alter one triplet, and since the  genetic code is degenerate , it may be that the new triplet encodes the same amino acid and that this mutation is silent and does not affect the individual. If the mutation creates a stop triplet it will generate a shorter protein. If it affects the stop codon, the synthesized protein will be longer, until another stop codon appears. If the mutation creates an amino acid other than the active center of the enzyme, the function of the protein will also be altered. If that protein improved the original, the individual would have an advantage over the rest of the population and could transmit it to their offspring.


    Gene mutation affecting the pattern of lecture

    The gene mutations affecting the reading frame are produced by insertions or deletions of nucleotides, but never three or multiples of three. They originate amino acid sequences different from the original ones or a termination codon, which prevents the protein from fulfilling its function.

    When a nucleotide is added or lost, all the triplets change, which is why it is said that they cause a shift in the reading frame, creating a completely different protein.

    These gene mutations are transmitted to all descendant cells of the cell in which the mutation occurred. If the mutation affects an intronic or other nonessential region of DNA, it would be a silent mutation. But if it is produced in an exonic (coding) region, the sequence of the gene will be altered, which will translate a different sequence of amino acids of a protein.

    If the new amino acid is similar to the substituted one, it may be that the functionality of the protein is maintained, but if they are very different or affect the active center of the protein, the new protein will not have its biological function.

    The effects of these mutations can be different:

    • Harmful, yet compatible with life, such as:

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    • Beneficial. It is rare, but it may be that the new protein has better properties and can perform its function better, so that the individuals that have it will have adaptive advantages over the rest of the species, and natural selection will make this one prevail over the original, causing evolution.
    • Lethal. They cause the death of the organism that suffers it. For example, in mutations produced during embryonic development that cause many of the natural abortions.

    The accumulation of mutations that occur throughout the life of an organism is related to the aging of its cells, and the appearance of certain types of cancer.

    Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

    Aragon. July 2020, question 8.

    8. According to an article published in February 2020 in the journal Genome Biology and Evolution, a group of scientists has discovered in the remains of mammoths found on Wrangel Island, that these animals accumulated a high number of mutations, responsible for numerous diseases, some of them serious. Answer the following questions: (2 marks)

    a) Define mutation . What consequences do they usually have for the individual? (0.4 points)
    b) Briefly explain the types of point (or gene) mutations that you are aware of (0.8 points)
    c) Suppose a mutation occurs in the DNA of a cell, what impact will it have on the transcribed mRNA ? What about the protein encoded by the affected gene? Explain the answer (0.4 points)
    d) Is there a difference if the mutation occurs in a somatic cell or in a germ cell? Reason for the answer. (0.4 points)

    Aragon. June 2006, option A. Issue 1.

    Short development topic (3 points): Gene or point mutations.

    Cantabria, July 2019, option 1, question 4.

    Explain why a DNA mutation consisting of a base substitution may have no apparent phenotypic effects . (Substitution: change from one base to a different one)

    Fundamental ideas about gene or point mutations

    The gene or point mutations can be of two types:

    • Base substitution mutations. A nucleotide with a nitrogenous base is exchanged for another with a different nitrogenous base, which causes the alteration of a single triplet of the gene. As the genetic code is degenerate, there are different triplets that can encode the same amino acid, so it is possible that the substitution of bases does not imply a change in the encoded amino acid. If this is not the case, one amino acid will be substituted for another or protein synthesis will be interrupted, so the synthesized protein will be different.
    • Nucleotide insertion or deletion (loss) mutations. When a nucleotide is inserted or lost, there is a change in the reading pattern, so that all the triplets that follow the site of the mutation are altered.