ChemTalk

Mutations

Core Concepts

In this article, you will be able to understand what mutations are and how they occur. After reading this article you’ll know their patterns and types.

Topics Covered in Other Articles

> Central Biology Dogma

> DNA replication

> Genetic Code

> Gene Expression

What is a Mutation?

Mutations are often associated with diseases and disorders, but it’s not completely true. Mutations are changes in the DNA sequence. A single change in your DNA could cause a rare genetic disorder, but it could also generate a beneficial adaptation, it could even have no effect whatsoever. Errors in DNA replication during cell division, exposure to mutagenic agents, or viral infection cause these modifications in the DNA.

There can be different types of mutations, depending particularly on the cells these changes occur. A change in the genes of the reproductive cell causes germ-line mutations. Children may inherit this kind of mutation. Somatic mutations, on the other hand, affect every other bodily cell and are not passed from parent to child, they are only passed to daughter cells through mitosis.

somatic vs germline mutations

The majority of mutations may therefore look harmful in the short term, yet they are essential to our long-term survival. Without mutation, there would be no change, and without change, especially, life could not evolve.

How Mutations Occur

Many factors can lead to DNA alterations. The most important factors are environmental, chemicals, errors during replication, or could even spontaneous.

Environmental factors

Sometimes interactions between DNA and the environment can harm genetic information. In reality, every time you go outside, you endanger the integrity of your DNA since the Sun’s ultraviolet (UV) rays can trigger changes to the DNA of the cells in your skin. A cytosine base that has been hydrolyzed to its hydrate form during the second round of replication mispairs with adenine before being eventually replaced by thymine. This is the genetic mutation that takes place. In fact, researchers have shown that genes linked to basal cell carcinoma, a kind of skin cancer, frequently have this UV-induced C-to-T mutation.

Chemicals

These types of DNA changes are mainly because of the presence of free radicals. These molecules have the ability to chemically alter nucleotides in a way that changes their capacity to link with bases.

Dioxin (Persistent Organic Pollutant) intercalates between base pairs, compromising the integrity of the DNA helix and increasing the susceptibility of that area to insertions or deletions. Similar to this, it has been shown that the cancer-causing chemical benzo[a]pyrene, which is found in cigarette smoke, causes lesions at the guanine bases of the tumor suppressor gene P53 at codons 157, 248, and 273.

Signature mutations are those mutations that are comparatively specific to a certain mutagen.

Spontaneous Mutations

Depurination is the process of hydrolyzing a nucleotide to remove a purine base while leaving the sugar-phosphate backbone intact. If DNA repair enzymes are unable to correct the mistake, depurination may result in the inclusion of an incorrect nucleotide during the succeeding round of replication.

Another possibility is deamination, which is the elimination of an amine group from a base. Cytosine is converted to uracil by deamination and this pairing with adenine rather than guanine during the following replication results in a base substitution. Repair enzymes may identify uracil as not being a component of DNA, and they will normally correct such a deficiency. If the cytosine residue in question is methylated, deamination will instead result in conversion to thymine.

Repair enzymes won’t notice this modification since thymine is a regular DNA component

Errors during Replication

DNA replication mistakes have a significant impact on some mutations, notably trinucleotide repeat (TNR) expansions. It is suggested that DNA polymerase slippage, which results in this enzyme sliding back and repeating replication of the previous segment, may be influenced by repeat sequences’ ability to produce secondary structures, such as intrastrand hairpins, during replication.

genetic code

Types of Mutations

  1. Point mutation
    • Substitution
    • Insertion
    • Deletion
  2. Chromosomal mutation
    • Inversion: Consists of the flipping and following reinsertion of a chromosomal segment. Causes Opitz-Kaveggia syndrome.
    •  Deletion: There are no genes present in a particular chromosomal region because of the loss of that portion of the chromosome. Causes cri-du-chat syndrome.
    •  Duplication: A repetitive chromosomal region increases the gene dosage and contributes to the development of certain cancers.
    •  Translocation: Consists of the attachment of a chromosomal region to another chromosome. Can cause types of leukemia.
  3. Copy number variation
    • Gene amplification: There are more tandem copies of a locus than typical. Causes types of breast cancer.
    •  Expanding trinucleotide repeat: Trinucleotide sequences repeat more frequently than they normally do. Causes Huntington’s disease and fragile X syndrome.

How can you inherit Mutations?

  • Autosomal Dominant: Just one parent has to pass the mutation to their children for them to inherit it. For example, Marfan Syndrome.
  •  Autosomal Recessive: Both parents have to pass the mutation to their child for them to inherit it. For example, Sickle Cell Disease.
  •  X-linked Dominant: Only one mutation on the X chromosome is needed to pass it from parent to child. For example, Fragile X Syndrome.
  •  X-linked Recessive: Male kids won’t be harmed if just the dad carries the gene, but all female offspring will be carriers. There is a 50% likelihood that female kids will be carriers and a 50% chance that male offspring will have the disorder if only the mom has the mutation. 50% of male kids will have the syndrome if both parents carry the mutation, whereas 100% of female offspring will be affected. For example, Color Blindness.
  •  X-linked: On the X chromosome, they can be passed on in a dominant or recessive manner. Example, Thrombocytopenia.
  •  Y-linked: Only males can inherit these mutations. For example, Webbed Toes.
  •  Co-dominant: Two parts make up each gene (one from the egg and one from the sperm). They frequently combine to produce a single characteristic. Nevertheless, occasionally they each function independently to create distinct forms of the feature. Example, Alpha-1 Antitrypsin Deficiency
  •  Mitochondrial: Only the egg’s mitochondria stay after fertilization when the two cells combine. As a result, the egg is the sole source of the maternal DNA in the embryo. Because of this, maternal inheritance is another name for mitochondrial inheritance. For example, Leber Hereditary Optic Neuropathy.

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