Gene Mutation: Definition, Causes, Types, Examples

Gene mutations can't really turn baby turtles into cartoon super heroes like the Teenage Mutant Ninja Turtles.

Genetic mutations are slight alterations of DNA or RNA nucleotides, genes or chromosomes that may occur during replication or cell division. Random, uncorrected errors may be beneficial or harmful in relationship to evolution.

Some effects of gene mutation go unnoticed.

The two types of mutations in biology mainly occur in germ (egg and sperm) cells and in somatic (body) cells.

Germline mutations that give rise to genetic disorders can be inherited due to alterations in DNA sequences. Somatic mutations like lung cancer associated with heavy smoking can't be passed down to subsequent generations.

Different mutations can prove deadly to an organism if gene regulation is severely disrupted. On the other hand, random mutations may give organisms with that mutated trait a competitive advantage.

For example, Charles Darwin found a correlation between the beak shape of finches and their prevalence in divergent habitats on the Galapagos Islands. Darwin's work led to the _theory of natural selection_.

Mutations frequently occur just before the process of mitosis when DNA is being replicated in the cell nucleus. During mitosis or meiosis, mishaps can occur when chromosomes are not lined up correctly or fail to separate properly. Chromosomal mutations in the germ cells can be inherited and passed along to the next generation.

Some gene mutations can interfere with the rate of normal cell growth and increase cancer risk. Mutations in non-reproductive cells can trigger benign growths or cancerous tumors such as melanoma in skin cells. Defective genes on chromosomes are passed on, as well as too many or too few chromosomes per cell when these mutations happen in germline cells.

Gene mutation examples include severe genetic disorders, cell overgrowth, tumor formation and heightened risk of breast cancer. Cells have a finely tuned mechanism for correcting mutations at checkpoints during cell division, which detects most mutations. Once DNA proofreading is completed, the cell proceeds to the next stage of the cell cycle.

Mutations can occur because of external factors, also known as induced mutations. Mutagens are external factors that can cause alterations to DNA. Examples of potentially harmful environmental factors include toxic chemicals, X-rays and pollution. Carcinogens are mutagens that cause cancer such as UV radiation.

Various types of spontaneous mutations happen due to mistakes in cell division or reproduction, as well as during DNA replication or transcription. During DNA replication, nucleotide bases can be added or deleted, or a segment of DNA may be translocated to the wrong place on a chromosome.

When the cell is dividing, mistakes can occur during chromosomal separation, resulting in abnormal numbers and types of chromosomes with different genes. Such mutations can also be passed on from parent to child.

The genetic code determines the order of codons that will create building blocks of amino acids and proteins. Mutations frequently occur, which isn't surprising given the billions of cells in the body that are perpetually dividing to replace old, worn out cells.

Most of the time, errors in DNA replication or segregation are quickly repaired by enzymes or the cell is destroyed before they can cause lasting damage. When DNA repair attempts fail, spontaneous mutations stay within the DNA. Benign spontaneous mutations increase the genetic variance and biodiversity of a population.

The following are some of the types of gene mutations that can occur:

• Tautomerism: This occurs during
replication of DNA in the cell nucleus. Tautomers are mismatched pairs of
nucleotide bases.

• Depurination: This is a chemical reaction that
happens when the bonds break between the deoxyribose sugar in DNA and the
purine base of guanine or adenine. Losing a purine base is a common spontaneous
mutation.

• Deamination: This occurs if enzymes remove a nitrogen group from an amino acid. For instance, the (temperature-dependent) hydrolic deamination of cytosine to uracil is a leading cause of single-site, spontaneous mutations, as reported in the
Proceedings of the National Academy of Sciences. 

• Transition and transversion: These mutations
are two types of DNA substitution errors that involve switching of nucleotide base
pairs.

• Transition: This occurs due to a genetic
shuffling of similarly shaped nucleotide bases. A transition mutation occurs when
a wild-type (normally occurring) base pair like adenine and thymine are
replaced by guanine and cytosine base pairs.

• Transversion: This refers to the interchange
of differently shaped purine and pyrimidine bases. For example, the mutated
segment of DNA may have adenine replacing thymine.

Changes in the number or type of nucleotides are called point mutations. The effects of point mutation can range from harmless to life threatening. Mispairing or reordering of nucleotide bases are considered silent mutations when the change doesn't affect cell functioning. The new amino acid may even perform the same functions as the one it replaced.

The following are types of point mutations that can occur:

• Missense mutation:
This happens when one nucleotide is replaced with another. Substitutions of bases can
interfere with normal protein syntheses and functioning. For instance, a single
point mutation on the hemoglobin beta (HBB) gene causes sickle cell anemia blood disorders.

• Nonsense mutations:
These occur when atypical base pairings produce a stop codon that may cause improper
functioning or impede functioning altogether.

• Frameshift mutations:
These are point mutations that result when a nucleotide pair is added or omitted in
a gene sequence that shifts how codons are read. Such mutations often result in
different amino acids being added to the protein being synthesized. An example is beta
thalassemia, a blood disorder caused by mutations to the HBB gene.
Diseases like cystic fibrosis involve gene mutation deletion – when
nucleotides, amino acids, base pairs or whole genes are removed.

Gene amplification is involved in the production of extra copies of genes with heightened expression. Duplication or amplification is seen in some breast cancers and other types of malignancies, for instance. Overproduction of repeated codons in a gene alters gene functioning.

For instance, Fragile X syndrome is an intellectual disability caused by a high number of trinucleotide repeats that impair DNA stability.

Significant mutations can result when the structure or number of chromosomes change. Chromosomal aberrations may occur during mitosis or meiosis.

Mutations can also involve sex chromosomes X and Y and can affect gender expression.

Errors in meiosis can result in deletion of chromosomal segments. For instance, cri du chat syndrome results from a missing piece of genetic material on the arm of chromosome 5. When a part of a chromosome breaks off, it may attach to another chromosome.

The following are a few examples:

• Duplications or amplifications:
These happen when a chromosome is added to a homologous chromosome that already
contains that sequence, as seen in some cancers.

• Inversions: These occur when part of
chromosome breaks off and then reattaches backwards. For instance,
Optiz-Kaveggia syndrome is linked to this type of mutation.

• Translocation: This is when a component of
a chromosome attaches to a non-homologous chromosome. A form of leukemia is
associated with translocation mutation.

• Nondisjunction: This is the failure of
chromosomal separation, causing reproductive cells to have too many or two few
chromosomes. Possible consequences can include miscarriage, Down syndrome and
Turner syndrome.

Prenatal diagnosis for high-risk populations and other types of genetic counseling, including DNA genetic testing kits, can provide helpful medical information for family planning. Screening and early detection leads to better treatment outcomes. There are also genetic benefits for certain disease carriers that are good to know.

Carriers of the sickle cell gene have a protective factor against malaria, which is particularly advantageous in tropical regions. Research suggests a similar evolutionary advantage to being a carrier of cystic fibrosis and cholera resistance. Carriers of the cystic fibrosis gene may be better able to retain fluid and recover if exposed to cholera, according to preliminary studies.