突然変異

Point mutations include silent, missense, nonsense, and frameshift mutations. Silent mutations don't alter amino acid sequences, so proteins function normally. Missense mutations substitute one amino acid for another, potentially causing malfunction. Nonsense mutations change amino acid codons to stop codons, truncating proteins prematurely. Frameshift mutations insert or delete nucleotides, shifting the reading frame and producing abnormal proteins.

Chromosomal alterations are large-scale mutations affecting entire chromosome regions, including deletions, duplications, inversions, or integrations of DNA segments. Unlike point mutations that change single nucleotides, chromosomal alterations encompass many genes and regulatory elements, making them typically far more serious. These mutations can be detected through karyotyping.

Only germline mutations present in egg and sperm cells transmit to offspring because somatic mutations occur in non-reproductive cells and aren't passed down. Germline mutations cause hereditary diseases, which can be autosomal or sex-linked. Examples include cystic fibrosis, an autosomal recessive disease, and Huntington's disease, an autosomal dominant disorder.

Ultraviolet radiation carries more energy than visible light and damages DNA by breaking bonds between base pairs. UV radiation causes thymine bases on the same DNA strand to pair with one another, forming characteristic thymine dimers. Natural sources include the sun, while artificial sources include tanning beds transmitting primarily UV-A rays with smaller amounts of UV-B.

Mutagens are external factors that damage DNA and cause mutations, including radiation and chemicals. Both somatic and germline mutations can arise spontaneously during DNA replication or be induced by mutagen exposure. If DNA damage occurs in regions regulating cell growth and division and isn't repaired before cell division, it can lead to cancer, especially in rapidly-dividing cells like skin cells.

Cells possess mechanisms to repair damaged DNA caused by environmental mutagens. However, repair doesn't always occur before the next cell division, particularly in rapidly-dividing cells such as skin cells. When DNA damage in growth-regulating regions escapes repair, mutations can accumulate and potentially lead to cancer development.

Mutations result from copying errors during DNA replication or physical and chemical damage to DNA. They can occur spontaneously or be induced by environmental exposure to radiation, chemicals, and other mutagens. Understanding mutation causes helps explain how genetic variation arises and contributes to eukaryotic evolution.