突变

Point mutations include silent, missense, nonsense, and frameshift mutations. Silent mutations don't change the amino acid sequence, so the protein functions normally. Missense mutations substitute one amino acid for another, potentially causing malfunction. Nonsense mutations create a stop codon, truncating the protein prematurely. Frameshift mutations shift the reading frame through nucleotide insertion or deletion, producing entirely different codons downstream.

Silent mutations change a codon without altering the amino acid it encodes. For example, changing CCA to CCG still codes for proline due to the redundancy of the genetic code. Since the amino acid sequence remains unchanged, the protein structure and function are unaffected, making these mutations phenotypically invisible to the organism.

Frameshift mutations occur when nucleotides are inserted or deleted, shifting how codons are read in groups of three. Unlike missense mutations that change single amino acids, frameshifts alter all downstream codons, creating a cascade of incorrect amino acids. This typically produces a completely abnormal, nonfunctional protein, making frameshift mutations particularly severe.

Somatic mutations occur in body cells and affect only that individual, while germline mutations occur in egg and sperm cells and can be inherited by offspring. Only germline mutations cause hereditary diseases like cystic fibrosis or Huntington's disease. Somatic mutations may cause cancer if they damage genes regulating cell growth, but they cannot be passed to future generations.

Ultraviolet radiation carries more energy than visible light and breaks bonds between base pairs in DNA. This damage creates thymine dimers, where thymine bases on the same strand pair abnormally. If cells divide before repairing this damage, mutations result. UV-A and UV-B rays from the sun and tanning beds reach Earth's surface and can damage skin cell DNA, potentially leading to cancer.

Chromosomal alterations involve large-scale changes like deletion, duplication, or inversion of DNA stretches, affecting many genes and regulatory elements simultaneously. Point mutations change only a single nucleotide, impacting one codon. Because chromosomal alterations encompass multiple genes and their regulatory regions, they typically cause far more severe consequences than individual point mutations.

Autosomal dominant diseases like Huntington's require only one mutated gene copy to develop, so inheriting the mutation from either parent causes disease. Autosomal recessive diseases like cystic fibrosis require two mutated copies; individuals with one mutated and one normal copy are carriers but unaffected. The inheritance pattern depends on whether the mutated protein is functional enough when only one copy is present.