The human DNA is a complex stretch of nucleotides that is compactly packaged into a very small space. During cell division, the DNA in the nucleus is replicated and some minor errors in the sequence occur by chance despite having an error-proof checking mechanism in place. DNA is also prone to damage by external sources like radiation – both UV rays and X-rays, carcinogenic substances like tobacco and other chemicals. Damage can also occur through sources within the cell such as reactive oxygen species and buildup of other metabolic by-products.
Mutations can be classified into two types- somatic and germline. Somatic mutations occur in somatic cells (any cell that is not an egg or sperm cell). These mutations cannot be passed on from parent to offspring. On the other hand, germline mutations are those that occur in germ cells (egg or sperm cells) that are inherited by the offspring even before birth. Interestingly, germline mutations are the reason why children look similar but not identical to their parents!
When you hear phrases like ‘cancer runs in the family’, its source can be linked to germline mutations. For instance, a mutation in the BRCA1/2 gene increases risk of developing breast cancer. However, not everyone who has such a mutation will develop cancer! This is because humans are diploid (have two copies of every gene) and even if one of them has a mutation, the other will function properly. Cancer can be induced only if both copies of the same gene are mutated. In fact, most cancers require mutations in multiple different genes for disease induction and propagation. Ultimately, the biggest risk factor for developing cancer is aging! As we age, we accumulate various mutations (somatic and/or germline) from the sources discussed above. However, the probability of developing cancer despite having cancerous mutations in one copy of the gene is low and individuals with germline mutations tend to develop cancer early on when compared to individuals who exclusively have somatic mutations.
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