MCAT Biochemistry
In the world of genetics, mutations are alterations in DNA that may lead to diverse consequences. They can result from different sources, such as exogenous sources (e.g. UV light, ionizing radiation, cigarette smoke, asbestos, and intercalating agents) and endogenous sources (e.g. reactive oxygen species, alkylating agents, DNA polymerase, and proofreading errors). Types of DNA mutations include single nucleotide or point mutations, which can be categorized as transitions (replacement of a pyrimidine with another pyrimidine, or a purine with another purine) and transversions (replacement of a purine with a pyrimidine or vice versa). Point mutations can also be classified as silent mutations (producing the same amino acid), missense mutations (producing a different amino acid), and nonsense mutations (producing a premature stop codon).
Other mutations include frameshift mutations (resulting from adding or deleting nucleotides not divisible by 3, usually causing altered and non-functional proteins, such as in Tay-Sachs disease and Duchenne muscular dystrophy) and splice site mutations (occurring in donor or acceptor splice sites, leading to the incorporation of introns or exclusion of exons in mature mRNA, often producing non-functional proteins). Unchecked mutations, especially affecting oncogenes and tumor suppressor genes, can eventually lead to the development of cancer due to the overexpression of oncogenes and underexpression of tumor suppressor genes.
Lesson Outline
<ul> <li>Causes of DNA mutations <ul> <li>Exogenous sources: UV light, ionizing radiation, cigarette smoke, asbestos, intercalating agents</li> <li>Endogenous sources: reactive oxygen species, alkylating agents, DNA polymerase and proofreading errors</li> </ul> </li> <li>Types of DNA mutations <ul> <li>Point mutations <ul> <li>Transitions and Transversions</li> <li>Silent mutations</li> <li>Missense mutations</li> <li>Nonsense mutations</li> </ul> </li> <li>Frameshift mutations <ul> <li>Causes and consequences</li> <li>Examples: Duchenne muscular dystrophy and Tay-Sachs disease</li> </ul> </li> <li>Splice site mutations <ul> <li>Occur in donor or acceptor splice sites of introns or adjacent non-coding regions to exons</li> <li>Result in altered mRNA and often non-functional protein</li> </ul> </li> </ul> </li> <li>Impact of DNA mutations <ul> <li>Oncogenes and Tumor Suppressor Genes <ul> <li>Role of proto-oncogenes and oncogenes in cancer development</li> <li>Role of tumor suppressor genes like p53 in preventing cancer development</li> </ul> </li> </ul> </li> </ul>
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FAQs
DNA mutations are alterations in the DNA sequence that can be caused by various factors. There are several types of DNA mutations, including point mutations (transitions and transversions), silent mutations, missense mutations, nonsense mutations, frameshift mutations, and splice site mutations. These mutations can occur due to errors during DNA replication by DNA polymerase or as a result of damage caused by endogenous and exogenous sources such as reactive oxygen species, chemicals, and radiation.
Transitions and transversions are both types of point mutations. Transitions involve the substitution of a purine base for another purine (A↔G), or a pyrimidine base for another pyrimidine (C↔T). On the other hand, transversions occur when a purine base is replaced by a pyrimidine or vice versa (A↔C, A↔T, G↔C, or G↔T). Transitions are more common than transversions, as they typically cause less DNA distortion and have fewer detrimental effects.
Endogenous sources of DNA mutations are intrinsic factors within the cell, such as errors during DNA replication, spontaneous deamination, and oxidative damage by reactive oxygen species produced as a byproduct of cellular metabolism. Exogenous sources of DNA mutations are external factors that can cause DNA damage, such as ultraviolet radiation, ionizing radiation, and exposure to mutagenic chemicals. Both endogenous and exogenous sources can lead to various types of DNA mutations, which may have different consequences on the function of genes and proteins.
A missense mutation occurs when a single nucleotide change results in a different amino acid being incorporated into the protein. This can lead to altered protein function or stability. A nonsense mutation is when a nucleotide change creates a premature stop codon, leading to a truncated protein. This usually results in a non-functional protein and can have severe consequences for the cell. Silent mutations are changes in DNA that do not alter the amino acid sequence of the protein, often occurring in the third (wobble) position of a codon. These mutations typically have little to no effect on protein function because they do not change the amino acid sequence.
Frameshift mutations occur when an insertion or deletion of nucleotides results in a shift in the reading frame of the genetic code, causing all the amino acids downstream of the mutation to be altered. This typically leads to the production of a non-functional protein. Splice site mutations affect the boundaries where the splicing machinery cuts and rejoins the RNA, potentially causing exon skipping, intron inclusion, or creating alternative splice isoforms. These mutations can also lead to the production of non-functional or compromised proteins, depending on the specific gene regions affected.
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