Answer & Explanation:
Two major types of mutations are deletions and substitutions
Deletion occurs when the DNA replication machinery misses or removes some nucleotides, meaning some of the sequence is missing from the new DNA. This can be just one nucleotide, a few, or lots of nucleotides.
Substitution occurs when the DNA replication machinery makes the wrong sequence, substituting one or several bases for others.
Both of these mutations can be deleterious, and occur at the level of the individual nucleotide. The effects of each of them greatly depend on the location and size of the mutation.
If a small deletion occurs at a relatively unimportant region, for example a non-coding region, or a part of the coding region that is not essential to the structure or function of a protein, then the deletion might not be too problematic to the cell. On the other hand, if even a small deletion were to affect the active site of an enzyme, a binding site, or destroy a key structural residue, then the protein could not perform its function properly. Depending on the role of the protein, this could be disastrous for the cell. The same principles apply for the substitution. If the substitution replaces important nucleotides for ones that change the amino acid sequence of important sites, then the protein could not perform its function. Alternatively, some regions might be more flexible, and the substitution could be inconsequential. Another way that a substitution might not be too problematic, is if the final amino acid that resulted from the substitution was either the same, or had similar properties, to the initial amino acid that was coded for. For example, if the initial and replacement amino acids were both uncharged, as opposed to if a uncharged residue was replaced with a negatively charged amino acid. Another difference in these mutations is that a deletion can cause a frameshift mutation. Frameshift mutations occur when the "triplet" nature of the genetic code is disrupted, such that the entire sequence is affected. I.e. if you remove one nuclelotide, then everything after that is read differently, as the machinery reads it in threesThis means that the entire protein after the deletion site is incorrect.
An example of a deletion mutation that causes a disease is in cystic fibrosis, where there is a deletion in the gene CFTR. The most common mutation, is a deletion of three nucleotides that causes a the loss of the amino acid phenylalanine at residue 508.
An example of a substitution mutation causing a disease is sickle cell anemia, which changes a glutamic acid to a valine in the haemoglobin protein.
