There are post-translational modifications that can occur. The signal sequences are removed and the protein is properly folded. There are other chemical modifications to the amino acids that can occur, such as phosphorylation, glycosylation, and methylation.
MUTATIONS At any point in time, a mutation can occur, which will change the protein made or will change the phenotype of the organism. Most mutations happen at the level of transcription, which may or may not affect the protein end product. There are several types of mutations. A point mutation affects just one base pair. An insertion involves the addition of one or more base pairs and a deletion involves the removal of a base pair. These will have differing effects on the protein. Some point mutations won’t affect the amino acid at all, leading to what’s called a silent mutation. A missense mutation happens when a different amino acid gets into the protein. Some are more severe than others, depending on the amino acid’s qualities. Conditional mutations are missense mutations that only sometimes affect the protein. A nonsense mutation turns an aminoacid codon into a stop codon. Frameshift mutations can be very severe because they cause each of the subsequent amino acids to be wrong. If the mutation affects a deletion that isn’t a multiple of three bases in a row, this leads to a frameshift mutation. Mutations can be spontaneous or can be caused by certain mutagens. Mutagens can be chemical or secondary to radiation. Most mutations are also considered carcinogens because they can cause cancer. Chemical mutagens include things called nucleoside analogs that are so similar to nucleosides that they get into the DNA molecule but don’t pair with other bases in the proper way. Intercalating agents get between base pairs, throwing off the sequence, leading to deletions or insertions. Radiation can be ionizing radiation or nonionizing radiation. Ionizing radiation includes gamma rays and x-rays. These can break bonds and can modify bases in the DNA molecule. Nonionizing radiation like ultraviolet light can cause dimer formation between two pyrimidine molecules. This will affect both transcription and replication. In most cases, DNA can repair itself. Most mistakes are fixed through a proofreading function. This happens because of DNA polymerase, which reads the base pair and 148
