DNA Point Mutations and Their Consequences in Coding Regions
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DNA Point Mutations and Their Consequences in Coding Regions

Readers should be able to learn about the different DNA point mutations that occur in eukaryotes and the types of consequences they have on coding regions. Readers should be able to define the different DNA point mutations and the consequences. Readers will also be able to understand how mutations lead to genetic variation in organisms.

The processes of mutation and recombination allow for genetic variation in organisms.  Mutation is considered one of the main reasons for evolutionary change and allows for new alleles to arise.  Recombination then groups these alleles into different combinations which then results in genetic variation.  Point mutations and their consequences will be discussed.

DNA point mutations usually refer to the small alteration (change) of one or more base pairs.  There are two main types of point mutations in DNA: 1) base substitutions and 2) base insertions or deletions.

There are two main base substitutions: 1) transition and 2) transversion.  In a transition mutation, the base is replaced by a similar base of the same chemical category.  Therefore, an example would be a purine (A or G) being replaced by another purine or a pyrimidine (T or C) being replaced by another pyrimidine. A transversion mutation is the opposite of a transition.  A base is replaced by a base with the opposite chemical category.  An example of this would be a purine (A or G) being replaced by a pyrimidine (T or C).   During base pair insertions or deletions (Indel mutations), one or more base pairs are inserted or deleted.

Consequences in coding regions

During single base substitutions all of the outcomes and consequences are due to the degeneracy of the genetic code and the existence of termination codons during translation.  Synonymous or silent mutations are a consequence of point mutation when the mutation in the codon still encodes for the same protein.  This is consequence due to the degeneracy of the genetic code. A nonsynonymous or missense mutation is when the codon for one amino acid is mutated into a codon that encodes for a different amino acid.  A nonsense mutation is when the codon for one amino acid is mutated into a stop or termination codon for translation.  While synonomous mutations do not alter the amino acid sequence of the polypeptide chain, missense and nonsense mutations may either cause conservative or nonconservative substitutions.  A missense mutation may replace an amino acid with a very similar amino acid chemically and structurally, this has less affect on the overall protein structure and function.  For nonconservative substitutions, one amino acid is replaced by a chemically and physically different amino acid. This substitution usually produces a major change in the protein structure and function such as nonsense mutations which completely terminate translation.  Indel mutations also cause a major change in protein structure and protein causing frame shift mutations.  Because mRNA is read three bases (one codon) at a time, an insertion or a deletion of one or more bases causes a complete change in the amino acids being produced. 

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