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Mutations Guided Reading
1. What is a mutation?
2. What is a gene mutation?
3. What is a chromosomal mutation?
4. What are the two classes of gene mutations?
5. What are the three types of point mutations?
6. List the two types of frameshift mutations and
describe how they affect the DNA sequence.
Now and then cells make mistakes in copying their DNA,
putting an incorrect nucleotide (A instead of G) or skipping a
new base completely. These mistakes are called mutations.
Mutations are changes in the genetic material of an
organism. Like the mistakes people make in their normal
lives, mutations come in many shapes and sizes. Mutations
that produce changes in a single gene are called gene
mutations. Mutations that produce changes in whole
chromosomes are known as chromosomal mutations.
Gene mutations that involve changes in only one or a few
nucleotides are called point mutations, because they occur
at a single point in the DNA sequence. There are two classes
of gene mutations: point mutations and frameshift
mutations. Point mutations are single base changes that do
not affect the reading frame; that is, the mutation only
makes a single change in a single codon, and everything else
is undisturbed.
There are three types of point mutations:
1. Silent Mutations: There is a base change, but the new
codon codes for the same amino acid, so it has no effect .
2. Missense Mutations: The mutation alters the meaning of
the codon, so it codes for a different amino acid.
3. Nonsense Mutations: This mutation changes the codon
to a stop codon, which prematurely ends translation when
the mRNA transcript is being read by the ribosomes.
Frameshift mutations alter the reading frame of the DNA.
There are two types of frameshift mutations:
1.Insertions: This mutation adds or inserts a base pair (or
more) into the DNA, shifting everything to the right (or left,
depending on your point of view) by one base pair.
2. Deletions: This mutation deletes a base pair (or more),
shifting everything the opposite direction of the insertion.
The effects of insertions and deletions can be much more
dramatic than point mutations. Remember that the genetic
code is read 3 bases at a time. If a nucleotide is added or
deleted, the bases are still read in groups of three but the
groupings have shifted.
7. Frameshift mutations (insertions and deletions) shift
the “reading frame” of the genetic message. How
can frameshift mutations affect proteins?
8. What impact do most mutations have?
9. What does a harmful mutation produce?
10. Describe how can a mutation be beneficial.
Insertions and deletions are called frameshift mutations
because they shift the “reading frame” of the genetic
message. By shifting the reading frame, they can change
every amino acid that follows the insertion or deletion.
Frameshift mutations can change a protein so much that it
cannot perform its normal function.
Chromosomal mutations involve changes in the number or
structure of chromosomes. Such mutations may change the
locations of genes on chromosomes and may even change
the number of copies of some genes.
Most mutations are neutral meaning they have little or no
effect on the expression of genes or the function of the
proteins they code for. Mutations that cause dramatic
changes in protein structure or gene activity are often
harmful, producing proteins that do not function. Harmful
mutations are the cause of many genetic disorders. Harmful
mutations are also associated with cancers. In contrast,
beneficial mutations may produce proteins with new or
altered amino acid sequences that can be useful to
organisms in changing environments. The ability to process
lactose as an adult is an example of a recently discovered
beneficial mutation. Having the ability to process lactose as
an adult helps to prevent diseases such as osteoporosis,
encourages muscle and bone health, and can provide milk
drinkers with longer and healthier lives.