In the living cell, DNA undergoes frequent chemical change, especially when it is being
Most of these changes are quickly repaired. Those that are not result in a mutation.
Thus mutation is a failure of DNA repair.
A mutation is any change in an organism’s DNA sequence. Proteins, encoded by the
genotype, produce the phenotype
Hence, DNA mutations affect phenotype only when the mutation is expressed (DNA
RNA protein) and the resulting protein functions abnormally.
Not all mutations affect the protein’s ability to function and thus do not generate a
One of the most common types of mutation is the point mutation, a change in a single
Point mutations can result from errors in DNA replication or from exposure to mutagenic
Silent Mutations - mutations that do not change the amino acid sequence of the protein
Missense Mutation - A point mutation that causes a change in the amino acid sequence
of the protein
Nonsense Mutation - A point mutation that creates a new stop codon
Frameshift mutation - is a result of addition or deletion of one or two nucleotides (or
multiple of them).
- Likely to create a new triplets of nucleotides
-New (useless) sequence of protein from the point of the mutation
Mutations can result from the incorporation of incorrect bases during DNA replication.
Most such spontaneous changes in DNA are temporary because they are immediately
corrected by processes collectively called DNA repair. Most damage to DNA is repaired by removal of the damaged bases followed by
resynthesis of the excised region.
-Mitochondria -The Mitochondria have many similarities to bacterial cell:
double membrane like some bacteria circular DNA
genes without introns (to be discussed) small ribosomes
similar size to prokaryotic cells
This has led many biologists to theorize that mitochondria are the descendants of
some bacteria which was endocytosed by a larger cell billions of years a