Variation • Variation – is a change or difference in something. • Variations can happen to organisms through: • Mutations • Reproduction (Mitosis and Meiosis) • Environment
Mutations • Mutations are a vital part of evolution, because they can change a gene, that changes the protein, to produce a totally new trait. • They occur constantly at a low rate. • Most are harmful, some are beneficial, and some are neutral.
Mutations • Spontaneous mutations • Can occur during DNA replication, recombination, or repair • Mutagens • Are physical or chemical agents that can cause mutations.
Wild-type hemoglobin DNA Mutant hemoglobin DNA In the DNA, the mutant template strand has an A where the wild-type template has a T. T T C A T C mRNA mRNA The mutant mRNA has a U instead of an A in one codon. G A A U A G Normal hemoglobin Sickle-cell hemoglobin The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu). Val Glu Example • The change of a single nucleotide in the DNA’s template strand leads to the production of an abnormal protein
Types of Point Mutations • Point mutations within a gene can be divided into 3 general categories • Base-pair substitution • Base-pair addition • Base-pair deletion
Results of Mutations • Silent – does not result in a change to the amino acid or protein. • Missense – causes one amino acid change and a change in the protein. • Nonsense - results in a stop codon instead of a codon for an amino acid. This results in translation being stopped before the protein is complete or may cause more than one amino acid in the chain to change. • Frameshift- When the number of nucleotides inserted or deleted is not a multiple of three, so that every codon beyond the mutation is read incorrectly during translation.
Substitutions • Substitution • Is the replacement of one nucleotide and its partner with another pair of nucleotides • Can cause silent, missense or nonsense results
Normal A A G G G G A U U U C U A A U mRNA Lys Protein Met Phe Gly Stop Base-pair substitution Silent - No effect on amino acid sequence U instead of C A U G A A G U U U G G U U A A Lys Met Phe Gly Stop Missense A instead of G A A U G A A G U U U A G U U A Lys Met Phe Ser Stop Nonsense U instead of A G A A G G U U G A A U U U U C Met Stop
Additions and Deletions • Additions and Deletions • Are additions or losses of nucleotide pairs in a gene • May produce frame shift, missense, nonsense results
Normal A A A G G G A G U U U U C U A mRNA Gly Met Lys Phe Protein Stop Amino end Carboxyl end Base-pair insertion or deletion Frameshift causing immediate nonsense Extra U A G A A G U U U U U G G C U A Met Stop Frameshift causing extensive missense Missing U A A A U A G U A G U U G G C Met Lys Ala Leu Insertion or deletion of 3 nucleotides: no frameshift but extra or missing amino acid Missing A A G A G G A A G U U U U U C Met Phe Gly Stop