mutations

Title the notes: Mutations Notes Add subject heading in the left hand margins. When we are finished taking notes, re-read the notes and write a brief summary of what you learned.

? elephant eleqhant The sequences of bases in DNA are like the letters of a coded message. If one or more of the bases were changed accidentally, would the cell still understand its meaning? For example: We know that the word ELEPHANT means an animal with a trunk. But if the P were changed to a Q, ELEQHANT would be meaningless. 

Now and then cells make mistakes in copying their own DNA, inserting the wrong base or even skipping a base as a strand is put together. These variations are called mutations. Mutations are changes in genetic information that can cause proteins to be made incorrectly.

A change in one or more DNA bases will be transcribed into an incorrect mRNA sequence, which could then be translated into an incorrect sequence of amino acids.

In a substitution, one base is changed to a different base In a substitution, one base is changed to a different base. Substitutions occur at a single point in the DNA sequence so they are also known as point mutations. Substitutions usually affect no more than a single amino acid, and sometimes they have no effect at all, as amino acids have more than one codon that can code for it.

For example:   BEFORE MUTATION AFTER MUTATION DNA GGGAAACCC AGGAAACCC mRNA Amino Acid

For example:   BEFORE MUTATION AFTER MUTATION DNA GGGAAACCC AGGAAACCC mRNA CCCUUUGGG UCCUUUGGG Amino Acid PRO – PHE - GLY SER – PHE - GLY This mutation would cause an incorrect amino acid sequence, with serine replacing proline.

  BEFORE MUTATION AFTER MUTATION DNA GGGAAACCC GGAAAACCC mRNA Amino Acid

  BEFORE MUTATION AFTER MUTATION DNA GGGAAACCC GGAAAACCC mRNA CCCUUUGGG CCUUUUGGG Amino Acid PRO – PHE - GLY Although a mutation occurred in this scenario, this mutation had no effect, as CCC and CCU code for the same amino acid, proline.

In an insertion, one base is inserted or added into the DNA sequence.   BEFORE MUTATION AFTER MUTATION DNA TACGCATGGAAA TACAGCATGGAAA mRNA Amino Acid

In an insertion, one base is inserted or added into the DNA sequence.   BEFORE MUTATION AFTER MUTATION DNA TACGCATGGAAA TACAGCATGGAAA mRNA AUGCGUACCUUU AUGUCGUACCUUU Amino Acid MET – ARG – THR - PHE MET – SER – TYR - LEU

In a deletion, one base is deleted or removed from a DNA sequence.   BEFORE MUTATION AFTER MUTATION DNA TACGCATGGAAA TACCATGGAAA mRNA Amino Acid

In a deletion, one base is deleted or removed from a DNA sequence.   BEFORE MUTATION AFTER MUTATION DNA TACGCATGGAAA TACCATGGAAA mRNA AUGCGUACCUUU AUGGUECCUUA Amino Acid MET – ARG – THR - PHE MET – VAL – PRO - ?

Insertions and deletions are also called frameshift mutations because they shift the “reading frame” of the genetic message. By shifting the reading frame, frameshift mutations can change every amino acid that follows the point of the mutation.

The effects of mutations on genes vary widely The effects of mutations on genes vary widely. Most mutations have little or no effect on the expression of genes or the function of the proteins for which they code.

Some genes negatively disrupt gene function by dramatically changing protein structure or gene activity.

However, some mutations produce beneficial variations However, some mutations produce beneficial variations. Without mutations, organisms could not evolve, because mutations are the source of genetic variability in a species.

Examples of Harmful Mutations: some cancers, sickle cell anemia Examples of Helpful Mutations: Chemical resistance to pesticides for insects, crops can be altered and strengthened by controlled mutations