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What is the substitution of bases

A nucleotide in a section of a DNA molecule is replaced by another nucleotide that has a different base


Depending on which New base is substituted for the original base, what are the three possible consequences

The formation of one of the three stop codons that mark the end of a polypeptide chain. The final protein would almost be certainly be significantly different and the protein could not perform it’s normal function

The formation of a codon for a different amino acid- the structure of the polypeptide produced would differ in a single amino acid. The protein of which this polypeptide is a part may differ in shape and not function properly

The formation of a different codon but one that produces a codon for the same amino acids as before - the genetic code is degenerate and so most amino acids have more than one codon. Mutation will have no effect


What’s the deletion of bases

The loss of a nucleotide base from a DNA sequence


What happens due to the frame shift in the deletion of bases

The gene is now read in the wrong three base forums and the coded information is altered.

Most triplets will be different as will the amino acids they code for.

The polypeptides will be different and lead to the production of a non functional protein that could considerably alter the phenotype


What impact would the deletion at the start of the sequence have rather then the end

One deleted base at the start of the sequence could alter every triplet in the sequence

One at the end is likely to have a smaller
Impact but can still have consequences


What are the other types of gene mutation

Addition of bases - an extra base becomes inserted in the sequence. Causes a frame shift to the right, all triplets become altered.

Duplication of bases - one or more bases are repeated. Frame shift to the right.

Inversion of bases - a group of bases become separated from the DNA sequence and rejoin at the same position but in inverse order (back to front). Affects amino acid sequence results

Translocation is bases- group of bases become separated from the dna sequence on one chromosome and become inserted into the dna Sequence of a different chromosome. Have significant effect on gene expression leading to abnormal phenotype


What does the degenerate nature of the genetic code mean

That some amino acids are coded for by more then one triplet


Basic mutation rate can be increased by outside factors known as mutagenic agents or mutagens. What are they?

High energy ionising radiation - these forms of radiation can disrupt the structure of dna

Chemicals - e.g. nitrogen dioxide may directly alter the structure of dna or interfere with transcription


A mutation causes three bases in the DNA of a gene to become duplicated. Explain how the effects of this mutation might differ if the duplicated bases are consecutive rather than in three separate locations

Where the duplicated bases are consecutive, the frame shift is three bases long and so the subsequent codons are not affected.

The polypeptide will have an additional amino acid but otherwise will be unchanged. If the bases are separate, the frame will initially be one base long, becoming two bases long after the second duplicate is added

Codons after both the duplications will be changed and the polypeptide will have different amino acids ( but not necessarily all degenerate code)

After the third duplicate base the codons will be unchanged


Suggest two reasons why the addition of a single base into a DNA sequence may not alter the amino acid sequence in the resultant polypeptide

Some codons will be changed to ones that code for the same amino acid (degenerate code). The frame shift might not alter some codons because the replacement bases are same as the originals