Chapter 12. Mutations: things that go bump in the night. Prepared by Woojoo Choi

Transcription

1 Chapter 12. Mutations: things that go bump in the night Prepared by Woojoo Choi

2 Mutations alter the DNA 1) Mutation: alteration in the genetic information 2) Mutation is a change in the base sequence of the DNA. 3) There are many possible changes Wild-type the cat sat on the mat Substitution the rat sat on the mat Insertion(single) the cat spat on the mat Insertion(multiple) the cattle sat on the mat Deletion(single) the ct sat on the mat Deletion(multiple) the cat the mat Inversion(small) the tac sat on the mat Inversion(large) tam eht no tas tac eht

3 Mutations alter the DNA 1) Altering the DNA base sequence has a variety of effects. In replication, any changes will be duplicated and passed on to the next generation. In transcription, the mutation in the DNA sequence will be passed on to the RNA molecule. In translation, an altered RNA sequence may be translated into an altered protein (or enzyme). 2) So, the final result of a change in the DNA sequence may be a defect in the operation of some vital reaction.

4 Silent mutation 1) A mutation with no observable effect on cell growth or survival 2) Silent mutation do not alter the phenotype. 3) The ways to get a silent mutation 1 The base change occurs in the non-coding DNA between genes.

5 Silent mutation 2 The base change occurs in introns except for few important bases at the splice recognition sites.

6 Silent mutation 3 The third main type is within the coding region of a gene and does get passed on to mrna. There are 64 different codons, most of the 20 possible amino acids have more than one codon. A base change that converts the orginal codon into another codon that codes for the same amino acid will have no effect on the final structure of the protein. Third base redundancy: since many amino acids have several codons, the third codon base can often be changed without changing the amino acid which is encoded. For example, since codons that code for the amino acid alanine have GC as the first two bases, any codon of the form GCX(X=any base) will give alanine.

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8 Nonsense and missense mutations 1) Missense mutation When a sequence change in DNA results in the replacement of one amino acid by another amino acid. Conservative substitution: replacement of an amino acid in a protein with another that is similar in its chemical properties(alanine->glycine) Changes will be relatively mild and usually non-lethal.

9 Nonsense and missense mutations Radical replacement: replacement of an amino acid in a protein with another that is very different in its properties(alanine->glutamic acid) Changes will seriously cripple or even totally incapacitate our protein.

10 2) Nonsense mutation Nonsense and missense mutations When a sequence change in DNA results in the replacement of the codon for an amino acid with a stop codon, thus producing a shortened protein The cell detects and digests a shortened and unfolded protein. The result is the total absence of this particular protein. Nonsense mutations are often lethal.

11 Deletion and indertions 1) So far we have been really rather restrained and only swapped a single base for another. 2) We can excise one or more bases of the DNA sequence. 3) Deletion: removal of one or many nucleotides from DNA

12 Deletion and indertions 4) The more bases we remove, the worse the mutation? Not necessarily!! with a single base deletion or insertion, the reading frame and protein sequence have been completely changed. Frameshift mutation: a mutation which changes the reading frame of the protein encoded by a gene Deletion or insertion of two bases would also change the reading frame.

13 Deletion and indertions Lets suppose three bases deletion. In this case, although an amino acid has been deleted, the correct reading frame has been preserved. Apart from the single amino acid we lost, the rest of the protein is unchanged. A three base insertion would also have the same effect.

14 Deletion and indertions If the deleted(or inserted) amino acid is in a relatively less vital region of the protein, we may actually get away with this and make a functional protein.

15 Deletion and indertions 5) A whole number of codons must be added or subtracted in order to avoid the horrible consequences of changing the reading frame.

16 Rearranging DNA: Inversion and translocations 1) Inversion when a segment of DNA is removed, flipped and reinserted, facing the opposite direction Inversions are definitely bad news.

17 Rearranging DNA: Inversion and translocations 2) Translocation when a segment of DNA is removed and reinserted in a different place, either on the same chromosome or on a completely different chromosome If an intact gene is merely moved from one place to another, little damage may result. If half of a gene is moved and stuck somewhere in the middle of another gene, the results will be chaotic and severely detrimental.

18 What causes mutations? 1) Induced mutations: mutations caused by mutagen Mutagen: agent that can cause mutation Two main types: toxic chemicals and radiation 2) Spontaneous mutation: mutations which happen without mutagens because of mistakes during DNA replication

19 What causes mutations? 3) The most common types of toxic chemicals EMS (ethyl methane sulfonate): add a methyl group to bases in DNA and changes their shape Nitrite: replaces amino groups with hydroxyl groups and converts the base cytosine to uracil

20 What causes mutations? 4) When the time comes for DNA replication, the DNA polymerase is confused by the altered bases and puts in wrong bases in the new strand of DNA.

21 What causes mutations? 5) Another type of chemical mutagen mimics the base found in natural DNA Base analog: chemical that resembles a base of a nucleic acid well enough to fool a cell into using it instead.(eg, bromouracil)

22 What causes mutations? 6) A more subtle form of chemical mimicry consists of imitating the structure of a base pair Intercalation: when a chemical agent inserts itself into DNA between two base pairs(eg, acridine orange) The DNA polymerase thinks the intercalating agent is a base pair and it puts in an extra base when making a new strand. Insertion of an extra base will change the reading frame and this will completely destroy the function of protein.

23 What causes mutations? 7) Teratogen: agent that can causes abnormal development of embyos leading to monstrosities (eg, thalidomide).

24 What causes mutations? 8) High frequency electromagnetic radiation(uv, X-rays and γ-rays) directly damages DNA UV radiation makes two neighboring thymine bases react with each other to give thymine dimers. X-rays tend to produce multiple mutations and often yield rearrangements of the DNA such as deletions, inversions and translocations. 그림추가

25 Mutations caused by insertion of transposons 1) Transposon: mobile segment of DNA that moves from place to place on a cell s chromosome(ch 20)

26 Genetically engineered gene disruption 1) Mutations that serve to completely inactivate a gene are useful in genetic analysis. 2) Sometimes foreign DNA is inserted into genes to disrupt them and then study the results.

27 1) 그림 추가 Genetically engineered gene disruption

28 Mutational hot spots 1) Hot spots: regions in the genetic material that are unusually susceptible to mutation 2) Most hot spots are due to the presence of occasional methylcytosine bases in the DNA. (spontaneously disintegrates to give methyluracil)

29 Reversion and suppression 1) Reversion a second mutation that restores original characteristics to a mutant organism It is a phenotypic term. 2) True revertant a revertant in which the original DNA base sequence is exactly restored It happens rare. 3) Second-site revertant a revertant in which a second change in DNA base sequence cancels out the effects of the first It happens more often.

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32 Detecting mutagenic chemicals by reversion 1) Chemical mutagens can be detected by the Ames test, used routinely by industry and government agencies to screen chemicals for possible mutagenic effects.

33 Detecting mutagenic chemicals by reversion 2) Many chemicals are checked by this test that examines chemical s effect on bacteria in culture. Mutants carrying mutations in the genes for histidine synthesis are used. These mutants cannot grow unless given histidine. When some mutants grow in the medium lacking histidine, these are revertants. The frequency of reversion is also increased by mutagenic agents.

34 DNA repair 1) There are several DNA repair systems 1 Mismatch repair: DNA repair system which recognizes wrongly paired bases

35 DNA repair 1 Mismatch repair We need to know which strand came from the mother cell and which was the recently synthesized (end error-carrying) daughter strand. In bacteria such as E. coli, the DNA is tagged by methyl group to indicate this. (eg, GATC bases) Different organisms use different tagging systems, but the principle remains the same.

36 DNA repair 2 Excision repair: cutting out a stretch of damaged DNA and replacing it with new DNA (DNA polymerase I)