15th Lecture: 18/11/14

Topic: Mutation

Notes:
1) Mutation is any inheritable change in the base sequence of DNA.
2) Mutant is a strain carrying such a change and different from parental strain. Parental strain also known as wild-type strain.
4) Base substitution is where the mutation happened  when a single base at one point in the DNA sequence is replace with a different base.
5) Base substitutions are divided into silent mutation, missense mutation and nonsense mutation.
6) Missense mutations may lead to significant changes in protein because instead of the original amino acid in the polypeptide chain, a different one is substituted.
7) Silent Mutation almost always occur in the third base of a codon and have no apparent effect. The third base of a codon replaced with another base but the codon is codes for the same amino acid whih results in no change of the protein translated. This happened because genetic code is degenerative.
8) Nonsense mutation is the mutation that forms stop codon. It change from codon codes for an amino acid to codon that codes for a stop codon. The translated protein may or may not be functional depending on degree of shortening.
9) Frameshift mutation include the insertion or deletion of a single base or bases in the DNA sequence.
10) Frameshift mutation will get termination of protein synthesis soon after the mutation because a totally different protein produced from the point of mutation.
11) Spontaneous mutation is a mutation that occurs without a mutagen while induced mutation is a mutation that requires chemical and physical agents to induce a mutation.
12) Spontaneous mutation make errors during DNA replication or just spontaneous alteration of a base.
13) There are four types of mutagens:

• Base analogues

- is a chemical that can substitute for a normal base in DNA sequence
- the common base analog is 5-bromouracil
- 5-bromouracil can substitutes for Thymine, it is most likely to pair with Adenine, however it can spontaneously pair with Guanine.
- In the next DNA replication, the Guanine will pair with Cytosine and this results in the change of one base pair of DNA.

• Chemical Mutagens

- substances what can alter a base that is already incorporated in DNA and change its hydrogen specificity
- chemically alters a base so that a new base pair appears in daughter cells in a later generation.

• Radiation

- Ionizing and nonionizing radiation
- Ionizing radiation have short wavelength, high penetrating power and more powerful. High dose can kill the cell.
- Non-ionizing radiation induce pyrimidine dimers and can kill about 90%-95% of cell. The survivors are normally mutant.

• Intercalating agents 

- planar, 3-ringed molecules whose dimensions are equivalent to purine and pyrimidine.
- may insert during replication between two base pair and pushing them apart.
- It is a frameshift mutation because it results addition or deletion of one or more base pairs.
- Examples: Acridines, Ethidium bromide

14) Mutation rate is the probability that a gene will be mutated in a single generation.
15) Mutation frequency is the frequency at which a specific kind of mutation is found in a population of individuals.
16) There are two ways to identify mutants:

• Positive Selection

-Detection is done by rejecting unwanted parental strain.
-Growing cells in plates containing antibiotics. Only those colonies that able to grow are resistant to antibiotics and we can identify them directly.

• Negative selection

- Negative selection is used to identify mutants that have lost the ability to perform a certain function that their parents had.
-Auxotrophic mutants, for example, are bacteria that have lost the ability to synthesize an essential nutrient.
-The replica-plating technique is used to identify mutants by negative selection.
-Inoculate a histidine enriched medium with bateria. Incubate so that cells can form colonies. This is the master plate.
-Press a sterile velvet surface into the colonies of the master plate. Some cells from each of the colonies adhere to the velvet.
-Prepare two mediums, one with histidine, the other without histidine. Transfer cells from the velvet to each plate.
-Colonies that grow on the histindine enriched medium but not on the medium lacking histidine are His- mutants.
17) Ames test can be carried out to revert mutant to wild type in the presence of a mutagen. Histidine auxotrophs of Salmonella are exposed to an enzymatically carcinogen and reversions to the nonmutant state are selected.

Activites :No other activity carried out except for teaching...^^

My own explorace:
1) Auxotrophy is the inability of an organism to synthesize a particular organic compound required for its growth while prototrophy is characterized by the ability to synthesize all the compounds needed for growth.
2) Nitrous acid causes A-T >< G-C transition. Nitrous acid will deaminates adenine to hypoxanthine, cytosine to uracil, and guanine to xanthine. Hypoxanthine pairs with cytosine rather than with thymine. Uracil pairs with adenine rather than with guanine. Xanthine, like guanine, pairs with cytosine.

Reflection:
Human really can't do anything when facing with mutation. No matter any changes in DNA are unable to cure with any medicines. That's why a lot of patient with genetic disorders are lost of hope because their diseases are unable to cure. The patient's family sometimes also feel so desperate because no much organisation provide help for patients with genetic disorders.