Lesson 12: Mutations

Point Mutations: A small-scale change in the nitrogenous base sequence of DNA. A point mutation is a failure by the replicating cell to copy the genetic information accurately. Point mutations may be beneficial, harmful, or neutral (having no effect on the organism). There are three major point mutations.

1) Base Pair Substitution: In a base-pair substitution, one nitrogenous base is accidentally replaced with a different base.

2) Insertions: In an insertion, one or more nitrogenous bases are inserted during the copying process.

3) Deletions: In a deletion, one or more nitrogenous bases are deleted during the copying process.

Chromosome Mutations: An error that involves an entire chromosome or a large part of a chromosome are known as chromosome mutations. An example of a chromosome mutation is non-disjunction. Non-disjunction occurs when sister chromatids fail to separate during meiosis which results in the production of gametes that contain too many or too few chromosomes.

Non-disjunction leading to chromosomal mutations

Inheritance of Mutations: Most mutations occur in parts of the DNA that are 'non-coding'. Many mutations can be corrected as the replication process continues, but as an organism gets older, the rate of mutations becomes more frequent potentially leading to cancer.

Inheritance of Mutations: Mutations that occur in somatic cells (body cells) will not get passed on to offspring, but mutations in the gametes (sex cells) do. Mutations that are dominant (such as Huntington's disease) will show up in the first generation of offspring, but mutations that are recessive may take several generations to show up phenotypically, and that too, only if by chance two individuals with the same mutation produce offspring.

Are all mutation harmful? Sickle Cell Anemia: Individuals with sickle cell anemia have inherited a mutated gene in which a single adenine base is substituted for a thymine. Are all mutation harmful?

Are all mutations harmful? Lactose Tolerance and Lactose Intolerance: About 75% of the adults worldwide are lactose intolerant. Those adults that can tolerate lactose can do so because of a genetic mutation. Individuals who have inherited the mutated gene for lactose tolerance continue to produce the enzymes that breakdown lactose. Are all mutations harmful?

Spontaneous or Induced Mutations: Spontaneous mutations occur naturally and randomly whereas induced mutations are caused by external, environmental factors. Induced mutations are a result of chemical or physical agents such as radiation or cigarette smoke.  

"Jumping Genes" or Transposons: Point mutations are mutations that occur within a gene in fixed locations. Barbara McClintock's work in genetics back in the 1940s, showed that genes do not always remain in fixed locations.

Since each kernel is an individual corn plant embryo, one cob produces a large sample of offspring; making it an ideal species for genetic research.

http://www.youtube.com/watch?v=91vR- FKBMT4

McClintock's research showed that an organism's genome is not static, but rather there are segments of DNA that can move as a unit from one location to another. These moveable segments are called transposons (or 'jumping genes'). Transposons are specific segments of DNA that can move along or between the chromosomes through the process of transposition.

McClintock learned that these transposons can affect the colour of the corn kernels. If a transposon is inserted into the gene for purple kernels, the gene is disrupted and purple pigment cannot be produced. Instead, the resulting kernel is white.

Hemophilia and Transposons: Hemophilia is an inherited X-linked disease that is caused by genetic mutation of the blood-clotting factor gene. In rare cases, hemophilia can also be caused by transposons. If a transposon inserts itself into a normal blood factor VIII gene, then the individual (if male) will be affected by hemophilia.

Microarray Technology Microarray technology simplifies the search for disease causing genes. A microarray consists of a small membrane or glass slide that contains samples of hundreds, or even thousands of DNA fragments arranged in a regular pattern. Each fragment corresponds to a particular gene. Samples with genetic material to be tested are spread over the microarray chip and interact with these gene fragments.

http://www.dnalc.org/resources/3d/26- microarray.html http://www.youtube.com/watch?v=VNsTh MNjKhM