Nov 22 Chapter 10 (continued)

 

ii)      Deletion or insertion of base (continued)

 

Reading frame – grouping of triplets

Deletion or insertion of a nucleotide within the coding sequence will result in the shift in the reading frame and will change the amino acid sequence after the mutation.  This is also known as frame-shift mutation.

 

Mutagenesis – creation of mutation

Mutagens – agents that induce mutagenesis

Examples: UV light, X-rays, cigarette smoke, etc.

Most mutagens are also carcinogens (cancer causing agents).

 

3. Virus

- Genes in packages

- Virus DNAs are packed in coat protein.

- Requires materials from host for its survival.

- Not considered as living organisms.

- Can become part of the host chromosome.

- May have derived from the host DNA originally.

 

a. Bacterial phage

- Viruses that infect bacteria.

- Two types of reproductive cycles: lytic and lysogenic cycles (Fig 10.17, p202)

Lytic cycle – break open bacteria (kills bacteria)

Lysogenic cycle – Phage DNA becomes a part of host chromosome and thus becomes prophage.  Prophage DNA does not kill bacteria.

- Bacteria phages are used commonly as a genetic tool to produce many copies of genes via recombinant DNA technology.

 

b. Viruses that cause human disease

 

cold virus, influenza virus, polio, chicken pox, herpesvirus, HIV (AIDS virus), Evola virus, hantavirus, West Nile virus, etc.

 

Features of disease causing viruses

enveloped virus

  

- Surrounded by a membrane envelope (enveloped virus), which allows its entry to animal cells.

- Genetic information may be coded in RNA instead of DNA.

- Can become a part of host chromosome (provirus).  Provirus is a latent form of virus.

- Some viruses evolve rapidly, making it difficult to develop vaccines.

            Examples: flu virus, HIV

- Virus that is harmless in one organism can be harmful to another organism.  Therefore, other organisms can serve as carriers of harmful viruses. 

Example: Hantavirus is common to rodents and does not kill the animals.  On the other hand, the infection of hantavirus to human can be fatal.

(This is one of the criticisms against xenotransplantation.)

- Retrovirus carries RNA as genetic information when they infect.  Once virus RNA is in a host cell, DNA is synthesized from RNA (reverse transcription), which is catalyzed by reverse transcriptase.

- HIV is an example of retrovirus.