23. Retroviruses

Learning objectives

• What are retroviruses?
• What does reverse transcriptase use as its primer?
• What is the function of RNA-dependent RNA polymerase?
• What are transposons?
• What are telomeres?
• What is the function of telomerase and what is its template?

Retroviruses

I threw in this picture for good measure.

Retroviruses are RNA viruses which use an enzyme called reverse transcriptase to integrate its RNA into the DNA of the host cell. The host cell then begins to transcribe this virus DNA, which produces more viruses.

The most famous retrovirus is HIV. HIV is treated with drugs which inhibit reverse transcriptase. Reverse transcriptase in HIV uses tRNALys as a primer for DNA synthesis.

RNA-dependent RNA polymerase

“Regular” RNA polymerase is DNA-dependent, meaning that it uses DNA as its template. Some viruses have their genetic material stored in RNA instead of DNA, and therefore use RNA-dependent RNA polymerase to transcribe its genes instead.

Transposons

Transposon or transposable element is a genetic sequences which can change its position within the genome. 45% of the human genome is comprised of transposons. There are two types of transposons, retrotransposons and DNA transposons.

Retrotransposons are “copy and paste”. These transposons are first transcribed to RNA, which is then reverse transcribed back to DNA, which will incorporate into the genome. The retrotransposons themselves code for reverse transcriptase, which will reverse transcribe the RNA back to DNA.

DNA transposons are “cut and paste”. These transposons simply move from one location in the genome to another.

When transposons move around they can move into an exon and disrupt it, causing mutations. For this reason transposons are generally negative. Eukaryotic cells use the RNA interference pathway to inhibit the activity of transposons. Some aspects of transposons may be beneficial, however.

Telomeres

Telomeres are located at the ends of a chromosome. They contain long (several thousand base pairs), non-coding, repetitive sequences. In humans the repeating sequence is TTAGGG.

With each cycle of DNA replication the chromosome is slightly shortened. The telomeres function as expendable regions of the chromosome, preventing important genetic information from being lost after DNA replication. In babies the telomeres are approximately 11 thousand base pairs in length; in elderly this length is reduced to 4 thousand base pairs. Due to this, it is hypothesized that telomere length is related to aging.

Telomerase is an enzyme which elongates the telomeres. It’s a special form of reverse transcriptase, and it uses an internal RNA template to synthesize more telomeres.

Summary

• What are retroviruses?
  • Retroviruses are viruses which use an enzyme called reverse transcriptase to transcribe their RNA genome into DNA, which is then inserted into the host cell genome
• What does reverse transcriptase use as its primer?
  • tRNALys
• What is the function of RNA-dependent RNA polymerase?
  • It transcribes RNA from the RNA genome of certain viruses
• What are transposons?
  • Transposon are genetic sequencess which can change their position within the genome
• What are telomeres?
  • Telomeres are long, non-coding regions at the ends of chromosomes
  • They prevent damage to the coding regions of the genome during DNA replications
• What is the function of telomerase and what is its template?
  • It’s an enzyme which elongates the telomeres.
  • It’s a special form of reverse transcriptase, and it uses an internal RNA template to synthesize more telomeres.