(BIO 103) Lesson 9: DNA Replication and Telomeres

What would happen if topoisomerase was inhibited?

DNA would be tightly coiled as replication began and the process could not proceed (DNA cannot open and be accessible to DNA polymerase)

There is an antibiotic that inhibits topoisomerase, why does this work to bacteria and not us when we take it?

There are different types/structures of topoisomerases and the one in bacteria is different from the one in humans. Same function but different structures

*Ciprofloxacin inhibits bacterial topoisomerase only*

How does AZT (Zidovudine)- a drug for HIV- work?

HIV has a single stranded RNA genome and uses a DNA polymerase called reverse transcriptase to become double stranded.

• Reverse transcriptase has a high affinity for AZT and can incorporate it into the HIV DNA in place of thymidine.

  • The 3’ end of AZT does not have a 3’OH for the reverse transcriptase to add to so it blocks polymer growth.

Compare telomerase expression in cancer cells vs normal cells

• RT-PCR was used to detect hTERT (human telomerase mRNA) in cancerous vs. normal cell lines, using the same primer.

Most cancer cells express hTERT, while normal cells do not, some cell lines make more hTERT than others.

• Telomerase- helps keep chromosomes long so that replication can continue to occur.

  • so it makes sense that cancer cells have hTERT mRNA

What happens when normal replication occurs and there is no telomerase?

• The leading strand will get shorter with each round of replication

• The lagging strand will get shorter with each round of replication

Exercise: Draw how the ends of chromosomes shorten with each round of replication and explain how telomerase prevents shortening.

Telomer _____ after each replication

shortens

What is Senescence?

A process by which a cell ages and permanently stops dividing, but doesn’t die.

Shortened telomeres are prone to what?

They are prone to DNA damage. If corrected cell can continue to live without dividing then it will be okay. If damage occurs to a cell, it may go through programmed death, apoptosis.

• If mutation in a protein known to help correct telomere DNA damage in senescent cells then,…death

WRN protein from Werner’s syndrome causes premature aging, this is why

Why can cancer cells continuously divide?

They’re able to maintain their length of their telomeres by expressing telomerase (which helps keep chromosomes long so that replication can continue to occur).

• Cancer cells do not senesce like non-cancer cells.