Lecture 6 (recording 9)
A quick key word review
1) Leading strand is synthesized continuously from Single RNA primer(s)
2) Lagging strand is synthesized discontinuously from multiple primer(s)
3) Okazaki fragments: RNA primer+DNA
4) DNA synthesis proceeds in 5’ to 3’ direcntion.
5) Primosome: helicase + primase_
6) The predominant helicase is on lagging strand
*For lagging strand the loss of sequence due to the shortenging of 5’ end of daughter DNA is a
problem at the end of chromosomes.
-In the majority of eukaryotes, this problem is solved by having repetitive DNA sequences at
the ends of the chromosomes. The ends are called: telomeres.
- The repetitive sequence that is added to the 3’ end of the parental strand (i.e. the
lagging strand template) is determined by the RNA template in telomerase. (telomerase, the
enzyme, already had RNA template, very simple, in it.
2) Resembles: Reverse Transcriptase (like a polymerase build primers)
3) Generates: G-rich ends
4) Adds nucleotides to: 3’ ends of parental strand
*Homeostatic control of telomere length
Getting old=telomere gets shorter since the enzyme, telomerase, decrease.=starts to loose
*Telomeres and Cancer
• Most cancer cells produce high levels of telomerase
• Modification of the telomerase RNA template interferes with cancer cell growth
• Prognoses of some cancers (eg. neuroblastoma) can be ascertained by telomerase levels
• Cell-targeted inhibitors of telomerase activity have been suggested as therapeutic *Apoptosis-cell suicide
Ppt14 How is DNA unwound?
* Winding problem
-Helicase separates the strands… but when the helix built up and form knots and supercoils,
helicase can’t progress any further b/s of the torsional stress…..
-replication introduces supercoiling in + direction.
*Stress released by Topoisomerase type I
1) Type of break: Single stranded
2) This allows DNA to rotate around the sugar-PO4 backbone of one strand
The free hydroxyl group attach to the phospho-backbone, letting the (–) supercoiling at the ends
of helix to loosen the torsional stress
(Slows down tumor growing)
*Topoisomerase type II to untangle and separate
1. Type of break: Double stranded
2. This allows: One double stranded helix to pass
The enzyme interrupts and opens a double helix letting another DNA helix to pass, then the helix
is shut to restore intact double helix
How mistakes found?
*The High Fidelity of DNA Replicati