Lecture 5 Hour 2

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25 Mar 2012
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Lecture 6 (Part I) Feb. 2/12
DNA Replication (II)
Linear DNA Replication (Eukaryotic)
Follows the same steps as circular DNA replication
Only differences include:
oDouble helix unwinding process
oTelomere issue at ends of replication strands
Problem at the ends of linear eukaryotic DNA
Occurs on lagging strands, sometimes the space at ends is too small
DNA primase cannot attach itself OR
If DNA primase does attach itself, the RNA primer will be removed
RNA primer is removed as there is no space for DNA polymerase to continue strand
Loss of genetic information
Telomere solution
Repetitive DNA sequence on 3’ end of parental lagging strands
Added by protein called telomerase
Adding on 3’ of lagging strand as is it reached last during lagging replication
Telomerase contains an RNA template that helps to elongate the DNA strand
Works in 5’-3’
This process of creating DNA strand from RNA template called reverse transcriptase
Telomeres are usually G-rich, meaning RNA template is C-rich
Telomeres are tucked in on 3’ ends
Telomere also helps protect DNA from proteins that splice single stranded DNA
Homeostatic control of telomere length
Tightly regulated by cells
Too many telomeres results in cancer
Aging is caused by deterioration of enzymes that monitor telomere length
Lecture 6 (Part I) Feb. 2/12
Telomere length is shortened
Loss of telomere genetic information = dying of cells = old age symptoms
How supercoiled DNA is unwound
Topoisomerase type 1
Makes a cut on a single strand of the double helix with splicing group
Phosphodiester bond broken
Strands of the double helix can now rotate out of helical formation, stress relieved
Rotation around sugar-phosphate backbone of one strand
Splicing group on topoisomerase and phosphodiester bond on DNA are returned
No ATP needed, operates in front of helicase
Topoisomerase type II
Allows one double stranded helix to pass through another
Makes a break on one double strand
Passes the other double strand through the break
Reforms the break after pass double strands through one another
Uses ATP