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BIO130H1 (500)

Jane Mitchell (60)

Lecture 5

Lecture 5 Hour 2

by OC13033

School

University of Toronto St. George

Department

Biology

Course Code

BIO130H1

Professor

Jane Mitchell

Lecture

Page:

of 2

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

(telomere)

•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