BSC-2010 Lecture Notes - Lecture 3: Dna Polymerase I, Okazaki Fragments, Dna Ligase

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5 Nov 2016
November 3rd, 2016 Chapter 16 & 17
You a ol uild o a 3 strad o DNA
“thesis of leadig strads is otiuous i 5 to 3 ed
The lagging strand elongates the other strand
- DNA polymerase must work in the direction away from the replication fork
- Primase must assemble new primers
- The lagging strand is synthesized by DNA polymerase III as a series of segments
called Okazaki fragments
- The leading strand is synthesized towards the replication fork; the lagging strand
is synthesized away from the replication fork
Resolution of DNA-RNA hybrid
- DNA polymerase I can remove RNA nucleotides and fill in the gap with DNA
Fills in the gaps with DNA nucleotides
- Ends of Okazaki fragments must be joined together
- DNA ligase can join the 5 ed of oe DNA oleule ith the 3 ed of aother
Termination of Replication
- Termination sequences in prokaryotes
Ter sites
Cause termination in vitro
- Eukaryote cells do not have termination sequences
How do Eukaryotes protect chromosome ends?
Every time you copy a chromosome, telomeres get shorter
- Eukaryotic chromosomal DNA molecules have repetitive, non-coding nucleotide
sequences at their ends called telomeres
- Telomeres do not prevent the shortening of DNA molecules, but they do postpone
the erosion of genes near the ends of DNA molecules
Telomerase is expressed in germ cells to maintain telomeres at their original length
About once every 100,000 nucleotides, DNA polymerase will add the wrong nucleotide
- DNA Polymerase I and III have 3 to 5 eoulease atiit hih auses the to
remove the mistaken nucleotide immediately after it is added
50 nucleotides per second human polymerase
500 nucleotides per second bacterial polymerase
A chromosome consists of a DNA molecule packed together with proteins
- The bacterial chromosome is double stranded, a circular DNA molecule, and a small
amount of protein. The DNA is supercoiled and found in the nucleoid
- Eukaryotic chromosomes have linear DNA molecules with a large amount of histone
proteins. Chromatin is a complex of DNA and protein found in the nucleus of
eukaryotic cells.
- Chromosomes fit into the nucleus through a multilevel system of packing
Two different versions of chromatin
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