Lecture 3 Flashcards
What direction does DNA synthesis proceed in?
5’ ⇒ 3’
So new nucleotides are added to the 3’OH end of the primer
Where does the energy for adding nucleotides come from?
The hydrolysis/splitting off of pyrophosphate
pyrophosphatase
enzyme that splits off more phosphate after the initial cleave & creates more energy
Why can’t DNA polymerases initiate DNA synthesis de novo?
They need a preexisting 3’-OH end
Primase
RNA polymerase that creates short sequences of RNA called primers
T/F most new strands of DNA initially begins with a primer
False
All new strands do
T/F these RNA primers stay in the final product,as they perform the same functions
False
They are replaced with DNA
What enzyme replaces RNA primer nucleotides with DNA nucleotides?
DNA Pol I (prokaryotes)
Flap endonuclease 1/FEN1 (eukaryotes)
What does dnaB do?
Encodes a helicase
Helicase
Binds to the origin of replication
Has a pore, which one strand goes through
Unwinds that strand in a 5’ ⇒ 3’ direction
Single Stranded Binding proteins
stabilize complex, prevent reannealing of the two strands
What does dnaG do
encodes primase
How long are primers?
~10 bases
Leading strand
The strand that is being synthesized in the same direction as the replication fork.
has continuous synthesis on 3’ end - initially started with a single RNA primer
Lagging strand
The strand that is being synthesized in the opposite direction as the replication fork
must be synthesized in okazaki fragments
DNA polymerase III
Enzyme that carries out DNA synthesis of long stretches of DNA
Okazaki fragments
short discontinuous segments of an RNA primer and replicated DNA
How are the rna primers removed?
exonuclease activity of DNA Polymerase I
How is the gap left by the removed rna primer filled?
DNA Polymerase I
DNA ligase
seals the discontinuous fragments into a single strand
Origin of replication
specific region of nucleotides that represents where replication begins
T/F there is a 5’ overhang left after replication
False
It’s a 3’ overhang
How many origins of replication are in bacteria
One
Synthesis of new strands occurs bidirectionally from two replication forks
How many origins of replication are in eukaryotes
Many, they are long and not well-defined
Describe the origin or replication in bacteria
There are AT-rich elements, and sites for DnaA protein to bind to
DnaA
Bind to the binding sites on orgin of replication and to each other
This bends the dna and separates the AT-rich region
How many replication forks are caused by a single origin of replication?
Two
Topisomerase
Relieves pressure on stands caused by helicase untwisting
DNA Polymerase III
Synthesizes the leading and lagging strands at speeds of 1000 bases/sec
B-clamp
Keeps Pol III attached to DNA
What’s the problem with having linear dna in nucleus?
the break at the end would activate the cell repair machinery, that would try and link that linear chromosome to another linear chromosome, which would be lethal
What are telomeres
long, simple repeat sequences at ends of linear dna
WRN protein
A part of the telomeric complex
Werner Syndrome
premature aging caused by impairment of WRN protein in telomeres
Increased occurrence in Japan and Sardinia
What does Telomerase do
Synthesizes repeat sequences of telomeres
Can bind to and extend these sequences
Structure of telomerase
multiple subunits and an RNA
Describe the RNA in telomerase
serves as a template for extending the single-stranded end of a chromosome
150-1300 bases long
TERT (Telomerase Reverse Transcriptase)
Synthesizes new DNA to complement the RNA in telomerase
T/F TERT doesn’t need a 3’OH end to synthesize DNA
False
It used the 3’OH end of the 3’ overhang and adds dna nucleotides to the end of that
T/F telomerase activity is linked to cancer and aging
True
