Class Notes (837,550)
Canada (510,314)
Biochemistry (847)
Chris Brandl (189)

Topic 20

6 Pages
Unlock Document

Biochemistry 2280A
Chris Brandl

Topic 20 Topic 20 – DNA Replication Makin’ Copies - cells must accurately copy your DNA billions of times in your lifetime  every time you make a new cell - during replication, each DNA strand serves as template for synthesis of complementary strand - DNA polymerase makes new strand complementary to template strand  adds new bases according to base pairing rules DNA replication is semi conservative - After replication, each of two daughter double helices contain one original strand and one newly synthesized strand Steps in DNA replication 1. Initiation 2. Priming 3. DNA synthesis 4. Proofreading (concurrent with DNA synthesis) 5. Ligation 1. Initiation - Occurs at specific DNA sequence called replication origin  E coli simple – one origin  Eucaryotes have more DNA and multiple origins - Initiator proteins recognize origin and pulls DNA strands apart - This sequence contains a lot of AT base pairs – weaker because only 2 H bonds - Make single stranded regions – need single strands for replication to happen - helicase enzyme that binds at place where double helix becomes single stranded. Helicase binds to ss DNA and unwinds double helix, consuming ATP - forms replication bubble - many copies of single strand binding protein bind to unwound DNA and prevent re annealing o think of as bouncers at bar preventing fight 2. Priming - DNA polymerase can’t start synthesizing using template strand, needs something to attach nucleotides too - needs a primer - primase synthesizes short (10-20nt) strand of RNA (primer), giving DNA something to work with 3. DNA synthesis 1 Topic 20 - DNA polymerase extends the RNA primer from its 3’ end (synthesis 5’ to 3’ direction) - DNA polymerase adds complementary base from deoxynucleoside triphosphates (dNTPs) to growing strand Template strand  3’ end top and 5’ end bottom  antiparallel to primer strand  5’ end being synthesized going down – always adding to 3’ end of growing strand  nucleoside triphosphate (3 phosphates) and free OH attacking, resulting in loss of a phosphate and new bond formed  base brought in making appropriate base pair reactions  polymerase grabs base and fits it in through a catalyzed reaction  the phosphate is released and a bond is formed  this is an energetically favourable process because energy is coming form dNTPs. Energy is used to form phosphodiester bond  polymerase will then move down strand and continue adding another base on 3’ end - Consequences for how DNA synthesis occurs  Synthesis occurs in both directions in replication forks  Helicase travels at front of fork, unwinding double helix  Two DNA polymerase complexes associate with helicase, each one synthesizing different template strand  DNA polymerase stays associated with template with help from sliding clamp protein – forms doughnut shaped dimer around DNA o Important because DNA dissociates easily unless another protein helps it stay on track  ddDNA is anti-parallel  replication in one template strands proceeds naturally  DNA travels with replication fork in 3’ to 5’ direction, synthesizing complementary DNA in 5’ to 3’ direction o Leading strand o Continuous synthesis – can continue for long distance (5x10^5 nt using 1 primer)  Other complementary stand, DNA synthesis must occur in direction opposite to replication fork moving 2 Topic 20 o Lagging strand o Discontinuous synthesis –form okazaki fragments, which are later connected by ligase th o In e coli, okazaki fragments are 1000-3000 nt long, but humans only 1/10 this length  Humans: 100-200 nt long o Helicase is moving along, exposing DNA (pulling strands apart). Polymerase can’t jump, so will have to dissociate and get another polymerase to start again o synthesis must be restarted repeatedly, DNA synthesis on lagging strand needs more primers than synthesis on leading strand  replication on leading and lagging strands occur concurrently  DNA helicase unwinds double helix. At some time later, bigger replication fork and helicase has to move to the right.  Polymerase can follow behind helicase and make one big strand o Leading strand  Top strand 3’  5’, left to right. Need to synthesize in opposite direction of helicase. Polymerase needs to constantly synthesize a bit, then jump back, forming okazaki fragments o Discontinuous bits o Lagging strand - It is thought that two DNA polymerase associate with each helicase  DNA is bent so it is not associating with primase – this is more efficient because polymerase can easily grab onto primer.
More Less

Related notes for Biochemistry 2280A

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.