Class Notes (808,765)
Canada (493,382)
Biology (2,348)
BIOLOGY 1A03 (716)
Lecture 2

BIOLOGY 1A03 Lecture 2: THEME 4

11 Pages
Unlock Document

McMaster University
Rosa Da Silva

Module 2 – Replication UNIT I: TRANSMISSION OF INFORMATION GENETIC INFORMATION IS COPIED • In 1953, Watson and Crick concluded that there is a copying mechanism for this genetic material MODEL FOR SEMI-CONSERVATIVE REPLICATION • In 1954, Watson and Crick proposed that DNA consists of a pair of complementary template chains • Stated that prior to replication, hydrogen bonds are broken between the strands, allowing for unwinding and separation COMPLEMENTARY: each strand contains the information necessary to reconstruct the other • Believed that when a cell copies DNA, each strand serves as a template for a new complementary strand • While replication begins with one parent helix, there are two helices in the end which are exact copies of the parent molecule SEMICONSERVATIVE MODEL: when a DNA double helix replicates, each of the two daughter molecules would have one old strand from the parental molecule, and one newly made strand UNIT II: EVIDENCE FOR THE SEMICONSERVATIVE MODEL OTHER DNA REPLICATION HYPOTHESES CONSEVATIVE: two complementary parental strands somehow fuse back together following replication DISPERSIVE: all four strands somehow combine into a mixture of old and new strands SUPPORT FOR SEMICONSERVATIVE REPLICATION • Matthew Meselson and Franklin Stahl demonstrated that DNA replicated in the previously proposed semiconservative manner • In the late 1950s, E.coli bacterial cells were cultured for generations in mediums containing nucleotide precursors with radioactively labeled heavy nitrogen isotopes (15N) • The bacteria was then transferred into a medium containing 14N • From that point, every new DNA strand was built containing 14N rather than 15N isotopes • DNA samples were extracted from the growing bacteria • Each sample was then centrifuged through a solution which separated DNA based on differing densities of the radioactively labeled molecules • Any DNA containing the 15N would be heavier and end up towards the bottom of the tube • DNA from bacteria growing in the 15N isotope medium had only one distinct band • Following transfer into the 14N medium/one round of replication, DNA also appeared as a single band but with lower density, meaning it was positioned above the 15N DNA in the tube indicating that it is a hybrid 14N/15N DNA • The conservative model of DNA was therefore rejected since no individual distinct band correlated with the 15N DNA • To test the semiconservative/dispersive model, bacteria was allowed to grow/divide for many generation in the 14N media after transfer from the 15N media • Extracted DNA could be separated into 2 distinct bands: one in the position containing only 14N, and another where the hybrid14N/15N hybrid was expected RESULTS • Consistent with the fact that each new double helix is made up of one old strand and one new strand • DNA is therefore replicated in a semiconservative fashion SEMICONSERVATIVE CHROMOSOME REPLICATION • Researchers are presently able to label individual nucleotides with fluorescent labels which allows for examination of replication in eukaryotic DNA • Discovered that chromosomes of eukaryotic cells could contain hybrid and fully labeled nucleotides, indicated by faintly/darkly fluorescing strands of labeled DNA (even within one chromosome) • Provides further support for semiconservative model • Mechanism of replication is similar in prokaryotes and eukaryotes UNIT III: REPLICATING DNA INITIATING REPLICATION • Begins in the S phase along the origins of replication • Initiation varies in prokaryotes and eukaryotes due to differing organizations of the genome • Replication in prokaryotes begins at a single origin of replication and continues around the circular chromosome from the one initiation site DNA SYNTHESIS OCCURS FROM 5' TO 3' • Similar to transcription • Template strand is copied from the 3' end to the 5' end and produces a daughter strand in a 5' to 3' direction • Incoming complementary nucleotide engage in a hydrogen bond with a nucleotide on the template strand and interact with the 3' hydroxyl of the forming polymer on the new daughter strand • A phosphodiester bond forms between growing daughter strand and new incoming nucleotide, allowing for it to become part of the new DNA backbone, and producing a pyrophosphate in the process BOTH STRANDS ARE SIMULTANEOUSLY TEMPLATES • Both strands serve as simultaneous templates since they run in an antiparallel fashion • Unwinding of the double helix results in separation of the parental strands at the replication forks within the origins of replication • Initiation requires that a short stretch of RNA/primer (5-10 nucleotides) be synthesized/base pair with the template • Primer is required since the enzymatic machinery which elongates the new strand can only do so from an existing piece of DNA/RNA • As elongation progresses, polymerization of each newly replicated strand is catalyzed by DNA polymerase in a 5' to 3' direction • DNA polymerase synthesizes a replicated DNA strand from the primers that anneal to the template CONTINUOUS/DISCONTINUOUS REPLICATION • DNA polymerization by DNA polymerase is limited to elongation in a 5' to 3' direction LEADING STRAND: replication of one
More Less

Related notes for BIOLOGY 1A03

Log In


Don't have an account?

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.