The Central Dogma of Molecular Genetics Replication DNA Transcription RNA Translation protein

DNA Replication An enzyme signal causes the weak hydrogen bonds to break & the DNA strands start to open up Free nucleotides fill in the open side of each base pair forming 2 identical strands of DNA Known as semi-conervative replication Because original parent strands are conserved but the original integtated molecule is not Each DNA strand as an “old” half & a “new” half

DNA does not leave the nucleus Only 2 complete copies of DNA in the nucleus of a somatic cell Not many ribosome could access/use a specific gene at the same time How does the ribosome synthesise the protein required if it does not have access to DNA itself? Messenger RNA

The human genome (all 46 strands of DNA in each cell) is estimated to contain 3 billion base pairs Mistakes can occur with mismatched pairs Called mutations

3 Stages of DNA Replication 1. Initiation When a portion of the double helix is unwound 2. Elongation When 2 new strands of DNA are assembled 3. Termination When the new DNA molecules re-form into helices New DNA molecule is proofed-read & corrected at the end of this process These stages take place simultaneously in the same DNA molecules

Enzymes – DNA Replication DNA gyrase Bacterial enzyme that relieves the tension produced by the unwinding of DNA during replication DNA helicase Unwinds double-helical DNA by distrupting hydrogen bonds DNA polymerase I Removes RNA primers and replicates them with the appropriate deoxyribonucleotides during DNA replication

Enzymes – DNA Replication DNA polymerase III Responsible for sunthesizing complementary strands of DNA during DNA replication DNA ligase Joins DNA fragments together by catalyzing the formation of a bond between the 3’ hydroxyl group & a 5’ phosphate group on the sugar-phosphate backbones Primase Builds RNA primers

DNA - Replication Separating the DNA Strands Helicase – unwinds double helix by breaking the H bonds SSB’s (single-stranded binding proteins) bind to DNA & keep it from annealing Gyrase – relieves the tension from the unwinding Replication fork forms where the two strands separate Replication of DNA will occur toward & away from fork Replication bubbles forms when 2 forks are close together

Building the Complimentary Strands DNA polymerase III Cannot initiate new complimentary strands on its own – uses RNA primers (built by RNA primase) Starts building complimentary strands from 5’ to 3’ Leading strand – built discontinuously in short Okazaki fragments away from the fork (from primer to primer) DNA polymerase I – removes RNA primers & replaces them with the proper DNA nucleotide (base), leaving a slight space between them DNA ligase joins fragments together

Leading vs Lagging Strands LeadingLagging Complementary strand is built toward replication fork Complementary strand is built away from replication fork Built continuously Built discontinuously in small sections known as fragments Primase needs to add only one RNA primer that DNA polymerase will use to build toward the replication fork Primase continuously adds primers as replication fork travels along DNA molecule Gaps between fragments are joined by DNA ligase

One Final Step After ligase joins the gaps between Okazaki fragments and the gaps where replication bubbles meet one more thing must happen DNA polymerase proofreads the new DNA strand to make sure that complementary base pairing is correct DNA polymerase cuts out any incorrect base pairing on the new strand and replaces it with the proper base

Wait! There’s a little problem! The RNA primer at the 5’ end of the daughter strand must be removed Once this is done there is nothing for any new DNA nucleotides to attach to to fill in the gap. This means the new DNA strand is slightly shorter (about 100 base pairs)