Molecular Genetics DNA Replication

DNA replication is essential in order for mitotic cell division to occur Is DNA replication semi-conservative or conservative?

Semi-conservative? (Image from:

Meselson & Stahl Matthew Meselson & Franklin Stahl experiment (1958) they grew Escherichia coli bacteria in a medium containing 15 N (heavy isotope of nitrogen) for 17 generations, ensuring that bacterial DNA was labelled with 15 N.

Meselson & Stahl Experiment they put the 15 N-labelled bacteria into a medium containing only 14 N and allowed it to grow they compared centrifuged samples of the 15 N-labelled bacteria: –before the switch to the new medium –after 1 replication in the new medium –after 2 replications in the new medium

Meselson & Stahl Experiment Animation at: Results of Meselson & Stahl experiments: What would the results have been if DNA replication was conservative?

DNA Replication Enzymes DNA helicaseprimase DNA gyraseDNA polymerase I DNA polymerase IIIDNA ligase

DNA Replication DNA helicase breaks H-bonds between complementary base pairs, unwinding the double helix single-stranded binding proteins (SSBs) bind to the unwound single strands of DNA to prevent the H-bonds from reforming

DNA Replication DNA gyrase relieves tension caused by the unwinding of the double helix (it cuts both strands and allows them to swivel around each other, then reseals them)

DNA Replication primase synthesizes an RNA primer of base pairs to the template strands DNA polymerase III adds complementary nucleotides in the 5’ to 3’ direction to the RNA primer –it adds free deoxyribonucleoside triphosphates

DNA Replication the leading strand is built toward the replication fork, uses the 3’ to 5’ template, and is built continuously the lagging strand is built away from the replication fork, and is built in short segments called Okazaki fragments

DNA Replication (Image from:

DNA Replication (Image from:

DNA Replication DNA polymerase I removes RNA primers (from both the leading and lagging strands) and replaces them with deoxyribonucleotides DNA ligase joins Okazaki fragments into one strand, through creation of phosphodiester bonds between them

DNA Replication DNA polymerase III and DNA polymerase I check new strands of DNA for errors as they are synthesized; they act as exonucleases (they excise incorrect nucleotides) and add the correct nucleotides to the strands

Self-Study I really like this interactive animation with questions. It’s a great self-study tool. Click on DNA Replication: r/ /animations/animations.ht m

Differences between Prokaryotic & Eukaryotic Cells there is usually only one replication origin in prokaryotic DNA, and more than one replication origin in eukaryotic DNA in prokaryotic cells, DNA polymerase I, II, and III function in replication and repair; in eukaryotic cells, there are 5 different types of DNA polymerase

Resources This is a fantastic website. At this part of the site there are multiple-choice problem sets. Try the problem sets on Meselson-Stahl and DNA replication at: McGraw-Hill narrated animation of DNA replication: hill.com/sites/ /student_view0/chapter14/animations.html

Resources (con’d) I’m not a huge fan of this site, but it shows the molecule structure of DNA and the enzymes in the animation of DNA replication: