DNA Replication 6.4

DNA Replication DNA replication is essential for cell division is DNA replication semi-conservative or conservative?

Semi-conservative?

Meselson & Stahl (1958) grew Escherichia coli bacteria medium containing 15 N (heavy isotope of nitrogen) for 17 generations ensured that bacterial DNA was labelled with 15 N.

Meselson & Stahl Experiment put the 15 N-labelled bacteria into a medium containing only 14 N and allowed it to grow compared centrifuged samples of the 15 N-labelled bacteria

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 topoisomerasesDNA 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 topoisomerases relieve tension caused by the unwinding of the double helix they cut both strands and allow them to swivel around each other, then reseal them

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

DNA Replication the leading strand is built continuously toward the replication fork the lagging strand is built away from the replication fork, and is built in short segments called Okazaki fragments

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

DNA Replication DNA polymerase II and DNA polymerase I check new strands of DNA for errors as they are synthesized act as exonucleases, excising incorrect nucleotides and adding the correct nucleotides to the strands

Differences between Prokaryotic & Eukaryotic Cells there is usually only one replication origin in prokaryotic DNA, and more than one replication origin in eukaryotic DNA

Differences between Prokaryotic & Eukaryotic Cells in prokaryotic cells, DNA polymerase I, II, and III function in replication and repair; in eukaryotic cells, there are more than 3 different types of DNA polymerase

Self-Study This interactive animation with questions is a good self-study tool. Click on DNA Replication: r/ /animations/animations.ht m

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: