Molecular Basis of Inheritance

DNA Studies Frederick Griffith – 1928 Frederick Griffith Streptococcus pneumoniae 2 strains – pathogenic & harmless Killed pathogenic  mixed with living harmless  living cells converted to pathogenic  offspring inherited pathogenic

DNA Studies Called process transformation Substance transforming was DNA

DNA Studies Hershey & Chase – 1952 Hershey & Chase Studied bacteriophages (viruses that infect bacteria) Virus – mostly DNA & protein Through radioactive isotopes, studied T2 (virus that infects E. coli in mammalian intestine)

DNA Studies Each separately added to E. coli: Radioactive sulfur for proteins Radioactive phosphorus for DNA Culture allowed to grow Bacteria broken from virus & studied – radioactivity in bacteria with phosphorus Result – DNA entered bacteria, not protein

DNA Studies Erwin Chargaff – determined A-T and G-C

DNA Structure James Watson & Francis Crick

DNA Structure Through use of Rosalind Franklin’s X-ray diffraction photograph, W&C determined double helix structure

DNA Structure Sugar-phosphate backbone Nitrogenous bases on inside (10 per turn of helix) Bond purine to pyrimidine A has 2 H bonds with T only G has 3 H bonds with C only

DNA Replication DNA untwists & unzips Complementary base pairs free in cytosol bond appropriately New strands re-zip and re-twist

DNA Replication Result – Two daughter DNA molecules One parent strand One new strand Called the semi-conservative model

Origins of Replication Where DNA replication begins Has specific nucleotide sequence Proteins recognize this at these sites  help separate DNA (open replication “bubble”)

Origins of Replication Can be hundreds of bubbles Extending from bubble – replication fork (where new strands are elongating) Replication in both directions until bubbles fuse

DNA Elongation DNA Polymerases – enzymes aiding elongation at a replication fork

Strand Arrangement Opposite sides of backbone run antiparallel (upside down) to each other 5’ end – phosphate 3’ end – hydroxyl group Phosphates connect from 5’ C of one sugar to 3’ C on next sugar

Strand Arrangement FYI…find nitrogenous base…that is 1’C… count clockwise to find others New nucleotides are added ALWAYS from 5’ end of the new DNA to 3’ end (3’-5’ of old strand)

Strand Arrangement When DNA unzips, the 3’-5’ strand can fill in easily – leading strand Copies toward replication fork Helped by DNA polymerase

Strand Arrangement The 5’-3’ strand fills in with pieces – lagging strand Pieces called Okazaki fragments DNA ligase helps to join sugars & phosphates together

How Does It Start? When replication starts, new chain begins with a primer Section of RNA Primase joins approx 10 RNA nucleotides together to start replication Later replaced by DNA with DNA polymerase help

How Does It Start? One primer for leading strand Each fragment of lagging strand is primed & then replaced by DNA

Other help Helicase – enzyme untwists DNA Single-strand binding proteins – keep DNA apart during process Overview of DNA Replication Overview of DNA Replication

Proofreading Mismatch repair Polymerase matches new nucleotide to parent strand  will remove if incorrect Each cell monitors DNA for new changes due to cell error or envi. Nucleotide Excision Repair New error found  a nuclease cuts it out  polymerase & ligase fill in proper pieces

Last fix Once the last RNA primer comes off, there is a gap that needs to be fixed

Last Fix Nucleotides can only add to the 3’ end of a preexisting polynucleotide No way to complete 5’ end Over time, DNA would progressively shorten – problem Solution – telomerase

Last Fix End of DNA – telomeres Repetitive expendable (non-coding) nucleotide sequence Protect major shortening of DNA Will shorten somewhat over time Telomerase will help lengthen

Last Fix Has RNA on it Serves as template to extend telomere at 3’ end of the telomere Telomeres Telomeres and Cancer Telomeres and Cancer