DNA: THE GENETIC MATERIAL CH 16

I. The Structure of DNA A. Levine – DNA is a polymer made of repeating monomers called nucleotides 1. Structure of a nucleotide 5 carbon sugar (deoxyribose) Phosphate group 1 of 4 different bases

2. Types of bases Purines: Adenine Guanine Pyrimidines: Cytosine Thymine Uracil (replaces T in RNA)

3. Nucleotides are linked via dehydration reactions The 5’ phosphate of one nucleotide is added to the 3’OH of the previous one

B. Chargaff’s rule Amount of A = amount of T Amount of C = amount of G

C. X-ray diffraction studies by Rosalind Franklin Showed that DNA has a shape of a helix with a uniform diameter

D. Watson and Crick Created a model of the structure of DNA

DNA STRUCTURE: DNA is double stranded 2 strands twisted into a helix Sugars and phosphates form the backbone Bases face inward 2 strands held together by hydrogen bonds between the bases The 2 strands are antiparallel A pairs with T C pairs with G

II. DNA Replication The Meselson- Stahl Experiment Showed that DNA replication is semiconservative – Each DNA molecule consists of one old strand and one new strand

A. General process of Replication DNA replication starts at origins of replication where DNA opens up forming replication fork Replication moves in both directions till whole molecule is copied

Helicase unzips DNA Single stranded binding proteins stabilize single strands DNA polymerase can only add a nucleotide to pre- existing chain so primase adds a small RNA primer DNA polymerase adds nucleotides to growing chain

B. Antiparallel Elongation DNA polymerase can only adds nucleotides to a free 3’-OH DNA replication can only move in the 5’→3’ direction DNA replication moving towards the fork is continuous DNA replication moving away from the fork is discontinuous occurring in Okazaki fragments

Proofreading and Repairing DNA DNA polymerase proofreads newly made DNA and fixes any mistakes Proofreading is not 100% efficient but is pretty high hill.com/sites/ /student_view0/ch apter3/animation__dna_replication__quiz_1_.html

III. Replicating the ends of DNA DNA polymerase cannot copy DNA to the end so several nulceotides are not copied Continuous replication shortens DNA Telomeres: short stretch of nucleotides at ends of DNA Don’t prevent shortening but prevent erosion of necessary DNA When telomeres get too short DNA replication and cell division stop. This limits # of cell divisions In fetal cells and germ cells that divide many times, telomerase replaces lost telomeres hill.com/sites/dl/free/ /126997/animation19.ht ml