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DNA & Genes CHAPTER 11

Objectives Analyze the structure of DNA. Determine how the structure of DNA enables it to reproduce itself accurately.

Hershey & Chase 1952: experimented to show that DNA, not protein, is the genetic material How did they show this? –Used bacteriophages (virus that infects bacteria) 1 bacteriophage made of only radioactive protein 1 bacteriophage made of radioactive DNA –Only the ones with DNA produced new viruses

DNA Structure Polymer of repeating subunits: NUCLEOTIDES –Sugar (deoxyribose) + phosphate + base NITROGEN BASES: -Adenine -Guanine -Thymine -Cytosine PURINES PYRIMIDINES

DNA Structure 1953: James Watson and Francis Crick published their DNA experiment –DNA is made of 2 chains of nucleotides held together by nitrogenous bases –DNA is shaped like a long zipper that is twisted into a coil like spring  shape is called a double helix

DNA Structure Double stranded Alternating sugar/phosphate backbone held together by covalent bonds Bases attach to the sugar Bases are on the interior of the double strand like teeth in a zipper.

Chargaff’s Rule Purine will always pair with a pyrimidine A T C G * In DNA, the amount of adenine (A) is ALWAYS equal to the amount of thymine, and the amount of guanine is ALWAYS equal to the amount of cytosine The 2 chains of nucleotides in DNA are held together by hydrogen bonds. Cytosine forms 3 hydrogen bonds with guanine, and thymine forms 2 hydrogen bonds with adenine

Sugar-Phosphate Backbone In each chain of nucleotides, the sugar of one is joined to the phosphate group of the next nucleotide by a covalent bond Complementary base paring produces a long, 2 stranded molecule that is often compared to a zipper. The sides of the zipper are formed by the sugar and phosphate units, while the teeth of the zipper are the pairs of bases Hydrogen bonds between nitrogen bases

DNA Replication Section 11.1

Mitosis/Meiosis Recall: During interphase, DNA replicates itself. Without DNA replication  new cells would only have ½ the DNA of their parents –Species could not survive and individuals could not grow or reproduce successfully.

Copying DNA Replication of DNA –Strands of double helix are complimentary base pairing allows each strand to serve as a template for a new strand new strand is 1/2 parent template & 1/2 new DNA Animation

Replication: 1st step Strands are separated DNA –helicase enzyme unzips part of DNA helix by breaking hydrogen bonds. stabilized by single-stranded binding proteins single-stranded binding proteins replication fork helicase

Replication: 2 nd Step Each side replicates the other with the help of DNA Polymerase (enzyme used to extend the strand of DNA). Adding bases –can only add nucleotides to 3 end of a growing DNA strand –strand only grows 5  3

Replication: 3 rd Step The lagging strand is synthesized discontinously. –Primase, RNA primer, synthesizes a short segments of DNA (Okazaki Fragments) extended by DNA polymerase

Animation

Result Two DNA molecules identical to the original and each other are made