Structure of DNA Unit 5B.1 2007-2008

Genetic information is stored and passed to subsequent generations through DNA molecules and, in some cases, RNA molecules. Prokaryotes, viruses and eukaryotes contain plasmids, which are small extra chromosomal, double stranded circular DNA molecules

Proof that DNA is the carrier of genetic information involved a number of important historical experiments

Hershey & Chase Viruses were made of only protein and DNA Performed an experiment using radioactively labeled viruses that infect bacteria. Viruses were made of only protein and DNA

Procedure Hershey and Chase labeled the virus DNA with a radioactive isotope and the virus protein with a different isotope. They then watched the virus infect bacteria cells and followed the radioactive isotopes.

Results When the viruses infected bacteria, only the DNA entered the bacteria cell Evidence that DNA is the genetic material of a cell DNA caused bacteria to produce more viruses.

Franklin & Wilkins Used X-ray crystallography to take pictures of DNA helped lead to the discovery of the shape of DNA

Watson & Crick Double helix shape DNA is made of two strands of DNA that twist together like a spiral staircase

Avery-MacLeod-McCarty Proved that DNA was the transforming agent that caused nonpathogenic bacteria to turn pathogenic and kill mice Found that enzymes that destroy DNA did not transform and that enzymes that destroyed only proteins could transform

STRUCTURE OF NUCLEIC ACIDS Arrow from: http://www.harrythecat.com/graphics/b/arrow48d.gif Built from NUCLEOTIDE SUBUNITS NITROGEN BASES CAN BE: ADENINE GUANINE CYTOSINE THYMINE URACIL Sugar can be DEOXYRIBOSE (DNA) RIBOSE (RNA) Image by: Riedell

DNA has no URACIL RNA has no THYMINE PURINES (A & G) have 2 RINGS PYRIMIDINES (T, C, & U) have 1 RING http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/genetics/DNA/DNA/fg4.html http://student.ccbcmd.edu/~gkaiser/biotutorials/dna/fg29.html

Directionality of DNA You need to number the carbons! nucleotide it matters! nucleotide PO4 N base 5 CH2 This will be IMPORTANT!! O 4 1 ribose 3 2 OH

The DNA backbone Made of phosphates and deoxyribose sugars 5 The DNA backbone PO4 Made of phosphates and deoxyribose sugars Phosphate on 5’ carbon attaches to 3’ carbon of next nucleotide base CH2 5 O 4 1 C 3 2 O –O P O O base CH2 5 O 4 1 3 2 OH 3

Double helix structure of DNA “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick

Anti-parallel strands Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons DNA molecule has “direction” complementary strand runs in opposite direction 5 3 3 5

Bonding in DNA 5 3 3 5 hydrogen bonds covalent phosphodiester ….strong or weak bonds? How do the bonds fit the mechanism for copying DNA?

Base pairing in DNA Purines Pyrimidines Pairing adenine (A) guanine (G) Pyrimidines thymine (T) cytosine (C) Pairing A : T 2 bonds C : G 3 bonds

CHARGAFF’s RULES Erwin Chargaff analyzed DNA from different organisms and found A = T G = C Now know its because: A always bonds with T G always bonds with C A Purine always bonds to a Pyrimidine

Chromosome Structure in Prokaryotes Approximately 5 million base pairs 3,000 genes Chromosome E. coli bacterium Bases on the chromosome DNA molecule in bacteria single DOUBLE STRANDED circular loop © Pearson Education Inc, publishing as Pearson Prentice Hall. All rights reserved