Chapter 5. Macromolecules: Nucleic Acids
Nucleic Acids Function: Examples: Structure: store & transmit hereditary information Examples: RNA (ribonucleic acid) DNA (deoxyribonucleic acid) Structure: monomers = nucleotides
Nucleotides 3 parts nitrogen base (C-N ring) pentose sugar (5C) ribose in RNA deoxyribose in DNA PO4 group 2005-2006
Types of nucleotides 2 types of nucleotides different Nitrogen bases purines double ring N base adenine (A) guanine (G) pyrimidines single ring N base cytosine (C) thymine (T) uracil (U) 2005-2006
Building the polymer 2005-2006
Nucleic polymer Backbone sugar to PO4 bond phosphodiester bond new base added to sugar of previous base polymer grows in one direction N bases hang off the sugar-phosphate backbone Why is this important? 2005-2006
RNA & DNA RNA DNA single nucleotide chain double nucleotide chain N bases bond in pairs across chains spiraled in a double helix double helix 1st proposed as structure of DNA in 1953 by James Watson & Francis Crick
Pairing of nucleotides Nucleotides bond between DNA strands H bonds purine :: pyrimidine A :: T 2 H bonds G ::: C 3 H bonds The 2 strands are complementary. One becomes the template of the other & each can be a template to recreate the whole molecule. Why is this important? 2005-2006
Information polymer Function series of bases encodes information like the letters of a book stored information is passed from parent to offspring need to copy accurately stored information = genes genetic information All other biomolecules we spoke about served physical or chemical functions. DNA & RNA are information storage molecules. DNA well-suited for an information storage molecule: chemically stable stores information in the varying sequence of nucleotides (the genetic code) its coded sequence can be copied exactly by the synthesis of complementary strands; easily unzipped & re-zipped without damage (weak H bonds) damage to one strand can be repaired by addition of bases that match the complementary strand 2005-2006
Isn’t this a great illustration!? 2005-2006
Why is it important that the strands are bonded by H bonds? DNA molecule Double helix H bonds between bases join the 2 strands A :: T C ::: G Why is it important that the strands are bonded by H bonds? H bonds = biology’s weak bond • easy to unzip double helix for replication and then re-zip for storage • easy to unzip to “read” gene and then re-zip for storage
Copying DNA Replication 2 strands of DNA helix are complementary have one, can build other have one, can rebuild the whole why is this a good system? when in the life of a cell does replication occur? when cells divide, they must duplicate DNA exactly for the new “daughter” cells Why is this a good system? mitosis meiosis
DNA replication “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” James Watson Francis Crick 1953 The greatest understatement in biology! 2005-2006
Watson and Crick … and others… 1953 | 1962 Watson and Crick … and others… Discovered & published in 1953 Nobel Prize in 1962: Watson, Crick, Wilkins 2005-2006
1953 | 1962 Maurice Wilkins… and…
Rosalind Franklin (1920-1958) A chemist by training, Franklin had made original and essential contributions to the understanding of the structure of graphite and other carbon compounds even before her appointment to King's College. Unfortunately, her reputation did not precede her. James Watson's unflattering portrayal of Franklin in his account of the discovery of DNA's structure, entitled "The Double Helix," depicts Franklin as an underling of Maurice Wilkins, when in fact Wilkins and Franklin were peers in the Randall laboratory. And it was Franklin alone whom Randall had given the task of elucidating DNA's structure. The technique with which Rosalind Franklin set out to do this is called X-ray crystallography. With this technique, the locations of atoms in any crystal can be precisely mapped by looking at the image of the crystal under an X-ray beam. By the early 1950s, scientists were just learning how to use this technique to study biological molecules. Rosalind Franklin applied her chemist's expertise to the unwieldy DNA molecule. After complicated analysis, she discovered (and was the first to state) that the sugar-phosphate backbone of DNA lies on the outside of the molecule. She also elucidated the basic helical structure of the molecule. After Randall presented Franklin's data and her unpublished conclusions at a routine seminar, her work was provided - without Randall's knowledge - to her competitors at Cambridge University, Watson and Crick. The scientists used her data and that of other scientists to build their ultimately correct and detailed description of DNA's structure in 1953. Franklin was not bitter, but pleased, and set out to publish a corroborating report of the Watson-Crick model. Her career was eventually cut short by illness. It is a tremendous shame that Franklin did not receive due credit for her essential role in this discovery, either during her lifetime or after her untimely death at age 37 due to cancer.
Interesting note… Ratio of A-T::G-C affects stability of DNA molecule 2 H bonds vs. 3 H bonds biotech procedures more G-C = need higher T° to separate strands high T° organisms many G-C parasites many A-T (don’t know why) At the foundation of biology is chemistry!!
Another interesting note… ATP Adenosine triphosphate modified nucleotide adenine ribose + Pi + Pi + Pi + + 2005-2006
Macromolecule Review
Carbohydrates Structure / monomer Function Examples monosaccharide energy raw materials energy storage structural compounds Examples glucose, starch, cellulose, glycogen glycosidic bond
Lipids Structure / building block Function Examples glycerol, fatty acid, cholesterol, H-C chains Function energy storage membranes hormones Examples fat, phospholipids, steroids ester bond (in a fat)
Proteins Structure / monomer Function Examples amino acids levels of structure Function enzymes u defense transport u structure signals u receptors Examples digestive enzymes, membrane channels, insulin hormone, actin peptide bond
Nucleic acids Structure / monomer Function Examples nucleotide information storage & transfer Examples DNA, RNA 2005-2006 phosphodiester bond