Molecular Biology 2.6 Structure of DNA and RNA

Nucleic Acids The nucleic acids DNA and RNA are polymers of nucleotides

Nucleic Acids  Nucleotides consist of 3 parts:  A sugar  5 carbon sugars = pentose sugars  A base  contains nitrogen  1 or 2 rings of atoms  1 ring = Pyrimidine  2 rings = Purine  A phosphate group  negatively charged  acidic

Nucleic Acids  Covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next.  This creates a “backbone” of alternating sugar and phosphate.

Nucleic Acids  The bonds holding the phosphate group to the sugar and the base to the sugar are both products of condensation reactions.  Water is eliminated when they form.  The oxygen to form the water has come from the sugar's -OH groups.

Nucleic Acids DNA differs from RNA in the number of strands normally present, the base composition and the type of pentose

Make a chart DNA  # of strands  Base composition  Sugar RNA

Nucleic Acids  DNA has 2 polymers of nucleotides (double-stranded)  RNA has 1 polymer (single-stranded)

Nucleic Acids  The sugar within DNA is deoxyribose  The sugar with RNA is ribose Draw both sugars. Compare the structures. Predict the reason for the difference in their names. Compare to the previously examined carbohydrate sugars.

Nucleic Acids  There are 4 different bases for each type of nucleic acid  These can be linked in any sequence Draw the 2 generic base types. Complete the “data- based question” p109

Nucleic Acids  Data-based #1

Nucleic Acids  Data-based question #2

Nucleic Acids  Data-based #3 and #4

Nucleic Acids The amino acid sequence of polypeptides is coded for by genes  The sequence of nucleotide bases provides the code  3 bases of the gene are needed to code for each amino acid More about this later

Nucleic Acids  The variety of combinations of bases is the key to nucleic acids acting as a store of genetic information.

Nucleic Acids DNA  Pyrimidines =  Thymine (T) and Cytosine (C)  Purines =  Adenine (A) and Guanine (G)

Nucleic Acids RNA  Pyrimidines =  Uracil (U) and Cytosine (C)  Purines =  Adenine (A) and Guanine (G) Note the change from T to U

Nucleic Acids DNA is a double helix made of 2 antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs

Nucleic Acids  The strands are held together by hydrogen bonds between the complementary base pairs.  Adenine always pairs with Thymine  Guanine always pairs with Cytosine  The 2 strands are parallel but run in opposite directions  one runs 5’ to 3’  the other 3’ to 5’

Nucleic Acids Draw simple diagrams of DNA using circles, pentagons, and rectangles to represent phosphates, pentoses, and bases.  The base is attached to the C 1  The phosphate is linked to the C 5  The phosphate of another nucleotide will link to the C 3

Review quiz  esources/cfb/quiz.cfm?qzid=15 esources/cfb/quiz.cfm?qzid=15