DNA Has a Structure That Suits Its Function

● ● ● ● Watson and Creek used chemical and physical evidence to deduce the structure of DNA ○ Data on the chemical makeup of DNA alongside X-ray diffraction data on the physical structure were used to solve DNA’s structure Chemical evidence from Base Composition ○ Scientists knew DNA was a polymer of nucleotides ■ Each nucleotide consists of a molecule of sugar deoxyribose, phosphate group, and a nitrogen-containing base ■ The only difference between the four nucleotides of DNA are their nitrogenous bases ● Purines ○ Adenine (A) ○ Guanine (G) ● Pyrimidines ○ Cytosine © ○ Thymine (t) ○ Erwin Chargaff and his colleagues ■ Created the rule ● The amount of A=T ● The amount of G=C ● A+T=C+G Physical Evidence from X-Ray Diffraction ○ The position of atoms in a crystalized substance can be inferred from the diffraction pattern of X-rays passing through the substance ○ Franklins images ■ Suggested that DNA was a double helix with 10 nucleotides in each full turn Watson and Crick’s Model (By using evidence form Franklin and Chargaff) ○ Had the nucleotides on the interior of the two strands ○ Had the sugar-phosphate backbone on the outside ■ Backbone had nucleotides attached by the deoxyribose sugar of one nucleotide and the phosphate on the next nucleotide ○ Two strands ran antiparallel ■ Ribose of one strand was across from a phosphate from the other strand at each end of the molecule ○ Always paired a double ring purine (A or G) on one strand and a single ring pyrimidine (T or C) on the other strand ○ Hydrogen bonds ■ Between the bases on opposite strands ○ Covalent bonds ■ Between the atoms that make up the nucleotides ■ Between the nucleotides in a DNA strand ○ Van der Waals forces ■ ● ● ● ● Between the flat bases that stack on top of each other within the double helix Five Key Features define the DNA structure ○ DNA is a double-stranded helix ■ Has a sugar-phosphate backbone on the outside ■ Base pairs lined up on the inside ○ DNA is a right-handed helix ■ If you curl your hand and put your thumb up the curve of the helix follows your fingers ○ DNA is antiparallel ■ Two strands run in opposite directions ○ DNA strands are held together by complementary base pairing ■ Hydrogen bonding between the bases on opposite sides ■ A=T; C=G ○ DNA has major and minor grooves ■ Outer edges of nitrogen bases are exposed The helix ○ Hydrogen bonds between complementary base pairs ■ A and T are paired by two hydrogen bonds ■ G and C are paired by three hydrogen bonds ○ Van der Waals Forces ■ Between adjacent bases on the same strand ■ When base rings come near each other they tend to stack poker chips because of these weak attractions Antiparallel Strands ○ The backbone of each DNA strand contains repeating units of the five-carbon monosaccharide deoxyribose attached to a phosphate group ○ The phosphate is attached to the 5’ end of the deoxyribose ■ Each phosphate group is connected to the 3’ carbon of the next deoxyribose ○ 5’ end ■ Has a free phosphate group ○ 3’ end ■ Has a free hydroxyl group The double-helical function of DNA is essential to its function ○ The genetic material stores an organism’s genetic information ■ The sequence of base pairs ○ The genetic material is susceptible to mutations in the information it encodes ○ The genetic material is precisely replicated in the cell division cycle ■ Using a complementary base pairing ○ The genetic material is expressed as the phenotype ■ Example: protein