UNIT 2:  MOLECULAR GENETICS

 

B.  The Structure of DNA

 

The Chemical Composition of DNA

·       by the 1860s, Miescher discovered that nuclein, was slightly acidic and contained large amounts of phosphorous and nitrogen

·       by the 1920s, it was discovered that DNA consisted of three main components:  deoxyribose sugar, a phosphate group that is negatively charged, and a nitrogenous base

·       four nitrogenous bases exist in DNA – adenine (A), guanine (G), cytosine (C), and thymine (T)

·       Figure 2, p. 210 illustrates the four nitrogenous bases

·       DNA is a polymer of nucleotide monomers

·       each monomer consists of a deoxyribose sugar (five-carbon cyclic ring structure) attached to a phosphate group and a nitrogenous base

·       Figure 3, p. 211 illustrates the five-carbon deoxyribose sugar, where each carbon is numbered clockwise

·       the first four of the five carbon atoms, together with an oxygen atom, form a five-membered ring, and the fifth carbon atom extends out from the ring

·       the first carbon is referred to as the “1 prime” carbon, the second is referred to as the “2 prime”, etc.

·       a deoxyribose sugar has a hydroxyl group on the 3’ carbon, and a hydrogen atom on the 2’ carbon

·       the nitrogenous base is attached to the 1’ carbon through a glycosyl bond and the phosphate group is attached to the 5’ carbon by an ester bond

·       Figure 9, p. 214 illustrates these connections

 

The Physical Structure of the DNA Molecule

·       Watson and Crick’s model consists of two antiparallel strands of nucleotides (see Figure 8, p. 214)

·       the bases of one strand are paired with bases of the complimentary strand

·       the bases face inward toward each other, such that a pyrimidine is always paired with a purine

·       A is always paired with T, and C is always paired with G

·       hydrogen bonds form between the base pairs such that two form between A and T, and three form between C and G

·       single hydrogen bonds are generally weak, however, since all of the base pairs running along the sugar-phosphate backbone of the entire structure are held together by H-bonds, their collective strength is high

·       this helps to maintain the stability of the entire molecule which is of extreme importance to the life of an organism

·       one complete helical turn is made every 10 nucleotides

·       the sugar-phosphate backbone adds to the stability of the molecule as well, as it supports the base position and maintains their position in the sequence

·       the two strands run antiparallel – one strand runs in the 5’ to 3’ direction, while the other strand runs in the 3’ to 5’ direction

·       the 3’ end terminates with a hydroxyl group of the deoxyribose sugar, and the 5’ end terminates with the phosphate group (see Figure 9, p. 214)

·       this means that every DNA molecule has an intrinsic directionality – for example, the sequence of bases is indicated in the following way:

5’ – ATGCCGTTA – 3’

3’ – TACGGCAAT – 5’

·         by convention, only the 5’ to 3’ strand is written, since the complimentary strand can be deduced from the pairing nature of the bases

 

Homework:              1-9, p. 216

 

For an excellent tutorial demonstrating the structure of DNA, click on http://www.dnaftb.org/dnaftb/