Molecular Genetics : DNA and RNA

DNA Structure

1. DNA (deoxyribonucleic acid) molecules make up the genetic material of living organisms. DNA is built up of many subunits, which are called nucleotides.

2. Each nucleotide consists of three parts:

1. a deoxyribose

2. a phosphate group

3. a nitrogenous base

3. There are 4 types of DNA bases: adenine, guanine, cytosine and thymine. A and G are purines; while C and T are pyrimidines.

4. To form a DNA molecule, nucleotides are linked together. The phosphate group of one nucleotide links to the deoxyribose of the next molecule to form a chain of nucleotides.

5. Because each nucleotide is linked to those on either side of it through carbons 3 and 5, that is why the linkages are called 3’-5’ linkage.

6. While the chain of sugar and phosphate forms the backbone of the DNA molecule, the sequence of bases in the chain will vary and it is this sequence that forms the genetic code determining the characteristics of an organism.

7. Two strands of nucleotides are linked by hydrogen bonds that form between the bases and this double strand makes up the double helix of a complete DNA molecule, a molecular spiral staircase.

8. The hydrogen bonding interactions are specific due to complementary base pairing:

1. Adenine always pairs with thymine (two hydrogen bonds)

2. Cytosine is paired with guanine (three hydrogen bonds)

9. Thus, a purine always pair with a pyrimidine. Because pyrimidines are monocyclic heterocycles and purines are bicyclic heterocycles, pyrimidines are smaller in size than purines. The pairing of a purine with a pyrimdine can ensure that the strands are always the same distance apart. It is worth remember the specificity of base pairing is governed by the 2 different sets of hydrogen-bonding interactions.

10. The result is that the two DNA chains run in opposite directions and are said to be antiparallel.

11. This is so because for the two polynucleotide strands, the deoxyribose moleucles are orientated in opposite directions : one strand runs in a 5’> 3’ direction whereas the other runs in a 3’> 5’ direction.

12. Thus for 2 complementary strands of DNA molecules, the sequences are palindromic. That reads for a given strand, reading the sequence in a certain direction will be the same as reading the other strand's sequence in the opposite direction. 

RNA Structure

1. RNA (ribonucleic acid) is quite similar to DNA except:

1. It contains the 5-carbon sugar ribose instead of deoxyribose,

2. It contains the bases adenine, guanine, cytosine and uracil.

3. It is a single-stranded molecule

2. Complementary base pairing also governs RNA structure,

1. uracil pairs with adenine

2. guanine pairs with cytosine

(To be continued....) 

by Ed Law