Nucleic Acids, Metabolism and the Living State: Quick Revision

A brief note on Nucleic Acids, Metabolism and the Living State. Read it before a test. Try each question at the end before you open its answer.

Nucleic acids

• A nucleotide is a nitrogenous base, a pentose sugar and a phosphate group. Base plus sugar is a nucleoside.
• Purines: adenine and guanine. Pyrimidines: cytosine, thymine and uracil.
• DNA: deoxyribose and thymine, usually double. RNA: ribose and uracil, usually single.
• Double helix (Watson and Crick): antiparallel strands; A with T by two hydrogen bonds, G with C by three.
• Pitch $3.4$ nm; 10 base pairs a turn; $0.34$ nm between base pairs.
• Chargaff: adenine equals thymine, and guanine equals cytosine.

Bonds between monomers

• Every bond that joins two monomers gives off one molecule of water. Hydrolysis breaks it by adding water.
• Peptide bond: amino acids. Glycosidic bond: sugars. Ester bond: glycerol and fatty acids. Phosphodiester bond: nucleotides.
• A chain of $n$ monomers has $n - 1$ bonds.

Metabolism and the living state

• Metabolism is all the reactions of life, in pathways of enzyme steps. Glucose to pyruvic acid takes ten steps.
• Anabolic: builds and uses energy. Catabolic: breaks down and releases energy, which is stored in ATP.
• ATP is adenosine triphosphate, the energy currency.
• Metabolic flux: the rate of flow through a pathway.
• A closed system reaches equilibrium, with no net flow and no work. A living cell is an open system in a steady state away from equilibrium.

A worked example

A piece of DNA has $600$ base pairs, and $45$ per cent of its bases are guanine or cytosine. How many hydrogen bonds hold its strands together?

1. Guanine-cytosine pairs. $\dfrac{45}{100} \times 600 = 270$ pairs, with three bonds each.
2. Adenine-thymine pairs. $600 - 270 = 330$ pairs, with two bonds each.
3. All the bonds. $3 \times 270 + 2 \times 330 = 810 + 660 = 1470$.

Practice with new numbers

A program to try

The program adds up the hydrogen bonds between a short strand and its partner.

Mistakes to avoid

• Pairing A with G. A pairs with T, and G pairs with C.
• Two bonds for G and C. They have three.
• Thymine in RNA. RNA has uracil.
• One bond for every monomer. A chain of $n$ monomers has $n - 1$ bonds.
• A steady state is not an equilibrium. A living cell keeps material flowing.

Check yourself

1. Which sugar is found in DNA?

2. How many hydrogen bonds join guanine and cytosine?

3. What is the bond between two amino acids called?

4. Why is ATP called the energy currency?

5. Is a living cell at equilibrium?