Biology 1st Paper (Botany) · Classes 11-12 · Chapter 1 · 1.4

Nucleic Acids and Their Components.

Your hair and your nose came down from your parents. But what carried them? The answer sits in a long chain inside the nucleus of the cell. In this note we take that chain apart: what it is built from, what each piece is called, and who found it and when.

1. Heredity and hereditary material

The passing of traits from parents to offspring is heredity, also called genetic transmission. The branch of biology that studies it is genetics.

The things that carry traits across are together called the hereditary material. Its main component is the chromosome, and the genes lie in order along the DNA of that chromosome.

2. The search for nucleic acid

In 1869 the Swiss physician Friedrich Miescher separated a new substance from the nuclei of white blood cells in wound pus. He named it nuclein.

In 1889 Altman noticed acid properties in nuclein and named it nucleic acid. In 1894 Albrecht Kossel identified the two kinds of nitrogen base (purine and pyrimidine), the sugar and the phosphoric acid, and won the 1910 Nobel Prize in Chemistry for it. In 1921 Levine found that nucleic acid comes in two kinds: DNA and RNA.

Nucleic acid is made of the elements C, H, O, N and P; 15% of it is nitrogen and 10% phosphorus. DNA usually lies in the chromatin of the nucleus. Of the RNA, 90% is in the cytoplasm and the other 10% in the nucleolus.

The acid built as a polymer of countless nucleotides is the nucleic acid. It is made of a nitrogen base, a pentose sugar and phosphoric acid, and it is the largest chemical molecule of the cell. Hydrolysis splits it back into exactly those three components. Open the branches below and see the layers one by one.

3. The three components: sugar, base, phosphate

A five-carbon sugar is a pentose. Nucleic acid is built from the ring forms β-D ribose or β-D deoxyribose. RNA has ribose; DNA has deoxyribose. Deoxy means without oxygen: deoxyribose lacks oxygen on carbon 2, and that is its only difference from ribose.

Bases are of two kinds by their rings. The two-ring purine has the formula C₅H₄N₄. The one-ring pyrimidine has the formula C₄H₄N₂.

There are two purines: adenine (A) and guanine (G). There are three pyrimidines: thymine (T), cytosine (C) and uracil (U). Uracil is only in RNA and thymine only in DNA. The book's memory rule says: the longer the name, the smaller the structure. Purine is the shorter name with the bigger ring; pyrimidine is the longer name with the smaller ring.

The names carry a story. Adenine and thymine are named after the thymus gland, where they were first separated (adeno means gland). Cytosine comes from cyto, meaning cell. Guanine comes from guano, the droppings of bats or seabirds.

The third component is phosphoric acid, formula H₃PO₄. Joined to one pentavalent phosphorus atom are three monovalent hydroxyl groups and one divalent oxygen.

4. From nucleoside to nucleotide

One nitrogen base joined to one pentose makes a glycoside compound called a nucleoside. The join is a glycosidic bond: nitrogen 1 of a pyrimidine, or nitrogen 9 of a purine, attaches to carbon 1 of the pentose.

Ribose gives four nucleosides: adenosine (A), guanosine (G), uridine (U) and cytidine (C). Deoxyribose gives four: deoxyadenosine, deoxyguanosine, deoxycytidine and deoxythymidine.

Build one yourself: pick a sugar and a base, join them, then add a phosphate. A pair that doesn't belong won't join.

Add one phosphate to a nucleoside and you get a nucleotide, a phosphate ester of the nucleoside. It is the building unit of nucleic acid. The phosphate sits on carbon 3 or 5 of the pentose. Ribose nucleotides: AMP, GMP, CMP, UMP. Deoxyribose nucleotides: dAMP, dGMP, dCMP, dTMP. Tap each row to match them up.

Add more phosphate and the nucleotide changes: AMP + P = ADP, then ADP + P = ATP. GMP, CMP and UMP do the same, giving GDP and GTP, CDP and CTP, UDP and UTP.

Nucleotides do more than build the framework of DNA and RNA. NAD⁺ and NADP⁺ work in intermediate metabolism, GTP in protein synthesis, ATP in respiration, and CTP in phospholipid synthesis.

5. Joined into a chain

When carbon 3 of one nucleotide's pentose joins carbon 5 of another through a phosphodiester bond, the result is a dinucleotide.

Many nucleotides joined this way, running 5'→3', make a long linear chain called a polynucleotide. Each single helix of DNA is one polynucleotide chain.

By pentose sugar, nucleic acids are of two kinds: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). One last sort: which piece belongs to which?

6. Check yourself

Three questions in the exam's style. Start with the first.

The second asks you to recall the pieces.

In the last, hold on to the definition.

The whole note on seven cards.

Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.4.