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

DNA: Structure, the Watson-Crick Model and Functions.

Every cell of a species carries the same amount of DNA, written with just four bases: A, T, G and C. Yet across the two strands, why is A always facing T and G always facing C? The answer hides in one small measurement. Let us dig it out.

1. What DNA is and where it sits

DNA is the short form of deoxyribonucleic acid. It holds and carries the hereditary traits of a living thing, which is why it is called the molecular core of life. Its building unit is the nucleotide. It has three ingredients: the sugar deoxyribose (S), the four bases A, G, C and T, and phosphoric acid (P). Every cell of one species holds the same amount of DNA.

In eukaryotic cells DNA is the main component of the chromosomes, and some viruses carry it too. It is usually thread-like, but circular in prokaryotes, mitochondria and chloroplasts. The amount of DNA in a cell is measured in picograms: 1 picogram = 10⁻¹² g.

2. Who found what

The German chemist Robert Feulgen devised the DNA staining method (Feulgen staining) in 1924. In 1950 Erwin Chargaff showed that in the DNA of any organism A equals T and G equals C. This is Chargaff's rule. So half the bases are purines (A, G) and half are pyrimidines (T, C). Then Maurice Wilkins and Rosalind Franklin used X-ray crystallography to give the DNA measures 2.0 nm, 0.34 nm and 3.4 nm. They said it was probably double-stranded, not one strand and not three.

In 1953 J. D. Watson and Francis H. C. Crick proposed the physical model of DNA, a twisted ladder. They were given the Nobel Prize for this model in 1962, together with Wilkins. Tap the six events one by one.

3. Why A goes with T

Watson and Crick did not build the model from nothing. They had six clues, in the order the book lists them. Now you put them in that order.

The logic is one of size. Two purines side by side need more than 2 nm, and two pyrimidines leave less than 2 nm. So one purine and one pyrimidine sit facing opposite ways, and the gap between the strands stays an even 2 nm. A and T join by two hydrogen bonds, G and C by three. The two strands are complementary, not identical.

Now zip them yourself. The top strand is given: drag the handle to close the bottom one like a zip, and count the bonds. The chips below show why a wrong pair doesn't fit.

With Chargaff's rule, knowing the percentage of one base gives you the other three: T = A, and G = C = (100 − 2A) ÷ 2. Change A and watch.

Now match the bond counts and the partners. Pick up a chip and drop it in the right box. One chip does not belong in DNA at all.

4. The double helix, laid out

The DNA molecule is double-stranded, a double helix like a twisted ladder. The book describes it as turned from right to left. The two strands run opposite to each other at an equal distance: one 5'→3', the other 3'→5'.

Each strand is a polynucleotide chain, built from alternating deoxyribose (S) and phosphate (P). Each rung between the strands is a pair of nitrogen bases, A=T or G≡C. Phosphate joins carbons 3' and 5' of deoxyribose, and the base joins carbon 1'. So the phosphate lies outside and the bases inside.

Base pairs form between O-HN, NH-N and NH-O of neighbouring bases. C and G can make all three, A and T make two.

The measures: each turn is 34 Å (3.4 nm) long with 10 steps, and the distance between two steps is 3.4 Å (0.34 nm). The helix is 20 Å (2 nm) wide. Each full turn makes one deep groove (major groove) and one shallow groove (minor groove). The molecular weight of DNA lies between 10⁶ and 10⁹. In the two drawings below, tap a dot or a chip and match each part with its measure.

5. Chemical make-up and jobs

DNA's chemical make-up has three things: the sugar deoxyribose, phosphoric acid and four nitrogen bases. Hydrolysing a nucleotide gives phosphoric acid and a nucleoside.

DNA does several jobs. It is the material of the chromosome and the molecular basis of heredity. It holds, controls and expresses traits and passes them to the next generation. It controls physiological work. It is also the basis of mutation, and it can repair its own damage.

6. Triplet, codon and the master molecule

The triplet is the basic unit of genetic information, and each triplet names one specific amino acid. On mRNA, the three consecutive bases complementary to a DNA triplet are called a codon.

DNA is the carrier of heredity because it makes exact copies of itself, makes specific proteins and carries all the biological signals. Its structure is stable and changes only by mutation. It also sends signals, and through mutation it gives variation.

DNA controls all the chemical reactions of the cell, so it is called the master molecule.

7. Check yourself

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

For the second, use the logic from this note.

In the last, match the numbers and names together.

The whole note on seven cards.

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