DNA as the Genetic Material: Experiments, Structure and Packaging: Quick Revision

A brief note on DNA as the Genetic Material: Experiments, Structure and Packaging. Read it before a test. Try each question at the end before you open its answer.

The three experiments

• Griffith (1928): Streptococcus pneumoniae. S cells have a capsule and kill mice. R cells have none. Heat-killed S cells with live R cells killed the mouse, and live S cells were found. A transforming principle had passed to the R cells.
• Avery, MacLeod and McCarty: protease and RNase did not stop the transformation. DNase did. So the principle is DNA.
• Hershey and Chase (1952): phages with 35S in the protein and 32P in the DNA. A blender and a centrifuge. The 32P was in the bacteria, and the 35S stayed in the liquid. So DNA enters the cell.
• A genetic material must copy itself, be stable, allow slow change and express itself. RNA is less stable, because of the 2′ OH.

The structure of DNA and RNA

• Nucleotide: a base, a sugar and a phosphate. A nucleoside is a base and a sugar.
• Purines: A and G. Pyrimidines: C, T and U.
• Bonds: N-glycosidic (base to sugar), phosphoester (sugar to phosphate), phosphodiester (between nucleotides, 3′ to 5′).
• Chargaff: A equals T, G equals C, purines equal pyrimidines. This holds only for a double strand.
• Double helix: antiparallel, right-handed. A-T has 2 hydrogen bonds. G-C has 3 hydrogen bonds. 0.34 nm between base pairs, 10 base pairs in a turn, pitch 3.4 nm.
• RNA: ribose, uracil, usually one strand.

Packaging

• Length of DNA: base pairs times 0.34 nm. A human cell has about 2.2 m.
• Bacteria: nucleoid, loops held by proteins, supercoiled.
• Eukaryotes: a core of 8 positively charged histones (rich in lysine and arginine), with about 200 base pairs of negative DNA wound round it, makes a nucleosome. Beads on a string make chromatin. The NHC proteins help to pack it further.
• Euchromatin: loose and active. Heterochromatin: dense and inactive.

A worked example

A DNA has 22 per cent guanine. What per cent is adenine?

1. C equals G, so G and C together are $44\%$.
2. The rest, $56\%$, is shared by A and T, so A is $28\%$.

Practice with new numbers

A program to try

The program finds the length of a human cell's DNA from its base pairs.

Mistakes to avoid

• Saying that Griffith found that DNA is the genetic material. He found a transforming principle.
• Mixing up 32P and 35S. Phosphorus is in DNA. Sulphur is in protein.
• Applying Chargaff's rule to a single strand.
• Saying that histones are negative. They are positive.

Check yourself

1. Which enzyme stopped the transformation in the experiment of Avery, MacLeod and McCarty?

2. How many hydrogen bonds does a G-C pair have?

3. What is a nucleosome?

4. Which chromatin is active, loose or dense?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 5, Molecular Basis of Inheritance.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 14, DNA Structure and Function.