Lesson 1 of 13 · 8 min

The DNA: structure of polynucleotides

NCERT § "The DNA"

Your β-globin gene is about 1,600 base pairs long. A base pair is a physical object you could, in principle, count one by one — so what exactly is being counted?

The story this chapter follows: One gene: β-globin

Every red blood cell you have was built using instructions from a single gene called β-globin, sitting on chromosome 11. This chapter opens that one gene end to end: how its DNA is built and packed, copied, read into RNA, translated into protein, and what happens when one base in it changes.

The lesson in notes

In short

DNA is a long polymer of deoxyribonucleotides, and its length is usually expressed as the number of nucleotides or base pairs. For example, bacteriophage φ×174 has 5386 nucleotides; bacteriophage lambda, 48502 bp; E. coli, 4.6 × 10⁶ bp; and the haploid human genome, 3.3 × 10⁹ bp.

A nucleotide has three parts: a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA) and a phosphate group.

The bases are purines (adenine and guanine) and pyrimidines (cytosine, thymine and uracil). Cytosine is common to DNA and RNA, thymine occurs in DNA, and uracil replaces thymine in RNA; thymine is 5-methyl uracil.

A base joins the 1′ carbon of the sugar through an N-glycosidic bond to form a nucleoside. Adding a phosphate to the 5′-OH through a phosphoester bond makes a nucleotide.

Nucleotides are joined by 3′–5′ phosphodiester bonds into a polynucleotide chain, which has a free phosphate at its 5′ end and a free 3′-OH at its 3′ end. Sugar and phosphate form the backbone; bases project from it.

In RNA every nucleotide has an extra -OH at the 2′ position of ribose.

Friedrich Miescher first identified DNA in 1869 as an acidic substance in the nucleus and called it 'nuclein'.