The chemical bond that holds the bases of the two strands of DNA together is

Step 1: Analyze the question The question asks which bond joins the bases of the two DNA strands to each other. This is about the "rungs" of the DNA ladder, not the backbone.

Step 2: Explain the concept DNA is a double helix made of two polynucleotide strands.

• Along each strand (the backbone): Sugar (deoxyribose) and phosphate groups are joined by strong covalent bonds (phosphodiester bonds).
• Between the two strands: The nitrogenous bases pair up through hydrogen bonds: • Adenine (A) pairs with Thymine (T) using 2 hydrogen bonds • Guanine (G) pairs with Cytosine (C) using 3 hydrogen bonds

Hydrogen bonds are weak individually, but there are millions of them in a DNA molecule, so together they keep the helix stable. Because they are weak, the strands can still be "unzipped" during DNA replication and transcription.

Step 3: Eliminate the wrong options

• A. Oxygen bond: This is not a recognised type of chemical bond.
• C. Electrovalent (ionic) bond: This involves transfer of electrons between ions, and it is not what pairs the bases.
• D. Covalent bond: This holds the sugar-phosphate backbone together within one strand, not the bases between strands.

Step 4: Answer The bases of the two strands are held together by hydrogen bonds, which is option B.

Exam tip: Remember "Hydrogen holds Halves together", meaning the two halves of the DNA ladder. Covalent bonds hold the Chain (backbone) of each strand. Also remember A=T (2 bonds) and G≡C (3 bonds).