Question

In a DNA double helix, guanine and cytosine bases, shown above are paired together by:

Answer 1

We know that there are four bases in DNA. They are adenine (A), Guanine (G), cytosine (C) and Thymine (T) which are present in the two strands of DNA. They are paired by a strong chemical bond. Here, we have to identify the chemical bond that pairs guanine and cytosine.

Complete step by step answer:
The structures of the bases of DNA are as follows:

In the DNA double helix structure, adenine pairs with the thymine by two hydrogen bonds and guanine paired with cytosine by 3 hydrogen bonds.

Therefore, by hydrogen bonds guanine and cytosine are paired.

Additional Information:
The differences between DNA and RNA are:

RNA	DNA

1. It consists of single stranded structure (exception to some virus)| 1) It consists of double stranded structure (exception to some virus)
   2)Have ribose sugar| 2) Have deoxyribose sugar
2. The pairing of adenine and uracil in RNA| 3) The pairing of adenine and thymine in DNA
3. It is genetic material to some viruses| 4) It is genetic material in all living organisms
4. It occurs in the form of three types in an organism, mRNA, rRNA and tRNA| 5) Only one form is present in an organism
5. Its length is short comprising few thousands nucleotides | 6) Its length is quite large comprising millions of nucleotides

Note: DNA consists of two strands which are wind around each other like that of a twisted ladder. Each strand of DNA has backbone made of sugar and phosphate groups. Each sugar is attached to four bases, adenine, guanine, thymine and cytosine. The hydrogen bonding between bases of two strands holds the two strands. The adenine forms base pairs with thymine and cytosine forms base pairs with guanine.