Question Video: Composing a Complementary Sequence to a Strand of DNA | Nagwa Question Video: Composing a Complementary Sequence to a Strand of DNA | Nagwa

Question Video: Composing a Complementary Sequence to a Strand of DNA Biology • First Year of Secondary School

A single strand of DNA has the following sequence: 5′-ATTATTGCGC-3′. Reading 3′ to 5′ on the complementary strand, what should the sequence of DNA bases be? You do not need to include the 3′ or 5′.

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Video Transcript

A single strand of DNA has the following sequence: five prime to three prime ATTATTGCGC. Reading three prime to five prime on the complementary strand, what should the sequence of DNA bases be? You do not need to include the three prime or five prime.

This question gives us the DNA sequence of one strand and asks us to find the DNA sequence of the complementary strand. Let’s start by considering what the letters in a DNA sequence actually represent. DNA is a polymer composed of monomers called nucleotides. Each nucleotide has a phosphate group, a deoxyribose sugar, and a nitrogenous base. The four types of nitrogenous bases found in DNA are adenine, guanine, cytosine, and thymine, which are usually represented by their initials. The deoxyribose sugars and phosphate groups of nucleotides are arranged along the outside of the DNA molecule in a structure called the sugar-phosphate backbone.

The particular order of these bases along the sugar-phosphate backbone contains the genetic information stored by DNA. So the term DNA sequence refers to the order of the nitrogenous bases along a strand of DNA. And we notate this order using the initials of the bases. Next, let’s consider what it means for two strands of DNA to be complementary. DNA is a double-stranded nucleic acid, and the two strands are held together by hydrogen bonds between their bases. The bases follow certain rules when bonding with each other, called base pairing rules.

In DNA, adenine always bonds to thymine, and guanine always bonds to cytosine. You may remember that the carbon atoms of pentose sugars are numbered one through five. The phosphate groups of the sugar-phosphate backbone bond to the third and fifth carbon atoms of deoxyribose. Because of this bonding pattern, we call this end of the DNA molecule the five prime end, and we call the other end the three prime end. As you can see in the diagram, the two strands of DNA are oriented in opposite directions. This opposite arrangement of DNA strands is known as antiparallel. So two strands of DNA are said to be complementary when their nitrogenous bases follow base pairing rules and they’re antiparallel.

Returning to our question, we have the base sequence ATTATTGCGC. The question asks us for the complementary sequence read from three prime to five prime, which is the direction that the antiparallel DNA strand would run. Following base pairing rules, adenine always bonds to thymine. So the complementary sequence will have an A when the given sequence has a T and a T where the given sequence has an A. Guanine always bonds to cytosine. So the complementary sequence will have a G when the given sequence has a C and a C where the given sequence has a G. The complementary sequence of DNA is TAATAACGCG.

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