Identify the key structural features of a DNA molecule.
The backbone of DNA is made of a sugar and a phosphate molecule.
Strong ionic bonds and hydrophobic interaction hold DNA together.
DNA contains the nucleotide bases adenine, thymine, guanine, cytosine, and uracil.
DNA strands are antiparallel and include a 5′ end and a 3′ end.
DNA is most often found as a left-handed helix, commonly referred to as A-DNA.
DNA bases are always paired using Watson-Crick base pairing.
The Correct Answer and Explanation is:
The correct key structural features of a DNA molecule are:
✅ The backbone of DNA is made of a sugar and a phosphate molecule.
✅ DNA strands are antiparallel and include a 5′ end and a 3′ end.
✅ DNA bases are always paired using Watson-Crick base pairing.
Explanation (300+ words):
DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. It is structured as a double helix, consisting of two long strands of nucleotides twisted around each other. Each nucleotide includes a phosphate group, a five-carbon sugar called deoxyribose, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C).
A key structural feature is the sugar-phosphate backbone, which forms the outer framework of each DNA strand. This backbone is created through covalent bonds between the phosphate of one nucleotide and the sugar of the next. This structure provides stability and orientation to the DNA molecule.
The strands of DNA are antiparallel, meaning one runs in a 5′ to 3′ direction and the other in 3′ to 5′. The numbers refer to the carbon positions in the sugar molecule, which are essential for enzymes that replicate or transcribe DNA.
The two strands are held together by hydrogen bonds, not ionic bonds or hydrophobic interactions, between the nitrogenous bases. These bases pair specifically: adenine pairs with thymine (A-T) using two hydrogen bonds, and guanine pairs with cytosine (G-C) using three hydrogen bonds. This base-pairing rule is known as Watson-Crick base pairing, named after the scientists who elucidated the DNA structure in 1953.
The statement that DNA is a left-handed helix is incorrect. DNA typically exists as a right-handed helix known as B-DNA, which is the most common form in living cells. A-DNA is another right-handed form found under dehydrated conditions, while Z-DNA is the rare left-handed form.
Also, uracil (U) is not found in DNA—it is found in RNA, replacing thymine.
In conclusion, the sugar-phosphate backbone, antiparallel strands with 5’ and 3’ ends, and Watson-Crick base pairing are all key structural features of DNA.