OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

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What is the shape of the DNA molecule?

Single strand
Circle
Double helix
Square

The correct answer is: Double helix

The shape of the DNA molecule is a double helix. This structure consists of two long strands of nucleotides twisted around each other, resembling a spiral staircase. The double helix is stabilized by hydrogen bonds between the complementary bases (adenine with thymine and cytosine with guanine) that connect the two strands. This unique shape allows DNA to store genetic information efficiently while also facilitating processes such as replication and transcription. The specific arrangement of the double helix means it can be compacted into chromosomes during cell division, ensuring that genetic material is passed on to new cells correctly. Other shapes mentioned, such as a single strand or circle, do not accurately depict the structure of DNA in its typical form; single-stranded DNA does exist in some contexts, such as during certain viral infections or in some nucleic acid processes, but predominant DNA in organisms is double-stranded. A circular shape describes DNA in prokaryotic organisms, such as bacteria, where the entire molecule is looped, but this is not the typical representation of eukaryotic DNA, which is linear and organized into chromosomes. The square shape is not a representation of DNA at all and has no biological significance in relation to DNA structure.