How is the structure of DNA suited to storing and copying genetic information?
The structure of DNA as a double-stranded antiparallel molecule of nucleotides, with complementary base pairing and a sugar-phosphate backbone, and the organisation of the genome in eukaryotes and prokaryotes.
An SQA Higher Biology answer on the structure of DNA, covering nucleotides, the antiparallel double helix, complementary base pairing, the sugar-phosphate backbone, and how the genome is organised in prokaryotes and eukaryotes.
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What this dot point is asking
The SQA wants you to describe DNA as a double-stranded molecule built from nucleotides, explain the antiparallel arrangement of the two strands and the rules of complementary base pairing, and describe how the genome is organised differently in prokaryotes and eukaryotes.
Nucleotides
Nucleotides join together through their sugar and phosphate groups to form a single strand. The phosphate of one nucleotide bonds to the deoxyribose sugar of the next, creating a strong sugar-phosphate backbone that runs the length of the strand. The bases project sideways from this backbone. Because each link is a sugar-to-phosphate bond, the backbone is chemically uniform and stable, which suits DNA to its job of storing information unchanged across many cell divisions.
The four bases fall into two chemical groups. Adenine and guanine are purines, which have a double-ring structure, while thymine and cytosine are pyrimidines, which have a single ring. A purine always pairs with a pyrimidine, which keeps the width of the double helix constant.
The double helix and base pairing
DNA is double-stranded. The two strands are held together by hydrogen bonds between the bases, following strict complementary base pairing:
The two strands are antiparallel: they run in opposite directions. One strand runs from its 3 prime end to its 5 prime end while the other runs 5 prime to 3 prime. The 3 prime and 5 prime labels refer to the carbon numbers on the deoxyribose sugar, and they set the direction in which a strand is read and copied. The antiparallel arrangement matters because the enzymes that copy and read DNA can only work in one direction along a strand.
Because A pairs with T and G with C, the amount of adenine in a double-stranded sample always equals the amount of thymine, and guanine equals cytosine. This is Chargaff's rule, and it lets you calculate any base percentage from the others.
Organisation of the genome
The genome is made up of sequences that code for proteins (genes) and sequences that do not code for proteins. Non-coding sequences are not junk: some regulate transcription (acting as switches that turn genes on or off), while others are transcribed into functional RNA molecules such as tRNA and rRNA that are essential for making proteins. In humans, only about 1.5 percent of the genome codes for protein, and the rest is non-coding.
The packaging differs between cell types:
- Prokaryotes (such as bacteria) have a single circular chromosome in the cytoplasm, often with additional small circular plasmids that carry a few extra genes.
- Eukaryotes have linear chromosomes held in the nucleus, and also carry circular chromosomes in the mitochondria and, in plants, the chloroplasts.
Examples in context
Example 1. The Human Genome Project. The international Human Genome Project (completed in 2003) sequenced all roughly 3.2 billion base pairs of the human genome. It confirmed that humans have only about 20,000 protein-coding genes, far fewer than expected, and that the great majority of the genome is non-coding. This work depended directly on understanding DNA as a double-stranded molecule with complementary base pairing, because the sequence of one strand can be used to check the sequence of the other.
Example 2. DNA stability and forensic samples. The strong sugar-phosphate backbone and the hydrogen bonds between paired bases make DNA stable enough to survive for years in dried blood or bone. Forensic scientists rely on this stability to recover and analyse DNA from old crime scenes, because the complementary base-pairing rule allows even a partial, single-stranded fragment to be matched and rebuilt.
Try this
Q1. Name the three components of a DNA nucleotide. [1 mark]
- Cue. Deoxyribose sugar, phosphate, and a base.
Q2. A DNA strand reads ATGCCA. Write the complementary strand. [1 mark]
- Cue. Apply A-T and G-C pairing: TACGGT.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20193 marksDescribe the structure of a DNA nucleotide and explain how nucleotides join to form a single DNA strand.Show worked answer →
A 3-mark describe-and-explain answer needs the three components and the linkage.
A nucleotide is made of a deoxyribose sugar, a phosphate group and one of four bases (adenine, thymine, guanine or cytosine). Markers award one mark for naming all three components.
Nucleotides join when the phosphate of one nucleotide bonds to the deoxyribose sugar of the next. Repeating this creates the strong sugar-phosphate backbone that runs along the strand. The bases project sideways from the backbone.
Markers reward (1) the three components, (2) the sugar-to-phosphate linkage and (3) the idea of a repeating sugar-phosphate backbone.
SQA Higher 20214 marksA sample of double-stranded DNA contains 22 percent adenine. Determine the percentage of guanine, and explain the base-pairing rule that allows this calculation.Show worked answer →
This is a Chargaff base-ratio calculation. Use complementary base pairing: A pairs with T and G pairs with C.
Step 1. Because A pairs with T, the percentage of thymine equals the percentage of adenine, so .
Step 2. Adenine plus thymine together is .
Step 3. The remaining bases must be guanine plus cytosine, so .
Step 4. Because G pairs with C in equal amounts, guanine is half of this: .
So guanine is 28 percent. Markers reward the correct rule (A with T, G with C), correct working and the final value with a percent sign.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)