Nucleic acids — IB Diploma Biology HL
IB Biology A1.2 Nucleic acids: nucleotides, DNA and RNA structure, base pairing, the genetic code, and HL directionality, nucleosomes and key evidence.
Revision notes
7 short notes, in the order of the specification. Each one in short:
DNA is the genetic material of all living organisms; some viruses use RNA, but viruses are not considered living. DNA and RNA are made of nucleotides, each with a phosphate, a pentose sugar and a nitrogenous base. Sugar–phosphate bonding links nucleotides into a strong covalent backbone, and the sequence of bases along it forms the genetic code.
RNA is a polymer formed by condensation of nucleotide monomers: each new bond between the phosphate of one nucleotide and the sugar of the next releases a water molecule. DNA is a double helix of two antiparallel strands of nucleotides, linked by hydrogen bonds between complementary base pairs, with adenine paired to thymine and guanine paired to cytosine.
DNA and RNA differ in three ways examiners look for. DNA is usually double-stranded, while RNA is single-stranded. DNA contains thymine, while RNA contains uracil in its place. DNA contains the sugar deoxyribose, while RNA contains ribose, which has an extra oxygen as an OH group on carbon 2. Examples of RNA include mRNA, tRNA and rRNA.
Complementary base pairing, based on hydrogen bonding, lets genetic information be replicated and expressed: each DNA strand acts as a template for a new strand, mRNA is copied from DNA, and tRNA anticodons pair with mRNA codons. Any length and base sequence is possible, giving DNA limitless storage capacity, and the conserved genetic code is evidence of universal common ancestry.
Nucleic acid strands have directionality because each phosphate links carbon 3' of one sugar to carbon 5' of the next, giving a 5' end and a 3' end. New strands are built only 5' to 3', and ribosomes read mRNA 5' to 3'. Each base pair is a purine with a pyrimidine, so the helix is uniform.
A nucleosome is a DNA molecule wrapped around a core of eight histone proteins, held together by an additional histone protein attached to linker DNA. Nucleosomes package the long DNA molecules of eukaryotic chromosomes. Positively charged amino acids on the histones attract the negatively charged phosphate groups of DNA, which binds the DNA tightly to the protein core.
The Hershey–Chase experiment showed DNA is the genetic material: viruses labelled with radioactive phosphorus in their DNA passed it into bacteria, while radioactive sulfur in their protein stayed outside. Chargaff's data showed that adenine equals thymine and guanine equals cytosine, but base proportions vary between species, falsifying the tetranucleotide hypothesis of a repeating four-base sequence.
Exam questions
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Quick check questions
What are the three components of a nucleotide?
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A phosphate group, a pentose sugar and a nitrogenous base.
State three differences between DNA and RNA.
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DNA has two strands, RNA one; DNA has thymine, RNA uracil; DNA has deoxyribose, RNA ribose.
What does antiparallel mean in DNA?
Show answer
The two strands run in opposite directions.
HL only Why does an A–T pair have the same width as a C–G pair?
Show answer
Each pair is one purine (two rings) bonded to one pyrimidine (one ring).
HL only Which isotope labelled DNA in the Hershey–Chase experiment?
Show answer
Radioactive phosphorus, ³²P.
Frequently asked questions
What is the structure of a DNA nucleotide?
A DNA nucleotide has a phosphate group, the pentose sugar deoxyribose and a nitrogenous base, which is adenine, thymine, guanine or cytosine. The phosphate and base are attached to the sugar on opposite sides. Nucleotides link through sugar–phosphate bonding to form a strong covalent backbone with the bases sticking out.
Why is DNA described as antiparallel?
DNA is described as antiparallel because its two strands run in opposite directions. The strands are held together by hydrogen bonds between complementary bases, adenine with thymine and guanine with cytosine. In diagrams, one strand is drawn upside down relative to the other to show this.
What are the main differences between DNA and RNA?
DNA is double-stranded, while RNA is single-stranded. DNA contains the base thymine, while RNA has uracil instead. DNA contains the sugar deoxyribose, while RNA contains ribose, which has an OH group on carbon 2 where deoxyribose has only hydrogen. Examples of RNA are mRNA, tRNA and rRNA.
Why is the genetic code evidence for common ancestry?
The genetic code is evidence for common ancestry because almost all organisms use the same codons for the same amino acids, with only minor variations. Such an arbitrary code is very unlikely to have arisen many times independently, so the best explanation is that all life inherited it from a universal common ancestor.
How did the Hershey–Chase experiment show DNA is the genetic material?
HL only Hershey and Chase labelled virus protein with radioactive sulfur and virus DNA with radioactive phosphorus. After infection, blending and centrifuging, the phosphorus was inside the bacteria and the sulfur stayed outside. Only DNA entered the bacteria, which then made new viruses, so DNA must carry the genetic instructions.
Written and checked against the IB Biology HL specification · Updated October 2026