IB Biology · Theme A: Unity and diversity · SL and HL
A1.2 Nucleic acids
A one-page summary of A1.2 Nucleic acids, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How does the structure of nucleic acids allow hereditary information to be stored?
How does the structure of DNA facilitate accurate replication?
What A1.2 covers
Building blocks
- A1.2.1The universal genetic material
- A1.2.2The nucleotide
- A1.2.3The sugar-phosphate backbone
- A1.2.4The bases are the code
- A1.2.5RNA is built the same way
The double helix and its jobs
- A1.2.6The double helix
- A1.2.7DNA compared with RNA
- A1.2.8Job one: replication
- A1.2.8Job two: transcription
- A1.2.9Vast storage
- A1.2.10One shared genetic code
Extension
- A1.2.11 · HLDirectionality: 5′ and 3′
- A1.2.12 · HLPurines, pyrimidines, and helix stability
- A1.2.13 · HLThe nucleosome
- A1.2.14 · HLThe Hershey-Chase experiment
- A1.2.15 · HLChargaff's data
A1.2 Nucleic acids: summary
The nucleotide
- Phosphate + sugar + base
- Base on 1′, phosphate on 5′, —OH on 3′
Backbone & code
- Phosphodiester bonds via condensation
- Base order is the information
DNA vs RNA
- Deoxyribose/ribose; T vs U
- Double- vs single-stranded
The double helix
- A-T (2 H-bonds), C-G (3 H-bonds)
- Antiparallel strands
Replication & code
- Each strand templates its complement
- Genetic code ≈ universal → common ancestry
HL · Structure & evidence
- Strands build 5′→3′ only
- Purine+pyrimidine = constant width
- Nucleosome: DNA + 8 histones
Key terms
- Nucleotide
- phosphate + sugar + base — the monomer of a nucleic acid.
- Phosphodiester bond
- the covalent bond joining one sugar's 3′ to the next nucleotide's 5′ phosphate.
- Condensation reaction
- a reaction that joins two molecules and releases water.
- Backbone
- the alternating chain of sugars and phosphates running the length of a strand.
- Gene
- a DNA sequence whose base order codes for one specific product.
- 5′ end / 3′ end
- the two different ends of a strand — free phosphate vs free —OH.
- Purine
- a base with two fused rings: adenine or guanine.
- Pyrimidine
- a base with a single ring: cytosine, thymine, or uracil.
- Chromatin
- DNA together with its histones, as it exists during interphase.
- Histone
- a positively charged protein that DNA winds around.
- Nucleosome
- DNA wound around a histone octamer, held by an H1 histone.
- Radioisotope
- a detectable form of an element that behaves normally in reactions.
Sample exam questions
Three of the 50 multiple-choice questions for A1.2. Try each one before opening the answer.
Question 1. What is the primary role of nucleic acids in living organisms?
- Providing structural support to the cell wall
- Storing and transmitting genetic information
- Catalysing the metabolic reactions of the cell
- Storing chemical energy for use in respiration
Show the answer
Answer: B. DNA (and RNA) store hereditary information as a sequence of bases and pass it on to new cells and offspring.
Question 2. Complementary base pairing is important because it allows DNA to:
- store chemical energy for the cell to use
- act as a template so each strand is copied
- speed up the metabolic reactions of the cell
- form a structural part of the cell membrane
Show the answer
Answer: B. Because A pairs only with T and G only with C, each strand specifies the sequence of its partner, which is the basis of accurate copying.
Question 3. A sample of double-stranded DNA contains 150 cytosine bases, which make up 15% of all the bases in the molecule. How many phosphate groups does the molecule contain?
- 150
- 1000
- 300
- 500
Show the answer
Answer: B. Each nucleotide contains exactly one phosphate, so the number of phosphates equals the number of nucleotides, which equals the total number of bases. If 150 cytosine bases are 15% of the total, the total is 150 divided by 0.15 = 1000.
Linking questions
Questions that connect A1.2 to other parts of the course, the kind that come up in Paper 2.
- How is the base sequence copied when DNA replicates before cell division? (see D1.1)
- How is the information in a DNA base sequence used to build a specific protein? (see D1.2)
- How do mutations change the base sequence, and what can be the consequences? (see D1.3)
- How do some viruses use RNA instead of DNA as their genetic material? (see A2.3)
Practise A1.2
Study notes, every question and full markschemes for A1.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.