Which bases pair together in DNA, and how many hydrogen bonds link each pair?

Adenine pairs with thymine through two hydrogen bonds. Cytosine pairs with guanine through three hydrogen bonds.

Describe how the two strands of a DNA molecule are arranged.

Two sugar-phosphate backbones run in opposite directions, so the strands are antiparallel. Hydrogen bonds between paired bases hold the strands together, and the pair twists into a double helix.

Why is the width of the DNA double helix the same all along its length?

A large two-ring base, adenine or guanine, always pairs with a small one-ring base, thymine or cytosine. Every rung of the ladder is therefore the same length.

CCEA A-Level Life and Health Sciences 0008

Experimental Techniques

Topic overview Standard solutions, titrations and qualitative tests Rates of reaction and organic preparations Physics measurements: oscillations, resistivity, lenses and critical angle Biological tests, colorimetry, chromatography and osmosis

Human Body Systems

Topic overview Blood vessels, the heart and cardiac output Heart sounds, ECG traces, pulse and blood pressure Body fluids, haemoglobin and the Bohr effect The respiratory system, gas exchange and lung disease Monitoring breathing and lung volumes Cellular respiration and ATP Homeostasis, blood glucose and thyroxine Sodium, blood pH and oxygen saturation A balanced diet and nutritional needs through life Cholesterol, minerals, vitamins, alcohol and exercise

Physical Chemistry in Industrial Processes

Topic overview Moles, reacting masses and percentage yield Titration types, indicators and standard solutions Titration calculations and molarity Enthalpy changes and calorimetry Hess's law and average bond enthalpies Collision theory, rates and the Maxwell–Boltzmann distribution Catalysts, chemisorption and catalytic converters Reversible reactions, the Haber process and the Contact process Scale-up, costs and siting a chemical plant

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Material ScienceDouble Award

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Medicine, Drugs and Clinical TrialsDouble Award

Topic overview Categories of medicine and routes to drug discovery Medicines from natural sources, drug names and the ATC system Drug development, preclinical testing and the therapeutic index Phases of clinical trials and trial design Licensing, UK regulators, Yellow Card and trial ethics Formulations, routes of administration and drug absorption Distribution, metabolism, elimination and sites of drug action Quantitative analysis of medicines and antibacterial bioassays

Scientific Method, Investigation, Analysis and Evaluation

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Sound and LightA2 option

Topic overview Transverse and longitudinal waves and wave graphs Phase difference and the electromagnetic spectrum Outer, middle and inner ear Sound intensity and the decibel scale Standing waves, harmonics and resonance tubes Hearing response, phons and equal loudness curves Structure of the eye, rods and cones Accommodation, lens power and correcting long sight Total internal reflection and fibre-optic communication Dipole antennas, wireless networks and radar Attenuation of radio waves and the Doppler effect

Genetics, Stem Cell Research and CloningA2 option

Topic overview DNA structure and the evidence for the double helix RNA, genes and the triplet code Genotype, phenotype and monohybrid inheritance Dihybrid crosses, epistasis and the chi-squared test DNA replication and the Meselson–Stahl experiment Meiosis and genetic variation Insulin, factor VIII and chymosin from genetically modified microbes Recombinant DNA technology and gene cloning Genetic engineering, food supply and ethics Gene therapy and cystic fibrosis Genetic disease, screening and counselling Genetic fingerprinting and gel electrophoresis Stem cells, therapeutic cloning and their ethics

MicrobiologyDouble Award

Topic overview Bacteria, fungi and protozoa Viruses, viroids and prions Classifying bacteria, fungi and viruses Light and electron microscopy Laboratory safety, biocontainment and quality assurance Aseptic technique, growth media and inoculation Counting and measuring microbial growth Growth requirements and growth inhibition Antibiotics, PCR, gene therapy and pharmaceutical microbiology The gut microbiome in health and disease

Oral Health and DentistryDouble Award

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Histology and PathologyDouble Award

Topic overview Specimen processing, data protection and safety law Clinical biochemistry tests and electrophoresis Blood smears and identifying blood cells Blood grouping, cross-matching and transfusion Diagnostic and surveillance work in hospital microbiology Preparing and staining tissue sections Recognising tissues, atheroma and emphysema on slides

Analytical Chemistry TechniquesDouble Award

Topic overview Volumetric analysis in industry and healthcare Applied titrations of vinegar, indigestion tablets and bleach Redox titration of iron tablets and evaluating titrations Principles of colorimetry Colorimetric analysis of copper ore, aspirin and iron Thin-layer chromatography and its uses Visualising chromatograms and identifying painkillers and pigments Calorimetry and combustion Molar enthalpy and bond energy calculations Qualitative tests for cations and gases

Enabling TechnologyDouble Award

Topic overview ECG and EEG machines in patient monitoring Ultrasound and IVF equipment in fertility treatment Prosthetic limbs, hearing aids and cochlear implants Workflow and equipment in a medical laboratory Ongoing monitoring of a long-term condition Health data collection, databases and data protection

Genetics, Stem Cell Research and Cloning

DNA structure and the evidence for the double helix

3 things to know
A2 5

Overview

Nucleotides and the sugar–phosphate backbone, base pairing and hydrogen bonds, the stability of the double helix, Chargaff's rule with worked percentages, and the work of Franklin, Wilkins, Watson and Crick.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why is DNA described as an information-carrying molecule?

Key term

Name the three components of a DNA nucleotide.

Key term

How are nucleotides joined to form a single strand of DNA?

Questions from this subtopic

Each one with the model answer underneath.

1
Why is DNA described as an information-carrying molecule?
Model answer
DNA stores information as the order of its bases along each strand. The base sequence determines the structure of the proteins a cell makes, including its enzymes.
2
Name the three components of a DNA nucleotide.
Model answer
A DNA nucleotide contains the sugar deoxyribose, a phosphate group and a nitrogen-containing base. The base is adenine, cytosine, guanine or thymine.
3
How are nucleotides joined to form a single strand of DNA?
Model answer
The phosphate of one nucleotide bonds to the deoxyribose of the next in a condensation reaction. The repeated sugar and phosphate groups form a sugar-phosphate backbone with the bases sticking out.
4
Which bases pair together in DNA, and how many hydrogen bonds link each pair?
Model answer
Adenine pairs with thymine through two hydrogen bonds. Cytosine pairs with guanine through three hydrogen bonds.
5
Describe how the two strands of a DNA molecule are arranged.
Model answer
Two sugar-phosphate backbones run in opposite directions, so the strands are antiparallel. Hydrogen bonds between paired bases hold the strands together, and the pair twists into a double helix.
6
Why is the width of the DNA double helix the same all along its length?
Model answer
A large two-ring base, adenine or guanine, always pairs with a small one-ring base, thymine or cytosine. Every rung of the ladder is therefore the same length.

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