DNA and anticancer drugsAQA A-Level Chemistry: Revision notes
Section 1
Nucleotides: the building blocks of DNA
A nucleotide is made of three parts joined together: a phosphate ion, the pentose sugar 2-deoxyribose, and one of four bases. The phosphate is bonded to the 2-deoxyribose, and the 2-deoxyribose is bonded to the base.
The four bases are adenine (A), cytosine (C), guanine (G) and thymine (T). The structures of the phosphate ion, 2-deoxyribose and the four bases are given in the Chemistry Data Booklet, so you do not need to memorise them, but you must be able to use them.
Use the Data Booklet for the structures, then describe the order: phosphate - sugar - base.
Section 2
A single strand of DNA
A single strand is a polymer of nucleotides. Nucleotides are joined by covalent bonds between the phosphate group of one nucleotide and the 2-deoxyribose of the next. This gives a sugar-phosphate-sugar-phosphate chain, called the backbone.
The bases are attached to the sugars and project out from the backbone. The order of bases along the chain is the genetic code.
The bases are bonded to the sugars, not to the phosphate groups.
Section 3
The double helix and base pairing
DNA exists as two strands wound into a double helix. The strands are held together by hydrogen bonds between bases on opposite strands.
Hydrogen bonding is specific because of the positions of the hydrogen-bonding groups on each base:
- adenine pairs with thymine (two hydrogen bonds)
- cytosine pairs with guanine (three hydrogen bonds)
This makes the two strands complementary: the base sequence on one strand determines the sequence on the other. Because hydrogen bonds are weak, the strands can separate for copying without breaking the covalent backbones.
The two strands are held by hydrogen bonds, not covalent bonds. In a base-pair question, state which bases pair and how many hydrogen bonds.
Section 4
Cisplatin: a Pt(II) anticancer drug
Cisplatin is the Pt(II) complex cis-Pt(NH₃)₂Cl₂. It is used to treat cancers such as testicular and ovarian cancer.
It works by a ligand replacement reaction with DNA. A chloride ligand is replaced by a nitrogen atom on guanine, which donates its lone pair to platinum and forms a dative covalent bond. Bonded to DNA, cisplatin distorts the helix and the strands cannot separate, so DNA replication is prevented. Cancer cells divide rapidly, so they cannot copy their DNA and stop dividing.
Name the three steps: ligand replaced, bond between Pt and N of guanine, replication prevented.
Section 5
Adverse effects and weighing benefits against harm
Cisplatin cannot tell cancer cells from healthy cells. It also binds to the DNA of healthy cells, especially those that divide quickly, such as bone marrow, hair follicle and gut lining cells. This causes adverse effects including hair loss, nausea and a weakened immune system, and the drug can also damage the kidneys.
Society needs to assess the balance between the benefits and the adverse effects. For a serious or life-threatening cancer, the benefit usually outweighs the harm, and doses are controlled to limit it.
In an evaluate question, always finish with a judgement that weighs benefit against harm.
Must Know
- A nucleotide = phosphate ion + 2-deoxyribose + base (A, C, G or T)
- A strand is a polymer: covalent bonds join phosphate to 2-deoxyribose, with bases on the sugars
- Two complementary strands form a double helix held by hydrogen bonds: A–T and C–G
- Cisplatin replaces a chloride ligand with N on guanine (dative bond), preventing DNA replication
- Adverse effects arise because healthy dividing cells are also affected
That's the notes covered.
Carry on to the next subtopic.
Exam questions on DNA and anticancer drugs
- DNA is the molecule that carries genetic information in the nucleus of a cell. A single strand of DNA is a polymer, built up from repeating units called nucleotides.Describe the structure of a single strand of DNA, including the position of the bases.2 marks
- In cells, DNA exists as two strands wound together in a double helix. The two strands are complementary, and each strand can be used as a template when the DNA is copied.Explain how hydrogen bonding between bases leads to the two strands of DNA being complementary.2 marks
- Cisplatin, cis-Pt(NH₃)₂Cl₂, is a Pt(II) complex given by injection to treat some cancers. A dose is calculated from body surface area: 75 mg of cisplatin for every square metre. A patient has a surface area of 1.80 m². Relative atomic masses: Pt = 195.1, N = 14.0, H = 1.0, Cl = 35.5.Cisplatin binds to DNA by a ligand replacement reaction. Explain what this means and how the bond between platinum and DNA forms.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).