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DNA

Lajoy Tucker

Teacher

Lajoy Tucker

Contents

Introduction to DNA

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Structure of DNA

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Base-pairing

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Cisplatin and DNA

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Practice Questions

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Introduction to DNA

DNA (Deoxyribonucleic Acid) is a large, double-stranded polymer made up of nucleotide monomers. These two complementary strands are arranged in the form of a double helix.

DNA Explainer Video

Structure of DNA

Diagram of the DNA double helix showing the sugar-phosphate backbone and complementary base pairs adenine-thymine and cytosine-guanine.

The monomers that make up DNA are nucleotides.

Each nucleotide in DNA consists of:

1. A phosphate group

Structural formula of a phosphate ion showing a central phosphorus atom bonded to four oxygen atoms, including one double bond and two hydroxyl groups.

2. A pentose sugar (2-deoxyribose)

Displayed structural formula of deoxyribose showing a five-membered sugar ring with hydroxyl groups and a CH₂OH side group.

3. A nitrogenous base

Structural formulas of the four DNA bases adenine, guanine, cytosine, and thymine used in complementary base pairing.


Example nucleotide with adenine as the base

Diagram of a DNA nucleotide showing a phosphate group, deoxyribose sugar, and nitrogenous base (adenine) joined together.

Nucleotides are joined by phosphodiester bonds between the phosphate group of one and the sugar of the next.

Diagram showing two nucleotides joining by a condensation reaction to form a phosphodiester bond in the DNA sugar-phosphate backbone, releasing water.

This results in a long strand of repeating nucleotides linked by covalent bonds in a sugar-phosphate-sugar-phosphate polymer chain.

Base-pairing

The double-stranded nature of DNA arises from bases on antiparallel polynucleotide strands pairing up. These base pairs are held together by hydrogen bonds.

Diagram showing the structure of DNA with two sugar-phosphate backbones joined by complementary base pairs, where adenine pairs with thymine and cytosine pairs with guanine.

Base pairs:

Adenine and Thymine (2 hydrogen bonds)

Diagram showing adenine and thymine joined by hydrogen bonds, illustrating complementary base pairing between nitrogenous bases in DNA.

Cytosine and Guanine (3 hydrogen bonds)

Diagram showing guanine and cytosine joined by three hydrogen bonds, illustrating complementary base pairing between nitrogenous bases in DNA.

Hydrogen bonds are formed between lone pairs on O or N atoms and atoms on the base of the neighbouring strand.

Cisplatin and DNA

Cisplatin is a platinum-based anti-cancer drug that binds to DNA, disrupting replication, therefore slowing the spread of cancer.

Mode of Action:

  • Cisplatin forms coordinate bonds with the nitrogen atoms on guanine bases

  • This distorts DNA shape, preventing replication and transcription

Ethical Issues Around Cisplatin

  • Benefits: Treats cancer effectively

  • Risks: Causes side effects like nausea, hair loss, kidney damage

  • Ethical balance: Patient consent, quality of life, long-term risks

Practice Questions

A section of a single strand of DNA has the following sequence of bases:

A T T G C A

a) Give the sequence of bases on the complementary strand.

b) State the total number of hydrogen bonds in this section of DNA.

Answer

a) T A A C G T

b) 14 (2 + 2 + 2 + 3 + 3 + 2)

Explain how two DNA strands are held together.

Answer

1. Hydrogen bonds between bases.

2. A pairs with T, G pairs with C.

3. Specific complementary base pairing.

Describe the role of cisplatin in cancer treatment and how it works at the molecular level.

Answer

1. Cisplatin binds to DNA.

2. Forms coordinate bonds with nitrogen in guanine bases.

3. Prevents replication/transcription of DNA.

4. Stops cell division leading to cancer cell death.

Proteins and Enzymes