DNA Structure
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of nucleic acids. You can test your knowledge on these below.
What are the monomers of DNA and what are they composed of?
Nucleotides, composed of a phosphate group, a deoxyribose sugar, and a nitrogenous base.
Describe the bonding between complementary base pairs in DNA.
Adenine (A) pairs with Thymine (T) via two hydrogen bonds, and Cytosine (C) pairs with Guanine (G) via three hydrogen bonds.
Why is DNA described as a polymer?
DNA is made from many repeated monomers, called nucleotides.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains DNA structure or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Overview & Nucleotide Structure
Overview
DNA (Deoxyribonucleic Acid) is the molecule that carries genetic information in cells. It is a double helix composed of two polynucleotide strands held together by hydrogen bonds between complementary base pairs.
Nucleotide Structure
Each DNA nucleotide consists of:
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A phosphate group
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A deoxyribose sugar (pentose sugar with 5 carbons)
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A nitrogenous base (Adenine, Thymine, Cytosine, Guanine)
Formation of DNA Strands & Complementary Base Pairing
Formation of DNA Strands
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Nucleotides join through phosphodiester bonds, formed by condensation reactions between the phosphate of one nucleotide and the deoxyribose sugar of the next.
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This forms a sugar-phosphate backbone, which provides structural stability.
Complementary Base Pairing
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Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
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Cytosine (C) pairs with Guanine (G) (3 hydrogen bonds)
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This pairing ensures accurate replication and stability of DNA.
Double Helix & Antiparallel Strands
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The two DNA strands run antiparallel (opposite directions).
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One strand is 5’ to 3’, and the other is 3’ to 5’, this refers to the carbon number on the deoxyribose sugar.
Function of DNA Structure
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Stable due to the strong, covalent phosphodiester bonds in the sugar-phosphate backbone.
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Complementary base pairing allows for accurate replication.
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Hydrogen bonds between bases allow easy strand separation for replication and transcription.
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Compact structure fits inside the nucleus while storing a vast amount of genetic information.
Key Terms
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Nucleotide: The monomer of DNA, composed of a phosphate, deoxyribose sugar, and nitrogenous base.
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Phosphodiester Bond: The bond linking nucleotides, forming the sugar-phosphate backbone.
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Complementary Base Pairing: The specific hydrogen bonding between A-T and C-G.
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Antiparallel: The opposite orientation of the two DNA strands (5’→3’ and 3’→5’).
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Double Helix: The spiral structure of DNA, discovered by Watson and Crick.
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Exam Tips
When explaining DNA structure in an exam, always refer to the phosphodiester bonds between adjacent nucleotides and the hydrogen bonds between complementary bases.
Avoid vague statements like "bases pair up"—instead, specify which bases pair.
No answer provided.
DNA is an important biological molecule. Describe its structure and explain how this structure allows it to carry out its function. (6 marks)
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DNA is a polymer of nucleotides.
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Each nucleotide contains a phosphate group, deoxyribose sugar, and a nitrogenous base
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DNA is double-stranded and forms a double helix. This gives DNA a compact structure which fits inside the nucleus while storing a vast amount of genetic information.
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The sugar-phosphate backbone is joined by phosphodiester bonds, giving DNA structural stability.
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Complementary base pairing occurs (A-T, C-G), which ensures accurate replication.
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The two strands run antiparallel (one runs 5’ to 3’, the other 3’ to 5’), allowing enzymes like DNA polymerase to function correctly.
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Hydrogen bonds between complementary bases allow strand separation, which is important for DNA replication and transcription.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!