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Visit this site to investigate the history, structure and role of DNA

WHAT DO WE ALREADY KNOW?

X-ray diffraction pattern of DNA In 1953, James Watson and Francis Crick published their findings on the discovery of the structure of DNA and were awarded a Nobel prize for their work Watson and Crick interpreted the X-ray diffraction patterns produced by Rosalind Franklin to propose a model for the molecule of life – Deoxyribose Nucleic Acid Using cutouts of the building blocks of DNA, Watson and Crick constructed a helical model of the DNA molecule and, in doing so, revolutionised the field of molecular genetics Discovering the Structure of DNA Click to play DNA complementary nucleotide game

DNA or Deoxyribose Nucleic Acid, is the molecule of heredity It is a long macromolecule or polymer consisting of TWO helical chains coiled around a common axis The building blocks or monomers of these chains are called NUCLEOTIDES Each nucleotide consists of a 5-carbon sugar, a phosphate group and a nitrogen containing organic base sugar phosphate group organic base A SINGLE NUCLEOTIDE The Structure of DNA

These two nucleotides both consist of a deoxyribose sugar, a phosphate group and a nitrogen-containing organic base These two nucleotides differ in that one of them contains a large base with the other containing a smaller base There are four different bases associated with DNA These bases are ADENINE, THYMINE, GUANINE and CYTOSINE (A, T, C and G ) T C A G The Nucleotides

T C A G These are the four different nucleotides or monomers that form the building blocks of DNA The nucleotides differ by virtue of their organic base Adenine and Guanine are the two larger bases and are described as PURINES Thymine and Cytosine are the two smaller bases and are described as PYRIMIDINES A G T C DOUBLE-RING (‘Purines’) SINGLE-RING (‘Pyrimidines’) The Nucleotides

Thymine is a PYRIMIDINE BASE - a SINGLE nitrogen containing ring is part of its structure Cytosine is a PYRIMIDINE BASE - a SINGLE nitrogen containing ring is part of its structure Thymine and Cytosine are PYRIMIDINE BASES The Nucleotides

Adenine is a PURINE BASE - TWO nitrogen containing rings are part of its structure Guanine is a PURINE BASE - TWO nitrogen containing rings are part of its structure Adenine and Guanine are PURINE BASES The Nucleotides

Strong bonds between nucleotides make a sugar-phosphate backbone The Structure of DNA DNA nucleotides are first joined together by…. CONDENSATION This makes a single polynucleotide strand

Second strand runs in the opposite direction to the first =Anti-parallel Weak hydrogen bonds between the base pairs hold the two polynucleotide strands together The whole molecule is then coiled into a DOUBLE HELIX The Structure of DNA Bases pair by Complementary base-pairing DNA consists of 2 polynucleotide strands

An important aspect of the DNA double helix is the specific way in which the bases pair together The distance between the two backbones does not vary along the length of the molecule This distance accommodates a PURINE-PYRIMIDINE PAIR Adenine (Purine) Thymine (pyrimidine) Weak hydrogen Bonds Adenine ALWAYS pairs with Thymine forming two hydrogen bonds The Base Pairing Rule

Cytosine ALWAYS pairs with Guanine forming three hydrogen bonds Cytosine (pyrimidine) Guanine (Purine) Weak hydrogen Bonds Adenine ALWAYS pairs with Thymine AT Cytosine ALWAYS pairs with Guanine C G This arrangement allows for the maximum number of hydrogen bonds between the bases The bases that pair together are described as complementary bases The Base Pairing Rule

deoxyribose sugar phosphate group base pair held together by weak hydrogen bonds anti-parallel polynucleotide strands held together by strong covalent bonds and forming the backbone of the molecule DNA is the material of heredity It stores genetic information It is self replicating It is able to express its genetic message DNA Structure - Summary

How is DNA adapted to carry out its functions? List adaptations from page 132 Answer summary questions page 132 Exam question 2 page 144

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