3.3 – DNA Structure 3.3.1 - Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate Nucleotides are formed from a pentose sugar, phosphate and a base. o

Phosphate links neighbouring sugars together (PO 43-)

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The sugar is either ribose for RNA or deoxyribose for DNA, which has one less oxygen

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Four types of bases, which comprise the coded genetic message 

Nitrogen-based ring structures 

Cytosine (C), Guanine (G), Adenine (A), Thymine (T)

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Nucleotides form sequences, which are instructions for the organism

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Changes to nucleotides cause mutations

3.3.2 - State the names of the four bases in DNA Purines - Two Ringed Bases 

Adenine (A)

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Guanine (G)

Pyrimidines - Single Ringed Bases 

Cytosine (C)

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Thymine (T)

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Uracil replaces Thymine in RNA (U)

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3.3.3 - Outline how DNA nucleotides are linked together by covalent bonds into a single strand 

DNA composed of two polynucleotide chains

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Nucleotides are covalently bonded

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Bond is a phosphodiester 

Two covalent bonds between the OH- and acidic phosphate group

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Nucleotides bond at the 3l (three prime) end of the molecule

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To form the polynucleotide, the nucleotides condense together one at a time, giving water in the reaction

3.3.4 - Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds Complementary means matching 

A double helix is made up of two anti-parallel polynucleotide chains

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Bases pair and are bonded with hydrogen bonds

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Adenine and thymine are the same distance apart as guanine and cytosine

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3.3.5 - Draw and label a simple diagram of the molecular structure of DNA 

DNA is made up of two anti-parallel polynucleotide chains

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They form the double helix ("spiral" structure)

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The DNA has a sugar-phosphate backbone

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Bonded with complementary base pairs 

Adenine to Thymine

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Cytosine to Guanine

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