3.3:DNA Structure

3.3:DNA Structure
3.3.1: Outline DNA nucleotide structure in terms of sugar (deoxyribose), base
and phosphate.
3.3.2: State the names of the four bases in DNA.
3.3.3: Outline how DNA nucleotides are linked together by covalent bonds
into a single strand.
3.3.4: Explain how a DNA double helix is formed using complementary base
pairing and hydrogen bonds.
3.3.5:Draw and label a simple diagram of
the molecular structure of DNA.
IB QUESTION: Draw a labeled diagram to show four DNA nucleotides, each with a different
base, linked together in two strands. [5]
Award [1] for each of these structures clearly drawn and labelled.
four nucleotides shown in diagram with one nucleotide clearly labelled;
base, phosphate and deoxyribose (shown as pentagon) connected between the
correct carbons and labelled at least once;
backbone labelled as covalent bond between nucleotides correctly shown as 3 to
5 bond;
two base pairs linked by hydrogen bonds drawn as dotted lines and labelled;
two H bonds between A and T and three H bonds between C and G;
adenine to thymine and cytosine to guanine; do not accept initials of bases
antiparallel orientation shown; [5]
IB Question: Draw and label a simple diagram to show how DNA is constructed from
sugars,phosphates and bases. [6]
Award [1] for each of the following clearly drawn and correctly labelled.
labelled phosphate linked to labelled sugar by a single bond;
labelled base linked to a labelled sugar by a single bond;
correctly linked base, sugar and phosphate labelled as nucleotide;
covalent bond(s) labelled between sugar and phosphate/between sugar and base;
at least two nucleotides linked by a single sugar-phosphate bond;
at least two nucleotides linked by base-base bonds;
hydrogen bonds labelled between bases;
A-T and/or G-C base pairing;
phosphate and base shown linked to correct carbon atoms on sugar; [6 max]
Draw a labelled diagram to show how two
nucleotides are joined together in a single
strand of DNA. [3]
3.4: DNA Replication
3.4.1: DNA Replication
3.4.1: Explain DNA replication in terms
of unwinding the double helix and
separation of the strands by helicase,
followed by formation of the new
complementary strands by DNA
3.4.2:Explain the significance of complementary base pairing in the conservation of the base
sequence of DNA.
IB Question: Outline the significance of complementary base-pairing in DNA.
when DNA replicates the 2 strands separate;
each single strand acts as template/serves as a base for base-pair matching;
free nucleotides of adenine/A bond only with thymine/T / vice versa and free
nucleotides of cytosine/C bond only with guanine/G / vice versa;
thus copying the opposite strand of the original DNA molecule;
replication is semi-conservative;
original order of bases is maintained/conserved;
new DNA identical to parent molecule;
(significance of base-pair matching) is that the information encoded in one DNA
molecule is passed to others;
DNA structure indicates the manner of DNA replication; [5 max]
IB Question: The structure of the DNA double helix was described by Watson and Crick in 1953.
Explain the structure of the DNA double helix, including its subunits and the way in which they
are bonded together.
subunits are nucleotides;
one base, one deoxyribose and one phosphate in each nucleotide;
description / diagram showing base linked to deoxyribose C1 and phosphate to C5 ;
four different bases – adenine, cytosine, guanine and thymine;
nucleotides linked up with sugar-phosphate bonds;
covalent / phosphodiester bonds;
two strands (of nucleotides) linked together;
base to base;
A to T and G to C;
hydrogen bonds between bases;
antiparallel strands;
double helix drawn or described; [8 max]
Accept any of the points above if clearly explained in a diagram
3.4.3: State that DNA replication is semi-conservative.

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