CELLS AND MOLECULES
DEOXYRIBONUCLEIC ACID (DNA )
You need to:
•
explain the structure and function of DNA.
Deoxyribonucleic Acid (DNA)
DNA is found in the nucleus of eukaryotic cells. It carries all
the information for making all the proteins that build an
organism. It therefore has to be copied before cell division. It
is the inherited molecule that carries the instructions for life.
One molecule of DNA is a polymer made up of many
nucleotide monomers.
Nucleotides
In DNA, nucleotides consist of a five
carbon sugar called deoxyribose, a
phosphate group and a nitrogenous
base all held together by H bonds.
There are four different types of
nitrogenous base present in DNA.
These are adenine (A), thymine (T),
guanine (G) and cytosine (C). A and G
are known as the larger purine bases.
C and T are known as the smaller
pyrimidine bases.
Nucleotides are joined together
during condensation reactions
between the phosphate group of
one nucleotide and the deoxyribose
of another.
The bond that forms between
nucleotides is called a phosphodiester
bond.
Many nucleotides join together in this
way to form a polynucleotide. The
combination and order of the
nucleotides in the sequence is specific
to the organism and passed on
through generations.
A simplified nucleotide
Base
Phosphate
One molecule of DNA consists of two
polynucleotides which lay side by side
running in opposite directions. The
nitrogenous bases of each strand face
one another and hydrogen bonds
form between each pair of bases. It’s
the weak hydrogen bonds that hold
the two strands together.When DNA is
copied - before cell division or during
protein synthesis - the hydrogen
bonds can be broken easily. The two
polynucleotide strands twist around
one another to form a double helix.
Double helix
Deoxyribose
Bonding between nucleotides
Question 1
(a) Circle the phosphodiester bonds in the diagram above.
(b) Which enzyme is responsible for joining two polynuceotide chains to form DNA?
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The structure
of DNA
Weak
hydrogen
bonds
between
base pairs
hold the two
strands
together
Complementary
base pairing
The DNA bases
adenine
guanine
thymine
cytosine
The space between the two DNA
strands allows the pairing of a purine
base with a pyrimidine base. Because
of the structures of the different
bases, adenine will only ever pair with
thymine, and guanine will only ever
pair with cytosine. This is known as
complementary base pairing.
Question 2
(a) If 30% of a DNA molecule is
guanine, what percentage of the
molecule is adenine?
(b) The bases of a section of one DNA
strand are shown below. Copy out
the sequence and write the
sequence of the other strand
underneath it.
AAGTTTCGCACAAG
Base pairing
label the DNA bases ...
(a) ________________
(c) ________________
(b) ________________
(d) ________________
This base pairing is held together by three hydrogen bonds.
This base pairing is held together by two hydrogen bonds.
your notes and answers ...
REVISION GUIDE / OCR AS UNIT 4
THE ADVANCED RESOURCE: APPLIED SCIENCE © 4SCIENCE 2006
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