The Structure of DNA
What I need to know:1. DNA units are called nucleotides
2. A nucleotide consists of a deoxyribose sugar,
phosphate and base
3. There are four bases (cytosine, guanine, adenine
and thymine). These bases have complimentary
base pairings which are joined by hydrogen bonds
4. Strong chemical bonds form between nucleotides
to make a strand of DNA
5. Two DNA strands run anti-parallel to each other
and twist into a double helix
6. DNA is arranged so that it tightly coils into a
chromosome
DNA
Do you remember what DNA stands for?
Where is it found?
What does it code for?
DNA
Each molecule of DNA consists of 2 strands,
where each strand is made up of repeating units
called NUCLEOTIDES.
The nucleotide
is made of :
•Deoxyribose
sugar
•Phosphate
•Organic Base
5C
1C
4C
3C
2C
Nucleotide
There are 4 different types of
nucleotides. This is
because there are 4 different bases:
GUANINE
CYTOSINE
THYMINE
ADENINE
Complementary base pairing
Each base can only join with one other base.
4 bases:
Thymine
Adenine
Cytosine
Guanine
This is always the case in DNA – G-C, TA pairs
STRONG
CHEMICAL
BOND links the
nucleotides
between the
phosphate and
sugar. They are
not easily broken
and join
nucleotides
together into a
permanent
strand
ADENINE
GUANINE
ADENINE
GUANINE
The 2 strands are
arranged in a twisted
THYMINE
coil called a “DOUBLE
HELIX”. It is often
likened to a spiral
ladder, where the
sugar-phosphate
backbone is the
railing and the base
CYTOSINE
are the steps
Weak hydrogen bond
5’
3’
ADENINE
GUANINE
3’
THYMINE
CYTOSINE
Notice how the two strands
run anti-parallel to each other
5’
Arrangement of DNA
DNA is tightly coiled and found on chromosomes
If you take the DNA from one cell it would
stretch out to 2 meters!
Therefore it is essential that the DNA is
arranged so that it fits within the nucleus of a
cell
It is tightly coiled and packaged around bundles
of protein (like beads on a string)
Now attempt the following questions
1) How many different types of base molecules are
found in DNA? Name each type.
2) Which type of bond forms between the bases of
adjacent strands of a DNA molecule?
3) Describe the base pairing rule
4) Draw the complementary strand for the following
strand or DNA
5’ C A T G C C A T G T A G
3’
• 5) What term is used to describe the two
strands of DNA that run in opposite
directions
• 6)What name is given to the twisted coil
arrangement typical of a DNA molecule?
• 7) If DNA is like a spiral ladder, which part
of it corresponds to the ladder’s (i) rungs
(ii)uprights?
Glossary
• COMPLEMENTARY – fitting together like a
hand in a glove; applied to DNA base pairing
• DOUBLE-STRANDED HELIX – describes the
spiral ladder shape of DNA molecules
• NUCLEOTIDE – unit which consists of a ribose
or deoxyribose sugar joined to a phosphate
group and to an organic base