Download COAS_B1_Ch08 Nucleic acids

Chapter 8
Nucleic acids
Background
e-Learning
Objectives
Polynucleotides
The structure of DNA
It is amazing to realise that until the middle
of the 20th century we did not even know that
DNA is the genetic material. Our DNA carries
the genetic code – a set of instructions telling the
cell the sequence in which to link together amino
acids when proteins are being synthesised. Slight
differences in the structure of these proteins
may result in slight differences in our metabolic
reactions. Partly for this reason, we are all slightly
different from one another.
You probably know that DNA is a ‘double
helix’. A DNA molecule is made of two long
chains of nucleotide molecules, linked together
to form a twisted ladder. Each chain is called a
polynucleotide.
DNA stands for deoxyribonucleic acid. When it
was discovered, it was given the name ‘nucleic acid’
because it was mostly found in the nuclei of cells
and is slightly acidic.
Each nucleotide in a DNA molecule contains:
a phosphate group
the five-carbon sugar, deoxyribose
an organic base.
Figure 8.1 shows the components of a nucleotide
in DNA. The base can be any one of four. These
are adenine, guanine, thymine and cytosine.
They are usually abbreviated to A, G, T and C.
Adenine and guanine each contain two rings in
their structure. They are known as purine bases.
Thymine and cytosine have only one ring. They are
known as pyrimidine bases.
phosphate
•
•
•
deoxyribose
organic bases
purine bases
adenine
guanine
pyrimidine bases
adenine
thymine
thymine
Figure 8.1 The components of a nucleotide in DNA.
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cytosine
Chapter 8: Nucleic acids
Figure 8.2 shows how these components are
linked in nucleotides and how the nucleotides
link together to form long chains called
polynucleotides.
You can see that the base in each nucleotide
sticks out sideways from the chain. In DNA, two
chains of nucleotides lie side by side, one chain
running one way and the other in the opposite
direction (Figure 8.3 and Figure 8.4). They are said
to be anti-parallel. The bases of one chain link up
with the bases of the other by means of hydrogen
bonds. The whole molecule twists to produce the
double helix shape.
nucleotide
phosphate
five-carbon
sugar
base
condensation
reaction
polynucleotide
The ‘direction’ of a polynucleotide
5´ end
5
1
4
3
2
3´ end
The carbon atoms of
the five-carbon sugar
are numbered. The top
phosphate is attached
to carbon 5, so is at
the 5´ end. The bottom
phosphate is attached
to carbon 3, so is at the
3´ end.
Figure 8.2 How nucleotides join to form a polynucleotide.
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Chapter 8: Nucleic acids
The key to the ability of DNA to hold and pass
on the code for making proteins in the cell is the
way in which these bases link up. There is just the
right amount of space for one large base – a purine
– to link with one smaller base – a pyrimidine. And
the linking is even more particular than that. A can
only link with T, and C can only link with G. This
is called complementary base pairing.
Complementary base pairing ensures that the
code carried on a molecule of DNA can be copied
perfectly over and over again, so that it can be
passed down from cell to cell and from generation
to generation. It is also what enables the code on
the DNA to be used to instruct the protein-making
machinery in a cell to construct exactly the right
proteins. You will find out much more about this if
you continue studying biology to A2 level.
antiparallel polynucleotide strands of DNA
3´ end
5´ end
5´ end
3´ end
The two polynucleotide strands are twisted round forming a double helix.
Figure 8.3 The structure of DNA.
5´ end
The polynucleotide strands are held together
by hydrogen bonding between the bases.
3´ end
G
C
hydrogen bonds between a T–A
complementary base pair
T
A
3´ end
5´ end
Figure 8.4 Hydrogen bonding joining the bases in DNA.
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Chapter 8: Nucleic acids
DNA replication
We have seen that, before a cell divides by mitosis,
its DNA replicates to produce two copies. One
copy is passed on to each daughter cell. DNA
replication takes place during interphase of the cell
cycle (Chapter 3).
Figure 8.5 and Figure 8.6 show how DNA
replication takes place. This method is called semiconservative replication, because each of the new
DNA molecules is made of one old strand and one
new strand of DNA.
1 Hydrogen bonds
between the
bases are broken.
SAQ
1 One end of a DNA strand is called the 5´
(‘five prime’) end, and the other is the 3´ end.
Why are they given these
Answer
names?
Extension
2 Free nucleotides
are present in
the nucleus.
3 Free nucleotides
pair up with
complementary
exposed bases.
4 The new strand
is linked together.
3´ end
5 There are now two
DNA molecules. Each
one contains one old strand
and one new one.
Figure 8.5 DNA replication.
DNA helicase unwinds and
unzips the DNA by breaking the
hydrogen bonds between bases.
3´ end
5´ end
DNA polymerase links the
newly arrived nucleotides by
forming covalent bonds between
phosphates and sugars. It only
does this if the nucleotides are
correctly paired.
Figure 8.6 Enzymes are involved in DNA replication.
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Chapter 8: Nucleic acids
The role of DNA
The structure of RNA
DNA carries a code that is used by the cell when
making proteins. The sequence of bases in the
DNA molecules determines the sequence of amino
acids that are strung together when a protein
molecule is made on the ribosomes.
A length of DNA that codes for making one
polypeptide is called a gene. It is thought that there
are around 30 000 genes in our cells (Figure 8.7).
The code is read in groups of three ‘letters’ –
that is, triplets of bases. As we have seen, there are
four bases in a DNA molecule, A, T, C and G. A
sequence of three bases in a DNA molecule codes
for one amino acid (Figure 8.8).
DNA is not the only polynucleotide in a cell.
There are also polynucleotides which contain
the sugar ribose rather than deoxyribose. They
are therefore called ribonucleic acids, or RNA for
short. Figure 8.9 shows the structure of RNA.
RNA is generally single stranded, while DNA is
generally double stranded. Another difference
between them is that RNA always contains the
base uracil (U) instead of thymine.
While DNA stores the genetic information
in the nucleus of a cell, RNA is involved with
using that information to make proteins.
U
ribose
uracil
G
C
Figure 8.7 The Human Genome Project has
worked out the base sequence in each human
chromosome.
These three bases
represent the amino
acid valine.
A
Figure 8.9 The structure of RNA.
These three bases
represent the amino
acid glutamate.
If this part of the DNA is being used, a polypeptide chain
is made with the amino acid glutamate joined to valine.
Figure 8.8 How DNA codes for amino acid sequences in proteins.
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RNA resembles
one polynucleotide
strand in DNA,
except that the
base uracil replaces
thymine and the
sugar is ribose.
Chapter 8: Nucleic acids
The sequence of bases on part of a DNA
molecule – a gene – is used to build an RNA
molecule with the complementary base sequence.
This RNA molecule then travels out into the
cytoplasm and attaches to a ribosome. Working
with other RNA molecules, the base sequence
is used to determine the sequence of amino
acids that are strung together to make a protein
molecule. The base sequence on the DNA therefore
determines the primary structure of the protein
that is made.
SAQ
2 Use a table, or a list of bullet points, to
summarise the differences
Answer
between DNA and RNA.
Summary
Extension
Glossary
is deoxyribonucleic acid. It is a double-stranded molecule made up of two strands of
•DNA
nucleotides.
DNA nucleotide is made up of a phosphate group, a five-carbon sugar called deoxyribose, and
•Aa base.
There are four bases in DNA – adenine, guanine, cytosine and thymine. They are usually
abbreviated to A, G, C and T.
•Adenine and guanine are purine bases. Cytosine and thymine are pyrimidine bases.
nucleotides in a strand of DNA are linked to each other by strong covalent bonds between the
•The
phosphate groups and deoxyribose. The phosphate groups bond to carbon 5 and to carbon 3 of the
deoxyribose ring. The end of the molecule where the phosphate is bonded to carbon 5 is called the 5
end, while the other is the 3 end.
two strands of a DNA molecule are linked to each other by weak hydrogen bonds between the
•The
bases. A always bonds with T, and C always bonds with G. A and T are linked by two hydrogen
bonds. C and G are linked by three hydrogen bonds.
two strands of a DNA molecule run in opposite directions. They are said to be anti-parallel.
•The
They twist around each other to form a double helix.
DNA molecules in a cell nucleus are replicated before cell division takes place. First, the two
•The
strands of the molecule are untwisted and unzipped. Free DNA nucleotides pair up with the exposed
bases on both strands. They are then linked together by the formation of bonds between their
deoxyribose and phosphate groups. This is catalsyed by the enzyme DNA polymerase. Two new
molecules are therefore formed, each identical to the original one. Each new molecule contains one
old strand and one new strand, so the process is called semi-conservative replication.
sequence of bases in a DNA molecule codes for the sequence of amino acids in a protein to be
•The
made on the ribosomes. Three bases code for one amino acid. A sequence of DNA nucleotides that
codes for one polypeptide is known as a gene.
is ribonucleic acid. There are several kinds of RNA. Most are single stranded. They contain
•RNA
the pentose sugar ribose, rather than deoxyribose. They contain the base uracil instead of thymine.
protein synthesis, an RNA molecule is built up against one of the DNA strands in a gene.
•During
The RNA then travels out of the nucleus to a ribosome, where its sequence of bases is used to
determine the sequence of amino acids in the polypeptide that is being constructed on the ribosome.
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Chapter 8: Nucleic acids
Questions
[3]
[1]
A
B
C
Figure 1
100
% DNA molecules
1 Figure 1 represents a nucleotide which forms part of a DNA
molecule.
a i Name A to C.
ii State which part of the nucleotide contains nitrogen.
During research into the mechanism of DNA replication,
bacteria were grown for many generations in a medium
containing only the ‘heavy’ isotope of nitrogen, 15N. This
resulted in all the DNA molecules containing only 15N.
This is illustrated in Figure 2.
75
50
25
0
DNA
DNA
DNA
containing containing containing
14
14
N only
N / 15N 15N only
Figure 2
The bacteria continued to grow in the ‘light’ nitrogen, 14N,
medium until the DNA had replicated once more. The DNA
molecules were analysed. The results are shown in Figure 4.
Figure 5 shows simple diagrams of DNA molecules,
indicating the nitrogen content of each.
Key
A
B
C
D
E
F
represents
DNA with 15N
% DNA molecules
100
75
50
25
0
DNA
DNA
DNA
containing containing containing
14
14
N only
N / 15N 15N only
Figure 3
100
% DNA molecules
These bacteria were then grown in a medium containing only
‘light’ nitrogen, 14N. After the time taken for the DNA to
replicate once, the DNA was analysed. The results are shown
in Figure 3.
b Explain how these data support the semi-conservative
hypothesis of DNA replication.
[3]
represents
DNA with 15N
75
50
25
0
DNA
DNA
DNA
containing containing containing
14
14
N only
N / 15N 15N only
Figure 4
Figure 5
100
% DNA molecules
c With reference to Figure 5, select the letter or letters which
best represent the bacterial DNA in Figure 2, Figure 3
and Figure 4.
The bacteria continued to grow in the ‘light’ nitrogen, 14N,
medium until the DNA had replicated once more. The DNA
molecules were analysed.
d Copy and complete the bar chart to the right to indicate
the expected results of the composition of these DNA
molecules.
[3]
OCR Biology AS (2801) January 2002
[3]
75
50
25
0
DNA
DNA
DNA
containing containing containing
14
14
N only
N / 15N 15N only
[Total 13]
Answer
130