Download DNA (Deoxyribonucleic acid)

DNA (Deoxyribonucleic acid)
A DNA molecule is a chain of nucleotides. A nucleotide is a 5-carbon sugar molecule
joined to a phosphate group (–PO3) and an organic base.
PO3
Adenine
phosphate
pentose sugar
(deoxyribose)
organic base
Above is an example of a nucleotide.
In DNA, the sugar is deoxyribose and the organic base is either:
A
T
-
adenine
thymine
C
G
-
cytosine
guanine
The nucleotides are joined by their phosphate groups to form a long chain: often thousands
of nucleotides long. The phosphate and sugar molecules are the same all the way down, but
the organic bases may be any of the four listed.
The sequence of bases down the DNA molecule forms a code, which instructs the cell to
make particular proteins. Proteins are made from linked amino acids. Different amino acids
produce different types of proteins.
A DNA molecule consists of two chains of nucleotides, which run anti-parallel to each
other: the two chains are joined by hydrogen bonds between the bases. This forms a double
helix (same shape as a twisted ladder).
DNA double helix:
Phosphate group
Bases
Pentose sugar
A
T
Hydrogen bonds
G
C
The ladder is twisted (double helix). Note the base pairings:
Adenosine only pairs with Thymine
Cytosine only pairs with Guanine.
From this, it is clear that if you know the pairings on one side, you can work out the pairings
on the other side (because of the specific complementary base pairings).
Fill in the organic bases for the following (only the first letter of each organic base should
be used):
A
C
T
A
The Genetic Code
C
-
DNA carries instructions (i.e. a code) for the assembly of amino acids (which
form proteins)
-
If one base was equal to one amino acid, the maximum codes would be 4.
If two bases was equal one amino acid, the maximum would be 16 codes.
If three bases equal one amino acid, the maximum codes would be 64.
There are at least 20 amino acids present in the human body.
the triplet code of bases provides sufficient amino acid codes for instructions to
produce or stop the assemble of proteins: three bases equal one amino acid.
-
some amino acids have more than one code
Mutation (as will be discussed later) can be explained by the fact that if a base is inserted
into the wrong place, the wrong amino acid is added. This results in a different protein being
produced.
The code is carried by a sequence of one of the DNA strands. This is called the “SENSE”
strand. The other strand is called the “NON-SENSE” strand. The DNA is universal: it is
interpreted by every organism in the same way.
DNA Replication – “the semi-conservative theory”
-
DNA helix unwinds
hydrogen bonds between the bases are broken by an enzyme called DNA
polymerase
new organic bases/nucleotides align themselves opposite the exposed bases
complementary base pairing is maintained (i.e. A with T and C with G)
the result is two identical strands
A
T
G
C
T
A
base pairs
A
T
G
C
T
A
sugar phosphate backbone
A
A
T
C
G
C
G
The enzyme DNA polymeraase breaks the
hydrogen bonds between the cases causing
the strands to separate (to unzip) from one
end.
One original molecule of DNA
A
T
T
A
G
T
C
T
A
G
C
T
A
T
C
G
A
A
A
T
C
T
G
G
C
C
G
G
T
C
A
G
C
Complementary bases join with hydrogen
bonds and free nucleotides link up to for a
new strand
The exposed bases attract the
complementary bases of free DNA
nucleotides. (e.g. C will only attract G)
A
T
G
C
T
A
original (old) DNA strand
A
T
G
C
T
A
A
T
C
G
new strand
Two exact copies of the
original DNA molecule have
been made
A
T
C
G
The sequence of the bases is
kept exactly the same due to
specific base pairing