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DNA _Deoxyribonucleic acid_


									SICM Tuition                                                                                         Biology AS
                                      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.


                                         pentose sugar                  organic base

Above is an example of a nucleotide.

In DNA, the sugar is deoxyribose and the organic base is either:

A       -         adenine                                           C   -       cytosine
T       -         thymine                                           G   -       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
                  Pentose sugar

                              A                        T

                                      Hydrogen bonds

                                  G                    C

                                                           Page 1
SICM Tuition                                                                         Biology AS
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):




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

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.

OK……so that was all GCSE stuff…let’s get on to the more advanced stuff….fun fun fun!!

                                            Page 2
SICM Tuition                                                                       Biology AS
The difference between DNA and RNA

       Deoxyribonucleic acid (DNA)                       Ribonucleic acid (RNA)
              Bases include:                                 Bases include :
                Adenine,                                        Adenine,
                Thymine,                                         Uracil,
                Cytosine,                                       Cytosine,
                 Guanine                                        Guanine
       Deoxyribose as pentose sugar                      Ribose as pentose sugar
             Double stranded                                 Single stranded
            Generally larger                                Generally smaller

Complementary base pairing
As we have previously mentioned, the bases pair together using hydrogen bonds. However,
the bonding between the pairs is different according to the class of base. There are two
different classes:
Purines        -     Adenine and Guanine
               -     these have two rings of carbon and nitrogen atoms

Pyrimidines -        Cytosine and Thymine/Uracil
               -     these have a single ring of carbon and nitrogen atoms


When A bonds with T, there are two hydrogen bonds holding them together.
When C bonds with G, there are three hydrogen bonds holding them together.

The importance of DNA
OK, so that’s all well and good…..but……WHO CARES?!

   -       ALL chemical reactions in a cell are controlled by enzymes
   -       Enzymes are proteins
   -       DNA codes for amino acids (which make up proteins)
   -       Therefore, DNA controls the activities of a cell

The way in which DNA codes for amino acids is complex. However, we MUST look at this
in detail (yay!)

                                            Page 3
SICM Tuition                                                                          Biology AS
The Triplet Code
   -       the sequence of bases in a DNA molecule code for a sequence of amino acids
   -       the code for each amino acid is three letters or a triplet code
   -       each sequence of three bases code for one amino acid
   -       three letters code for one amino acid
   -       three bases together code for one amino acid
   -       that’s right…I’ve said it enough times…hopefully you have it in your head now!

   -       the three bases together are called “codons”
   -       a codon codes for an amino acid:
           o For example:
                      C-A-A (on the sense strand) = VALINE
                      T-T-T (on the sense strand) = LYSINE

The genetic code is non overlapping and degenerate
   -       there is a specific codon that indicates the start of the code
   -       this is always AUG
   -       AUG codes for methionine
   -       by having this start codon, it ensures that the code is always read correctly and “in

E.g.   AUCGUAUGTTTCAA…. will code for methionine and then lysine and valine

       -       some amino acids are coded for by more than one codon
       -       e.g. GUC, GUG, GUA and GUU all code for Valine
                  which one of these is the code from the valine sense strand shown above?
       -       there are 20 different amino acids even though there are 64 different
               arrangements of A, U, C and G

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