Nucleic acids
Information molecules throughout the living world
 Code containing the information in nucleic acids known as the genetic code is
universal
 Makes sense in all organisms
 Two types of nucleic acid – DNA and RNA
 Day to day control of cells and organisms
 Built up from repeating units called nucleotides
 A nucleotide consists of 3 substances combined together
a phosphate group
, which is negatively charged, and gives nucleic acids
their acidic properties.
• a pentose sugar, which has 5 carbon atoms in it. By convention the carbon atoms
are numbered as shown (1', 2', etc, read as "one prime", "two prime", etc), to
distinguish them from the carbon atoms in the base. If carbon 2' has a hydroxyl
group attached (as shown), then the sugar is ribose, found in RNA. If the carbon 2'
just has a hydrogen atom attached instead, then the sugar is deoxyribose, found in
DNA.
• a nitrogenous base. There are five different bases all containing the elements
carbon, hydrogen, oxygen and nitrogen. Since there are five bases, there are five
different nucleotides
o Adenine (A) Cytosine (C) Guanine (G) Thymine (T) Uracil (U)
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Nucleotides can have one, two or three phosphate groups. So for instance you
can have adenosine monophosphate (AMP), adenosine diphosphate (ADP) and
adenosine triphosphate (ATP). These nucleotides are very common in cells and
have many roles other than just part of DNA. ATP is used as an energy store,
while AMP and GTP are used as messenger chemicals
Nucleotides are formed by condensation reactions see OHP
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Nucleotides polymerise by forming phosphodiester bonds between carbon 3' of
the sugar and an oxygen atom of the phosphate.
This is a condensation polymerisation reaction.
The bases do not take part in the polymerisation, so there is a sugar-phosphate
backbone with the bases extending off it.
This means that the nucleotides can join together in any order along the chain.
Two nucleotides form a dinucleotide, three form a trinucleotide, a few form an
oligonucleotide, and many form a polynucleotide.
A polynucleotide has a free phosphate group at one end, called the 5' end
because the phosphate is attached to carbon 5' of the sugar, and a free OH
group at the other end, called the 3' end because it's on carbon 3' of the sugar.
The terms 3' and 5' are often used to denote the different ends of a DNA
molecule.
The three-dimensional structure of DNA was discovered in 1953 by Watson and
Crick in Cambridge, using experimental data of Wilkins and Franklin in London,
for which work they won a Nobel prize.
The main features of the structure are:
o DNA is double-stranded, so there are two polynucleotide stands
alongside each other.
o The strands are antiparallel, i.e. they run in opposite directions.
o The two strands are wound round each other to form a double helix (not
a spiral, despite what some textbooks say)
o The two strands are joined together by hydrogen bonds between the
bases.
o The bases therefore form base pairs, which are like rungs of a ladder.
o The base pairs are specific. A only binds to T (and T with A), and C only
binds to G (and G with C). These are called complementary base pairs (or
sometimes Watson-Crick base pairs).
o This means that whatever the sequence of bases along one strand, the
sequence of bases on the other stand must be complementary to it.
Differences between DNA and RNA
o Deoxyribose sugar and ribose sugar
o 3 different types – messenger, transfer, ribosomal
o Thymine in DNA is replaced with Uracil in RNA
o RNA is usually single stranded, but can fold into 3-dimentional structures, like
proteins.
o RNA is usually shorter than DNA
o Amount of DNA in a cell is constant whereas amount of RNA is variable
Messenger RNA (mRNA)
o mRNA carries the "message" that codes for a particular protein from the
nucleus (where the DNA master copy is) to the cytoplasm (where proteins are
synthesised).
o It is single stranded and just long enough to contain one gene only and pass
through the nuclear pore
o It has a short lifetime and is degraded soon after it is used.
Ribosomal RNA (rRNA)
o rRNA, together with proteins, form ribosomes, which are the site of mRNA
translation and protein synthesis.
o Ribosomes have two subunits, small and large, and are assembled in the nucleolus
of the nucleus and exported into the cytoplasm.
o rRNA is coded for by numerous genes in many different chromosomes.
o Ribosomes free in the cytoplasm make proteins for use in the cell, while those
attached to the RER make proteins for export.
Transfer RNA (tRNA)
o tRNA is an “adapter” that matches amino acids to their codon.
o tRNA is only about 80 nucleotides long, and it folds up by complementary base
pairing to form a looped clover-leaf structure.
o Each tRNA molecule has a binding site at one end where a specific amino acid
attaches itself to the bases there
o Each tRNA molecule also has a specific sequence of three bases at one end of it
called an anticodon to pair with codon on mRNA
o 20 forms for 20 different of amino acids