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Nucleic acids and protein synthesis
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AS Bio notes Raida 12DNucleic acids and protein synthesis
6.2 The structure of DNA and RNA
DNA stands for deoxyribonucleic acid
RNA stands for ribonucleic acid
DNA and RNA are macromolecules
They are known as nucleic acids because they were originally found in the nucleus
[proteins and polysaccharides are also macromolecules]
The monomers from which DNA and RNA molecules are made from are nucleotides
DNA and RNA are therefore polynucleotides
Nucleotides:
-> nucleotides are made up of 3 smaller components, these are:
A nitrogen-containing base
A pentose sugar
A phosphate group
Nitrogen-containing base:
There are 4 different nitrogen-containing bases found in DNA and 4 in RNA
In DNA the bases are: Adenine, Cytosine, Guanine and Thymine
In RNA the Thymine is replaced by a similar base called Uracil
The bases are A, G, C, and U
Adenine and Guanine are related to the chemical Purine and are referred to as purines
Cytosine and Thymine are related to the chemical Pyrimidines and are referred to as
pyrimidines
Purines have two rings in their structure
Pyrimidines have one ring in their structure
Pentose sugar:
Sugars with five carbon atoms are called Pentoses
Two pentoses are found in nucleic acids, ribose and deoxyribose
A nucleic acid containing Deoxyribose is called Deoxyribonucleic acid (DNA)
A nucleic acid containing Ribose is called Ribonucleic acid (RNA)
Deoxyribose is almost the same as Ribose except that it has one fewer oxygen atoms in
its molecule
Phosphate group:
The phosphate group gives nucleic acids their acid nature
The structure of ATP
ATP (adenosine triphosphate) is also a nucleotide
Structure of ATP consists of three components; adenosine, ribose and phosphate
, Adenine plus ribose forms a sugar-base called Adenosine
Adenosine can be combined with one, two or three phosphate groups to give; Adenosine
monophosphate, Adenosine diphosphate and Adenosine triphosphate (AMP, ADP, and
ATP)
DO NOT confuse Adenine with Adenosine; Adenosine is adenine with a sugar joined to
it
DO NOT confuse Thymine with Thiamine; Thiamine is a vitamin
Dinucleotides and Polynucleotides
Two nucleotides can be joined together by a condensation reaction
The molecule formed by joining two nucleotides is called a Dinucleotide
The bond formed is called a Phosphodiester bond
“-diester” is used because the phosphate group involved now has two ester bonds, one
to each sugar it is connected to
The sugars and phosphates are linked by the phosphodiester bonds to form a backbone
from which the bases stick out sideways at right angles to the backbone
-> Dinucleotide: two nucleotides joined together by a phosphodiester bond
-> Phosphodiester bond: a bond joining two nucleotides together there are two ester bonds, one
from the shared phosphate group at each of the sugars either side of it
Structure of DNA
The idea of the molecule having two polynucleotide chains, not one
The two chains could be held together by hydrogen bonding between the bases of the
two chains
The only way this could be done would be to twist each of the two strands into a helical
shape - a double helix - and to run the strands in opposite directions
Features of the DNA molecule
The 5’ and 3’ (five-prime and three-prime) refer to carbon atoms 5 and 3 of the sugar
The two ends of a DNA strand are called the 5’ end and 3’ end
At the 5’ end is phosphate and 3’ end is sugar
-> The DNA molecule has the following features:
Made of two polynucleotide chains
Each chain is a right-handed helix
The two chains coil around each other to form a double helix
The chains run in opposite directions (they are said to be antiparallel)
Each chain has a sugar-phosphate backbone with bases projecting at right-angles
The bases in one chain are attracted to the bases of the other chain by hydrogen
bonding between the bases (this holds the chains together)
Adenine ( A ) always binds with Thymine ( T )
Cytosine ( C ) always binds with Guanine ( G )
-> This is referred to as Complementary base pairing; the hydrogen bonding of A with T or U,
and of C with G in nucleic acids
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