Hydrolysis reactions occur every time food is digested. These reactions involve breaking down polysaccharides, polypeptides and triglycerides into the smaller units of which they are made. Water molecules are used in hydrolysis reactions – they are the reverse of condensation reactions. Once again, enzymes are required to catalyse the reactions.
•
Hydrolysis of starch (a polysaccharide) uses water and produces many molecules of glucose.•
Hydrolysis of protein (made of polypeptide chains) uses water and produces many amino acids.•
Hydrolysis of a triglyceride (a lipid) uses water and produces fatty acids and glycerol molecules.3 THE CHEMISTRY OF LIFE 47 Figure 3.9 The structure of the four nucleotides in DNA.
DNA (deoxyribonucleic acid) molecules make up the genetic material of living organisms. DNA is an extremely long molecule but, like proteins and carbohydrates, it is built up of many subunits. The subunits of DNA are called nucleotides.
Each nucleotide consists of three parts – a sugar (deoxyribose), a phosphate group and a nitrogenous base (Figure 3.8). DNA contains four diff erent bases: adenine, guanine, cytosine and thymine. These are usually known by their letters: A, G, C and T (Figure 3.9).
To form a DNA molecule, nucleotides are linked together. The phosphate group of one nucleotide links to the deoxyribose of the next molecule to form a chain of nucleotides, as shown in Figure 3.10 (overleaf). The sugar and phosphate groups are identical all the way along the chain and form the backbone of the DNA molecule. The sequence of bases in the chain will vary and it is this sequence that forms the genetic code determining the characteristics of an organism.
Two strands of nucleotides are linked by hydrogen bonds that form between the bases and this double strand makes up the double helix of a complete DNA molecule (Figure 3.10). Adenine always pairs with thymine and is bonded with two hydrogen bonds, while cytosine is paired with guanine by three hydrogen bonds. The arrangement is known as complementary base pairing. Notice that the two DNA chains run in opposite directions and are said to be antiparallel.
You can imagine the molecule rather like a rope ladder with the sugar–
phosphate backbone being the sides of the ladder and the rungs being formed by the hydrogen-bonded base pairs. To form the characteristic double helix of a DNA molecule, the ladder must be twisted to resemble a spiral staircase.
Figure 3.8 The general structure of a DNA nucleotide.
phosphate pentose sugar
organic base
thymine
cytosine
adenine
guanine purines
pyrimidines
T
C
A
G
phosphate pentose sugar
organic base
thymine
cytosine
adenine
guanine purines
pyrimidines
T
C
A
G phosphate
deoxyribose
base
3.4 DNA replication
Assessment statements
•
Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase.•
Explain the signifi cance of complementary base pairing in the conservation of the base sequence of DNA.•
State that DNA replication is semi-conservative.An essential feature of DNA is that it must be able to replicate itself accurately, so that when a cell divides the genetic code it carries can be passed on to the daughter cells. DNA replication copies DNA precisely so that new molecules are produced with exactly the same sequence of bases as the original strands. DNA replication takes place in the nucleus during the S phase of the interphase of the cell cycle when DNA is not tightly coiled.
As Figure 3.11 shows, this process does not occur in a haphazard manner. An enzyme called helicase unzips one region of the DNA molecule and nucleotides are added in a step-by-step process that links them to one another and to their complementary bases in an area known as the replication fork.
1 The fi rst step in the process is the ‘unzipping’ of the two strands.
Helicase moves along the double helix, unwinding the two strands, which separate from one another as the relatively weak hydrogen bonds between the bases are broken.
Figure 3.10 The structure of DNA.
How nucleotides join together.
Each nucleotide is linked to the next by covalent bonds between phosphates and sugars.
Part of a DNA molecule. Two DNA strands, running in opposite directions, are held together
by hydrogen bonds between the bases. The DNA double helix. It has a
‘backbone’ of alternating sugar–phosphate units, with the bases projecting into the centre creating base pairs.
C links to G by three hydrogen bonds
A links to T by two hydrogen bonds one
DNA strand
one DNA strand
complementary base pair sugar-phosphate
‘backbone’
G
A G
A
T C
C T
3 THE CHEMISTRY OF LIFE 49 2 The unpaired nucleotides are exposed and each single strand now acts
as a template for the formation of a new complementary strand. Free nucleotides move into place: C pairs with G and A pairs with T.
3 The free nucleotide bases form complementary pairs with the bases on the single DNA strands. DNA polymerase is the enzyme involved in linking the new nucleotides into place. Finally, the two new DNA molecules are rewound, each one forming a new double helix.
The two new DNA strands that are produced are absolutely identical to the original strands. Complementary base pairing between the template strand and the new strand ensures that an accurate copy of the original DNA is made every time replication occurs. DNA replication is said to be semi-conservative because no DNA molecule is ever completely new.
Every double helix contains one ‘original’ and one ‘new’ strand.
3.5 Transcription and translation
Figure 3.11 DNA replication.
Assessment statements
•
Compare the structure of RNA and DNA.•
Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.•
Describe the genetic code in terms of codons composed of triplets of bases.•
Explain the process of translation leading to polypeptide formation.•
Discuss the relationship between one gene and one polypeptide.two new strands 1 hydrogen bonds
between base pairs are broken - DNA
‘unzips’
2 free nucleotides diffuse into position
3 base pairing between the bases on opposite strands, and condensation reactions between pentose and phosphate in the new strand make new polynucleotide strands - one strand acting as a template for the other
T A
T A
T A
C G C
C G G
C
G T
A T
A T
A C
G
G
G A
C
T A
C C G
C C C
G T
A A
T
A G
T C T
G G
1 hydrogen bonds between base pairs broken - ‘unzipping’
2 free nucleotides diffuse into position
3 base pairing reactions form an mRNA strand – the reference strand of the DNA acts as a template
mRNA strand reference strand of DNA
The main role of DNA is to direct the activities of the cell. It does this by controlling the proteins that the cell produces. Enzymes, hormones and many other important biochemical molecules are proteins, which control what the cell becomes, what it synthesises and how it functions.
Protein synthesis can be divided into two sets of reactions: the fi rst is