NUCLEIC ACIDS
AS Biology
BY1
NUCLEIC ACIDS - MADE IN THE NUCLEUS AND SLIGHTLY ACIDIC
 Long
molecules (polymers) made up of
nucleotides
 Linked by condensation reactions
 2 main types:
DNA = deoxyribonucleic acid
 RNA = ribonucleic acid

NUCLEOTIDES
 Nucleotides
•
•
•
consist of a:
Pentose sugar
Phosphate group
Nitrogen-containing base
•
(purine or pyrimidine)
INTRODUCTION TO NUCLEOTIDES
All nucleotides contain the following three groups:
a
phosphate
group
a nitrogencontaining
base
a pentose
sugar
In DNA the sugar is deoxyribose, whereas in RNA the
sugar is ribose.
FORMATION OF NUCLEOTIDES
DNA
 Has
deoxyribose as the sugar
 Exists as a double helix of 2 chains
linked by H bonds between
complementary bases
 Contains the nucleotides:
Adenine
 Thymine
 Cytosine
 Guanine

Figure 2.16
COMPLEMENTARY BASE PAIRING
A
bonds with T
 C bonds with G
:
BASES
There are five bases, split into two types:


adenine (A)
and guanine
(G) are purine
bases.
thymine (T),
cytosine (C) and
uracil (U) are
pyrimidine
bases.
A
T
G
C
DNA contains A, G, T and C, whereas RNA
contains A, G, U and C.
U
IDENTIFYING BASES
BASE PAIRING RULES
FORMATION OF POLYNUCLEOTIDES
DNA STRUCTURE
Held by
covalent
bonds
DNA
2
major functions:
replication, in dividing
cells
2. carrying the
information for protein
synthesis in all cells
1.
 Replication
allows
accurate copying of
DNA for cell division
DETERMINING THE STRUCTURE OF DNA
The double-stranded
structure of DNA was
determined in 1953 by the
American biologist James
Watson and the British
physicist Francis Crick.
X-ray diffraction studies by
British biophysicist Rosalind
Franklin strongly suggested
that DNA was a helical
structure.
The Austrian chemist Erwin Chargraff had earlier showed that DNA
contained a 1:1 ratio of pyrimidine:purine bases.
STRUCTURE OF DNA
HOW IS DNA PACKAGED?
In eukaryotic cells:
• DNA is packaged as chromosomes in the nucleus
•It is tightly coiled and folded so
that all 2m can fit in the nucleus
•It is wrapped around proteins
called histones
•DNA + Histones = chromatin
In prokaryotic cells:
• DNA is loose in the cytoplasm –
there are no histones or
chromosomes.
STRUCTURE OF EUKARYOTIC
CHROMOSOMES
RNA
 Has
ribose as the sugar
 Is single-stranded
 Uracil replaces
Adenine
A
can hydrogen bond
with U
 C can hydrogen bond
with G
RNA
3
types:
Ribosomal (rRNA)
 Messenger (mRNA)
 Transfer (tRNA)

RNA is made by making a complementary
copy of a section of the DNA
DNA
RNA
RIBOSOMAL RNA
•Makes
up part of the
ribosome – made in
the nucleolus
ribosome
mRNA
strand
TRANSFER RNA
amino acid
attachment site
3’ end
•Found
in the
cytoplasm
•Used to carry amino
acids to the
ribosome during
protein synthesis
5’ end
hydrogen bond
nucleotides
anticodon
DNA
MESSENGER RNA
•Is
a mobile copy of a
gene that codes for a
protein
•It moves out of the
nucleus to the
ribosome to act as a
template for protein
synthesis
mRNA
ribosome
mRNA
strand
ATP – ADENOSINE TRIPHOSPHATE

Has ribose sugar, adenine, and 3 phosphate groups
(Has 2 extra phosphates on the adenine containing nucleotide
ATP
Is made by condensation reactions
(dehydration synthesis)
 Is broken by hydrolysis reaction to liberate useful
energy for the cell

DNA REPLICATION
DNA REPLICATION
•Watson
and Crick provided evidence that
complementary base pairing was key to DNA’s
ability to replicate
•The
DNA ‘unzips’ as hydrogen bonds
between base pairs are broken
DNA REPLICATION
•New
polynucleotide strands can
then be synthesized using the
originals as a template
•Replication
allows accurate
copying of the DNA for cell
division (mitosis and meiosis)
DNA REPLICATION
WHAT’S THE KEYWORD?
MULTIPLE-CHOICE QUIZ