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CARBOHYDRATES
-compounds of C, H and O
-originally thought of as ‘hydrates of carbon’
e.g. glucose C6H12O6 thought to be C(H2O)
carbohydrates:
-are poly-hydroxylated aldehydes and ketones
-can cyclise
-can form polymeric chains
CARBOHYDRATES
• are synthesized by plants in photosynthesis
(sun’s energy converts CO2 and water into glucose and O2)
• when glucose is metabolised (oxidised):
multistep process that forms carbon dioxide, water, and a great
deal of energy
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Monosaccharides
• simplest carbohydrates are monosaccharides
(simple sugars)
• have three to seven carbon atoms in a chain,
with a C=O at either the terminal carbon (C1)
or C2
• other carbon atoms bear OH group
• monosaccharides usually drawn with C=O
near top of structure
Physical Properties of Monosaccharides
• sweet tasting (relative sweetness varies)
• many hydroxy groups:
⇒ hydrogen bonding
⇒ polar
⇒ high melting points
⇒ insoluble in organic solvents like ether
⇒ highly water soluble
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Naming Monosaccharides
-we classify monosaccharides by the type of carbonyl they contain
(aldehyde or ketone):
Classifying Monosaccharides
…and by the number of carbon atoms they contain:
-six carbon
aldo sugar
-six carbon
keto sugar
-five carbon
aldo sugar
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Cyclic Forms of Monosaccharides
• The hydroxy and carbonyl groups of monosaccharides can react
intramolecularly to give cyclic form:
Disaccharides
-some carbohydrates are complex e.g. table sugar (sucrose) is a
disaccharide.
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Disaccharides
-some carbohydrates are complex e.g. table sugar (sucrose) is a
disaccharide.
Can be hydrolysed into monosaccharides e.g.
1 sucrose
H3O+
1 glucose
+
1 fructose
(monosaccharide)
(monosaccharide)
(disaccharide)
Polysaccharides
-some carbohydrates are complex e.g. table sugar sucrose is a
disaccharide. Can be hydrolysed into monosaccharides e.g.
starch and cellulose are polymers
of glucose:
1 cellulose
H3O+
> 2000
glucose
HO
CH2OH
OH
O
CH2OH
O
HO
OH
O
OH
HO
D-galactose-D-glucose
Lactose
(a disaccharide)
Starch
(a polysaccharide)
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Sugars in Nucleic Acids
• two sugars, D-ribose and 2-deoxy-D-ribose form the building
blocks of RNA and DNA, respectively.
NUCLEIC ACIDS (Ch. 25.4)
Nucleic acids are very high
molar mass biopolymers that
play an essential role in protein
synthesis.
ribonucleic acid
(RNA)
Deoxy-ribonucleic acid
(DNA)
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What is a nucleotide?
-DNA strands are composed of nucleotides
single strand
of DNA
one
nucleotide
-phosphoester bonds link sugar-base units together
What is a nucleotide?
-a sugar-phosphate-base unit is called
a nucleotide:
a nucleotide
(fig. 25.18)
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Bases in DNA
-five bases are found in nucleic acids:
(A)
(G)
(C)
(T)
(U)
-base composition always follows ‘pairing’ pattern e.g.:
Clostridium bacteria: 37.2% A 37.2% T 12.8% G 12.8% C
Bases in DNA
Chargaff’s Rules (p. 1054)
amount of A = amount of T
amount of G = amount of C
-base composition always follows ‘pairing’ pattern e.g.:
Clostridium bacteria: 37.2% A 37.2% T 12.8% G
12.8% C
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Base pair formation in DNA
-adenine (A) hydrogen bonds only to thymine (T)
-C only forms complementary hydrogen bonds to G
Base pair formation in DNA
two-stranded
DNA double helix
-two strands of DNA hydrogen
bond to and coil around each
other forming a double helix:
(base
pair)
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Base pair formation in DNA
-two strands of DNA hydrogen
bond to and coil around each
other forming a double helix:
(base
pair)
two-stranded
DNA double helix
Chargaff’s Rules
Q: DNA from sea urchins is found to contain 18% G nucleotides.
Use Chargaff’s Rules to deduce the percentage abundance of C, T
and A nucleotides.
A: In DNA, G must pair with C and A must pair with T so:
amount of G = 18%
amount of G-C base pairs = 36%
amount of T-A base pairs = 100% - 36% = 64%
amount of T = amount of A = 32%
Chargaff’s Rules (p. 1054)
amount of A = amount of T
amount of G = amount of C
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Chargaff’s Rules
Q: An 18 picomole sample of nucleotides from bacterial DNA
yielded 3.9 pmol of adenine (A). How much C is expected to be
found in the sample?
A: In DNA, G must pair with C and A must pair with T so:
amount of A = 3.9 pmol (= 21.67%)
amount of T-A base pairs = 43.33%
amount of G-C base pairs = 100% - 43.33% = 56.67%
amount of G = amount of C = 28.33% (5.1 pmol)
Chargaff’s Rules (p. 1054)
amount of A = amount of T
amount of G = amount of C
Replication of DNA
-two strands of DNA are not identical but complementary
-to produce copies of itself :
1. DNA uncoils into two separate strands
2. complimentary bases are attached
new ‘daughter’ DNA strand is
produced featuring identical sequence
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Replication of DNA
Q: What DNA base sequence is complementary to
(5’) TCGGACATCGT (3’)?
A: In DNA, G must pair with C and A must pair with T so:
(5’) TCGGACATCGT (3’)
(3’) AGCCTGTAGCA (5’)
...is the complementary sequence
Replication of DNA
Q: What DNA base sequence is complementary to
(5’) GTGACGACTGC (3’)?
A: In DNA, G must pair with C and A must pair with T so:
(5’) GTGACGACTGC (3’)
(3’) CACTGCTGACG (5’)
...is the complementary sequence
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Transcription of DNA
-DNA provides template for synthesis of all cell’s components
-how are proteins made?
-DNA is transcribed into another nucleic acid, RNA
-messenger RNA (mRNA) is a single stranded nucleic acid featuring
same nucleotides as DNA except base thymine (T) is replaced by
uracil (U) (see fig. 25.4)
Transcription of DNA
-to produce RNA readout of a gene, DNA uncoils
-complimentary bases assemble to form mRNA readout of the gene
Translation of DNA
-mRNA sequence becomes translated in ribosomes where enzymes
(proteins) are assembled from amino acid building blocks
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Chemistry In Action: DNA Fingerprinting
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