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NUCLEIC ACIDS (DNA)
Structure and function

Helix
• Most DNA has a righthand twist with 10 base
pairs in a complete turn
• Left twisted DNA is
called Z-DNA or
southpaw DNA
• Hot spots occur where
right and left twisted
DNA meet producing
mutations

copyright cmassengale

4

The structure of DNA and RNA
• Genetic material of living organisms is either
DNA or RNA.
• DNA – Deoxyribonucleic acid
• RNA – Ribonucleic acid
• Genes are lengths of DNA that code for
particular proteins.

A BRIEF HISTORY ON NUCLEIC
ACID
• Frederich Meischer (1844-1895)
– Extracted pus from wounds and
bandages
– Isolated a substance from white blood
cells which he named “nuclein” (because
it was found in the nucleus of cells).

• Edmund Beecher Wilson (1856-1939)
– Establishes that “mother’s” nucleus contain the same
number of chromosomes as “father’s nucleus”, and
both are present in the offspring…
– Therefore half of the information is received from
each parent!

• Oscar Hertwig (1849-1922)
– Suggests nuclein is needed to inherit characteristics
from parents.

• Richard Altmann (1852-1900)
– Determines nuclein is actually acidic and changes
the name to “nucelic acid”

A picture of a DNA molecule using
Crystallography by Franklin

• Phoebus Aaron Levene (1869-1940)
– Suggested nucleic acids had
a highly repetitive sequence…
(although he could not describe
the repetition patterns)

• Albrecht Kossel (1853-1927)
– Worked with yeast (why?)
– Showed there were 2 types of nucleic acids
(and won a Nobel prize for this!)

So what are nucleic acids?
• There are 2 types:
– DNA
(deoxyribonucleic acid)
– RNA
(ribonucleic acid)

• Polymers
(i.e made of many
monomers joined ogether)

More about nucleic acids
• They are in charge of:

STORAGE

TRANSMISSION
USE
»Of genetic information

More on nucleic acids
• Composed of monomers called nucleotides

• Each nucleotide has:
– A pentose (5 carbon) sugar
– A phosphate group
– A nitrogen-containing base

More on nucleic acids (remember there
are 2 types: DNA and RNA)

DNA bases

RNA bases

Thymine (T)

Uracil (U)

Adenine (A)

Adenine (A)

Cytosine (C)

Cytosine (C)

Guanine (G)

Guanine (G)

Each nucleotide must have:
1. Phosphate group

pyrimidines

purines

DNA’s nucleotides

DNA is unique because of the
nucleotide sequence
• Only 4 nitrogenous bases – thus only 4
nucleotides are found in DNA

• What does this mean?

1. DNA sequences are unique
• Nucleotides can join to each other in 2 ways:
- As a sequence
- By complementary base pairing (not e shown below)

A

C

C

G

T

A

T

A

G

The sequence is called the “genetic code” and is
UNIQUE to each individual.

How do nucleotides join in a sequence?
•

Nucleotides can join through
covalent bonds between their

sugar group
AND
phosphate group

Forming the
sugar-phosphate backbone
bases are not involved directly in
this type of bonding.

Nitrogenous bases – Two types
Pyrimidines
• Have single ring

Thymine - T
Cytosine - C
Uracil - U

Purines
• Have double rings of
Carbon and Nitrogen
atom

Adenine - A
Guanine - G

Base-Pairings: Purines only pair with Pyrimidines
AS Biology. Gnetic control of protein structure and

2. Complementary base pairing
• RNA consists of a single strand, but DNA consists
of a double strand.
• In a double strand, BASES bond to each other.
• This IS NOT AT RANDOM, but occurs through
COMPLEMENTARY BASE PAIRING
• A purine will always
bond a pyrimidine.

Complementary base-pairing
A

T

C

G

Always
adenine – thymine
cytosine - guanine

Exercise: Which is the complementary
strand?
A

T

T

A

C

G

C

G

G

C

C

G

T

A

•

Hydrogen bonds
attach nucleotide
bases to each other,
and determine the
bases that CAN join
(i.e complementary
bases)

•

C and G make 3 H
bonds.

•

A and T make 2 H
bonds.

The hydrogen bonding is one of the causes leading a
DNA molecule to twist (like a double helix)

• This was
discovered by
James Watson
and Francis
Crick (both of
whom won a
Nobel Prize for
this).

The 2 strands run anti-parallel to each other

Questions:
1. Physically, why can’t a guanine (G) in one strand
bond with an adenine (A) in another strand?
 Guanine and Adenine are both

purines.
 This means they are slightly
larger molecules than Thymine
and Cytosine.
 If they pair up, the distance
between one strand and the other
is larger than the average, and
surrounding nucleotides could not
bond.

1. Physically, why can’t a guanine (G) in one strand
bond with an adenine (A) in another strand?
 Guanine

can
make 3 hydrogen
bonds, whereas Adenine can only make
2.
This makes the bonding unstable as
Guanine ends up with a “lose” end (nonbonded hydrogen)

Questions
2. Why does DNA “need” to have a “coiled”
shape?
DNA carries ALL the information
that makes up an organism.
It is present in EVERY nucleus
of EVERY cell of the organism.
(If DNA was extended side by
side, the DNA in our bodies would
be long enough to go around the
earth!)
By coiling DNA (in fact, SUPERCOILING it), DNA can actually fit
into the nucleus of each cell.

2. Why does DNA “need” to have a “coiled”
shape?
 DNA carries the hereditary information.
 By COILING it, the “strong” part
(sugar-phosphate backbone) is
exposed, rather than the bases.
 This confers some “protection” to the
information

Summary on DNA structure:
 DNA is a ______ helix.
 Each strand runs _____________ to the other.
 Alternating ______ and _________ molecules form a backbone
for each strand.
 The two strands backbone sugar and phosphate molecules are
held together by ___________ bonds.
 The two strands are held together by _________ bonds between
complementary base pairs.
 There are four nitrogenous bases: two _______, Adenine (A) and
Guanine (G) and two __________, Thymine (T) and Cytosine (C).
 Adenine and ________ always bond through 2 hydrogen bonds.
 Cytosine and _______ always bond through 3 hydrogen bonds.

SUMMERY ON DNA STRUCTURE:
 DNA is a double helix.
 Each strand runs anti-parallel to the other.
 Alternating sugar and phosphate molecules form a backbone for
each strand.
 The two strands backbone sugar and phosphate molecules are
held together by covalent bonds.
 The two strands are held together by hydrogen bonds between
complementary base pairs.
 There are four nitrogenous bases: two purines, Adenine (A) and
Guanine (G) and two pyrimidines, Thymine (T) and Cytosine (C).
 Adenine and Thymine always bond through 2 hydrogen bonds.
 Cytosine and Guanine always bond through 3 hydrogen bonds.

SO HOW DOES DNA

STORE
TRANSMIT

USE
genetic information?

REFERENCE
• Gavin40 accessed from
http://www.slideshare.net/gavin40/nucleic-acids29117862
• Ihmcbiology1213 accessed from
http://www.slideshare.net/ihmcbiology1213/dnastructure-15152681
• Campbell & Reece: Chapters, 5, 16.

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NUCLEIC ACIDS (DNA)

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