1. DNA is made up of nucleotides that form a double helix structure. Each nucleotide contains a phosphate group, sugar, and one of four nitrogenous bases. 2. The two strands of the DNA helix run in opposite directions and are held together by hydrogen bonds between complementary base pairs on each strand. Adenine bonds with thymine and cytosine bonds with guanine. 3. The coiled and double helix structure of DNA allows it to efficiently store the genetic code within the nucleus of cells while protecting the genetic information through its structure.

1. NUCLEIC ACIDS
Structure and function

2. A brief history of nucleic acids
• 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).

3. • 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!

4. • 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”

5. • 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!)

6. So what are nucleic acids?
• There are 2 types:
– DNA
(deoxyribonucleic acid)
– RNA
(ribonucleic acid)
• Polymers
(i.e made of many
monomers joined ogether)

7. More about nucleic acids
• They are in charge of:
STORAGE
TRANSMISSION
USE
»Of genetic information

8. More on nucleic acids
• Composed of monomers called nucleotides
• Each nucleotide has:
– A pentose (5 carbon) sugar
– A phosphate group
– A nitrogen-containing base

9. 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)

10. Each nucleotide must have:
1. Phosphate group

11. 2. A single sugar

12. A single base

13. pyrimidines
purines
DNA’s nucleotides

14. DNA is unique because of the
nucleotide sequence
• Only 4 nitrogenous bases – thus only 4
nucleotides are found in DNA
• What does this mean?

15. 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.

16. 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.

17. 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.

18. Complementary base-pairing
A
T
C
G
Always
adenine – thymine
cytosine - guanine

19. Exercise: Which is the complementary
strand?
A
T
T
A
C
G
C
G
G
C
C
G
T
A

20. •
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.

21. 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).

22. The 2 strands run anti-parallel to each other

23. 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.

24. 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)

25. 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.

26. 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

27. 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.

28. Summary 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.

29. So how does DNA
STORE
TRANSMIT
USE
genetic information?