THE NUCLEIC ACIDS

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Friedrich Miescher in 1869

• Isolated what he called

nuclein from the nuclei of

pus cells

•Nuclein was shown to

have acidic properties,

hence it became called

nucleic acid

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Two types of nucleic acid are found

•Deoxyribonucleic acid (DNA)

•Ribonucleic acid (RNA)

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The distribution of nucleic acids in the

eukaryotic cell

• DNA is found in the nucleus

with small amounts in mitochondria and

chloroplasts

• RNA is found throughout the cell

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DNA as genetic material: The

circumstantial evidence1. Present in all cells and virtually restricted to the

nucleus

2. The amount of DNA in somatic cells (body cells) of any

given species is constant (like the number of

chromosomes)

3. The DNA content of gametes (sex cells) is half that of

somatic cells.

In cases of polyploidy (multiple sets of chromosomes)

the DNA content increases by a proportional factor

4. The mutagenic effect of UV light peaks at 253.7nm.

The peak for the absorption of UV light by DNA

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NUCLEIC ACID STRUCTURE

•Nucleic acids are

polynucleotides

•Their building blocks are

nucleotides

© 2016 Paul Billiet ODWS

P

P

P

P

P

P

G

C

C

A

T

T

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NUCLEOTIDE STRUCTURE

PHOSPHATE

SUGAR

Ribose or

Deoxyribose

BASE

PURINES PYRIMIDINES

Adenine (A)

Guanine(G)

Cytocine (C)

Thymine (T)

Uracil (U)

P

P

P

P

P

P

G

C

C

A

T

T

Types of nucleotides

Deoxyribose or Ribose is a pentose

C1

C5

C4

C3 C2

O

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P

P

P

P

P

G

C

C

A

T

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RIBOSE DEOXYRIBOSE

CH2OH

H

OH

C

C

OH OH

C

O

H HH

C

CH2OH

H

OH

C

C

OH H

C

O

H HH

C

Spot the difference

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THE SUGAR-PHOSPHATE

BACKBONE

•The nucleotides are all

orientated in the same

direction

•The phosphate group joins

the 3rd Carbon of one sugar

to the 5th Carbon of the next

in line.

P

P

P

P

P

P

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ADDING IN THE BASES

•The bases are attached to

the 1st Carbon

•Their order is important

It determines the genetic

information of the molecule

P

P

P

P

P

P

G

C

C

A

T

T© 2016 Paul Billiet ODWS

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DNA IS MADE OF

TWO STRANDS OF

POLYNUCLEOTIDE

P

P

P

P

P

P

C

G

G

T

A

A

P

P

P

P

P

P

G

C

C

A

T

T

Hydrogen bonds

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DNA IS MADE OF TWO STRANDS OF

POLYNUCLEOTIDE

• The sister strands of the DNA molecule run in opposite directions (antiparallel)

• They are joined by the bases

• Each base is paired with a specific partner:

A is always paired with T

G is always paired with C

Purine with Pyrimidine

• Thus the sister strands are complementarybut not identical

• The bases are joined by hydrogen bonds, individually weak but collectively strong.

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C

G

G

T

A

A

P

P

P

P

P

P

G

C

C

A

T

T

Hydrogen bonds

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Purines & Pyrimidines

Adenine

CytosineGuanine

Thymine

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Structure of nucleotidesP

P

P

P

P

P

G

C

C

A

T

T

Nucleotide Structure - 4Base-Sugar-PO4

2-

N

N

5

61

2

3

4

OC

5’

1’4’

3’ 2’

OH

Nucleoside

Nucleotide Structure - 4Base-Sugar-PO4

2-

P

O

O

O

O

N

N

5

61

2

3

4

OC

5’

1’4’

3’ 2’

OH

Nucleoside Monophosphate

+ HPO4 2-O N

N

N

N

NH2

HO OH

OPO

O

O

P

O

O

O

PO

O

O

ATP

H2O O N

N

N

N

NH2

HO OH

OPO

O

O

P

O

O

O

ADP

+ ~31 KJ/mol (7.4 Kcal/mol)

20

sugar base

sugar base

phosphate

sugar base

phosphate

sugar base

phosphate

sugar base

phosphate

nucleoside nucleotides

nucleic acids

Nucleoside, nucleotides and nucleic acids

The chemical linkage between monomer units in nucleic acids

is a phosphodiester

Deoxyribonucleotides of nucleic acids

PP

(PPi)

Nucleic Acid StructurePolymerization

P P P

S

N

C

P P P

S

N

C

+

P P P

S

N

C

P

S

N

C

Phosphodiesterase

Formation of polynucleotide

Nucleic Acid StructureThe double helix

Major

Groove

Minor

Groove

Physical Structure (cont’d)

Chains are anti-parallel (i.e in opposite directions)

Diameter and periodicity are consistent 2.0 nm

10 bases/ turn

3.4 nm/ turn

Width consistent because of pyrimidine/purine pairing

Wilkins & Franklin (1952): X-ray crystallography

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Erwin Chargaff’s Data (1950-51)

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Watson & Crick Base pairing

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The Double Helix (1953)

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