NUCLEIC ACIDS

Watson CrickDied in 2004

Watson and Crick described the

structure of DNA: 1953

Nucleic Acids are:

DNA (deoxyribonucleic acid): - giant polymers that carry

instructions for making proteins

RNA (ribonucleic acid):- interpret and carry out the instructions

coded in the DNA

Nucleic acids store the

information to make proteins

Monomers of nucleic acids:nucleotides

A nucleotide consists of a:

Nitrogenous base

Pentose sugar

Phosphate

DNA

DeoxyriboseRibose

Sugars in:RNA

A NUCLEOSIDE is a base + sugar:

A nucleoside + a phosphate =

Why are two water molecules formed?

As two bonds form.

Formation of an RNA nucleotide

Phosphoester link

Many nucleotides in DNA :

Formation of a dinucleotide

Phosphodiester bridge

REMEMBER:

CytosineThymine

PURINE

PYRIMINDINE

AdenineGuanine

In DNA:CytosineThymine

In RNA:CytosineURACIL

Two nucleotides join together to form a dinucleotide and many form a polynucleotide

The nucleotides are joined by:

sugar of one nucleotide

phosphate of the next

phosphodiester linkages between the:

DNA is like a ladder

Bases are like the rungs of the ladder.

Backbones of RNA and DNA consist of:

alternating sugars and phosphates

Bases:- are attached to sugars- project from the side

5´

5´ 3´

3´ 3´

3´5´

5´

T

G

C

T

T

T

C

G

A

C

T

G

T

T

G

A

C

G

A

A

G

C

G

T

A

A

C

A

C

A

T A

G C

GC

A T

TA

C G

G

A T

T A

C G

T A

A T

C G

A T

Cartoon ofbase pairing

Cartoon of double helix

DNA is a double helix.

A DNA molecule:consists of two polynucleotide strands each coiled in a right-handed helix (cylindrical spiral)

The two complementary polynucleotide strands are held

together by hydrogen bonding

between the nitrogenous bases

of adjacent nucleotides

opposite directions - they are antiparallel

In a molecule the two strands run in :

A DNA molecule:is of uniform diameter

RNA is a single strand that may fold

RNA has larger grooves:

easier to be attacked by enzymes

Complementary base pairing: particular bases pair only with certain

bases e.g. adenine – thymine makes it possible to copy DNA

molecules faithfully

Chargaff’s rule

Erwin Chargaff

In human DNA, the four bases are present in these percentages: A=30.9% and T=29.4%; G=19.9% and C=19.8%.

Chargaff’s rule

Chargaff’s rule:A+G = T+C

e.g. If there is 31% adenine in DNA, find the percentage of guanine.

A = T = 31 + 31 = 62%.So C + G = 100 – 62 = 38%. Guanine = 38/2 = 19%

Chargaff also found out that:the composition of DNA varies from one species to another, in particular in the relative amounts

of A, G, T, and C bases.

OrganismPercentage of each base

Adenine Guanine Cytosine ThymineHuman 31.2 18.8 18.8 31.2Cow 27.9 22.1 22.1 27.9Salmon 29.4 20.6 20.6 29.4Rat 28.6 21.4 21.4 28.6

The virus has single-stranded DNA as its genetic material. Explain the evidence from the table which suggests that the DNA is single-stranded. (2)

OrganismPercentage of each base

Adenine Guanine Cytosine ThymineHuman 31.2 18.8 18.8 31.2Cow 27.9 22.1 22.1 27.9Salmon 29.4 20.6 20.6 29.4Rat 28.6 21.4 21.4 28.6 Virus 24.7 24.1 18.5 32.7

amounts of A and T /C and G/complementary bases different; therefore no base-pairing;

Differences between RNA & DNA

RNA DNAA single-stranded molecule in most of its biological roles

Double-stranded molecule

A shorter chain of nucleotides

A long chain of nucleotides

RNA DNApentose sugar: ribose

pentose sugar: deoxyribose

bases: adenine, guanine, cytosineuracil

bases: adenineguaninecytosinethymine

RNA DNAchemically less stable

chemically very stable

Three reasons:[see next two slides]

RNA DNARibose sugar is more reactive because of C-OH (hydroxyl) bonds.

Not stable in alkaline conditions.

Deoxyribose sugar in DNA is less reactive because of C-H bonds.

Stable in alkaline conditions.

RNA DNA

RNA has larger grooves, which makes it easier to be attacked by enzymes.

DNA has smaller grooves, which makes it harder for enzymes to "attack" DNA.

RNA DNAratio of:adenine & uracil to cytosine &guanine varies

ratio of:adenine & thymine to cytosine & guanine is one

RNA DNAmanufactures in the nucleus but found throughout the cell

found almost entirely in the nucleus [occurs in mitochondria & chloroplasts]

amount varies from cell to cell (and within a cell according to metabolic activity)

amount is constant for all cells of a species (except gametes and spores)

RNA DNARNA strands are continually made, broken down and reused

RNA is more resistant to damage by ultra-violet rays.

DNA is completely protected by the body, i.e., the body destroys enzymes that cleave DNA.

DNA can be damaged by exposure to ultra-violet rays.

UV radiation generates thymine dimers between adjacent thymine bases in DNA, which disrupts base pairing between DNA double strands. This can give rise to mutations during DNA replication.

RNA DNA Can act as an enzyme [called ribozyme].

Does not act as an enzyme.

Ribozyme structure

RNA DNARNA is synthesized from DNA when needed

DNA is self-replicating

RNA DNAmaybe temporary - existing for short periods

permanent

3 basic forms: messenger, transfer and ribosomal RNA

only one basic form, but with an almost infinite variety within that form

Keep the following in mind when studying this topic:

Nucleic Acids

Carbohydrates

Lipids

Proteins

What they look like

What they do/Where are they found

What are they made up of- at the level of atom

OverviewA. WATERB. THE KINDS OF MOLECULES THAT

CHARACTERISE ORGANISMSC. CARBOHYDRATESD. LIPIDSE. PROTEINSF. NUCLEIC ACIDSG. IMPORTANCE OF CARBON IN BIOLOGICAL

MOLECULES

Living organisms on Earth are made of primarily:

1. water 2. molecules containing carbon

Carbon occurs in EVERY cell:

Carbon: an element found in all organic molecules

Organic describes molecules that have:a C skeleton some H atoms

Inorganic molecules include:CO2

all molecules without C, e.g. H2O

Hydrocarbons:molecules consisting only of C & H

Carbon has four bonding sitesThis makes carbon: versatile & stable

H H C C

H

H

14 electrons: 2 , 8 , 4

H H C C

H

H

Carbon-hydrogen covalent bonds (C-H bonds) are:

nonpolar (equal sharing of electrons) contain large amounts of energy

14 electrons: 2 , 8 , 4

Carbon may bond to:1) itself forming carbon chains

Chains Branches Rings

Carbon chains: form the skeletons of most

organic molecules vary in length and shape

Carbon may bond to:2) other elements e.g. H, O, N

Fatty acid

Organic Molecules contain at least one C-C bond

What are the two most important characteristics of carbon as a basis for the

chemistry of life ?

it has four valence bonds

the energy required to make or break a bond is

just at an appropriate level for building

molecules which are: i) stable ii) reactive

1 2

Carbon is the basic building block of life. Discuss. [SEP, 2002]

Despite the relatively restricted number of chemical elements found in living organisms, the molecules they form are structurally very diverse and complex. Discuss. [MAY, 2009]

Essay Titles

THE END