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

Many nucleotides in DNA :

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

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

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: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%

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 DNApentose sugar: ribose

pentose sugar: deoxyribose

bases: adenine, guanine, cytosineuracil

bases: adenineguaninecytosinethymine

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]

interpret and carry out the instructions coded in the DNA

carry instructions for making proteins

RNA DNARNA strands are continually made, broken down and reused

DNA is completely protected by the body

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

THE END