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Outline :1) Structures of nucleotides2) The formation of phosphodiester bonds in

a polynucleotide3) Distinguish between DNA and RNA4) Structure of DNA based on Watson and

Crick Model

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Found in nucleus of a cell, is the important

genetic material of all living organisms. Elements : carbon, hydrogen, oxygen,

phosphorus and nitrogen are bound up toform different compounds which are then

linked up to produce different types ofnucleotides. Polynucleotides are biopolymers of

nucleotides that include DNA and RNA.

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DNA (deoxyribonucleic acid) Double-stranded helical spiral (twisted

ladder)

Serves as genetic information center

In chromosomes RNA (ribonucleic acid) Part single-stranded, part double-

stranded

Serves primarily in assembly of proteins

In nucleus and cytoplasm of cell

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Consists of 3 components :

A pentose sugar, a phosphate group & anitrogenous base 3 components are linked together through

condensation reactions. Pentose sugar and the nitrogenous base

are joined together to form a nucleoside.

The 5th

carbon of the pentose sugar linked with phosphate to form a phosphodiester

bond.

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5

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Deoxyribose has the same structure asribose except that it does not have anoxygen atom at its second carbon.

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Single ringcarbon-

nitrogen

compound

Double

ringcarbon-

nitrogen

compound

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Double helix form Was first discovered by Watson and Crick

(1953)

(evidences : X-ray diffraction photographs ofDNA & chemical data on the base compositions

of DNA) Diameter of DNA helix is 2nm.

The distance between pairs is 0.34 nm

A complete turn of the DNA helix consists of 10base pairs with a distance of 3.4 nm.

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a polymer consists of2 polynucleotide strandsheld together by hydrogen bonding

2 strands are antiparallel, 5 to 3 & 3 to 5

4 different bases : adenine(A), guanine (G),cytosine (C) & thymine (T).

AT (2 hydrogen bonds) CG (3 hydrogen bonds) 2 polynucleotide strands are complementary.

Sequence of bases in DNA forms thegeneticcode thatcontrolsthe activities of the cells &determines the characteristic of an organism.

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O

O=P-OO

PhosphateGroup

NNitrogenous base

(A, G, C, or T)

CH2

O

C1C4

C3 C2

5

Sugar

(deoxyribose)

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consists a single polynucleotide strand

shorter than in DNA

Each nucleotide containsa ribose sugar, aphosphate & one of the 4 nitrogenous bases A,

uracil (U), C & G Uracil in RNAreplacesthymine in DNA

3 main types of RNA:

Messenger RNA (mRNA) Ribosomal RNA (rRNA)

Transfer RNA (tRNA)

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consists a single RNA polynucleotide strand

mRNA contains complementary codons formedby the transcription of a single cistron (gene)on a single DNA strand.

Carries the genetic code transcripted from DNAin the nucleus to the ribosomes in thecytoplasm where protein is synthesised.

About 3 5% of the total RNA of the cell ismRNA.

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The smallest RNA molecules

Different tRNAs to carry different amino acidsin the cytoplasm to the ribosomes for thepolymerisation of polypeptide chains.

A single strand polynucleotide folded into athree-dimensional clover-leaf shape held byhydrogen bonds.

The 3-end always ends in the base sequenceof CCA and acts as the amino acid binding site.

The anticodon consists of a base tripletsequence and is directly related to the specificamino acid carried by the tRNA molecule.

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Consists of a single polyuncleotide strandsynthesised in the nucleolus.

rRNA form aggregates with proteins to form thelarge ribosomal subunit and small ribosomalsubunit .

The subunits leave the nucleus via nuclear poreinto the cytoplasm.

The subunits combine to form a ribosome. It makes up about 80% of the total RNA of the

cell.

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Feature DNA RNASugar deoxyribose ribose

Bases C, G, A, T C, G, A, U

Strands Double-strandedPairing across strand

Mostly single stranded

Helix Double helix form No double helix form

Types Only one type of

DNA

3 types (mRNA, tRNA,

rRNA)

Function Heredity, cellularcontrol center

Interprets genetic info,protein synthesis

Where Chromosomes of cell

nucleus

Cell nucleus and

cytoplasm