1

1.2 CARBOHYDRATES

1.4 PROTEINS

1.3 LIPIDS

1.5 NUCLEIC ACIDS

1.1 WATER

2

1.5 NUCLEIC ACIDS

1.5 Nucleic Acids (11/2 hours)

Objectives :

• Describe the structure of nucleotide as the basic composition of nucleic acid (DNA & RNA).

• Describe the structure of DNA based on the Watson & Crick Model.

• State the types and function of RNA.

• State the differences of DNA & RNA.

NUCLEIC ACIDS

Structure of DNA

Differences ofDNA & RNA

Structure of nucleotide

Types & functionOf RNA

mRNA, rRNA, tRNA

DNAdeoxyribonucleic

acid

RNAribonucleic

acid

• 2 types

• Complex organic molecules• Polynucleotides (polymers of nucleotides)

NUCLEIC ACIDS

Monomers / basic composition of nucleic acids

Each nucleotide composed of:i. Phosphate group

ii. Pentose sugariii. Nitrogenous base

STRUCTURE OF NUCLEOTIDE

(nitrogenous base)

STRUCTURE OF NUCLEOTIDE

phosphate group

STRUCTURE OF NUCLEOTIDE

A hydrogen atom attached

to its 2nd carbon

A hydroxyl group attached

to its 2nd carbon

Pentose sugar

Deoxyribose ( in DNA ) C5H10O4 Ribose ( in RNA ) C5H10O5

Ring structures for the 2 pentose sugars

*In a nucleotide :- phosphate group is joined to the 5th C of pentose- nitrogenous base is joined to the 1st C of pentose

STRUCTURE OF NUCLEOTIDE

Nitrogenous base2 groups

pyrimidine purine

6-membered ring6-membered ring fused to a 5-membered ring

Cytosine

Thymine (in DNA)

Uracil (in RNA)

Adenine Guanine

Nitrogenous base(a) Pyrimidines (Single ring)

(b) Purines (Double ring)

Cytosine (C) Thymine (T) in DNA Uracil (U) in RNA

Adenine (A) Guanine (G)

• Nucleotides are combined by condensation reaction

• forming a polynucleotide (DNA or RNA)

FORMATION OF POLYNUCLEOTIDE

• 2 nucleotides are joined by a phosphodiester bond - between a

phosphate group (of a nucleotide)

& the 3rd C in a pentose (of the other nucleotide)

FORMATION OF POLYNUCLEOTIDE

123

45

Phosphodiester bond

• Forming a backbone with repeating sugar-phosphate units

• Breakdown of polynucleotide may occur by hydrolysis

FORMATION OF POLYNUCLEOTIDE

Organic bases

Pentose sugars

• Watson & Crick proposed the double helix model in 1953 for DNA

• 1 DNA molecule consists of 2 helical polynucleotide strands

• The pentose sugar: deoxyribose • The nucleotides in each strand

are joined by phosphodiester bonds

STRUCTURE OF DNA

• Contains 4 types of organic bases:- adenine (A)- guanine (G)- cytosine (C)- thymine (T)

STRUCTURE OF DNA

• Organic bases of each strand are paired in the interior of the helix

• The helix shape is maintained by H bonds between organic bases of opposite strands

STRUCTURE OF DNA

• Organic bases of each strand are paired in the interior of the helix

• The helix shape is maintained by H bonds between organic bases of opposite strands

STRUCTURE OF DNA• The 2 strands must be

complementary to each other

• A must pair with T with 2 H bonds

• G must pair with C with 3 H bonds

• For a DNA, amount of A = T ; G = C

Hydrogen bonds

Hydrogen bonds

• The 2 strands must run in opposite directions (anti-parallel)~1 strand runs

5’ 3’ while another runs 3’ 5’

STRUCTURE OF DNA

5’

5’

3’

3’

Sugar-phosphate backbone Sugar-phosphate backboneBase pairs

OH

OH

The double helixThe helix is “right-handed”, curving up to right. The two strands are held together by hydrogen bonds (dotted lines) between the nitrogenous bases, which are paired in the interior of the double helix.

• DNA functions as the hereditary genetic material

–as it consists of nucleotides with certain sequence of organic bases

IMPORTANCE OF DNA

• Single stranded polynucleotide• Pentose sugar: ribose• Nitrogenous bases - Adenine (A)

- Guanine (G)

- Cytosine (C)

- Uracil (U)

RNA

Organic base is uracil instead of thymine

Onenucleotide

RNA

• 3 types of RNA :–Messenger RNA (mRNA)

–Ribosomal RNA (rRNA)

–Transfer RNA (tRNA)

TYPES OF RNA

i. mRNA • it carries the genetic code transcript

(copied from DNA)• carries it from the nucleus to the site of

protein synthesis in the cytoplasm

TYPES & FUNCTION OF RNA

Involvement of mRNA in protein synthesis

ii. rRNA• manufactured in the nucleolus• forms the ribosomal subunits

(together with ribosomal proteins)

TYPES & FUNCTION OF RNA

Involvement of rRNA in protein synthesis

iii. tRNA

• smallest RNA molecules containing 75 - 80 nucleotides

• different tRNAs to transfer (carry) the different amino acids to a ribosome during protein synthesis

TYPES & FUNCTION OF RNA

tRNA

Involvement of tRNA in protein synthesis

• Mainly functions in protein synthesis

• For certain virus, RNA may become the hereditary genetic material

FUNCTIONS OF RNA

• number of polynucleotide strands• types of:

–pentose sugar–nitrogenous bases

• location in cells• chemical stability & existing • basic form• function • molecular mass• ratio of nitrogenous bases

DIFFERENCES OF DNA & RNA

Differences between DNA and RNA DNA RNADouble stranded ?

Larger molecular mass ?

Deoxyribose as pentose sugar ?

Organic bases: A,T,C,G ?

Found mainly in nucleus Manufactured in nucleus but found throughout the cell

Chemically very stable Chemically unstable

Permanent Temporary existing

Only 1 basic form ?

Ratio of A to T & G to C is one Ratio of A to U & G to C varies

SEMESTER 1SESSION 2009/2010

Question • Differentiate between RNA and DNA.

[10 marks]

SESSION 1999/2000

Question

• Elaborate on the structure of DNA according to Watson and Crick.

[10 marks]

References :

• Campbell, 8th edition• Solomon, 9th edition

Next Topic….• 2.0 Cell Structure & Functions

43

2.2 Plant & Animal Cells

2.4 Cell Transport

2.3 Cell Membrane & Organelles

2.5 Cells are grouped

into tissues

2.1 Prokaryotic & Eukaryotic

Cells