Genetic Material

The Plant Cell

Nucleic acids

1. Ribonucleic acids: RNA2. Deoxyribonucleic acids: DNA, the genetic blueprint of life

RNA and DNA are the polymers of nucleotides.

Nucleotide: phosphate, a ribose sugar and a nitrogenous base

N-base

Phosphate

Deoxyribose sugar

There are two types of N-bases

1. Pyrimidine 2. Purine

AdenineGuanine

CytosineThymineUracil

O

P

O

O -----Sugar -Base

Sugar----Base

O

Phospho-diester bond

Polymerization of nucleotides occurs by the formation of phospho-diester bond

Discovery of DNA

1. Friedrich Miescher in 1868 isolated a phosphorus containing compound, which he named nuclein (today nucleoprotein).

2. Miescher in 1872 isolated an acidic compound (nucleic acids) from salmon sperms.

3. Altman in 1889 described a method for preparing protein free nucleic acids.

4. Several scientists discovered the presence of four bases (ACGT) in nucleic acids.

5. Chargaff in 1951 revealed a simple mathematical relationship between the proportion of bases in any one sample of DNA. Molarity of A = T and C = G

Structure of DNA

6. Wilkins and Franklin in early 1950s collected X-ray crystallographic data of DNA.

7. In 1953 Watson and Crick proposed double helix structure of DNA.

Watson and Crick proposed that

a. DNA comprises of two polynucleotide chains.

b. The N-bases are stacked on the inside of the helix, while the sugar-phosphate forming the backbone of the molecule outside.

c. The polynucleotide chains interact by hydrogen bonding.

d. The two strand of the double helix are anti-parallel (5’ 3 and 3’ 5’)

5’ 3’

123

4

5

5’ contains phosphate group 3’ contains hydroxyl group

DNA replicationGenetic material is faithfully inherited from generation to generation.Therefore, nucleotide sequence of DNA must be faithfully copied fromoriginal DNA molecules to the daughter molecules. This is done by anorchestrated process called DNA replication.

The hydrogen bonds break to form a replication fork

New DNA synthesis occurs 5’ to 3’

5’

5’

3’

3’ Leading strand

Lagging strand

Major steps involved in DNA replication

1. Separation of two DNA strands to form a replication fork or replication bubble.

2. Formation of a small RNA fragment called primer by enzyme Primase.

3. Extension of primer (formation of long fragments) by DNA polymerase.

4. Joining long fragments by enzyme DNA ligase to make one full strand of the molecule.

5’

3’5’

3’

Replication fork or bubble

3’3’

5’

5’

DNA ligase

5’5’3’

3’3’5’5’3’

Daughter DNA molecules

DNA replication is remarkably accurate!!It progresses in semi-conservative mode.