NUCLEIC ACIDS

(DNA and RNA)

They are large, complex molecules of high molecular weight.

They contain C, H, O, N and P.

Their monomers are nucleotides.

The Structure of a Nucleotide

PO4

5C Sugar

(ribose or deoxyribose)

Nitrogenous bases

(adenine, guanine, thymine,cytosine, uracil)

1

2

3

Nucleic acids are polymers of nucleotides.

Phosphoester bond Glycoside bond

Nitrogenous base

+5C

Sugar +Phosphate group

Nucleotides

Nucleic acidsNucleotide

Adenine =

Cytosine Guanine

Thymine

Number of H bonds between bases

Adenine

Guanine

Cytosine

Thymine

Purine bases

Pyrimidine bases

A

T= 1

G

C= 1

T+C

A+G = 1 Purines

Pyrimidines

= 1

Nucleoside

A+G T+C= =

Number of nucleotide in DNA

Number of nucleotide in DNA

1/2

A+G= =

T+C 1

Number of nucleotide in one strand

Number of nucleotide in one strand

Number of H bonds

= ( A X 2 ) + ( G X 3 )

Number of H bonds

= ( T X 2 ) + ( CX 3 )

Number of phosphodiester bonds

=Number of nucleotide in DNA - 2

Number of phosphodiester bonds

=Number of nucleotide in one strand - 1 X 2

• ratio is specific for species. If the ratio is smaller and it is resist to heating.

• In prokaryotes, DNA is found in cytoplasm and naked. In eukaryotes, histones are found in the structure of DNA. DNA is located in nucleus, mitochondrion, chloroplast.

• All kind of biological process are directed by DNA.

• DNA is replicated during interphase.• DNA undergoes mutations. If a mutation occurs

within the sex cell, it is inherited to next generation.

A+T

G+C

Nucleic Acids

DNA (Deoxyribonucleic acid)

RNA (Ribonucleic acid)

DNA: Found in nucleus, mitochondria and chloroplast in eukaryotes

Is the hereditary material that is transmitted from one generation to the next, during reproductionContains 5 C sugar(deoxyribose), phosphate group (PO4), nitrogenous bases adenine (A)

guanine (G)

cytosine (C)

thymine (T)

DNA is double stranded (double helix)

Crick and Watson walking along the Backs. 1953

RNA:

Found in nucleus and cytoplasm

Works with DNA, involved in protein synthesis

Contains 5 C sugar (ribose), phosphate group (PO4) and nitrogenous bases adenine (A)

guanine (G)

cytosine (C)

uracil (U)

RNA is single stranded.

DNA RNA

It is double stranded ( in some viruses, it is single)

It is single stranded ( but in some RNA viruses, it is double helix)

5C sugar is deoxyribose 5C sugar is ribose

Nitrogenous bases are A, G, C, T

Nitrogenous bases are A, G, C, U

Location:

prokaryotes: in cytoplasm

eukaryotes: nucleus, mitochondrion, chloroplast

Location:

prokaryotes: in cytoplasm

eukaryotes: nucleus, cytoplasm mitochondrion, chloroplast, ribosome

DNA RNA

Function: is the primary hereditary material, controls the structure of proteins synthesized and this way controls all cellular activities

Function: vital for protein synthesis

can replicate itself is transcribed by DNA

DNA RNA

Enzymes for replication:

DNA polymerase

DNA ligase

DNA nucleotide sentetase

Enzyme for production:

RNA polymerase

RNA nucleotide sentetase

Enzymes for depolymerization:

DNAse ( deoxyribonuclease)

Enzymes for depolymerization:

RNAse ( ribonuclease)

GRIFFITH’S EXPERIMENT

Griffith was trying to find out a vaccine against pneumonia

transformation

Blood analysis showed the presence of some live S- type bacteria

THE CHEMICAL BASES OF HEREDITY

HERSEY AND CHASE EXPERIMENT

Hersey and Chase worked with bacteriophage because it is analog of chromosome

 

The Hershey-Chase Experiment – Bacteriophage

1. Hershey and Chase forced one population of phages to synthesize DNA using radioactive phosphorous. 2. The radioactive phosphorous "labeled" the DNA.3. They forced another group of phages to synthesize protein using radioactive sulfur.4. The radioactive sulfur "labeled" the protein.5. Bacteria infected by phages containing radioactive protein did not show any radioactivity.6. Bacteria infected by phages containing radioactive DNA became radioactive.7. This showed that it was the DNA, not the protein, that was the molecule of heredity.

DNA REPLICATION

Three possible mechanisms of DNA replication

a) Semiconservative replication: The two parental strands seperate, each forms a template for new strand.

b) Conservative replication: Each of the two strands of parent DNA is replicated, without strand seperation.

c) Dispersive replication: During replication, parent chains break at intervals, and replicated segments are combined into strands with segments from parent chains. All daughter helixes are part old, part new.

DNA REPLICATION

DNA is labelled with 15 N isotope

Normal or 14 N compound medium

14 15 1514

F1 generation

100% hybrid DNA

15 15

Normal or 14 N compound medium 14 14 14 14 14 141515

50% hybrid DNA

15 N - 14 N

50% normal DNA

14 N - 14 N

F2 generation

If normal DNA is given;

2n

2 hybrid and 2 pure heavy DNA are formed

If hybrid DNA is given;

2n

one of the DNA is always hybrid the others are pure

DNA REPLICATION

Replication number

Replication number

DNA REPLICATION MECHANISM

DNA REPLICATION

Replication occurs in three stages:

1. UNWINDING: Helicase enzymes seperate the parental double helix by breaking down the H bonds, forming the replication fork.

Replication

fork

2. CONTINUOUS SYNTHESIS: The “leading strand” is assembled continuously in the 5' to 3' direction by DNA polymerase, using the single parental strand as a template, it adds nucleotides to the growing 3' end

DNA REPLICATION

C

A T

G 5'

5'

5'

3'

3'

Replication is continuous

DNA polymerase can work only in the 5' - 3' direction

3'

5'

5'

5'

3'

3'

3'

With okazaki fragments replication is discontinuous

DNA ligase links the okazaki fragments which are 1000- 2000 nucleotide long fragments in prokaryotes and 100-300 nucleotide long in eukaryotes

3. DISCONTINUOUS SYNTHESIS: The “lagging strand” is assembled discontinuously. It is produced as a series of short segments ( Okazaki fragments), each of which is synthesized in the 5' to 3' direction by DNA polymerase, using the single parental strand as a template

DNA REPLICATION

OKAZAKI FRAGMENTS

• In bacteria, the Okazaki fragments are each 1000-2000 nucleotides long

• In eukaryotes, they are 100 to 300 nucleotides length

• Finally, the fragments are joined to the 5' end of the growing chain by a DNA ligase enzyme