MOLECULAR BASIS OF INHERITANCE

1. Watson and Crick model of DNA/Double helical model of DNA

 Double helical model of DNA was given by Watson and Crick
 DNA is made up of 2 polynucleotide strands which runs anti parallel to each other and right
handed coiled.
 Backbone of DNA is made up of sugar and phosphate.
 Two strands of DNA are joined by hydrogen bond.
Adenine pairs with the thymine by 2 hydrogen bonds and cytosine pairs with granite by 3
hydrogen bonds.
 Diameter of DNA is 20Ao/2nm
 The pitch of the helix is 34Ao/3.4 nm has 10 bp. Distance between successive base pairs is
3.4Ao/0.34nm.
 The plane of one base pair stacks over the other.

2. Packaging of DNA in eukaryotes/nucleosome model

 Nucleosome model explains about packaging of DNA in eukaryotes.

 DNA in eukaryotes associated with the positively charged proteins called histones.
 Histones are made up of basic amino acids Lysine and Arginine. Histones are of 5 types .They are
H1,H2A,H2B,H3,H4.Two molecules each of H2A,H2B,H3,H4 forms octamer.
 Histone octamer is wrapped by 2 coils of DNA which forms nucleosome.
 Nucleosomes are interconnected by linker DNA which has H1 histone.
 When it is observedunder microscope it appears like beads on the string .Where beads represent
nucleosome and string represents DNA.
 Higher level of packaging requires non histone chromosomal proteins(NHC)
3. Griffith experiment/Transforming principle
 In 1928, Griffith conducted experiments on Streptococcus pneumoniae which causes
pneumonia.
 Two strains of bacteria –S Strain and R strain. S strain have mucous coat and it is pathogenic.
R Strain don’t have mucous coat and it is non pathogenic.
1. S strain injected into mice mice die
2. R strain injected into mice mice alive
3. S strain (heat killed) injected into mice mice alive
4. R strain + heat killed S strain Injected into mice mice die
He concluded that R strain transformed into S strain in the presence of heat killed S strain. This is called
transforming principle.

4. Hershey and chase experiment/unequivocal proof for DNA as genetic

material.

 Hershey and chase grew some viruses on a medium that contained radioactive phosphorus
(32P)and some on radioactive sulphur(35S)
 Viruses grown in the presence of radioactive phosphorus contained radioactive DNA and viruses
grown on radioactive sulphur contained radioactive protein coat.
 Radioactive phages were allowed to attach E.coli bacteria .Then, as the infection proceeded, the
viral coats were removed from the bacteria. Bacteria that were infected with viruses grown in 35S
shows radioactivity in supernatant.
 Bacteria that were infected with viruses grown in 32P shows radioactivity in bacteria cells.DNA
is therefore the genetic material that is passed from viruses to bacteria.
5. Stahl and Messelson experiment/Experimental proof for semi conservative
method of replication

 Stahl and Messelson grew E.coli in a medium containing 15NH 4Cl, then the 15Nwas
incorporated into newly synthesized DNA.
 This heavy DNA molecule can be distinguished from the normal DNA by centrifugation in
caesium chloride medium.
 Then they transferred the cells into 14NH4Cl and took samples at various intervals .
 Thus the DNA that was extracted from the culture one generation after the transfer from 15N to
14N medium has hybrid DNA.
 DNA extracted from the culture after another generation (40 min) was composed of equal amount
of hybrid and light DNA.
 One of the parental strand is conserved in the newly produced DNA. Hence it is called semi
conservative method of replication.

6. Process of replication/ semi conservative method of replication

  Origin- DNA replication starts at specific point in the DNA called origin.
 Helicase –unwinding of double helix.
 Gyrase/ topoisomerase-Relieving of supercoils.
 Primase-synthesis of primers which binds to the exposed strand of DNA.
 DNA polymerase III-Addition of DNA nucleotides in complementary to the template strand .
It is functional in 5’ to 3’ direction hence it produces leading or continuous strand in 5’to 3’
direction and lagging strand or discontinuous strand in 3’to 5’ direction.DNA nucleotides
present in the lagging strand of DNA are called okazaki fragments.
 DNA polymerase I- removal of primers.
 DNA polymerase II-Proof reading
 DNA ligase- Joining of DNA nucleotides.

7. Process of transcription

 Initiation-RNA polymerase binds to promoter by the help of sigma factor and initiates
transcription.
 Elongation- RNA polymerase catalysed the addition of ribonucleotides in complementary to the
template strand.
 Termination-Polymerase reaches the terminator region, the nascent RNA falls off, by the help of
Rho factor.
8. Post transcriptional changes in eukaryotes
 Splicing-removal of introns and Joining of exons in hnRNA
 Capping-addition of methyl guanosine triphosphate to the 5’ End of hnRNA.
 Tailing-addition of adenylate residues to the 3’end of RNA.

9. Transcription unit

 Promoter and terminator flank the structural gene in transcription unit.

 The promoter is said to be located towards 5’ end (upstream)of the structural gene and
terminator is located towards 3’ end(downstream) of coding strand which defines the end of the
process of transcription.
 Promoter provides binding site for the addition of RNA polymerase.

10. Salient features of genetic code

 Codon is triplet.61 codons codes for amino acids and 3 codons do not code for any amino acids,
hence they function as stop codons.
 One codon codes for only one amino acid, hence it is unambiguous and specific.
 Some amino acids are coded by more than one codon, hence the code is degenerate.
 The codon is read in mRNA in a continuous manner .There are no punctuations.
 The code is nearly universal for example from bacteria to human UUU would code for
Phenylalanine.
 AUG has dual functions. It codes for methionine and it also act as initiator codon.

11. Mutation and types of mutation

Sudden change in the genetic make up of an organism is called mutation.
Types
1. Point mutation
Change in the single base of a codon is called point mutation.ex- Sickel cell anemia.
2. Frame shift mutation
Insertion or deletion of one or two bases changes the reading frame from the point of insertion or
deletion is called frame shift mutation.
12. Structure of tRNA-the adapter molecule

 Structure of tRNA was explained by clover leaf model.

 tRNA consists of 4 arms made up of stem with paired bases and loop with unpaired bases.
 Amino acid arm is loop less arm which binds to amino acid.
 Tpsi C arm binds to ribosome during translation.
 Anticodon arm binds to mRNA.
 DHU arm binds to amino acyl tRNA synthetase.
 Some of the tRNA has extra variable arm between T psi C and anticodon arm.

13. Translation/Process of protein synthesis

 Activation of amino acid and charging of tRNA- Amino acids are activated by ATP and binds to
the tRNA forms amino acyl tRNA.
 Initiation-The ribosome binds to the mRNA at the start codon (AUG) that is recognized only by
the initiator tRNA.
 Eongation- Amino acid linked to tRNA sequentially binds to the appropriate codon in mRNA
.The ribosome moves from codon to codon along the mRNA. Amino acids are added one by one
is translated into protein.
 Termination-Release factor binds to the stop codon, terminating translation and releasing the
complete polypeptide from the ribosome.
14. What is UTR? Write its significance.
mRNA has some additional sequences that are not translated and are referred as untranslated region
(UTR).The UTRs are present at both 5’ end (before start codon) and 3’ end ( after stop codon).They are
required for efficient translation.

15. What are the different levels of regulation of gene expression in

eukaryotes?
1. transcription level
2. processing level
3. transport of mRNA from nucleus to the cytoplasm.
4. translation level

16. Regulation of gene expression in prokaryotes/lac operon concept.

Lac operon concept was given by Jacob and Monad explains about gene regulation in prokaryotes.
Absence of inducer(switched off)

 Regulatory gene is transcribed to produce repressor.

 Repressor binds with the operator and prevents the binding of RNA polymerase to promoter.
 No transcription of structural gene.
 No enzymes are produced.
Presence of inducer(switched on)

 Regulatory gene is transcribed to produce repressor.

 Repressor gets inactivated by inducer(lactose)
 Inactivated repressor doesn’t binds to operator.
 Binding of RNA polymerase to promoter.
  Transcription of structural gene (z,y,a) produces beta galactosidase, permease and trans
acetylase.
 Beta galactisidaserequired for hydrolysis of lactose.
 Permease required for transport of lactose from the medium into cell.

16. What is negative regulation?

Regulation of lac operon by repressor is referred to negative regulation.

17. What is operon?

Operon is polycistronic structural gene regulated by a common promoter and regulatory gene.

18.Write the criteria for a genetic material.

 It should be able to generate its replica (Replication)
 It should be chemically and structurally stable.
 It should provide the scope for slow changes (mutation)that are required for evolution.
 It should be able to express itself in the form of Mendelian characters.

19. Distinguish between heterochromatin and euchromatin.

Heterochromatin-Highly condensed region of chromatin.
Transcriptionaly inactive .Darkly stained.
Euchromatin - less condensed region of chromatin.
Transcriptionaly active. Less stained .

20. What is central dogma of molecular biology?

Central dogma of molecular biology given by Franscis Crick.
Unidirectional flow of information from DNA to protein is called central dogma of molecular biology.

21. What is Chargaff’s rule?

Number of purine is equal to number of pyrimidine in ds DNA is called Chargaff’a rule.

22. Process of replication is called semi conservative. Give reason.

Process of replication is called semi conservative because one of the parental strand is conserved in the
newly produced DNA .

23. DNA is better genetic material than RNA.Give reason.

DNA is better genetic material than RNA because RNA has ribose sugar which is more reactive.

24. RNA polymerase in eukaryotes shows division of labour. Substantiate.

RNA polymerase I- transcription of rRNA
RNA polymerase II-trannscription of hnRNA which is the precursor of mRNA.
RNA polymerase III- transcription of tRNA

25. What is transcription?

The process of copying genetic information from one strand of the DNA into RNA is termed as
transcription.