Maharashtra Board. Class XII Science (Biology).

1. •
MOLECULAR BASIS OF INHERITANCE
Biology –
Chapter 4
(part 2)
By – Dr. Janaki V.
Pandey (Ph.D.)

2. PROTEIN SYNTHESIS
• The process of protein synthesis includes
Trancription and Translation.
• Transcription-process of copying genetic
information from one strand of DNA into a
single stranded RNA.
• Translation- process of formation of protein
from RNA.

3. CENTRAL DOGMA OF PROTEIN SYNTHESIS
• Ds DNA molecule ------» mRNA (messenger).
• mRNA synthesize protein.
• This is called as central dogma of protein synthesis.
• It was postulated by Crick in 1958.
DNA Transcription mRNA Translation Polypeptide
• Temin (1970) and Baltimore(1970) gave concept
of retroviruses/ riboviruses.
DNA Transcription mRNA Translation Polypeptide
Reverse Transcription
Enzyme used- RNA dependent DNA polymerase

4. TRANSCRIPTION
• Only one strand of DNA is copied into RNA --------> Template
Enzyme RNA polymerase (RNAP).
• In Eukaryotes, Transcription takes place in nucleus
and Translation occurs in cytoplasm.
• DNA ----------> mRNA ---moves-------> Ribosomes.
• Transcription occurs in the nucleus during G1 and G2
phases of cell cycle.
• DNA has promotor (P) and terminator (T) sites. Transcription
starts at (P) site and stops at (T) site.
• The process of Transcription, involves three stages viz.
Initiation, Elongation and Termination.

5. Transcription Unit
• Each transcribed segment of DNA is called
transcription unit.
• It consists of
(i)Promotor
(ii) The structural gene
(iii) Terminator.

6. i. Promotor :-(DNA sequence)---Binding site for
enzyme RNAP.
RNAP binds to specific Promotor.
It is present towards 5' end i.e upstream.
In prokaryotes, the enzyme recognizes the
promotor by its sigma factor sub unit.

7. ii. Structural genes:- (DNA sequence)-----> Codes for the
synthesis of proteins.
• The DNA strand used for synthesis of RNA is called
Template strand and oriented in 3‘-5' direction.
• It is also called Antisense stand.
• The DNA strand which is not involved in synthesis of
RNA is called Coding strand and oriented in 5‘-
3'direction.
• It is also called Sense strand.

8. iii. Terminator :- (DNA sequence )---------> Terminates the
transcription.
Itispresenttowards3'end/downstream.

9. 1) Initiation-The enzyme RNA polymerase binds to the
promotor site and starts a the initiation process.
Mechanism of Transcription

10. • Elongation
• RNAP moves along the DNA and separate them.
• One strand (antisense strand) acts as template
and forms mRNA.
• rNTPs join to the DNA template.
• Hybrid DNA-RNA molecule dissociates.
• The mRNA
molecule is
free.

11. Termination-
RNAP reaches the terminator site and leaves DNA strand.
Fully formed mRNA is also released.

12. • In bacteria, mRNA does not require any
processing because it has no introns.
• Prokaryotes posses only one type of RNAP
• In Eukaryotes, there are three types of
RNAPs i.e
• RNA polymerase-I transcribes rRNA.
• RNA polymerase-II transcribes mRNA and
hnRNA (heterogeneous nuclear RNA).
• RNA polymerase-III transcribes tRNA and
snRNA(small nuclear RNA)

13. Transcription unit and the gene
• Cistron is a segment of DNA coding for a polypeptide.
• It can be monocistronic or polycistronic.
Monocistronic
A single structural gene is present in a transcription
unit.
Polycistronic
Many structural genes are present in one transcription
unit.

14. Structural genes in eukaryotes have introns
and exons.
• Introns – noncoding sequence and splicied in
processed mRNA
• Exons- coding sequences and appear in
processed
mRNA

15. Processing of hnRNA
hnRNA undergoes 3 process.
• Splicing- introns are
removed and exons are
joined by DNA ligase.
• Capping-methylated
guanosine triphosphate
(MeGTP)is added to the 5’
end
• Tailing- adenyl residue (A) is
added to 3’ end
(polyadenylation).
• Now, the fully processed
hnRNA is called mRNA.

16. GENETIC CODE
• DNA molecule has 4 types of nitrogen bases
(A,G,C,T/U in RNA).
• About, 20 different types of amino acids are
involved in the process of synthesis of proteins.
• These 20 types of amino acids are encoded by 4
types of nitrogen bases.
• This information is stored in the form of coded
language (Cryptogram) called genetic code.
• It is in form of triplet codons ( AUG, UUA, etc.).
• Each Codons codes for a specific amino acids.
• Eg. AUG codes for methionine.

17. Dictionary of genetic code
4 nitrogen bases, triplet is to be formed.
43 = 64 codons are formed which codes 20 different amino acids.

18. Characteristics of Genetic code
i. Genetic code is a triplet code :
• Sequence of three consecutive bases
constitute codon (AUG). Each codon
specifies one particular amino acid.
• In every living organism genetic code is a
triplet code.

19. ii) Genetic code has distinct polarity :
• Genetic code shows definite polarity i.e.
direction.
• It is always read in 5' 3' direction.
• e.g. 5' AUG 3'
5’ 3’

20. iii. Genetic code is non-overlapping:
• Code is non overlapping .
• Each single base is a part of only one codon.
Adjacent codons do not overlap.
• (If non-overlapping, then with 6 consecutive
bases only two amino acid molecules will be in
the chain. If it was overlapping type, with 6
bases, there would be 4 amino acid molecules
in a chain. )

21. iv) Genetic code is commaless:
• There is no gap or punctuation mark between
successive /consecutive codons.
v) Genetic code is universal :
• In all living organisms the specific codon
specifies same amino acid.
• E.g. codon AUG always specifies amino acid
methionine in all organisms from bacteria up
to humans.

22. vi) Genetic code has degeneracy :-
• Usually single amino acid is encoded by single
codon. However, some amino acids are encoded
by more than one codons. E.g. Cysteine has two
codons, while Isoleucine has three codons. This is
called degeneracy of the code.

23. vii. Genetic code is non-ambiguous
• Specific amino acid is encoded by a particular
codon. Alternatively, two different amino acids will
never be encoded by the same codon.
viii) Codon and Anticodon
• Codon is a part of mRNA (DNA). e.g. AUG is
codon. It is always represented as 5' AUG 3'.
• Anticodon is a part of tRNA. It is always
represented as 3' UAC 5'.

24. ix) Initiation codon and Termination codon:-
• AUG is always an initiation codon in mRNA.
AUG codes for amino acid methionine.
• 3 codons viz. UAA, UAG and UGA are
termination codons which terminate/stop the
process of elongation of polypeptide chain,
as they do not code for any amino acid.

25. Mutations and Genetic Code
• Mutation is a sudden change in the DNA sequence.
• It change genotype (i.e. character) and leads to
variations.
• Mutation occurs due to deletion or insertion/
duplication of a segment of DNA.

26. Types of mutation:
1) Point mutation.
• Eg. Sickle cell anaemia.
• It occurs due to change in
a single base pair of DNA.
2) Frame shift or deletion mutation-
• occurs due to deletion or
insertion of base pair.

27. (Out of frame deletion)-
• Insertion or deletion of one
or two bases change the
reading frame from the point
of insertion or deletion.
(In frame deletion) -
• Insertion or deletion of 3 or
multiple of 3 bases (insert /
delete) results in insertion/
deletion of amino acids.
• It does not change the reading
frame and other amino acid
remains unchanged.
met lys leu Ala
Met Stop
met Phe Gly stop
Frame shift Mutation

28. t-RNA- the adapter molecule
• tRNA reads the codon and
simultaneously binds the amino
acid.
• Cloverleaf structure of tRNA
possess anticodon
• It shows amino acid acceptor
end at (3' end).They also have
unpaired CCA bases for binding
amino acids.
• For every amino acid, there is
specific tRNA.
• Initiator tRNA is specific to
methionine. There is no tRNA’s
for stop codons.

29. • It is the mechanism in which the codons on the
mRNA are translated and specific amino acids are
added to form a polypeptide chain on ribosomes.
• It involves 3 steps.
1. Initiation
2. Elongation
3. Termination
Translation

30. 1. Initiation of polypeptide chain-
• A) (Addition of amino acid to initiator codon).
• Amino acid is activated by ATP. Small subunit of ribosome (30s)
binds to the mRNA at 5’ end. Initiator codon AUG get attach to
mRNA. t-RNA having methionine binds with initiation codon (AUG)
by its anticodon (UAC) through H-bond in eukaryotes. While in
prokaryotes it binds with formyl methionine.
• B) (Ribosomes subunits join)
• (50 s) & (30s) ribosomes join.
-help of Mg ++ ions.
• C) (Positioning)
• initiator charged t-RNA (1st tRNA)
occupies P-site of ribo-
some and A-site is vacant.

31. 2.Elongation of polypeptide chain
• Addition of amino acids to
Methionine.
• Formation of tRNA complex.
(amino acid, t-RNA and ATP).
• (Entry and binding of new tRNA complex.)
• t-RNA amino acid complex enter
ribosome at A-site.
• 1st Amino acid on the initiator t-RNA at
P-site and 2nd amino acid on t-RNA at A-
site join by polypeptide bond with the
help of ribozyme.
• (Exit of old tRNA) .
• 1st t-RNA moves to the E-site.
• The 2nd t-RNA at A-site carrying a
dipeptide moves to the P-site leaving A-
site vacant. This process is translocation.

32. • Both the sub units of the ribosome move along in relation to tRNA and
mRNA.
• The next charged tRNA molecule carrying amino acid will occupy the
vacant A-site
• 1st uncharged tRNA is removed from the E-site.
• The process repeated as amino acids are added to the polypeptide.
Time required is less than 0.1 sec for formation of peptide bond.
• 3rd amino acid with charged tRNA binds at A-site of ribosome.
Anticodon and codon binds by H-bond. Polypeptide bond is formed.
• 2nd tRNA is discharged from P-site to E-site and leaves ribosome.
• The events are repeated and all codons on mRNA are exposed one
by one for translation.
5’ 3’
1st
2nd
3rd

33. • (Binding to stop codon)
• A stop codon (UAA, UAG or UGA) is present at the end of
mRNA.
• When stop codon exposed at A-site, it cannot be read and
joined by anticodon.
• (Binding of release factor)
• The release factor binds to the stop codon and terminates
the translation process.
• Polypeptide and mRNA is released in cytoplasm and
ribosome subunits dissociates.
3. Termination and release of polypeptide

34. Regulation of gene expression
A process by which a
Gene is regulated and its
product (polypeptide) is
synthesized.
• Genes of a cell are
expressed to perform
different functions.
For eg. In E.coli-- β-
galactosidase converts
Lactose ---------->
Galactose + Glucose
In eukaroytes the regulation can be at
different levels like--

35. • Gene expression is regulated by metabolic or
physiological or environmental conditions of the
surrounding medium.
• Regulation takes place with the help of inducible
enzymes.
• This phenomenon is called induction and small
molecule responsible for this, is known as inducer.

36. Operon Concept
The clusters of gene with related functions are called operon.
It consists of –
1) Operator gene
2) Promoter gene
3) Structural gene
4) Regulator gene
The gene expression depends on whether operator is switched on or off.

37. Lac operon
• It is inducible operon in E. coli.
• Operon is switched on when lactose (inducer) is
present in the medium.
• It consists-
1) Regulator gene
(repressor gene)
• It control operator and
produce a repressor protein.
• Repressor protein binds with operator
and stops its action.

38. 2) Promoter gene
• It is present adjacent to operator.
• RNAP enzyme binds to the promoter.
• When operator is turned
on RNAP transcripts
structural gene and when
turned off RNAP cannot
transcript structural gene.
3) Operator gene
• Present adjacent to structural gene and control its function.
• When operator is turned on by inducer (lactose), structural
gene produce mRNA.

39. 4) Structural gene
• In presence of lactose, it
produce mRNA.
• mRNA produce polypeptide
(protein).
• Protein act as enzymes to
catalyze lactose in the cell.
• The 3 structural genes produce 3
enzymes
• lac Z- β- galactosidase-- digest lactose
• lac Y- β- galactoside permease--
permits entry of lactose
• lac A-transacetylase—transfer acetyl group.

40. 5) Inducer
• It inactivates the repressor –
allolactose.
• When lac operon is
switched on, then inducer
binds with repressor protein
and prevents binding of
repressor to the operator
gene.
• Operator is free and RNAP
enzyme move from
promoter to structural gene
via. Operator gene.

41. Role of lactose
• Enz. Permease allows the lactose to enter the cell.
• A small amount of permease enz. is present even when
operon is switched off.
• Lactose (inducer) binds to the repressor and do not
allow repressor to join to operator gene.
• Operator is switched on and structural gene produce all
enzymes and degrade lactose.
• Thus, lactose
acts as a inducer
of its own
breakdown.

42. • When the inducer level falls, operator is blocked
again by repressor and structural gene are
inactivated.

43. Genomics
• It is a complete copy of genetic
information (DNA) or one complete
set of chromosomes (monoploid or
haploid) of an organism.
• Genomics study may be classified
into two types:
a. Structural genomics: It involves
mapping, sequencing and analysis
of genome.
b. Functional genomics: It deals with
the study of functions of all gene
sequences and their expression in
organisms.

44. Application of genomics
1. In a number of different sectors like medicine,
biotechnology and social sciences.
2. It helps in the treatment of genetic disorders through
gene therapy. eg. Sickle cell anaemia, haemophilia.
3. Genomics is used in agriculture to develop transgenic
crops (eg. Bt Cotton) having more desirable characters.
4. Genetic markers developed in genomics, have
applications in forensic analysis.
5. Genomics can lead to introduce new gene in microbes to
produce enzymes(insulin), therapeutic proteins(to treat
various cancer) and even biofuels (sugarcane to produce
biodiesel).

45. Human Genome Project
• The project was initiated in 1990 and was completed in 2003.
• The research project determine the genomic structure of
humans.
• The Project was associated with Bioinformatics.
• It provides a complete and accurate sequence of the 3 billion DNA
base pairs of humane genome.
• The project has estimated about 33000 genes present in humans.
• The work of human genome project has allowed researches to
understand the blueprint in building and constructing the human
genome.
• Such studies will lead to understand human evolution.

46. The main aim of the projects are—
1. Mapping the entire human genome at the level of
nucleotide sequences.
2. To store the information collected from the project in
databases.
3. To develop tools and techniques for analysis of the data.
4. To Transfer the related technologies to the private
sectors, such as industries.
5. To Take care of the legal, ethical and social issues which
may arise from project.
6. To sequence the genomes of several organisms such as
bacteria, fungi, insects, plants, etc to study gene
function.

47. DNA Fingerprinting
• Every individual has its unique genetic make-up called
as Fingerprint.
• The technique developed to identify a person with the
help of DNA restriction analysis, is known as DNA
profiling or DNA fingerprinting.
• The technique of fingerprinting was first given by Dr.
Alec Jeffreys in 1984.
• DNA fingerprinting technique is based on identification of
nucleotide sequence present in DNA molecule.
• About 99.9% of nucleotide sequence in all persons, is
same. Only some short sequences of nucleotides differ
from person to person.

48. • This short sequence is called VNTRs (Variable
Number of Tandem Repeats) which is about 20-
100 bp.
• The length of the regions having VNTRs is
different in each individual and hence is the key
factor in
DNA Profiling.

49. Steps involved in DNA finger printing
• Isolation of DNA: the DNA is recovered from the tissues like
blood, hair roots, skin,etc. from the body (host).
• Restriction digestion : the isolated DNA is treated with
restriction enzymes. It cut the DNA into small fragments
having variable lengths. This phenomenon is called
Restriction Fragment Length polymorphism (RFLP).

50. 3. Gel electrophoresis :
• The DNA samples are loaded for agarose gel
electrophoresis under an electric influence.
• The DNA fragments, which are negatively charged
move to the positive pole. The movement of these
fragments depends on length of the fragments.
• This results in formation of
bands.
• dsDNA splits into ssDNA by
alkali treatment.

51. 4. Southern blotting :
• The separated DNA fragments are transferred
to a nylon membrane or a nitrocellulose filter
paper by placing it over the gel and soaking
them with filter paper overnight.

52. 5. Selection of DNA Probe :
• A known sequence of ssDNA
called DNA Probe is prepared.
• DNA Probe is obtained from
organisms or prepared by
cDNA preparation method.
• The DNA Probe is labelled with
radioactive isotopes.
Radioactive
labelled

53. 6.Hybridization :
• Probe DNA is added to the
nitrocellulose filter paper
containing host DNA.
• The ss DNA probe pairs with
the complementary base
sequence of the host DNA
strand.
• As a result DNA-DNA
hybrids are formed on the
nitrocellulose filter paper.
Remaining single stranded
DNA probe fragments are
washed off.

54. 7.Photography :
The nitrocellulose filter paper is photographed on an X-
ray film by autoradiography. The film is analysed to
determine the presence of hybrid DNA.

55. • Used in forensic science to
identify potential crime suspect.
• Used to establish paternity and
relationships within family.
• To identify and protect the
commercial varieties of crops
and livestock.
• To find out evolutionary history
of organism and trace out the
links btw different groups of
organisms.
• Used in pedigree analysis in
cats, dogs, horses and humans.
Application of DNA Fingerprinting