Transcription and introns and exons

1. • Transcription
• Monocistronic
• Polycistronic
• Introns
• Exons..

2. Transcription

3. Transcription
• The process of copying genetic information from one
strand of the DNA into RNA is termed as transcription.
• Enzyme involved in transcription is DNA-dependent RNA
polymerase..

4. • The principle of complementarity governs the process of
transcription.
• (Remember complementarity does not mean identical)
• In transcription adenosine pair with uracil instead of
thymine.

5. • IN replication, total DNA of an organism gets duplicated,
• But in transcription only a segment of DNA and only one
of the strands is copied into RNA..

6. Why Both the strands are not copied during
transcription
• If both strands act as a template, they would code for
RNA molecule with different sequences..
• In turn, RNA molecules code for proteins, the sequence
of amino acids in the proteins would be different.

7. • If both strands act as a template , one segment of the
DNA would be coding for two different proteins.
• This would complicate the genetic information transfer
machinery..
If both strands in DNA act as a template ..?

8. • Second, the two RNA molecules if produced
simultaneously would be complementary to each other.
• Hence would form a double stranded RNA.
• This would prevent RNA from being translated into
protein
If both strands in DNA act as a template ..?

9. Transcription Unit

10. Transcription unit
• A transcription unit in DNA is defined primarily by the
three regions in the DNA:
• (i) A Promoter
• (ii) The Structural gene
• (iii) A Terminator..

12. • DNA-dependent RNA polymerase also catalyse the
polymerisation in only one direction, that is, 5'→3‘.
• RNA polymerase polymerises Ribose Nucleotides (RATP,
RGTP, RUTP, RCTP)..
• The strand that has the polarity 3'→5' acts as a template
strand.
Template strand.

13. • The other strand which has the polarity (5'→3') is
referred to as coding strand.
• Coding strand sequence same as RNA (except thymine at
the place of uracil)..
• Coding strand which does not code for anything..
Coding strand

14. 3' -ATGCATGCA-5' Template Strand
5' -TACGTACGT-3' Coding Strand

16. Promoter
• The promoter and terminator flank the structural gene in
a transcription unit.
• The promoter is said to be located towards 5' -end
(upstream) of the structural gene…
• It is a DNA sequence that provides binding site for RNA
polymerase..

17. • Terminator:
• The terminator is located towards 3' -end (downstream)
of the coding strand.
• Terminator defines the end of the process of
transcription

18. Transcription Unit and the Gene
• A gene is defined as the functional unit of
inheritance.
• There is no ambiguity that the genes are located
on the DNA.

19. Gene and Cistron
• GENE:
• The DNA sequence coding for tRNA or rRNA
molecule also define a gene.
• CISTRON:
• A cistron as a segment of DNA coding for a
polypeptide or protein.

20. Monocistronic
• The structural gene in a transcription unit could
be said as monocistronic..
• (In eukaryotes) …
Promoter S.T. Gene Terminator

21. Polycistronic
• The structural genes in a transcription unit could be said as
Polycistronic…
• (mostly in bacteria or prokaryotes).
Promoter GENE
Z
GENE
Y
Gene
A
Terminator

22. Exons and introns
• In eukaryotes, the monocistronic structural genes have
interrupted coding sequences – the genes in eukaryotes
are split.
• The coding sequences or expressed sequences are
defined as exons.
Promoter Exon Intron Exon Intron Exon Terminator
Promoter Structural gene Terminator

23. Exons
• Exons are said to be those sequence that appear in
mature or processed RNA.
• The exons are interrupted by introns…
Promoter Exon Intron Exon Intron Exon Terminator

24. Introns:
• Introns defined as Non-Coding DNA sequence.
• Introns or intervening sequences do not appear in
mature or processed RNA.
• Introns present in Eukaryotic DNA only.

25. • Inheritance of a character is also affected by promoter
and regulatory sequences of a structural gene.
• Sometime the regulatory sequences are loosely defined
as regulatory genes,…

26. Types of RNA and the process of Transcription

27. TRANSCRPTION

28. Types of RNA
• In Prokaryotes and Eukaryotes there are three major
types of RNAs:
• mRNA (messenger RNA),
• tRNA (transfer RNA),
• rRNA (ribosomal RNA).
• All three RNAs are needed to synthesise a protein in a
cell..

29. • The mRNA : provides the template.
• tRNA: brings aminoacids and reads the genetic code.
• rRNAs: play structural (Ribosomes) and catalytic role
(Enzyme) during translation.

30. • In bacteria a single DNA-dependent RNA polymerase
that catalyses transcription of all types of RNA in
bacteria.

31. The process of Transcription..
• Initiation
• Elongation
• Termination.
• RNA polymerases able to catalyse all the three steps?
• The RNA polymerase is only capable of catalysing the
process of elongation.

32. Initiation:
• RNA polymerase binds to promoter and initiates
transcription (Initiation).
• Initiation require binding of initiation-factor (σ) to RNA
Polymerase.

33. Elongation

34. Elongation
• RNA polymerase uses Ribose nucleoside triphosphates as
substrate and polymerises in a template depended fashion..
• Only a short stretch of RNA remains bound to the enzyme.

35. Termination

36. Termination
• Once the polymerases reaches the terminator region,
the nascent RNA falls off and RNA polymerase release
out. Nascent= Freshly generated

37. • Termination process require binding of termination-
factor (ρ) to RNA polymerase.

38. • In bacteria, since the mRNA does not require any
processing to become active.
• The transcription and translation take place in the same
compartment (Cytoplasm).

39. • Many times the translation can begin much before the
mRNA is fully transcribed.

40. • In eukaryotes, Transcription has
• two additional complexities –

41. (i) There are at least three RNA polymerases in
the nucleus of Eukaryotes..
• The RNA polymerase I: transcribes rRNAs
• (28S, 18S, and 5.8S).

42. • The RNA polymerase II: transcribes precursor of mRNA,
that is the heterogeneous nuclear RNA (hnRNA).

43. • RNA polymerase III: is responsible for transcription of
tRNA, 5srRNA, and snRNAs (small nuclear RNAs).

44. • ii) The second complexity is that the primary transcripts
(hnRNA) contain both the exons and the introns.
• Intorns are non-functional.
• Thats why hnRNA subject to processing in nucleus.
• Splicing
• Capping
• Tailing

45. hnRNA processing in Nucleus
• Splicing : where the introns are removed and exons are
joined in a defined order.

46. hnRNA processing in Nucleus
• Capping: an unusual nucleotide (methyl guanosine
triphosphate) is added to the 5'-end of hnRNA.

47. hnRNA processing in Nucleus
• Tailing: adenylate residues (200-300) are added at 3'-end
in a template independent manner. (Polyadenylation)..

48. • After splicing, capping, tailing the processed hnRNA,
now called mRNA.
• The m-RNA is transported out of the nucleus in to
nucleus for translation…

50. • The split-gene represent probably an ancient feature of
the genome.
• The presence of introns is reminiscent of antiquity, .
• The process of splicing represents the dominance of
RNA-world.
•

51. • Reminiscent of antiquity: In past introns are important
pieces of information used to form m-RNA, but now no
importance..

52. Dr. HarinathaReddy Aswartha
Assistant professor
Department of Microbiology
ANDHRAPRADESH
INDIA