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THE CENTRAL DOGMA OF
MOLECULAR BIOLOGY
by
Sadeeq Sheshe
Cancer Genetics Lab, Department of Bioscience,
COMSATS Institute of Information Technology, Islamabad
 The central dogma is a concept in molecular biology
that attempts to explains the key processes inside the
nucleus of the cell
 According to this concept, genetic information in DNA
is being replicated, later transcribed to RNA and then
translated into the protein
 It was earlier thought to be an irreversible process,
however the concept of reverse transcription proved
otherwise
Central Dogma
 DNA replication is the process of making new strands of
DNA using another DNA as a template
 DNA replication is catalyzed by the enzyme, DNA
polymerase whose structure is quite different in both
prokaryotic and Eukaryotic cells
 However, the process is semi-conservative in both
prokaryotic and eukaryotic organisms
 This means after the replication, each double stranded
DNA is made up of one original strand (template) and a
newly synthesized strand.
DNA Replication
Figure 1
 DNA replication occurs in 5’-3’ direction
 This means the hydroxyl (OH) on the 3’ on the
starting nucleotide attacks the alpha phosphate on
the incoming nucleotide thus establishing a phospho-
diester bond
 DNA replication does not start de novo but require
short strings of RNA oligos called primers
 Primers are synthesized by the enzyme primase
DNA replication
 DNA replication consists of 3 steps
 Initiation, Elongation and Termination
 Prokaryotic replication starts at an Origin of
replication
 These consensus sequences are recognised by the
different proteins involved in replication, forming the
pre-initiation complex
 The double strands unwinds and primers are
synthesized
Replication
 Certain proteins are assoicated with the replication
process
 Primase, Helicase, Topoisomerases and SSB proteins
 Primase synthesizes primers
 Helicase unwinds double stranded DNA
 Topoisomerases relieve tensional force generated due to
winding
 SSB holds single strands together preventing them to fold
and form helical structures
 Termination is achieved with the bi-directional synthesis
converging at a single point called TER
Replication
Figure 2
 This is the process of synthesizing an RNA molecule using
DNA as template
 It is an enzymatic process catalyzed by a DNA dependent
RNA polymerase
 RNA polymerase is also structurally different in both
prokaryotic and eukaryotic cells
 Also moves along5’-3’ direction but does not require
primer
 Question: why RNA polymerase does not require primer
and DNA polymerase do?
Transcription
Figure : promoter regions
 Just like the replication process, transcription also has 3 steps;
Initiation, Elongation and termination
 Transcription initiates by recognition of certain sequences on the
template Strand called promoter sequences. This are the start
point for transcription.
 This follows with the assembly of the transcription machinery
 And the elongation follows, in 5’-3’ direction, releasing
pyrophosphates
 The pyrophosphate is hydrolyzed to 2 inorganic phosphates with
release of energy
 This energy is the driving force for the process, just like in
REPLICATION
Transcription
Figure
Figure: Elongation
 The termination step is of two types
 The rho dependent and rho independent
 Rho is a protein that is part of the polymerase transcription
complex
 It searches for specific sequences and halt the polymease,
hence stopping the transcription
 While in rho independent termination, the transcriptional
machinery including the polymerase recognized certain
sequences along the template and stop
Transcription
 This is otherwise known as protein synthesis
 The information on the DNA that has been
transcribed into RNA is finally translated into a
protein
 The information on the RNA are recognized as codons
which are triplets of bases.
 Each amino acid has a codon although, one amino
acid can have more than one codon
 This is degeneracy of the protein code
Translation
Figure
 Translation takes place outside the nucleus, specifically on
the famous organelle, ribosome
 Although, the ribosome has two different structural
subunits in both prokaryotic and eukaryotic cells, it is quite
structurally distinct in both cells
 These sub units together with various proteins that assist
in translation form the translational machinery
 Each amino acids is first activated by attachment to an
Adapter RNA, tRNA catalysed aminoacyl-tRNA synthetases
Translation
Figure
 Translation also occurs sequentially in steps:
Initiation, elongation and termination
 Initiation factors help assemble the machinery and
allow the small subunits to search for the start codon,
AUG
 In prokaryotic cells, the Methionine for the starting
AUG is formylated to differentiate from other AUGs in
the subsequent parts of the sequence
 This distinction is far more complex in eukaryotic cells
Translation
 The process continues with the aminoacid-tRNA
entering the A site of the ribosome and the used
tRNA leaving via the E site
 The growing peptide keeps elongating in the P site of
the ribosome
 The termination is achieved by recognition of the stop
codons.
 There are 3 stop codons in all organisms
Translation
 After translation, the protein is premature
 It is modified were it becomes fully functional
 This include folding, cleavage, attachment of other
groups e.t.c
 This is the post translational modification of proteins
Translation
 Give at least 3 differences between replication and transcription
 Why is synthesis by both DNA and RNA polymerases occur in 5’-3’ direction?
Why not in 3’-5’ direction?
 What will happen if there is a mistake in replication?
 What is the difference between RNA transcript and its DNA template?
 What will happen if there is a mistake in translation?
 What is the difference between CODON and ANTICODON?
 Where does translation takes place? Why does it takes place in that place?
 How can you compare and contrast between the 3 process: translation,
transcription and replication?
 How can you differentiate between prokaryotic and Eukaryotic replication?
 Why is replication more complex in Eukaryotes than in prokaryotes?
Questions

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Sadeeqsheshe the central dogma of molecular biology

  • 1. THE CENTRAL DOGMA OF MOLECULAR BIOLOGY by Sadeeq Sheshe Cancer Genetics Lab, Department of Bioscience, COMSATS Institute of Information Technology, Islamabad
  • 2.  The central dogma is a concept in molecular biology that attempts to explains the key processes inside the nucleus of the cell  According to this concept, genetic information in DNA is being replicated, later transcribed to RNA and then translated into the protein  It was earlier thought to be an irreversible process, however the concept of reverse transcription proved otherwise Central Dogma
  • 3.  DNA replication is the process of making new strands of DNA using another DNA as a template  DNA replication is catalyzed by the enzyme, DNA polymerase whose structure is quite different in both prokaryotic and Eukaryotic cells  However, the process is semi-conservative in both prokaryotic and eukaryotic organisms  This means after the replication, each double stranded DNA is made up of one original strand (template) and a newly synthesized strand. DNA Replication
  • 5.  DNA replication occurs in 5’-3’ direction  This means the hydroxyl (OH) on the 3’ on the starting nucleotide attacks the alpha phosphate on the incoming nucleotide thus establishing a phospho- diester bond  DNA replication does not start de novo but require short strings of RNA oligos called primers  Primers are synthesized by the enzyme primase DNA replication
  • 6.  DNA replication consists of 3 steps  Initiation, Elongation and Termination  Prokaryotic replication starts at an Origin of replication  These consensus sequences are recognised by the different proteins involved in replication, forming the pre-initiation complex  The double strands unwinds and primers are synthesized Replication
  • 7.  Certain proteins are assoicated with the replication process  Primase, Helicase, Topoisomerases and SSB proteins  Primase synthesizes primers  Helicase unwinds double stranded DNA  Topoisomerases relieve tensional force generated due to winding  SSB holds single strands together preventing them to fold and form helical structures  Termination is achieved with the bi-directional synthesis converging at a single point called TER Replication
  • 9.  This is the process of synthesizing an RNA molecule using DNA as template  It is an enzymatic process catalyzed by a DNA dependent RNA polymerase  RNA polymerase is also structurally different in both prokaryotic and eukaryotic cells  Also moves along5’-3’ direction but does not require primer  Question: why RNA polymerase does not require primer and DNA polymerase do? Transcription
  • 10. Figure : promoter regions
  • 11.  Just like the replication process, transcription also has 3 steps; Initiation, Elongation and termination  Transcription initiates by recognition of certain sequences on the template Strand called promoter sequences. This are the start point for transcription.  This follows with the assembly of the transcription machinery  And the elongation follows, in 5’-3’ direction, releasing pyrophosphates  The pyrophosphate is hydrolyzed to 2 inorganic phosphates with release of energy  This energy is the driving force for the process, just like in REPLICATION Transcription
  • 14.  The termination step is of two types  The rho dependent and rho independent  Rho is a protein that is part of the polymerase transcription complex  It searches for specific sequences and halt the polymease, hence stopping the transcription  While in rho independent termination, the transcriptional machinery including the polymerase recognized certain sequences along the template and stop Transcription
  • 15.  This is otherwise known as protein synthesis  The information on the DNA that has been transcribed into RNA is finally translated into a protein  The information on the RNA are recognized as codons which are triplets of bases.  Each amino acid has a codon although, one amino acid can have more than one codon  This is degeneracy of the protein code Translation
  • 17.  Translation takes place outside the nucleus, specifically on the famous organelle, ribosome  Although, the ribosome has two different structural subunits in both prokaryotic and eukaryotic cells, it is quite structurally distinct in both cells  These sub units together with various proteins that assist in translation form the translational machinery  Each amino acids is first activated by attachment to an Adapter RNA, tRNA catalysed aminoacyl-tRNA synthetases Translation
  • 19.  Translation also occurs sequentially in steps: Initiation, elongation and termination  Initiation factors help assemble the machinery and allow the small subunits to search for the start codon, AUG  In prokaryotic cells, the Methionine for the starting AUG is formylated to differentiate from other AUGs in the subsequent parts of the sequence  This distinction is far more complex in eukaryotic cells Translation
  • 20.  The process continues with the aminoacid-tRNA entering the A site of the ribosome and the used tRNA leaving via the E site  The growing peptide keeps elongating in the P site of the ribosome  The termination is achieved by recognition of the stop codons.  There are 3 stop codons in all organisms Translation
  • 21.  After translation, the protein is premature  It is modified were it becomes fully functional  This include folding, cleavage, attachment of other groups e.t.c  This is the post translational modification of proteins Translation
  • 22.  Give at least 3 differences between replication and transcription  Why is synthesis by both DNA and RNA polymerases occur in 5’-3’ direction? Why not in 3’-5’ direction?  What will happen if there is a mistake in replication?  What is the difference between RNA transcript and its DNA template?  What will happen if there is a mistake in translation?  What is the difference between CODON and ANTICODON?  Where does translation takes place? Why does it takes place in that place?  How can you compare and contrast between the 3 process: translation, transcription and replication?  How can you differentiate between prokaryotic and Eukaryotic replication?  Why is replication more complex in Eukaryotes than in prokaryotes? Questions