THE CENTRAL DOGMA OF MOLECULAR BIOLOGYby

Sadeeq ShesheCancer 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