Biology 335 - Molecular Genetics

Central Dogma

 
The Central Dogma

illustrates the pathway by which information stored in the DNA sequence
is expressed - first as a transient message and then as a protein with cellular function...

The molecules of the pathway are linked by arrows to indicate the direction of information flow...

 
Replication and Recombination

Information Storehouse. So far we have mostly considered DNA structure in light of its function in the transmission of heritable information.
Expression of that information involves several processes
Not orginally part of the Central Dogma,
the arrow from RNA to DNA illustrates the flow of information used by a class of viruses called 'retroviruses'. This arrow represents the enzymatic activity of
revese transcriptase
which makes a DNA copy of the viral RNA genome
(or cDNA from mRNA)

Transcription

The enzyme RNA polymerase recognizes template promoter sequences and initiates, elongates and terminates the synthesis of an unstable intermediate RNA molecule
In some cases this RNA is functional (ie rRNAs, tRNAs) but in most cases it is a transient message due to RNA's inherent instability.
Transient Intermediate
The primary transcript in prokarytoes is the mature mRNA.
In eukaryotes several post-transcriptional modifications - including addition of the 5' CAP,
the removal of introns and joining of exons to form a contiguous coding sequence, and cleavage and addition of the 3' poly-A tail.
Translation
The ribosome translates the nucleotide sequence of the mRNA into an amino acid polymer using tRNAs to translate triplets of nucleotides into the amino acids specified by the genetic code.

As translation proceeds, the newly synthesized protein folds through a series of transient states to eventually achieve its mature 3D shape.

The protein represents the functional expression of the infomation stored in the heritable storehouse. The surfaces of the mature protein are responsible for molecular recognition and signaling, enzymatic and structural functions.
The circular arrow leading back to protein was proposed to explain the protein based transmission of information used by a class of 'slow viruses' or prions responsible for
kuru (a human disease contracted through the ritual consumption of human brains),
Creudzfeldt-Jacob disease
(a heritable disease similar to kuru),
scrappie (a disease common in sheep) and
Bovine Spongiform Encephalopathy (BSE)
a scrappie-like disease of cattle linked to feed
(a kuru for cows - cows aren't supposed to eat cows...)

 

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Transcription

Review
Replication
Review
Recombination