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Lecture 6: The 'central dogma' of molecular biology

Information stored as the base pair sequence of DNA encodes the amino acid sequences of the
thousands of proteins present in any given cell. The coded information in a gene sequence is first
TRANSCRIBED into an RNA nucleotide sequence and then TRANSLATED…

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'RNA' as if it were a homogeneous substance but there are different functional types of RNA.
Some are enzymes (!) others have unknown functions.


Messenger RNA (mRNA).
This acts as genetic intermediate, carrying information from the gene (i.e. DNA) to the protein
synthesis machinery (i.e.ribosomes).
In eukaryotes, this means that…

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In this context, what is meant by the term COGNATE?
a tRNA and its corresponding amino acid




Transcription (RNA synthesis) in E. coli
RNA polymerase is a highly processive enzyme that catalysis the reaction:
(RNA)n + NTP (RNA)n+1 + PPi


RNA polymerase requires NTPs plus Mg 2+ and a double-stranded…

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In bacteria, a single type of RNA polymerase makes all of the different kinds of RNA. In
eukaryotes, there are multiple RNA polymerases each making a different type of RNA.


Both strands of DNA are used as templates for transcription but not over their whole length, so that,
at any…

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If the heat-denatured DNA is then cooled rapidly, the strands will stay apart because they will
not have time to line up correctly and ANNEAL together again.
Annealing (a process also referred to as RENATURATION or HYBRIDISATION) will occur, however,
if the DNA is cooled slowly.
This can be utilised…

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Following the action of E. coli RNA polymerase on phage DNA, the resultant
TRANSCRIPT (i.e. the RNA product) was found to hybridise with only one
Strand of the DNA.
Thus, RNA polymerase makes a complimentary copy
Of only one DNA strand during transcription.




In summary, each RNA molecule (messenger, ribosomal…

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subunit helps find sites on the DNA where transcription begins ( promoter site)
at these sites participate in the initiation of RNA synthesis and then dissociates to from
the rest of the enzyme
random starts on foreign DNA



Both forms can transcribe 'foreign' DNA, but they do so with random…

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Post-transcriptional modification


RNA synthesis involves TRANSCRIPTION plus POST-TRANSCRIPTIONAL MODIFICATION. Understand the
meaning of: PRIMARY TRANSCRIPT; RNA PROCESSING; MATURE RNA. Here, we consider processing of
tRNA andrRNA. Processing of eukaryotic mRNA will be discussed in the next lecture.
Bacterial mRNA is not processed at all.


The primary transcript ('precursor tRNA') from…

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Mature tRNAs and the long rRNAs from both prokaryotes and eukaryotes are chemically
modified post-transcriptionally.
This occurs mainly on the primary transcripts although some modifications take place after
length-shortening.
The principal forms of chemical modification are: pseudouridine formation (Greek letter psi,
) via isomerisation of specific (not all) uridine
nucleosides…

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Rifamycin (semi-synthetic derivative = rifampicin) and Actinomycin D are specific inhibitors of
prokaryotic transcription although they act in fundamentally different ways.


How do the modes of action of these drugs differ?
Rifampicin is a compound derived from a strain of Streptomyces
Specifically inhibits the initiation of RNA synthesis
Interferes with…

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