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Transcript
The Central Dogma of Molecular
Biology
by
E. Börje Lindström
This learning object has been funded by the European Commissions FP6 BioMinE project
The flow of information
DNA molecule
• General structure:
- double stranded
- complementary
- helical
- antiparallel
• Strands:
- backbone of alternating phosphate and
deoxyribos units
- four different bases; adenine (A), guanine (G),
cytosine ( C ), and thymine (T).
• Double helix:
• Major and Minor groove
- due to base pairing: A=T and GC
DNA molecule, cont.
• Size:
- units: kilobase (kb) or kilobase pairs (kb pairs)
- E. coli chromosome  4 700 kb pairs
• Form:
- closed chromosome molecule (in bacteria)
-  1 mm long  packing problem in bacteria
- solved by supercoiling
• DNA binding proteins:
Un-specific:
- histones
Specific:
-Repressors
- RNA polymerase
- restriction enzymes
- modification enzymes
DNA molecule, cont.
DNA molecule, cont.
DNA replication
General
• Semi conservative:
-new DNA molecules contain:
1 old strand and
1 new strand
• use a ’template’:
- one of the strand is used
• ’primers’:
-usually a piece of RNA
- DNA-polymerase unable
to start replication
Initiation of replication
• Start point:
-only one (1) on the chromosome (300 bp)
- origin (ori)
ori
• bidirectional:
- both directions
Synthesis of DNA (replication)
• several enzymes involved (~ 20 pc)
- DNA helicase
Unwinding the molecule
- DNA gyrase
(topoisemerase II)
Open up (cut)
the strands
- DNA-binding enzymes
Protect ss-DNA from nucleases
- Primase
Synthesises the RNA primer
- DNA-plymerase III
Synthesis in direction 5’3’
There are 3 enz. in E. coli; pol I, II and III
- DNA-plymerase I
Removes the primer
Repair any missing bp in DNA
- DNA ligase
Makes a phospho-di-ester bond
(glueing)
Synthesis of DNA, cont.
• ’leading’ and ’lagging’ strands:
- leading: continous synthesis
- lagging: dis-continous synthesis
• proof-reading:
- checking if any mitakes has been made
- pol. III removes the wrong nucleotides (3’ 5’)
Figures, DNA replication
RNA transcription
Three types of RNA:
• mRNA (genetical)
• tRNA (aa-carrier)
• rRNA (structural)
Structure:
-ss-stranded (internal ds secundary structures)
- ribose
- four different bases; adenine (A), guanine
(G), cytosine ( C ), and uracile (U).
Synthesis of RNA
• ds DNA is the template:
- only one of the strands
• RNA polymerase:
- consists of four different subunits
- a2bb’s
- a2bb’ = core enzyme
- s recognises the start site
• Direction of synthesis:
- 3’5’
Start and stop of RNA synthesis
• Where is the start ?
- Note! No primers necessary!
- The polymerase binds to the promoter
- s recognises and attaches to the promoter region
- ds-DNA opens up and the synthesis starts
- s is detached and the core enzyme continues
• Where does the synthesis stop?
-termination at special DNA-sequenses, terminators
- inverted repeates in DNA  ’stem-loop’-structures in
RNA
Promoters
A sequence in DNA upstreams a structural gene:
P
-35bp
• -10 sequence
SG
-10bp
Pribnow box
• Strong promoters bind s effective
mRNA
• Short half-time
• Polycistronic (in bacteria)
- information from several structural genes
• Definitions:
- operator (O): a gene that can be effected by a repressor protein
- operon: structural genes with the same repressor
P
O
SG1
SG2
SG3
Translation
Necessary substances:
• mRNA
• ribosomes
• tRNA + aa  tRNAaa (attached aa)
• different factors
• enzymes
• energy
tRNA
• DNA-genes:
- Linear tRNA form (primary) 
- cloverleaf structure (secundary)
• Two peoperties:
- binds aa (enzymatic)
- binds to mRNA (codon) with its anti-codon
tRNA, cont.
Synthesis of proteins
A four (4) step process:
• Initiation
• Elongation
• Termination-release
• Peptide folding
• Initiation:
-a complex of
- 30S subunit,
- f-meth-tRNA, (start codon AUG in mRNA)
- mRNA and
- initiation factors are formed
• Shine-Delgarno sequence
-3-9 bases in mRNA
- complementary to 16S rRNA
- addition of 50S subunit
Synthesis of proteins, cont.
• Elongation:
-several elongation factors are needed
- Next aa-tRNA is added to the A-site (ribosome)
- a peptide bond is created
- the peptide is moved to the A-site
- translocation to the P-site during
- movement of the ribosome forward
- a free A-site is created …
-Etc.
• polysomes:
- mRNA with several ribosomes
Synthesis of proteins, cont.
• Termination:
-stop codes in mRNA
- UAA, UAG and UGA; nonsence codes
- no tRNA for these codes exist
- release factors RF1-3 release the protein
- the ribosomes disintegrate
• The genetic code:
- in mRNA
• 3 bases
- 1 aa
• 43 = combinations
-but only 24 aa
- degenerated code
- the aa has several codes
Reading frame
• Open reading frame (ORF):
S D-G
AUG
• Codon usage:
- a gene
UAG
-The code (tripletts) does not mean the
same in all organisms
- The mRNA or ORF give different
products
The wobble concept