The Central Dogma of Molecular Biology

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The Central Dogma of Molecular Biology

• by
• E. Börje Lindström

This learning object has been funded by the
European Commissions FP6 BioMinE project
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The flow of information
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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

- due to base pairing AT and G?C

• Major and Minor groove

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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

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DNA molecule, cont.
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DNA molecule, cont.
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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

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Initiation of replication

• Start point

• only one (1) on the chromosome (300 bp)
• origin (ori)

ori

• bidirectional

- both directions
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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)
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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)

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Figures, DNA replication
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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).

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Synthesis of RNA

• ds DNA is the template

- only one of the strands

• consists of four different subunits
• a2bbs
• a2bb core enzyme
• s recognises the start site

• RNA polymerase

• Direction of synthesis

- 3?5
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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

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Promoters
A sequence in DNA upstreams a structural gene

• -10 sequence Pribnow box

• Strong promoters bind s effective

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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

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Translation
Necessary substances

• mRNA
• ribosomes
• tRNA aa ? tRNAaa (attached aa)
• different factors
• enzymes
• energy

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tRNA

• DNA-genes

- Linear tRNA form (primary) ?
- cloverleaf structure (secundary)

• Two peoperties

• binds aa (enzymatic)
• binds to mRNA (codon) with its anti-codon

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tRNA, cont.
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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

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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
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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

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Reading frame

• Open reading frame (ORF)

- a gene

• Codon usage

• The code (tripletts) does not mean the same in
  all organisms
• The mRNA or ORF give different products

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The wobble concept