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The Central Dogma of Molecular Biology

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  1. The Central Dogma of Molecular Biology by E. Börje Lindström This learning object has been funded by the European Commissions FP6 BioMinE project

  2. The flow of information

  3. DNA molecule • General structure: • double stranded • complementary • helical • antiparallel • Strands: • backbone of alternating phosphate and deoxyribosunits • four different bases; adenine (A), guanine (G), cytosine ( C ), and thymine (T). • Double helix: - due to base pairing: A=T and GC • Major and Minor groove

  4. 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

  5. DNA molecule, cont.

  6. DNA molecule, cont.

  7. 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

  8.  Initiation of replication • Start point: • only one (1) on the chromosome (300 bp) • origin (ori) ori • bidirectional: - both directions

  9. 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)

  10. 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’)

  11. Figures, DNA replication

  12. 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).

  13. Synthesis of RNA • ds DNA is the template: - only one of the strands • consists of four different subunits • a2bb’s • a2bb’ = core enzyme • s recognises the start site • RNA polymerase: • Direction of synthesis: - 3’5’

  14. 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

  15. P SG -35bp -10bp Promoters A sequence in DNA upstreams a structural gene: • -10 sequence Pribnow box • Strong promoters bind s effective

  16. P O SG1 SG2 SG3 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

  17. Translation Necessary substances: • mRNA • ribosomes • tRNA + aa  tRNAaa (attached aa) • different factors • enzymes • energy

  18. tRNA • DNA-genes: - Linear tRNA form (primary)  - cloverleaf structure (secundary) • Two peoperties: • binds aa (enzymatic) • binds to mRNA (codon) with its anti-codon

  19. tRNA, cont.

  20. 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

  21. 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

  22. 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

  23. S D-G AUG UAG 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

  24. The wobble concept