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Mv management of genetic information


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  • 1. Management of GeneticInformation
  • 2. Learning objectives Understand the mechanism of DNA replication, RNA synthesis and protein synthesis
  • 3. Flow of genetic information
  • 4. Two possible models of the DNAreplication
  • 5. Expt by Meselson-Stahl proved thesemiconservative model of replication
  • 6. Which direction does replication go? Major enzyme: DNA polymerase III  DNA double helix unwinds at a specific point called an origin of replication  Polynucleotide chains are synthesized in both directions from the origin of replication; DNA replication is bidirectional in most organisms  At each origin of replication, there are two replication forks, points at which new polynucleotide chains are formed  There is one origin of replication and two replication forks in the circular DNA of prokaryotes  In replication of a eukaryotic chromosome, there are several origins of replication and two replication forks at each origin
  • 7. Replication inprokaryotes
  • 8. Replication ineukaryotes
  • 9. DNA synthesis based on two template strands: leading strand andlagging strand templates; mechanism in prokaryotes is presented DNA is synthesized from its 5’ -> 3’ end (from the 3’ -> 5’ direction of the template)  the leading strand is synthesized continuously in the 5’ -> 3’ direction toward the replication fork  the lagging strand is synthesized semidiscontinuously (Okazaki fragments) also in the 5’ -> 3’ direction, but away from the replication fork  lagging strand fragments are joined by the enzyme DNA ligase
  • 10. Replication fork
  • 11. Enzymes and proteins in DNA replication
  • 12. The action of DNA polymerase Why 53’ direction?
  • 13. Start of DNA replication
  • 14. Unwinding DNA gyrase introduces a swivel point in advance of the replication fork a helicase binds at the replication fork and promotes unwinding single-stranded binding (SSB) protein protects exposed regions of single-stranded DNA
  • 15.  Primase catalyzes the synthesis of RNA primer Synthesis  catalyzed by Pol III  primer removed by Pol I  DNA ligase seals remaining nicks
  • 16. Summary of DNA replication inprokaryotes DNA synthesis is bidirectional DNA synthesis is in the 5’ -> 3’ direction  the leading strand is formed continuously  the lagging strand is formed as a series of Okazaki fragments which are later joined
  • 17. DNA polymerases Five DNA polymerases have been found to exist in E. coli  Pol I is involved in synthesis and repair  Pol II, IV, and V are for repair under unique conditions  Pol III is primarily responsible for new synthesis
  • 18. Eukaryotic DNA replication Not as understood as prokaryotic. Due in no small part to higher level of complexity. Cell growth and division divided into phases: M, G1, S, and G2 DNA replication occurs during the S phase
  • 19. RNA synthesis Transcription Template is DNA Major enzyme: DNA directed RNA polymerase No need for primers 5’ to 3’ direction
  • 20. RNA synthesis Requires a promoter region in the template DNA to which the RNA polymerse will bind Promoter 40 base pairs upstream (-40) away from the start site (+1) Three stages:initiation, elongation, termination Termination may be  rho factor dependent – rho factor terminates synthesis  or rho factor independent – formation of a stable hairpin loop
  • 21. Promoter 40 base pairs upstream (-40)away from the start site (+1)
  • 26. Eukarotic transcription have 3 classes ofRNA polymerases RNA pol I transcribes large ribosomal RNA genes RNA pol II transcribes protein encoding gene RNA pol III transcribes small RNAs (including tRNA and 5SRNA)
  • 27. Post transcriptional modification of theeukaryotic mRNA Capping – methyl guanosine attachment at the 5’ end to protect the cleavage of the RNA by exonucleases as RNA moves out of the nucleus Addition of poly A at the 3’ end (200-250 long) helps to stabilize the mRNA structure; increases resistance to cellular nucleases Splicing – removal of non coding sequences (introns)
  • 28. Protein synthesis Translation Based on the m-RNA sequence, genetic code Starts from 5’ end of the transcript Occurs in the ribosomes Activation of amino acids – attachment to the tRNA Initiation, elongation, termination
  • 29. Genetic code Triplet nucleotide – one amino acid Nonoverlapping No punctuation Degenerate Almost universal
  • 30. Initiation Initiation factors Shine-Dalgarno sequence in mRNA 30S ribosome N-formylmet
  • 31. Inhibitors of protein synthesis
  • 32. Postranslational modification Protein folding –chaperones Proteolytic cleavage (zymogens) – hydrolytic enzymes in the gut Amino acid modifications Attachment of carbohydrates Addition of prosthetic groups
  • 33. Regulation of protein synthesis and geneexpression 20K to 25K genes in the human genome Only a fraction of the genes are expressed at any given time Two types of gene expression: constitutive and inducible Inducible genes are highly regulated – regulatory proteins, hormones and metabolites