1. Management of Genetic
Information

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 DNA
replication

5. Expt by Meselson-Stahl proved the
semiconservative 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 in
prokaryotes

8. Replication in
eukaryotes

9. DNA synthesis based on two template strands: leading strand and
lagging 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

16.  Primase catalyzes the synthesis of RNA primer
 Synthesis
 catalyzed by Pol III
 primer removed by Pol I
 DNA ligase seals remaining nicks

21. Summary of DNA replication in
prokaryotes
 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

22. 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

23. 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

25. RNA synthesis
 Transcription
 Template is DNA
 Major enzyme: DNA directed RNA polymerase
 No need for primers
 5’ to 3’ direction

26. 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

27. Promoter 40 base pairs upstream (-40)
away from the start site (+1)

28. INITIATION STEP

29. ELONGATION STEP

30. TERMINATION STEP

31. ρ-FACTOR INDEPENDENT- FORMATION OF HAIRPIN LOOP

32. Eukarotic transcription have 3 classes of
RNA 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)

33. Post transcriptional modification of the
eukaryotic 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)

37. 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

38. Genetic code
 Triplet nucleotide – one amino acid
 Nonoverlapping
 No punctuation
 Degenerate
 Almost universal

41. Initiation
 Initiation factors
 Shine-Dalgarno sequence in mRNA
 30S ribosome
 N-formylmet

44. Inhibitors of protein synthesis

45. Postranslational modification
 Protein folding –chaperones
 Proteolytic cleavage (zymogens) – hydrolytic
enzymes in the gut
 Amino acid modifications
 Attachment of carbohydrates
 Addition of prosthetic groups

46. Regulation of protein synthesis and gene
expression
 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