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Differential Gene Expression in Cells w/ Identical DNA:

⁃ Post-Transcriptional Control:
⁃ once an mRNA is made, a series of events must occur if the final product is going
to affect the cell
⁃ each of these events offers an opportunity to regulate gene expression
⁃ the 3 major control points @ this stage are as follows
⁃ splicing mRNAs in various ways
⁃ altering the rate @ which translation is initiated
⁃ modifying the life span of mRNAs & proteins after translation has
occurred
⁃ Alternative Splicing of mRNAs: diff
...
types of exons
⁃ results in diff
...
cell types
⁃ How Translation is Controlled:
⁃ many miRNAs (micro RNAs) interfere directly w/ translation
⁃ in other cases, mechanisms that do not involve miRNAs are responsible for
controlling the timing or rate of translation
⁃ for example: regulatory proteins may bind to mRNAs or ribosomes to regulate
translation
⁃ in addition, in response to viral infection, the translation can be slowed or
stopped by phosphorylation of a certain ribosomal protein
⁃ Post-Translational Control:
⁃ mechanisms for post-translational regulation allow the cell to respond to new
conditions rapidly by activating or inactivating existing proteins
⁃ regulatory mechanisms occurring late in the flow of info from DNA to RNA to
protein involve a trade-off between speed & resource use
⁃ We end up w/ the same DNA in all of our cells b/c of DNA replication (mitosis)
⁃ in order for cells to divide, the DNA in the “mother” cell must be faithfully replicated in
order to ensure both “daughter” cells have a full complement of chromosomes

⁃ DNA Replication:
⁃ during replication the 2 parental strands separate
⁃ the bases are paired in the parental duplex either A-T or G-C
⁃ each parental strand serves as a template for the synthesis of a new daughter
strand
⁃ Proteins involved in DNA Replication:
⁃ topoisomerase II: relieves the stress of unwinding
⁃ helicase: enzyme that unwinds the DNA duplex
⁃ DNA polymerase: extends an RNA primer
⁃ single-stranded binding proteins: stabilize single strands of DNA



⁃ DNA ligase: connects the gaps on DNA
⁃ Primase: lays down RNA primers
⁃ DNA can only be synthesized from 5’ to 3’
⁃ incoming nucleotides are accepted if they correctly base pair w/ the template
⁃ the 3’ OH of the growing strand attacks the phosphate bond of the incoming
nucleotide, providing energy to drive the reaction
⁃ a nucleotide can only be added to a 3’ end of a DNA strand
⁃ replication of one strand is discontinuous (fragmented) but another grows
continuously
Leading strand: continuous strand
Lagging strand: fragmented strand
⁃ RNA primase transcribes the DNA & lays down an RNA primer
DNA synthesis has to primed by a short stretch of nucleic acids
primers: short stretch of nucleic acids
⁃ primers are removed & replaced w/ DNA & the fragments of the discontinuous
(lagging) strand are ligated where they meet
⁃ DNA polymerase extends the RNA primer
⁃ a diff

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