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Lecture 2

Lecture 2 - DNA

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Western University
Biology 2581B
Jim Karagiannis

LECTURE 1: DNA, THE MOLECULE OF HEREDITYAND BIOLOGICAL INFORMATION Key Concepts 1. What is information 2. Quantitating information 3. DNAis the genetic material 4. Chemical structure of DNA How much Information is in a DNASequence, a Gene, or a Genome? • For example – the gene coding for insulin: o Coding sequence comprises 1789 base pairs o Codes for a hormone o Secreted by islet cells in the pancreas o Important in maintaining blood glucose levels Quantitating Information • Simple game – round 1 gives us information (the number is higher than 10); however, round 2 does not present us with new data, as we have already found out that the number is higher than 10 o Data does not necessarily equate with information • Hypothetical devices – a message cannot be conveyed for device 1, as there is only 1 letter that is produced; however with device 2, a message can now be conveyed, as there are 2 symbols od uncertainty (Acan be one outcome, whereas B can be another outcome) o Uncertainty must be present in order to convey a message • Uncertainty = l2g (M), where M is the number of possible symbols • Example: Device #3 o Uncertainty = l2g (4) = 2 bits o How much information could device #3 generate? o It depends on the length of the sequence • Maximum information content of any sequence = L[l2g (M)], where Lis the length of the sequence • Bits = unit of measurement • Choice of base is arbitrary, but generally speaking, when talking about information, we use a base of 2 Putting it all Together • For DNA– 4 possible symbolsA, G, C or T o Uncertainty= lo2 (4) = 2 bits • For the insulin gene, the information content is 2789[log (4)] = 3578 bits • Note: these calculations assume an equal probability (0.25) of seeing each symbol in the sequence Why Bits? • If you were to unambiguously convert a DNAsequence into a string 1’s and 0’s, you would need, on average, 2 bits/symbol • For example, using 0 =A, 1 = G, 10 = C, 11 = T would result in 0110101011 (which could be interpreted asAGCCCT orAGGAGACT or…) whereas, 2 bit/symbol would work perfectly to convert a DNAsequence (shown to the right) • Thus, a DNAsequence has the capacity to store information, but can this property be shown experimentally? Experiment • Two different types of colonies can emerge with Streptococcus pneumonia – formation of smooth colonies and rough colonies • Smooth colonies are virulent and rough colonies are avirulent • Experiment 1 – virulent form injected into mouse, mouse dies • Experiment 2 – rough colony is isolated from virulent form and injected into mouse, mouse stays alive • Experiment 3 – virulent form is heated, heat kills cells leaving cellular debris, injected into mouse and mouse stays alive • Experiments 1-3 were control experiments; critical observation is listed as experiment 4 • Experiment 4 – virulent form is heated, heat kills cells leaving cellular debris, which is mixed with rough colony and injected into mouse (mixture of dead S cells and avirulent R cells), mouse dies – when tissue was analyzed, living smooth colonies were recovered • Interpretation is that something in the cellular debris derived from the S form, passed on information to the R cells which were able to use that information and convert themselves to S, killing the mouse as a result • This implies that the unit of biological information was present in the cellular debris DNACarries Biological Information • What specific component of the debris held the unit of information? • It was shown that the transfer of information was independent of the mouse – mouse was not needed at all for the transformation of R to S form • Cellular debris was purified into different components • Purified component that held almost 100% DNAwas the component that was able to transform R to S • Another experiment worked with bacteria phages and viruses, which provided convincing evidence that genes are made of DNA • Phage particles consists of DNAcontained with a protein
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