<p>Okay. I can be wrong - it’s been known to happen frequently, and in this case I will not hesitate to admit it. :)</p>
<p>I’m fairly certain that it was sodium; it was more of a pure guess than anything, but I seem to recall that the membrane potential of the sodium ion is +50-60 mV.</p>
<p>Membrane potential includes both K+ and Na+ ions, and is ~55 mV at rest.</p>
<p>I thought that the only answer choice that was anywhere near 58mV was sodium potential after plugging in the values into the nernst equation. hmmm…</p>
<p>!!! No, resting membrane potential is -70 mV when the potentials of the sodium, potassium, chlorine, calcium, and polypeptide/DNA components are all considered. The membrane potential of the SODIUM ION ONLY is about +50-60 mV.</p>
<p>And mjin, that sounds reasonable based on my knowledge.</p>
<p>what was the answer to the frequency of errors in dna after proofreading and the problem that states that the F2 plants have 4/1000 as 12cm and 4/1000 as 40 cm and they want you to find out the fraction of plants that are 28 cm tall (the mean height is 24cm)</p>
<p>And also what is the true statement about Hardy-weinberg equilibrium? (what does dN/dt=0 mean)</p>
<p>I have no idea how to find frequency of recombination. I need to review genetics before the semifinal exam.</p>
<p>dN/dt is the derivative of population with respect to time. In English, dN/dt = 0 means that the total change in population is zero for that time period. This is not <em>necessarily</em> a true statement about Hardy-Weinberg equilibrium, as a randomly mating population with no forces of natural selection and no immigration or emigration can increase in size but still be in equilibrium.</p>
<p>^One of the conditions for Hardy-Weinberg is that the size of the population remains constant. (I think)</p>
<p>I wasnt sure if the answer was dN/dt=0 or if the genotypes are stable… Also does DNA get messed up 1 in every 1x10^6 bases?</p>
<p>It was 1 every 10^9.</p>
<p>The conditions are:</p>
<p>The population must be very large in size.
It must be isolated from other populations. (no gene flow)
No mutations.
Random mating.
No natural selection.</p>
<p>I would assume that the “genotypes are stable” answer would be right. dN/dt = 0 implies no population growth; it is perfectly fine for populations to grow as long as the other principles are observed.</p>
<p>are semifinals particularly impressive?
are they very impressive? I mean… it says you’re among the top ~500 in the US. But i feel that it’s pretty erratic, considering I broke but have only taken honors bio and have an Bio 770 SAT</p>
<p>@mjin: the plants problem requires quantitative genetics & it’s a real pain in the arse.</p>
<p>How does one prepare for semifinal round? Thank you in advance!</p>
<p>the cee website said that Part A is “approx 40 minutes” and B&C “approx 8o minutes”. does this mean that I’m given a 2 hour block of time where I have to pace myself?</p>
<p>also, compared to the open exam, how difficult/obscure are the semi problems?
If i only read campbell and skimmed parts of raven&lodish, would that suffice?</p>
<p>I always thought that the Semis test less random knowledge than the Open exams do. The Semis require you to do some analysis and synthesis of the random knowledge that you do know. But I’ve never taken semis before, so I could be wrong.</p>
<p>Semis is kinda supposed to be more “thinking” problems. I’d say, from my experience part C of semis is the hardest written assessment the USABO has (the finals theoretical isn’t too bad). Essentially, what they’re trying to test in semis is 1. Basic knowledge (parts A & B) and problem solving (similar to what you’d encounter in the finals practical in part c)</p>
<p>Just took it this morning.</p>
<p>Basically, a whole bunch of multiple choice, plus a short-answer section. Really not a great deal different from the Open Exam in execution.</p>
<p>Also, @ samgunno: You have 2 hours for all three parts. The multiple choice will take you FAR more than 40 minutes.</p>
<p>thanks guys for answering quick, here are some follow up qs:</p>
<p>phospho: how basic is “basic”? The open exam was also supposed to test some “basic” knowledge, but lots of people will agree that the stuff we took last february had ridiculously obscure questions not covered in campbell.</p>
<p>oldguy: did you have enough time for all three parts? or were you rushed at the end?</p>
<p>sorry if i’m too inquisitive, but i want a better sense of what to expect after that killer open exam.</p>
<p>“Basic” as in USABO basic, meaning campbell & some extra plants etc.</p>
<p>Quick Q for oldguy: Were there and “Pick all that apply” Qs? And how would you compare difficulty to the Open difficulty ( i remember last year everyone thought the open was rly hard & they made the semi easier)?</p>