Tutor profile: Imad H.
What are the irregulars for the informal second-person singular imperative, and how can I easily remember them?
Remember: Ven DiSal Haz Ten Ve Pon Sé? Venir: Ven Decir: Di Salir: Sal Hacer: Haz Tener: Ten Ir: Ve Poner: Pon Ser: Sé
Subject: Organic Chemistry
Out of all the resonance structures possible for a molecule, which one is the best?
Though you will find variations in the way this is explained, there are essentially 4 factors that make a contributor the best contributor (listed in order of importance): 1. All atoms should have "a closed shell," meaning all elements have access to eight electrons. The only exception is hydrogen, which can only have two. 2. There should be minimal formal charges, which cannot exceed -1 or +1. If you have one contributor that has four formal charges and another has two, then the one with only two is better. 3. Negative formal charges should be on the more electronegative atoms, and positive charges should be on more electropositive atoms. Oxygen is more electronegative than nitrogen, so oxygen "prefers" a negative charge over nitrogen, and nitrogen "prefers" a positive charge over oxygen. 4. The same charges (+ +, - -) should be as far as possible from each other, and opposites (+ -) should be as close as possible.
Why is DNA stable despite the large negative charge?
Histones contain metals that carry positive charge that stabilize the negative charge on the DNA. As a result, the molecule does not unravel. Also, during replication, DNA polymerase (for example) has Mg2+ metal cofactors which allow for the enzyme to both attach to and stabilize the DNA. Furthermore, there is a stacking effect from the hydrogen bonds between the nitrogenous bases which keep the two strands together. Something important to keep in mind is that the negative charges are necessary for replication and transcription, because the strands can straighten out and become accessible to the necessary enzymes.
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