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Sharon W.
UCLA Alumni Tutor
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Organic Chemistry
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Question:

Explain why an SN1 reaction produces a racemic mixture.

Sharon W.
Answer:

During an SN1 reaction (also called a unimolecular nucleophilic substitution), the leaving group (usually a weak, stable base) leaves the electrophile to form a carbocation. A carbocation is unstable (a positive charge on a not-very-electrophilic Carbon?? That's not so good!) and becomes trigonal planar. Why does it become trigonal planar? Well, once the leaving group leaves, there's only three more things attached to Carbon. Because electrons repel each other, they'll want to form the best conformation by being as far away as from each other as possible which happens to be trigonal planar. Because the carbocation is trigonal planar (hint: the keyword is planar), the molecule is flat. The nucleophile can attack from either side of the carbocation. This 50/50 chance of being attacked from either side leads to a racemic mixture in SN1.

Psychology
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Question:

Jane, Alfred, and Kim were assigned to a group project in their social studies class. While all three of them are normally diligent workers, Kim started procrastinating on her parts of the project. Kim even asked Alfred if he could finish researching half of her questions. This behavior is an example of:

Sharon W.
Answer:

This is an example of social loafing. Social loafing is the tendency for individuals to put in less effort when they're in a group. In this case, Kim has been procrastinating on her part of the project and has even asked Alfred to work on her project responsibilities. Kim is normally a hard worker, but she's putting in less effort because she's part of a group. Social loafing is similar to the concept of the bystander effect. In both social psychological phenomena, individuals feel less personal responsibility (also called the diffusion of responsibility). They assume that other group members will take up the responsibility instead.

Biology
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Question:

Which of the following mutations are the most harmful to an organism? A. Silent Mutation B. Missense Mutation C. Frameshift Mutation

Sharon W.
Answer:

C. Frameshift Mutations Recall that mutations are changes in a DNA sequence. Many of these mutations occur at the level of a single nucleotide, which are called point mutations. (I.e. If the original DNA sequence was something like AGTCGCGCTATGAGT and any one of those letters have changed, then a point mutation has occurred.) Once an mRNA strand is synthesized from a DNA strand, mRNA is read in three letter sequences called "codons" (I.e. If we take that same DNA sequence from above and divide it into three, these become "codons" AGT-CGC-GCT-ATG-AGT). The cell takes these "codons" and uses the genetic code (i.e. usually a big table that tells you which three letters makes what amino acid) to translate each codon into one amino acid. If no mutation occurs, then the normal amino acid will be produced from the codon. Now let's look at each choice individually! A. Silent Mutations. Silent mutations are mutations where the change in a nucleotide has no impact on the amino acid produced. Remember, there's something called "wobble" or degeneracy in the genetic code. This simply means a lot of different DNA/mRNA sequences can code for the same amino acid (i.e. AAA and AAG both make the amino acid Lysine). Because the normal protein would still be produced, this is unlikely to be harmful to the organism. B. Missense Mutation. This happens when a change in a nucleotide results in a change in the amino acid. If you think about it, this would only affect that ONE amino acid, not the whole protein. While it's not ideal, it's still not the worst thing for an organism. C. Frameshift Mutation. Frameshift mutations happen when a nucleotide is inserted or deleted in the genome. Remember how I mentioned that mRNA is read three nucleotides at a time? Well imagine if you inserted or deleted one nucleotide! It would shift the entire sequence by one, affecting every single codon after that! (i.e. If the original sequence was THE DOG RAN, and one nucleotide was deleted, it would change the sequence to TED OGR AN) This is why it's called a frameshift mutation - it shifts the entire frame!

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