Bulletin Board for C3032: Genetics - Fall 2006 - Hamilton 702

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Describe how the E. coli use the negative and positive controls of the lac operon to economize on RNA and protein synthesis.

You can find detailed answers to this question in any molecular biology textbook (Gene VIII).

In short, in the prescence of lactose, allolactose binds to the lacI repressor and derepresses the operon and enables the transcription of genes necessary for processing lactose. That's the positive regulation. In the prescence of glucose, cAMP level is low, which is necessary to bind CAP and promote the transcription of the operon. Therefore glucose represses the operon. That is the negative regulation.

what is a balancer chromosome? what is its purpose?

A balancer chromosome contains a dominant "marker", a recessive lethal gene that "balances" the recessive lethal allele of the gene interested on the other chromosome, and multiple inversions that inhibits recombination. The presence of a balancer chromosome ensures that the frequency of the recessive lethal allel is the same in each generation by killing the WT animals (because WT animals are homozygous for the balancer, which is also recessive lethal).

What are nonsense suppressor tRNAs?

Nonsense suppressor tRNAs are mutations in tRNA anticodons that generates tRNAs capable of pairing with stop codons. These tRNAs would be able to bypass a nonsense mutation in a gene by binding to the mutant stop codon. Therefore instead of a shorter protein, which is caused by the nonsense mutation, a protein of possibly normal length would be produced.

Wild type vs. Wild-type

Wild type animals are the animals that are "wild", etc. not manipulated in a laboratory. Wild-type animals are the animals that we use as standards for evaluating mutant phenotypes. Unlike Wild Type, Wild-type is an arbitrary concept that people use to make genetic analysis more convenient.

What is a M1 division pattern

For a specific allele a/+, after Meiosis I, M1 division pattern is seen when both daughter cells contain either a or + (not both of them). If both a and + are seen in both daughter cells, M2 division pattern has occured.

How 3:5 tetrads (half conversion) are formed?

Half conversions are formed when one of the two homologous chromosomes do not undergo mismatch repair. When this mismatch is replicated during mitosis after meiosis has finished, the two daughter cells will have different allels at this locus. If this happens to one of the two chromosomes that has gone through recombination, and the other chromosome is repaired, you would be able to see a 3:5 ratio

Update: 08/28/2006