When evolutionary biologists examine DNA sequences of protein-coding genes, they observe that every third nucleotide changes more rapidly over time compare to other nucleotides. Based on the properties of the genetic code, why might this occur?
What will be an ideal response?
Answer: Answers should explain that the genetic code uses 4 nucleotides in three-base codons to designate the order of amino acids in a protein, but that there are 64 different codons and only 20 standard amino acids. Thus the genetic code is degenerate, with multiple codons designating the same amino acid. For many of the amino acids, the first two letters of the codon are the same, but the third base of the codon is variable. Mutations in the third base are silent, or have no effect on the protein or the organism; therefore, no selective pressure is applied to mutation of the third base and more mutations accumulate in the third position over evolutionary time.
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A plant secretes ____ when it needs to respond defensively to a pathogen
a. salicylic acid b. systemin c. brassinosteroids d. mineralocorticoids e. nitric oxide
Why is it easy to measure allele frequencies in a codominant allele system?
A. Such systems are rare. B. Phenotypes are equivalent to genotypes. C. Most alleles are present in homozygous condition. D. They are present in a 1:2:1 ratio in all populations. E. None of these is true.
Leptin is a circulating hormone that is produced by fat cells and plays a role in body metabolism and obesity. It normally binds to receptors in the brain and inhibits appetite. Studies have demonstrated that when obese mice that are leptin deficient are injected with leptin they quickly lose their excess weight. Interestingly, however, many overweight people have high levels of leptin in their bloodstream. Why do you think that the high levels of leptin in obese individuals are insufficient to curb their appetite?
A. In obese people with high levels of leptin, the leptin is a non-functional second messenger. B. The form of leptin released by obese people may lack protein kinase activity. C. Obese people who have high levels of leptin may have a leptin receptor with decreased sensitivity. D. Paracrine signaling may be disrupted in obese people with high levels of leptin.
What is the source of CO2 released into the atmosphere?
A. Volcanic activity B. Anaerobic microbial activity C. Respiration D. Fuel combustion E. All of the above.