Phenomenon: Miss Westbrook visited family in Aspen last spring. Aspen is in Colorado and has an elevation of 8,000-9,000 feet. The Bay Area has an elevation of 0-100 feet. Miss Westbrook’s breathing and heart rate both sped up every time she went hiking or skiing in Aspen. Why do you think this happened? Hint: Your brain regulates the rate and depth of your breathing to match the needs of your body for 02 intake and CO2 removal.

Breathing rate refers to the number of breaths per minute.

step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.