Lternative splicing can process the same pre-mrna into different forms of mrna. which one of the following reasons best explains why this pre-mrna can't be processed into a mrna containing only (exon 1-intron a-exon 3)?
a. the spliceosome can not excise rna sequences bounded by two 5' splice sites.
b. the spliceosome can not excise rna sequences bounded by two 3' splice sites.
c. the spliceosome can not excise rna sequences bounded by an upstream 5' splice site and a downstream 3' splice site.
d. the spliceosome can not excise rna sequences bounded by an upstream 3' splice site and a downstream 5' splice site.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.

i think that it is plants

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!