DNA is composed of nucleotides, which are composed of three parts:
1. A five carbon carbohydrate (pentose)
Pentose is classified as a monosaccharide (simple carbohydrate), and its molecular formula is represented by 
. They are water soluble molecules with sweet taste. Monosaccharides may have from three carbon atoms (trioses) to six (hexoses). In DNA is present pentose (5 carbon atoms)
2. A nitrogenous base
Nitrogenous bases are ring-shaped compounds that contain nitrogen in their molecular structure. They are classified into two groups: purines and pyrimidines
The purine bases are adenine (A) and guanine (G). They are larger and contain more than one ring in their structure,.
The pyrimidines, cytosine (C), uracil (U) and thymine (T), are smaller and composed of a single ring.
3. One or more phosphate groups
A phosphate consists of a trivalent anion that contains one phosphorus atom and four oxygen atoms. Its molecular formula is expressed by 
.
almost any living organisms like animals and humans. Im not so sure about plants though. happy studying! :)
The correct answer is d. both excitation and inhibition.
There is a process which is called summation and it refers to the process that determines whether or not an action potential (on postsynaptic neuron) will be generated by the combined effects of excitatory and inhibitory signals (from the presynaptic neurons). Depending on the sum total of inputs, summation may or may not reach the threshold voltage to trigger an action potential (firing of postsynaptic potential).
The combination of drought and poor land use practice created the the dust bowl.
