The lower the level of the pyramid, the more important the needs are. On the most bottom part, you will see physiological needs such as the need for food and water, for shelter and for warmth, while higher up at the top you'll see things such as being esteemed as someone great or being successful at what you do. Those thing are higher because they rare less important for us than physiological needs.
naturally occurring compounds, which form the differentiating component of nucleotides; five bases occur in nature, three of which are common to RNA and DNA