An hypothesis is a possible explanation for a set of observations or answer to a scientific question. A supposition or proposed explanation made on the
basis of limited evidence as a starting point for further
investigation.
And practically, an hypothesis is used as the basis for further experiments, where the hypothesis is rigorously tested under varying conditions. The hypothesis that is tested and modified under different conditions may become a general
theory
.The hypothesis that has the least number of variables sufficient to explain the phenomenon is preferred.
<span>prophase I. the chromosomes condense, and the nuclear envelope breaks down. ...Metaphase I. pairs of homologous chromosomes move to the equator of the cell.Anaphase I. homologous chromosomes move to opposite poles of the cell.Telophase I and Cytokinesis. ...Prophase II. ...Metaphase II. ...Anaphase II. ...<span>Telophase II and Cytokinesis.</span></span>
Smoking has been statistically identified as a <em><u>causative</u></em> element in the development of cancers of the lung, tongue, esophagus, and stomach.
It's certainly not a <em>prevention</em> in any way.
The most important idea is that the genetic material of any organism must be able to accurately replicate itself at least every generation (or for multicellular organisms at each cell division).
Base pairing (A-T or U and C-G)allows DNA and RNA (eg in polio virus, see Wikipedia page on RNA dependent RNA polymerase) to create a copy of themselves, when the appropriate enzymes are present. Proteins have no way of making a copy of themselves.
Stability is probably the main reason DNA is the most common genetic material. DNA has no enzymatic activity and was probably selected for to maintain the integrity of the genetic material (rather than having to perform a function for the cell/virus, during which it may be destroyed). The double helix structure also protects its integrity, and proofreading enzymes have also evolved which correct most of the mistakes made at DNA replication. RNA viruses don't have this mechanism- which could be said to be an advantage (as they can rapidly change and therefore avoid their hosts' immune systems), however in non-parasitic organisms most mutations in a gene would lead to a loss of an essential function and the extinction of that genome.
I don't think either of these reasons are relevant, but I think the main reasons retroviruses convert their RNA to DNA are so they can use the host cell's replication machinery (this was they do not need to encode as many genes), and secondly they need avoid the antiviral mechanisms of the cell, which would destroy any double stranded RNA molecules found (even if the virus was single stranded, dsRNA would have to be produced at replication).
