<p>I have three questions and I can't figure them out...</p>
<p>1)Why do you suppose humans circulate the monosaccharide, glucose, in their blood, rather than employing a disaccharide like sucrose as a transport sugar, as plants do?
The closest I can come up is that it has something to do with absorption of sugar in humans, but I don't know if that's right and if it is, then why.</p>
<p>2) If you were now to mix these two RNA single-strand copies together, do you think they would forma double stranded RNA molecule?</p>
<p>3) Why is DNA double-stranded, while RNA is generally single-stranded?
I know about transcription and all that, but I keep on thinking this has something to do w/ evolution or some chemistry reason...</p>
<p>2) The RNA strands would only form a double-stranded RNA molecule if they had complementary nitrogenous bases or at least if enough bases were complementary for the cumulative strength of the H bonds to keep the RNA molecules together.</p>
<p>1) Just the way evolution took its course. Now, I'm just making an educated guess, but it could also be because a disaccharide like sucrose could not be taken up by the body as quickly as a monosaccharide like glucose. It would need to go through an extra step (breaking down sucrose with the enzyme sucrase) before it could be taken up by cells and broken down further via cellular respiration. This would be detrimental if energy were needed quickly.</p>
<p>2) Like Odin said, only if the two strands were complementary. Double stranded RNA does exist in nature, including certain viruses and certain functions in human cells.</p>
<p>3) You're mostly right about the transcription bit. Also, double stranded DNA is a good form of redundancy and protection; there are two strands, and copies are made rather than altering the original during replication and transcription. Additionally, the double helix works well chemically because it puts the relatively hydrophobic nitrogenous bases in the middle of the DNA molecule, shielded from the aqueous environment of the cell.
Single stranded RNA also allows for translation of DNA into proteins. And last but not least, single stranded RNA allows certain molecules of RNA to form ribozymes; single stranded RNA molecules that act as enzymes. This is made possible by different segments of the same RNA molecule having complementary base pairs, allowing these segments of the RNA molecule to bind to one another and make different conformations that allow it to act as an enzyme.</p>
<p>1) simply what goldshadow says, glucose is more basic and more basic is better in terms of BASIC cell needs. when cells first developed they werent very advanced and needed a BASIC form of sustenance. also, like he said, the extra step needed to break down a disaccharide would simply decrease efficiency</p>
<p>2) Yes exactly what he said. when does dsRNA exist in human cells? i can only think of certain viruses that have that. not doubting u, im just wondering</p>
<p>3) Goldshadow gave a good description. i also think that it provides extra proofreading.</p>