The difference between nucleotides and nucleosides is that nucleotides contain a phosphate group whereas nucleosides do not. This means that a nucleoside is made up of just a pentose sugar and a nitrogenous base. The DNA and RNA differ from each other based on their pentose sugars (DNA has deoxyribose and RNA has ribose). Another difference between RNA and DNA is that uracil nitrogenous base is only found in RNA whereas thymine nitrogenous base is only found in DNA.

tRNA is synthesized by RNA polymerase III, mRNA is synthesized by RNA polymerase II, and rRNA is synthesized by RNA polymerase I. DNA polymerase I is involved in DNA synthesis, and specifically, has a 5' to 3' exonuclease functionality, which removes RNA primers laid down by primase and replaces them with DNA nucleotides. 

Two orthophosphates are connected via anhydride linkage to form the high energy pyrophosphate.  This is the "" bond. Glycosidic linkage describes a bond between two or more sugar molecules, not between orthophosphates. This anhydride linkage is made up of single bonds, and not double or triple bonds. An amide bond is the specific chemical name for a peptide bond.

The  (melting temperature) of DNA is defined as the temperature at which half of the DNA strands would become denatured. It is known that GC base pairs are kept together via hydrogen bonding at three different locations, compared to hydrogen bonding at just two locations in AT base pairs. Because of this additional interaction, a DNA strand with a higher component of GC base pairs will have a higher  than one with a higher component of AT base pairs.

Eukaryotes have so much DNA that it needs to be compacted to make it less bulky and more protected. The lowest level of organization is known as a coil. Predictably, multiple coils together form a super-coil. Next, the DNA is wrapped around proteins known as histones. 8 histones together form a nucleosome. Finally, fully packed and organized DNA is known as chromatin.

Heterochromatin is dark, dense, tightly packed, and rich in repeating segments. It is often in cells that are inactive or less active. As such, heterochromatin is sometimes referred to as "non-coding DNA". Euchromatin, on the other hand, is less tightly packed, and more readily coded. Centromeres are the part of the chromosome that attach to kinetochores during cellular division. Finally, The ends of chromosomes are known as telomeres. Of note, centromeres and telomeres are actually both composed of heterochromatin.

Guanine is a purine, not a pyrimidine. The purines are guanine and adenine, while the pyrimidines are cytosine, thymine, and uracil. Uracil is present only in RNA, and thymine only in DNA. The rest of the bases are present in both DNA and RNA.

It is true that DNA is more stable than RNA. While the exact chemical reasons for this are complex, it is useful to know RNA is readily hydrolyzed in basic conditions. In order to make sense of this, remember that DNA acts as the genetic code, and must hold that genetic information for relatively long periods of time. While there are several types of RNA, it typically acts as a messenger, and is degraded after completing its task. While DNA does contain the base thymine and RNA does contain uracil, this is unrelated to the relative stabilities of the two nucleic acids. 

While it could be useful to know that the bases are at the core of the DNA while the sugar-phosphate chains are on the outside, it is possible to answer the question without having memorized that fact. The bases (thymine, cytosine, guanine, adenine) are non-polar, and will cluster in the middle of the chain, away from water. They also connect to each other via hydrogen bonding. On the other hand, the sugar-phosphate groups are hydrophilic, and will cluster towards the outside of the molecule.

All DNA contains 50% purines and 50% pyrimidines due to Watson-Crick base pairing.

However, doing the calculations based on the information given can also give the correct answer. Adenine (A) and guanine (G) are purines. Cytosine (C) and thymine (T) are pyrimidines. Chargaff's rules state that in DNA, G = C and A = T. If the sample is 15% G, then it must also be 15% C. This leaves 70% for A and T, or 35% each. 15% C +  35% T = 50% pyrimidines.