Nucleosomes represent the first level of chromosome organization, and occur in the interphase of the cell cycle. The nucleosome core has an eight histone-DNA complex, called a histone octamer. The region between two cores includes DNA up to a length of 80 base pairs. This packs the DNA tightly, to about one-third its initial length (a decrease of two-thirds). Histones all have are folded in a manner containing three alpha-helices as two loops. The DNA wraps around the core in a left-handed coil.

Nucleic acids such as DNA and RNA contain a series of nucleotides, each of which contains a sugar, a nitrogenous base, and a phosphate group. The sugar and phosphate group form the backbone of the linear chain, while the nitrogenous bases are able to protrude out. When hydridizing with other nucleic acid strands, only certain nitrogenous bases can pair with others. This base pairing is due to hydrogen bonds that form between the two nitrogenous bases, and the number of hydrogen bonds differs depending on what bases are involved.

For both DNA and RNA strands, guanine and cytosine will pair with one another via three hydrogen bonds.

In RNA, the nucleotide thymine is absent, but in its place is the nucleotide uracil. Uracil is able to base pair with adenine via two hydrogen bonds, but this only happens in RNA, not DNA!

In DNA, thymine is present rather than uracil. The thymine found in DNA is also able to base pair with adenine via two hydrogen bonds. Thus, out of all the answer choices, this is the only correct one that can occur within DNA.

Nucleic acids are made up of nucleotides that contain a pentose sugar, a phosphate group, and a nitrogenous base. There are two types of nucleic acids; DNA and RNA. The difference between these two nucleic acids is their pentose sugar. DNA has deoxyribose whereas RNA has ribose sugar residues. Note that both are types of pentose sugar; therefore, all types of nucleic acids have pentose sugars.

Nitrogenous bases found in nucleic acids are classified as either purines or pyrimidines. Free purines in the blood are broken down to uric acid in the liver. Recall that purines (guanine and adenine) are found in both RNA and DNA; therefore, breakdown of either type of nucleic acid will lead to release of purines and, subsequently, formation of uric acid in the liver.

As mentioned, nucleic acids are made up of monomers called nucleotides. Nucleosides, on the other hand, only have a pentose sugar and nitrogenous base (they lack the phosphate group); therefore, they are not the monomers of nucleic acids.

To answer this question we need to recall that the monomer of nucleic acids is a nucleotide. Nucleotides are made up of three molecules: pentose sugar, phosphate group and nitrogenous base. The question tells us that the monomer has a phosphate group and a guanine (nitrogenous base); however, we are not given any information regarding the pentose sugar. Recall that the two types of nucleic acids (DNA and RNA) are distinguished from each other by the pentose sugar. DNA has deoxyribose whereas RNA has ribose. Since we do not know the type of pentose sugar, we cannot determine the monomer’s identity.

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.