Adenine and guanine are purines, while cytosine and thymine are pyrimidines.  A helpful mnemonic for this is Pure (purine) As (adenine) Gold (guanine).  Note that uracil is a pyrimidine, but is only seen in RNA.

The 80S eukaryotic ribosome is made up of 40S and 60S subunits.

The 70S prokaryotic ribosome is made up of 30S and 50S subunits.

IF-3 binds to the 30S subunit first. Then IF-2 forms the ternary complex, and it binds with IF-1 to the 30S subunit. Upon release of IF-1 and IF-3, the 50S subunit will bind to prepare for translation.

The nitrogenous bases that are considered to be pyrimidine derivatives are thymine, cytosine, and uracil. Thus, the purines are adenine and guanine. The mnemonic Pure As Gold (Purines = Adenine and Guanine) can be used to remember which bases are purines.

Nucleotides are monomers of nucleic acids and are made up of three molecules: nitrogenous base (uracil, thymine, cytosine, adenine or guanine), pentose sugar (deoxyribose for DNA and ribose for RNA) and phosphate group. Nucleosides, on the other hand, are molecules that only contain nitrogenous base and pentose sugar; they do not contain phosphate groups.

The question states that molecule A is heavier than molecule B. This could result from several reasons. First, molecule A could be a double stranded DNA and molecule B could be a single stranded RNA. This will allow the DNA molecule to be a lot heavier than its RNA counterpart. Second, molecule A could contain more purines (adenine and guanine) than molecule B). Recall that purines are two membered rings whereas pyrimidines (uracil, cytosine and thymine) are one membered rings. This means purines have more atoms and, therefore, have a higher molecular weight.

We are not given enough information to determine the identity or the distribution of nitrogenous bases in molecule A and B; therefore, we cannot conclude any of the given statements.

DNA and RNA molecules contain four types of nitrogenous bases. Guanine, cytosine and adenine are found in both whereas thymine is only found in DNA and uracil is only found in RNA. This means that DNA molecules will have more Thymine bases than RNA molecules. Another difference between DNA and RNA molecule is the type of pentose sugar. RNA contains ribose sugar whereas DNA contains deoxyribose. Deoxyribose contains one less hydroxyl group than ribose; therefore, DNA molecules have more deoxygenated sugars than RNA.

Think of it as this. CUT the PY (pie) PYrimidines. PURe As Gold. PURines. A will pair with T (and U in RNA). G will pair with C. The deamination of cytosine makes uracil, which is in RNA. 

In a biochemistry course, we often rely on important information that was learned during general and organic chemistry. One important concept to remember is chirality. A molecule is chiral if it is not superimposable on its mirror image. These molecules lack a plane of symmetry.  

It is a common mistake for students to think that all amino acids are chiral. While most amino acids are chiral, glycine is in fact achiral. Remember, the side chain on glycine is simply a hydrogen. The central carbon therefore is connected to two hydrogens, and is not a stereocenter.