Uracil is a nucleic acid found in RNA but not DNA. Therefore, if a question has uracil as one of the nucleic acids in the sequence, you know that you are dealing with a piece of RNA. RNA is genetic material that has been transcribed from a piece of DNA.

For DNA base-pairings, A-T must be paired up (2 hydrogen bonds) and G-C must be paired up (3 hydrogen bonds). For RNA base-pairings, A-U pair up instead of A-T.

The 5' to 3' linkages of DNA residues are via phosphodiester bonds, which are a type of covalent bond.

The following are the major steps in gene product modification:

1. Transcription of DNA into mRNA (primary transcript)

2. Pre-mRNA processes (mature transcript)

3. Translation of mRNA by tRNA (proteins)

4. Post-translational processing (modified protein)

Austrian chemist Erwin Chargaff discovered that the amount of cytosine was equal to the amount of guanine in the cell, and the amount of adenine was equal to the amount of thymine in the cell. This discovery was important in determining that cytosine always pairs with guanine and that adenine always pairs with thymine. 

In RNA, uracil pairs with adenine and cytosine pairs with guanine, whereas in DNA, thymine pairs with adenine and cytosine pairs with guanine. Uracil is unique to RNA and thymine is unique to DNA. 

G-C is held together by three hydrogen bonds, while A-T is held together by two hydrogen bonds. Having more hydrogen bonds means that boiling points would be higher, so the set of base pairs with the most C-G bonds will have the highest boiling point.

Since adenine pairs with thymine in DNA, this would mean there are also 270 thymine bases. Similarly, cytosine and guanine pair up as well. This would mean there are also 430 cytosine bases. Adding all those together would give you 1400 bases total.

A nucleotide comprises of a purine/pyrimidine base, sugar, and a phosphate group. This is easy remember by focusing on the "t" in the terms "nucleotide" and "phosphate."

A nucleoside comprises only a purine/pyrimidine base and sugar. There is no "t" in "nucleoside" and hence, no phosphate.

All of these are actual nucleic acids, except xDNA, which does not exist. Therefore, we can eliminate this answer right away. mtDNA, or mitochondrial DNA, is a small portion of a person's DNA that is housed within the mitochondria. It is significant in that it is inherited solely from the mother and is thought to have evolved separately from "normal" DNA. However, it isn't involved with amino acids at all, so it too can be eliminated as a choice.

RNA (ribonucleic acid) is heavily involved in protein synthesis (the creation of new proteins, which are just long chains of amino acids). The first part of the process is transcription, in which the double-stranded DNA (deoxyribonucleic acid) is "unzipped" by the enzyme helicase. Another enzyme, RNA polymerase, "reads" this single strand and produces a chain of nucleotides exactly opposite to the DNA's nucleotides, and this chain, in its mature form, is called mRNA (messenger RNA). Living up to its name, mRNA travels outside the nucleus, where DNA is housed, and out into the rest of the cell, to the ribosome.

At this point, translation begins. Every trio of nucleotides on the mRNA, called a codon, represents a specific amino acid. Another type of RNA, tRNA (transfer RNA) can "translate" the codon into the appropriate amino acid, since it carries a three-nucleotide-long anticodon on one segment, which can form a bond with the corresponding codon on the mRNA, and an amino acid on the other segment. In this way tRNA brings amino acids to the growing protein chain. Another involved nucleic acid is rRNA (ribosomal RNA) which makes up a large part of the ribosome itself and is responsible for helping to properly attach each amino acid that the tRNA brings to the amino acid sequence being built.

Based on the question, which asks about the molecule that retrieves amino acids so that they can be added to the protein being built, the correct answer is tRNA.