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.

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

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, translationbegins. 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.

Pyrimidine bases include cytosine, thymine, and uracil (RNA), and are monocyclic. Purine bases include adenine and guanine, and are bicylic with two rings in the molecular structure.

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.

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 correct answer here is uracil. Remember that in DNA replication, the nucleic acids present are TCGA. When we switch over to RNA production, the thyamine is replaced by uracil to form UCGA. If you chose any of the other options, remember that each one has a pair (AT and CG) and in RNA it switches over to UA and CG. If you remember that, you will always recall that uracil replaces thyamine in RNA sequencing

Other than the sites of hydrogen bonding, the nitrogenous rings are hydrophobic in nature. This property leads them to stick toward the center of the helix and away from the polar solvents around them. Once they are correctly base paired, their hydrophilic sites are mitigated by the hydrogen bonding leading to increased stability.

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.

The 5' end of a DNA strand is defined by the presence of the 5' carbon on the deoxyribose sugar towards that end. At the 5' carbon, there is a phosphate group which allows it to be added to the growing nucleotide chain.

A deoxyribose sugar is a major component of DNA, but it is not the functional group located at the 5' end of the DNA strand. The nitrogenous bases are attached to the 1' carbon. The poly A tail is a post transcriptional modification of mRNA.