Uracil is one of the nucleotide bases that composes RNA. It is replaced by thymine in DNA.

Uracil, thymine, and cytosine are pyrimidine residues, capable of bonding and pairing with the purines adenine and guanine via hydrogen bonding. During DNA replication, thymine matches with adenine. During transcription, uracil matches with adenine.

Nucleotides (DNA monomers) and ribonucleotides (RNA monomers) are formed from a pentose sugar, phosphate group, and nitrogenous base. Each nitrogenous base has a complement that allows it to form hydrogen bonds to the template strand. This allows for the proper sequence of genetic code in DNA replication and RNA transcription.

Purine residues will always pair with pyrimidine residues. The purines are adenine and guanine. The pyrimidines are cytosine and thymine in DNA, and cytosine and uracil in RNA. Adenine will match with thymine or uracil, forming two hydrogen bonds, while cytosine will match with guanine to form three hydrogen bonds.

Template strand cytosine and adenine are methylated in DNA replication, which allows DNA mismatch repair enzymes to distinguish between old and new DNA strands.

In contrast, histone acetylation relaxes DNA coiling and allows for the DNA to be transcribed.

You can remember that methylation makes DNA mute, and acetylation makes DNA active.

Purines are defined by their two-ring structure. A six-member ring with two amine groups and a five-member ring with two amino groups join to form each purine molecule. Addition substituents on the rings (often ketones or other amines) determine purine identity.

Glycine, aspartate, and glutamine are necessary for purine synthesis, along with phosphoribosyl pyrophosphate (PRPP). Glycine is incorporated into the final purine product structure, while glutamine is converted to glutamate and aspartate is converted to fumarate. The final purine product is used to make useful molecules, such as adenine and guanine for nucleotide synthesis.

Membrane fluidity can be buffered by cholesterol in both warm and cold environments. At high temperatures cholesterol raises the melting point, while at lower temperatures cholesterol prevents the formation of crystalline structures between phospholipids.

The correct answer is a glycerol head containing a phosphate group and two fatty acids. Phospho refers to the phosphate group and lipid refers to the glycerol backbone attached to two fatty acid chains.

Omega-3 fatty acids are named after the double bond that starts on the third-to-last carbon in the fatty acid. The carboxylic acid carbon is considered the first carbon, while the last methyl group carbon is considered the "omega" carbon. As a result, the double bond will be three carbons away from the end.

 bonds are on the same plane, while  bonds are not on the same plane; therefore, the  bonds are much more useful for making branch points off of existing glycogen chains.

linkages cannot be easily broken down by eukaryotes and animals. Glycogen must be easily accessible as an energy source, and does not contain any glycosidic linkages.