Scientists have indeed counted about 20,000 proteins coded for by the genome. Coding sequences are only about 2% or less of the genome. The definition of paralogs is genes related by duplication within a genome. Within the genome, not about 5%, but rather about 50%, of DNA sequences are repeated.

The pentose phosphate pathway (also known as the hexose monophosphate shunt or HMS), which mainly serves to produce  for anabolic reduction reactions and ribose-5-phosphate for nucleic acid production, takes place in the cytosol of hepatic cells.

From the carbon skeleton of oxaloacetate, methionine can be created.  However, glutamine comes from alpha ketoglutarate, valine and leucine come from pyruvate, and histidine comes from ribose-5-phosphate.

The reaction for the conversion of glutamine into glutamate is:

As seen in the reaction above, carbon dioxide is uninvolved.

When a change results in an early stop codon, nonsense mutation occurs and the protein is done being read early, often resulting in a nonfunctional protein. When a base change results into a different amino acid, this is a missense mutation. When a base change occurs but results in the same amino acid being read, this is considered a silent mutation. 

To answer this question, it's essential to have an understanding of what a signal sequence is.

A signal sequence (also sometimes called a signal peptide) is a specific sequence of amino acids on a polypeptide that appears near the beginning of translation. When this signal sequence is present, it causes a temporary halt in the translation process. Meanwhile, another protein called a signal recognition particle (SRP) comes along and binds to the ribosome that is translating the polypeptide. Together, this polypeptide-ribosome-SRP complex is transferred from the cytosol to the surface of the endoplasmic reticulum (ER). Once there, the complex allows the polypeptide to resume synthesis, but in doing so, causes it to be synthesized into the inner lumen of the endoplasmic reticulum. Consequently, this polypeptide will go on to be modified within the ER and also the Golgi apparatus. Afterwards, it will be sent off within a vesicle, where is will either be A) secreted outside of the cell or B) incorporated into the endomembrane system of the cell (in other words, the peptide will be inserted into a membrane such as the plasma membrane, ER membrane, Golgi membrane, etc.). Lastly, it is the nuclear localization sequence (NLS) that, when added to a protein, allows it to enter the nucleus through the nuclear membrane.

Just as there is an initiation codon regulating translation, there are termination codons that code for the end of translation. The three termination codons are UAA, UAG, and UGA. 

A helpful mnemonic for these are the phrases:

You are annoying (UAA)

You are gross (UAG)

You go away (UGA)

Procollagen has amino and carboxy procollagen extension propeptides that make it soluble. The preprocollagen undergoes both hydroxylation and glycosylation at specific aminoacid residues to form procollagen. Once secreted extracellularly, proteinases remove the extension peptides from procollagen to form the final collagen molecule.

Biotin moves carboxyl groups in the enzyme acetyl-CoA carboxylase. Tetrahydrofolate and S-adenylosyl methionine move methyl groups in amino acid synthesis and post-translational modifications such as DNA methylation. B12 cobalamin is a cofactor in the reactions producing succinyl-CoA and methionine, where it transfers methyl groups to complete the products.