Disulfide bridges are formed between the sulfhydryl (-SH) groups of two cysteine residues. These bonds are covalent, and are important in stabilizing the tertiary structure of many proteins.

Proteins have different levels of structure. Primary structure is the sequence of amino acids, joined by peptide bonds. Secondary structure is determined by hydrogen bonding in the amino acid chain backbone. Tertiary structure is the entire protein's shape, determined by R-group interaction and hydrophobic forces. Quaternary structure is only found in certain proteins, and results from the joining of multiple polypeptide subunits into a functional protein.

Chaperones are proteins that are vital for the proper folding of some polypeptide chains. Without chaperones, proteins may be folded incorrectly and become nonfunctional. Chaperones are particularly essential to tertiary and quaternary structure.

The other answers describe functions of other types of proteins.

The generation of new protein begins in the nucleus with the transcription of a gene’s DNA sequence into mRNA. The mRNA is then translated into protein by ribosomes in proximity to the rough endoplasmic reticulum. The rough endoplasmic reticulum functions to accept the growing protein sequence and ensure that it is appropriately folded and modified (i.e. addition of carbohydrate) appropriately. The protein is then transported to the Golgi apparatus for additional maturation of the protein, such as carbohydrate modifications. Lastly, the protein is transported to the cell surface based on signals or motifs in protein sequence that determine where it is transported.

Note that only proteins that are transported in vesicles (i.e. membrane proteins and secreted proteins) require intervention by the rough endoplasmic reticulum and Golgi apparatus. Cytoplasmic proteins can be translated by free-floating ribosomes and folded by chaperone proteins.

When a point mutation on the DNA strand creates a premature stop codon the RNA template will not be completely translated, resulting in a protein with a lower molecular weight due to fewer amino acid residues. As a result, the protein will also likely be nonfunctional. This is an example of a nonsense point mutation.

A slightly altered protein with the same molecular weight would be an example of a missense point mutation, resulting in the substitution of one amino acid for another.

Denaturation of a protein means that the structure of the protein has changed, rendering it non-functional. The presence of an enzyme can alter the structure of a substrate protein, but is only likely to affect a small region of the protein structure. In contrast, most denaturing processes involve environmental changes that affect the protein as a whole.

A drop in pH can cause denaturation, as it can de-protonate or re-protonate the protein causing a conformational change. This results in changes in the polar structure of the amino acid and can lead to hydrophobic shifts in tertiary structure to decrease functionality. Change in basicity can cause denaturation for the same reason, since this is essentially an increase in pH. Temperature change can also cause denaturation by disrupting internal bonds of the protein used to create secondary and tertiary structure.

Note that primary structure is not affected by denaturation, which is why proteins can re-fold and regain function after denaturation.

There are four levels of protein structure. Primary structure involves the linear arrangement of amino acids. It is simply the linear sequence of amino acids created by the ribosome during translation. Secondary structure involves the hydrogen bonding of the backbone and can form alpha-helices and beta-pleated sheets. Tertiary structure involves the interaction between amino acid side chains, or R-groups. These interactions can be hydrogen bonds, hydrophobic interactions, or disulfide bridges. Quaternary structure involves the interaction between two or more folded subunits, and is not present in every protein structure.

The primary structure of a protein is a linear sequence of amino acids. Amino acids are joined by peptide bonds between the N terminus of one amino acid and the C terminus of another amino acid through a condensation reaction, which results in the release of a water molecule. 

The secondary structure of protein folding is two-dimensional and can take two forms: alpha helices and beta-pleated sheets. The secondary structure is characterized by hydrogen bonds between peptide groups.

The tertiary structure of protein folding has a polypeptide chain backbone and a number of protein secondary structures: alpha helices and beta-pleated sheets. The tertiary structure is three-dimensional. The protein folding that causes the formation of the tertiary structure is influenced by hydrophobic interactions, disulfide bridges, hydrogen bonds, and salt bridges that create hydrophobic and hydrophilic regions.