1. Proteins are polymers -- loong and chain-like moleculars.
2. mRNA encodes a polypeptide. (mRNA codes fro a polypeptide)
3. A linear chain of amino acid residues is called a polypeptide.
4. Short polypeptides, containing less than 20–30 residues, are rarely considered to be proteins and are commonly called peptides.
5. The amino acids in a polypeptide chain are linked by peptide bonds between amino (-NH2) and carboxyl group (-COOH).
6. An individual amino acid in a chain is called a residue.
7. A polypeptide chain ends with a free amino group, known as the N-terminus or amino terminus (positively charged), and a free carboxyl group, known as the C-terminus or carboxy terminus(negatively charged). polarity
8. All proteins are made of 21 amino acids.

These amino acids can be divided into three groups based on their properties determnined by their side chains:

• Hydrophobic: carbon-rich side chains which don't interact well with water.
• Hydrophilic/Polar: iteract well with water.
• Charged: iteract with oppositely charded amino acids or other molecules.

11. A protein can be composed of one or more polypeptides.
12. beta-strand vs beta-sheet
13. Primary structure

Linear sequence of amino acids encoded by DNA. Amino acids are joined by peptide bonds, happending between -COOH and -NH2. A water H2O molecule is released each time a bond is formed. The linked series of carbon, nitrogen and oxygen atoms make up the protein backbone

14. Secondary structure

Alpha helix and Beta sheet

15. Tertiary structure

3D shape of the protein

16. Quaternary structure

Two or more polypeptide chains come together to form one fucntional molecule wih several subunits.

reference

• Molecular biology (Molecular Biology 5th (Fifth) Edition, Robert Weavers, amazon)
• https://en.wikipedia.org/wiki/Protein
• https://pdb101.rcsb.org/learn/videos