Polypeptides, the precursors of proteins, are chains of amino acids. The two ends of a polypeptide are called the N-terminus, or amino end, and the C-terminus, or carboxyl end. For many (but not all) chloroplast proteins encoded by nuclear genes, cleavable transit peptides are added to the N-termini of the polypeptides, which are used to help direct the polypeptide to the chloroplast for import (N-terminal transit peptides are also used to direct polypeptides to plant mitochondria).
N-terminal transit sequences are also called presequences because they are located at the "front" end of a polypeptide—ribosomes synthesize polypeptides from the N-terminus to the C-terminus.
Chloroplast transit peptides exhibit huge variation in length and amino acid sequence. They can be from 20 to 150 amino acids long—an unusually long length, suggesting that transit peptides are actually collections of domains with different functions. Transit peptides tend to be positively charged, rich in hydroxylated amino acids such as serine, threonine, and proline, and poor in acidic amino acids like aspartic acid and glutamic acid. In an aqueous solution, the transit sequence forms a random coil.
Not all chloroplast proteins include a N-terminal cleavable transit peptide though. Some include the transit sequence within the functional part of the protein itself. A few have their transit sequence appended to their C-terminus instead. Most of the polypeptides that lack N-terminal targeting sequences are the ones that are sent to the outer chloroplast membrane, plus at least one sent to the inner chloroplast membrane.