After centrifugation, the contents of the tube are collected as fractions from the top (smaller, slower traveling) to bottom (bigger, faster traveling) and the optical density of the fractions is determined. The first fractions removed have a large amount of relatively small molecules, such as tRNAs, individual proteins, etc. It is possible to use this technique to study the overall degree of translation in cells (for examples), but it can be used much more specifically to study individual proteins and their mRNAs. As an example shown in the lower portion of the figure, a protein that composes part of the small subunit can first be detected in the 40S fraction, then nearly disappears from the 60S fraction (the separations on these gradients are not absolute), then reappears in the 80S and polysome fractions. This indicates that there is at most very little of the protein found in the cell that is not part of the small subunit. In contrast, in the upper row of the immunoblot figure, a soluble protein appears in the soluble fractions and associated with ribosomes and polysomes. The particular protein is a chaperone protein, which (in brief) helps to fold the nascent peptide as it is being extruded from the ribosome. As other work in the paper showed, there is a direct association of the chaperone with the ribosome. The technique can also be used to study the degree of translation of a particular mRNA In these experiments, 5' and 3' sequences of an mRNA were investigated for their effects on amount of mRNA produced and how well the mRNAs were translated. As shown, not all mRNA isoforms are translated with the same efficiency "even though" their coding sequences are the same.