Out of the Cell: toward the ribosome

Question

Which is the cellular organelle that has two parts: Big and Small?

Answer 1

I'm not clear on exactly what your question is, but I can tell you that the ribosome has two subunits, one large and one small. I'll explain a little about that below, and if you have any follow-up questions (or if I totally misunderstood what you're asking) please follow-up!

Ribosomes are comprised of two parts - a large subunit, and a small subunit. In eukaryotic cells, like human cells, the ribosome can be described as 80S, which describes the sedimentation of the ribosome in a unit called Svedberg units, and can be thought of as something approximating size - the larger the molecule, the faster it will sediment, and so the larger the sedimentation coefficient. Prokaryotic ribosomes, like bacteria, are 70S, which means that they take larger to sediment and are smaller (with a molecular mass of about 2 x 10⁶ Da, compared to eukaryotic ribosomes which have a molecular mass of 3.2 x 10⁶ Da).

The 80S eukaryotic ribosome has a large 60S subunit and a smaller 40S subunit, while the 70S prokaryotic ribosome has a large 50S subunit and a smaller 30S subunit. You'll notice that the sedimentation coefficients of the subunits don't add up to the sedimentation coefficient of the entire ribosome (i.e. 60S + 40S versus 80S), and this is because, as discussed above, the Svedberg unit approximates mass but is actually a measure of the sedimentation rate.

One way these subunits are relevant is that the small subunit is the part of the ribosome that initiates translation of RNA.