2.3. Bridging the gap: Molecular devices[[Student version, December 8, 2002]] 57
Figure 2.30: (Schematic.) (a),(b)Passive ion channels, leading to the Ohmic part of membrane conductances
(see Chapter 11). When open, the individual ion channels for sodium and potassium have similar conductances,
but the latter are about 25 times more plentiful in living cells. (c)Schematic of the sodium–potassium pump, also
discussed in Chapter 11. The sketch has been simplified; actually the pump is believed to bind three Na+ions and
aphosphate before its main conformational change, which expels the Na+’s. Then it binds two K+ions, releases
ADP and phosphate, pulls the K+’s in and releases them. At this point the pump is ready to begin its cycle anew.
[Copyrighted figure; permission pending.]
of various needed proteins. A higher organism may have tens of thousands of distinct genes,
whileE. colihas fewer than 5000. (The simplest known organism,Mycoplasma genitalium,
has fewer than 500!) In addition to the genes, the DNA contains a rich array of regulatory
sequences for the binding of regulatory proteins, along with immense stretches with no known
function.
2.Another molecular machine calledRNA polymerasereads the master copy in a process called
transcription(Figure 2.32). RNA polymerase is a combination of walking motor and enzyme;
it attaches to the DNA near the start of a gene, then pulls the polymer chain through a
slot, simultaneously adding successive monomers to a growing “transcript” made of RNA
(Section 2.2.3). The transcript is also calledmessenger RNAor “mRNA.” In eukaryotic cells
it leaves the nucleus through pores in its bounding membrane (see Figure 2.6), thus entering
the cytosol. The energy needed to drive RNA polymerase comes from the added nucleotides
themselves, which arrive in the high-energy NTP form (Section 2.2.1); the polymerase clips off
twoofthe three phosphate groups from each nucleotide as it incorporates it into the growing
transcript (Figure 2.14).
3.In the cytosol, a complex of devices collectively called theribosomebinds the transcript and
again walks along it, successively building up a polypeptide, based on instructions encoded
in the transcript. The ribosome accomplishes thistranslationbyorchestrating the sequential
attachment oftransfer RNA(or “tRNA”) molecules, each binding to a particular triplet
of monomers in the transcript and each carrying the corresponding amino acid monomer
(residue) to be added to the growing polypeptide chain (Figure 2.33).
4.The polypeptide may spontaneously fold into a functioning protein, or may so fold with the
help of other auxiliary devices picturesquely calledchaperones.Additional chemical bonds
(“disulfide bonds” between residues containing sulfur atoms) can form to cross-link monomers