CRISPR-Cas9: Another Tool for Editing Genomes

Some bacteria have a system for recognizing an invading virus and destroying it. The system is called CRISPR (for clustered regularly interspaced short palindromic repeats) [Link to a discussion of palindromes.] It is described on a separate page — Link to it.

Like restriction enzymes (also discovered in bacteria), CRISPR provides a way of targeting a particular DNA sequence. The CRISPR

Chlorophylls and Carotenoids


Two types of chlorophyll are found in plants and the green algae.

  • chlorophyll a and
  • chlorophyll b

The difference in their structures is shown in the figure (red disks).

In the chloroplast, both types are associated with integral membrane proteins in the thylakoid membrane.

Note the system of alternating single and double bonds (white bars) that run around the porphyrin ring. Although I am forced to draw the single and double bonds in fixed


Structure of heme. It consists of a porphyrin ring surrounding an atom of iron (Fe).

Heme serves as the prosthetic group of the cytochrome enzymes, as well as of myoglobin and hemoglobin. Compare its structure with that of chlorophyll.