Prokaryotic Cell Structure: Ribosomes

II. THE PROKARYOTIC CELL: BACTERIA

B. PROKARYOTIC CELL STRUCTURE

3. Structures Located Within the Cytoplasm

d. Ribosomes

The overall purpose of this Learning Object is:
1) to learn the chemical makeup and the functions associated with bacterial ribosomes; and
2) introduce how some antibiotics function by interfering with the functions of bacterial ribosomes.

LEARNING OBJECTIVES FOR THIS SECTION

In this section on Prokaryotic Cell Structure we are looking at the various organelles or structures that make up a bacterium. As mentioned in the introduction to this section, a typical bacterium usually consists of:

a cytoplasmic membrane surrounded by a peptidoglycan cell wall and maybe an outer membrane;
a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and
often various external structures such as a glycocalyx, flagella, and pili.

Structures located within the cytoplasm of bacteria include the nucleoid, ribosomes, and in some bacteria, plasmids, endospores, inclusion bodies, and organelles used for photosynthesis. We will now look at bacterial ribosomes.

Ribosomes (def)

A. Structure and Composition

Ribosomes are composed of ribosomal RNA (rRNA) and protein. Prokaryotic cells have three types of rRNA: 16S rRNA, 23S rRNA, and 5S rRNA. Like transfer RNA (tRNA), rRNAs use intrastrand H-bonding between comlementary nucleotide bases to form complex folded structures.

Ribosomes are composed of two subunits with densities of 50S and 30S. ("S" refers to a unit of density called the Svedberg unit (def).) The 30S subunit contains 16S rRNA and 21 proteins; the 50S subunit contains 5S and 23S rRNA and 31 proteins.

- ribosomal subunit densities: 50S and 30S

The two subunits combine during protein synthesis to form a complete 70S ribosome about 25nm in diameter. A typical bacterium may have as many as 15,000 ribosomes.

B. Functions

Ribosomes function as a workbench for protein synthesis, that is, they receive and translate genetic instructions for the formation of specific proteins. During protein synthesis, mRNA attaches to the 30s subunit and amino acid-carrying transfer RNAs (tRNA) attach to the 50s subunit (see Fig. 1). Protein synthesis is discussed in detail in Unit 6.

To view electron micrograph showing the nucleoid and ribosomes in a Streptococcus, see the Rockefeller University home page.

Flash animation summarizing translation.

html5 version of animation for iPad summarizing translation.

For More Information: RNA from Unit 6

For More Information: Transcription from Unit 6

For More Information: Translation from Unit 6

YouTube movie illustrating DNA replication, transcription, and translation.

YouTube movie illustrating transcription and translation.

YouTube movie illustrating translation (1)

YouTube movie illustrating translation (2)

YouTube movie illustrating ribosomes in action.

3D animation illustrating transcription.

From Drew Berry, wehi.edu.au. This animation takes some time to load.

3D animation illustrating translation.

From Drew Berry, wehi.edu.au. This animation takes some time to load.

3D animation illustrating elongation during translation.

From A.H. Whiting, J. Frank, R. Agarwal , Howard Hughes Medical Institute. This animation takes some time to load.

TPS Question

C. Antimicrobial Agents that Alter Prokaryotic Ribosomal Subunits and Block Translation in Bacteria
Many antibiotics alter bacterial ribosomes (def), interfering with translation (def) and thereby causing faulty protein synthesis. The portion of the ribosome to which the antibiotic binds determines how translation is effected. For example:

The tetracyclines (tetracycline, doxycycline, demeclocycline, minocycline, etc.) bind reversibly to the 30S subunit, distorting it in such a way that the anticodons of charged tRNAs (def) cannot align properly with the codons of the mRNA (def).

Flash animation illustrating how tetracyclines bind to the 30S ribosomal subunit and block translation.

html5 version of animation for iPad illustrating how tetracyclines bind to the 30S ribosomal subunit and block translation.

The macrolides (erythromycin, azithromycin, clarithromycin, dirithromycin, troleandomycin, etc.) bind reversibly to the 50S subunit. They appear to inhibit elongation of the protein by preventing the enzyme peptidyltransferase from forming peptide bonds between the amino acids. They may also prevent the transfer of the peptidyl tRNA from the A-site to the P-site.

Flash animation illustrating how macrolides bind to the 50S ribosomal subunit and block translation by blocking peptidyltransferase.

Flash animation illustrating how macrolides bind to the 50S ribosomal subunit and block translation by preventing the transfer of the peptidyl tRNA from the A-site to the P-site.

html5 version of animation for iPad illustrating how macrolides bind to the 50S ribosomal subunit and block translation by blocking peptidyltransferase.

html5 version of animation for iPad illustrating how macrolides bind to the 50S ribosomal subunit and block translation by preventing the transfer of the peptidyl tRNA from the A-site to the P-site.

Antimicrobial chemotherapy will be discussed in greater detail later in Unit 2 under Control of Bacteria by Using Antibiotics and Disinfectants.

Preview of "Chemotherapeutic Control of Bacteria" from Unit 2.

Concept map for Ribosomes