An antibiotic can wipe out bacteria and leave your own cells alone, even though both are busy making protein. How is that possible? The answer sits in a tiny granule called the ribosome.
1. What a ribosome is, and where it sits
The tiny, nearly round granules that lie free in the cytoplasm or stick to the ER are ribosomes. Protein is made on them. On rough ER they sit in rows on both faces.
Free ribosomes are a notable feature of prokaryotic cells, and a free ribosome has no covering membrane. When several ribosomes lie on one mRNA like a string of pearls, the whole group is a polyribosome, or polysome.
2. How it was found
In 1954 Albert Claude spun the cytoplasm of liver cells in a centrifuge, pulled out small RNA-rich particles and named them microsomes. A year later, in 1955, George Palade discovered the ribosome. The name itself came in 1958, from Richard B. Roberts.
Small ribosome-like particles have also been found in chloroplasts, mitochondria and nucleoplasm. They are called ribonucleoprotein particles, RNP for short.
3. Two kinds, and their subunits
Ribosomes are of two kinds. Prokaryotes carry 70S, with a molecular weight of 2.7 × 10⁶ daltons. Eukaryotes carry 80S, at 4.0 × 10⁶ daltons. The 70S splits into 50S and 30S subunits, and the 80S into 60S and 40S. Normally the two subunits stay apart and come together only while protein is being made.
What does the S stand for? The rate at which particles of different mass settle in a centrifuge is the Svedberg unit. Its symbol S honours the Swedish biochemist Theodor Svedberg.
4. Size, parts and sites
A ribosome is 22 nm wide and 20 nm high. Small as it is, about 22% of the dry weight of an E. coli cell is ribosome.
Ribosomes are made mostly of rRNA and protein, in a ratio of about 1 : 1. The 70S has three rRNAs (23S, 16S, 5S) and 52 kinds of protein. The 80S has four rRNAs (28S, 18S, 5.8S, 5S) and 80 kinds of protein. These numbers are where the exam looks. Drop the ten chips into the right bins.
sort themtap a chip, then pick a bin
Whose ribosome?
Pick up a chip, then drop it in a bin.
Right. Next one.
Not yet. Remember the pairs: 70S goes with 50S and 30S, and 80S with 60S and 40S.
All in place. Prokaryotes have 70S, with 50S and 30S, three rRNAs and 52 proteins. Eukaryotes have 80S, with 60S and 40S, four rRNAs and 80 proteins.
During protein synthesis a ribosome shows several sites: the aminoacyl (A) site, the peptidyl (P) site, the exit (E) site and an mRNA binding site. Figure 1.7 shows the two subunits of an 80S ribosome with the mRNA, a tRNA and the new protein. Tap each point.
Figure 1.7tap a point
Build of a ribosome
The 40S subunit. The small subunit of the 80S ribosome.
The 60S subunit. The large subunit of the 80S ribosome.
mRNA. The template for protein synthesis.
tRNA. Brings the amino acid.
The new protein. The polypeptide chain.
All five seen. The mRNA binds the small subunit first, and the large one joins afterwards.
Drawn by hand after the book's figure, not to scale.
5. Where it is made, and what it does
In prokaryotes ribosomes arise from the DNA, the primitive chromosome. In eukaryotes the two subunits are built separately inside the nucleus and then move out to the cytoplasm.
The two subunits of a ribosome are not stuck together all the time. Run it and see who comes first and who joins later.
processpress run
The subunits join
The chief job of the ribosome is protein synthesis, which is why it is called the cell's protein factory. At the start the mRNA binds the small subunit first: the 30S in prokaryotes, the 40S in eukaryotes. Then the large subunit joins and the 70S or 80S unit forms. Put the steps in order.
in orderput the steps in order
The ribosome joins up
Before work begins, what state is the ribosome in?
Yes. Then?
Not yet. The large subunit comes only after the mRNA has bound the small one.
Right order. The subunits stay apart and join only while protein is being made.
Now back to the opening question. Prokaryotic ribosomes are chemically different. So antibiotics such as streptomycin stop protein synthesis in bacteria and kill them, but do not disturb protein synthesis in the human body.
The book also states that ribosomes make cytochromes, which carry electrons in cellular respiration, and that glucose phosphorylation occurs in ribosomes.
6. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
A granular particle lying free in the cytoplasm or on the endoplasmic reticulum, where protein synthesis takes place. The main chemical components of the particle are:
i. DNA ii. RNA iii. Protein
i and ii: i is wrong, since the ribosome has no DNA.
i and iii: i is wrong.
Right. The main components of the ribosome are RNA and protein, in about a 1 : 1 ratio.
Not all three, because i is wrong.
Think a little on the second.
check yourselfpick one answer
Question 2
Who discovered the ribosome?
Right. George Palade discovered ribosomes in 1955.
Garnier did not discover the ribosome.
Garth Nicolson is linked with membrane models, not the ribosome.
Thomas Thomson did not discover the ribosome.
In the last one, keep an eye on the numbers.
check yourselfpick one answer
Question 3
Which are the subunits of a prokaryotic ribosome?
These are the subunits of the eukaryotic (80S) ribosome.
No such pair exists; 40S belongs to eukaryotes and 50S to prokaryotes.
70S is the total size of the prokaryotic ribosome, not its subunits.
Right. The prokaryotic 70S ribosome consists of 50S and 30S subunits.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhat is a ribosome and where does it lie?
AnswerGranules free in the cytoplasm or attached to the ER where protein is synthesised; they sit in rows on both faces of rough ER.
QuestionHow many kinds of ribosome are there and where does each occur?
AnswerTwo: 70S in prokaryotes and 80S in eukaryotes.
QuestionInto which subunits do 70S and 80S ribosomes divide?
Answer70S = 50S + 30S; 80S = 60S + 40S.
QuestionHow many rRNAs, and of what sizes, does each ribosome have?
Answer70S: three (23S, 16S, 5S) + 52 proteins. 80S: four (28S, 18S, 5.8S, 5S) + 80 proteins.
QuestionWhere are ribosomes made in prokaryotic and eukaryotic cells?
AnswerProkaryotes: from the DNA (primitive chromosome). Eukaryotes: the two subunits are made separately inside the nucleus.
QuestionWhy are ribosomes called the "protein factory" of the cell?
AnswerBecause their chief job is protein synthesis.
QuestionWhy does streptomycin kill bacteria but not disturb human protein synthesis?
AnswerProkaryotic ribosomes are chemically different, so it blocks bacterial protein synthesis but not that of human cells.
Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.3.