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Ribosomes - Protein Construction Teams
Cells need to make proteins. Those proteins might be used as enzymes or as
support for other cell functions. When you need to make proteins, you look for
ribosomes. Ribosomes are the protein builders or the protein synthesizers of
the cell. They are like construction guys who connect one amino acid at a time
and build long chains.
Ribosomes are found in many places around the cell. You might find them
floating in the cytoplasm (cytosol). Those floating ribosomes make proteins that
will be used inside of the cell. Other ribosomes are found on the endoplasmic
reticulum. Endoplasmic reticulum with attached ribosomes is called rough. It
looks bumpy under a microscope. Those attached ribosomes make proteins that
will be used inside the cell and proteins made for export out of the cell.
Two Pieces Make the Whole
A ribosome is not just one piece. There are two pieces or subunits. Scientists
named them 60-S (large) and 40-S (small). When the cell needs to make protein,
mRNA is created in the nucleus. The mRNA is then sent into the cell and the
ribosomes. When it is time to make the protein, the two subunits come together
and combine with the mRNA. The subunits lock onto the mRNA and start the
protein synthesis.
The 60-S/ 40-S model works fine for eukaryotic cells. Prokaryotic cells have
ribosomes made of 50-S and 30-S subunits. It's a small difference, but one of
many you will find in the two different types of cells. Scientists have used this
difference in ribosome size to develop drugs that can kill prokaryotic
microorganisms that cause disease.
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Mixing and Matching Amino Acids
The process of making proteins is quite simple. We just explained that mRNA is
made in the nucleus and sent into the cell. The mRNA then combines with the
ribosome subunits. Another nucleic acid lives in the cell - tRNA, which stands for
transfer RNA. tRNA is bonded to the amino acids floating around the cell. With
the mRNA offering instructions, the ribosome connects to a tRNA and pulls off
one amino acid. Slowly the ribosome makes a long amino acid chain that will be
part of a larger protein.