BIOL 1500 Study Guide - Midterm Guide: Endoplasmic Reticulum, Nuclear Membrane, Nuclear Pore
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Structure of Ribosome
The nucleolus is the site in which new ribosomes are assembled.
How do you make a ribosome? Some chromosomes have sections of DNA that encode
ribosomal RNA, a type of structural RNA that combines with proteins to make the
ribosome. In the nucleolus, new ribosomal RNA combines with proteins to form the
subunits of the ribosome. The newly made subunits are transported out through the
nuclear pores to the cytoplasm, where they can do their job.
Some cell types have more than one nucleolus inside the nucleus.
Prokaryotes, which do not have a nucleus, don't have nucleoli and build their ribosomes
in the cytosol.
Ribosomes are the molecular machines responsible for protein synthesis. A ribosome is
made out of RNA and proteins, and each ribosome consists of two separate RNA-protein
complexes, known as the small and large subunits. The large subunit sits on top of the
small subunit, with an RNA template sandwiched between the two. (A ribosome looks a
little like a hamburger with a puffy bun on top, an RNA “patty” threading through it.)
In eukaryotes, ribosomes get their orders for protein synthesis from the nucleus, where
portions of DNA (genes) are transcribed to make messenger RNAs (mRNAs). An mRNA
travels to the ribosome, which uses the information it contains to build a protein with a
specific amino acid sequence. This process is called translation. Prokaryotes lack a
nucleus, so their mRNAs are transcribed in the cytoplasm and can be translated by
Eukaryotic ribosomes may be either free, meaning that they are floating around in the
cytoplasm, or bound, meaning that they are attached to the endoplasmic reticulum or
the outside of the nuclear envelope. Because protein synthesis is an essential function
of all cells, ribosomes are found in practically every cell type of multicellular organisms,
as well as in prokaryotes such as bacteria. However, eukaryotic cells that specialize in
producing proteins have particularly large numbers of ribosomes. For example, the
pancreas is responsible for producing and secreting large amounts of digestive enzymes,
so the pancreatic cells that make these enzymes have an unusually high number of
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