9.3
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The subunits of eukaryotic ribosomes are protein-synthesizing structures formed inside the nucleolus.
The eukaryotic ribosome is called an 80S ribosome. It is made up of two subunits: a large 60S subunit and a small 40S subunit.
The 60S subunit consists of 28S, 5.8S, and 5S rRNAs and about 49 proteins.
The 40S subunit consists of an 18S rRNA and about 33 proteins.
Both subunits are exported from the nucleus through the nuclear pores. They join on an mRNA to make proteins—either free in the cytosol or attached to the outer nuclear envelope or the rough endoplasmic reticulum.
The mRNA binding site is located in the small subunit. The three tRNA binding sites, E, P, and A, are formed at the interface of the two subunits.
During translation, the initiator tRNA binds directly to the P, or peptidyl, site. During the elongation stage, new tRNAs enter the A or aminoacyl site, and empty tRNAs leave from the E or exit site.
The rRNA in the large subunit also catalyzes peptide bond formation between amino acids, allowing the growing polypeptide chain to form.
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