Ribosomes

Eukaryotic ribosomes are protein-synthesizing structures formed inside the nucleolus. The 80S ribosome is divided into large and small subunits.

The large subunit, called the 60S ribosome, consists of 28S, 5.8S, and 5S rRNAs and 49 proteins. The smaller subunit, the 40S ribosome, consists of an 18S rRNA and 33 proteins.

Both subunits are exported out of the nucleus through the nuclear pores. They join for protein synthesis either while floating in the cytoplasm or when attached to the outer nuclear envelope or the rough endoplasmic reticulum.

The mRNA binding site lies within the small ribosomal subunit. The three tRNA binding sites, E, P, and A, are formed at the interface of both ribosomal subunits.

Here, an initiator tRNA is placed at the P or peptidyl site, the new tRNAs are received at the A or aminoacyl site, and the empty tRNAs are released from the E or exit site.

rRNA in the large subunit also catalyzes the peptide bond formation between amino acids.