Protein synthesis

Transfer RNAs (tRNAS) have two jobs:  1. They carry a protein building block (an amino acid) to a cell's protein assembly machine, called a ribosome. 2. They translate the genetic code of messenger RNA (mRNA) into the amino acid sequence of proteins.

tRNAs fold up into an upside-down L shape. The top end of the L has an amino acid bound to it, and the bottom end has three bases (nucleotides) that bind to mRNA when a protein is made.  mRNA carries the genetic code for a protein in a sequence of nucleotides. Every three nucleotides of mRNA are recognized by a particular tRNA that carries a particular amino acid.

When making a protein, the ribosome holds mRNA in place. When a tRNA enters the ribosome, it binds to a complementary section of mRNA. At this moment, the tRNA releases its amino acid to be incorporated into a growing chain of amino acids that will become a protein. The empty tRNA leaves the ribosome to make space for another amino acid-carrying tRNA.

RNA cutters, or ribonucleases (RNases), are enzymes made of RNA that break down other RNA molecules by chopping them up into smaller pieces.

RNases perform many roles, including: getting rid of unwanted RNA, refining RNA molecules by chopping off unwanted sections of RNA, and serving as a first defense against infection from RNA viruses by breaking down viral RNA.

RNases work like recycling plants. They stop cells from accumulating unwanted RNAs by breaking them down into pieces that can be recycled into new RNA molecules.

There are two types of RNase: the first bites off one nucleotide at a time from the ends of an RNA molecule; the other works in the middle of an RNA molecule, splitting it apart or chopping out chunks of RNA.

RNases break down all the unprotected RNAs that they encounter in a cell. An RNA can protect itself from RNase attack by forming complexes with proteins or by adding particular chemical groups to either of its ends. And an RNase can be stopped in its tracks when an RNase inhibitor protein binds to it.

All living things that have been studied contain RNases. So, RNases must have evolved in early life-forms and been conserved through evolution. This tells us that breaking down RNA is a very important process.

Ribosomes are a cell’s protein-assembly machines. Their job is to link protein building blocks (amino acids) together to make proteins in an order spelled out in messenger RNA (mRNA).

Ribosomes are made of two parts: a small subunit and a large subunit. These subunits are made of ribosomal RNA (rRNA) and proteins.

When making a protein, the small ribosome subunit holds mRNA in place. Transfer RNAs (tRNA) enter the ribosome carrying individual amino acids. The large ribosome subunit bonds these amino acids together to make a protein. When a tRNA releases its amino acid, it exits the ribosome and another tRNA takes its place to add its amino acid to the growing protein. When a ribosome finishes translating an mRNA, the small and large ribosome subunits split apart.

It’s the rRNA in the ribosome that links amino acids together when making a protein, which means that rRNA is an RNA enzyme. The role of the ribosome proteins seems to be to stabilize the ribosome structure.

All living things that have been studied contain rRNA. So rRNA must have evolved in early life-forms and been conserved through evolution. Evolutionarily speaking, rRNAs are far older than the proteins in a ribosome. This suggests that in very early life-forms, ribosomes operated without proteins to stabilize them.

This is good evidence for the RNA World hypothesis, which suggests that RNA allowed life to form on Earth, and that it came before DNA and proteins. Evolution of rRNA as a cell’s protein-assembly machine would have made the existence of proteins possible.

mRNA-splicing machines chop out unwanted sections of newly made messenger RNA (mRNA) to create mature mRNA that can be translated to make a protein.

When a strand of mRNA is first copied from DNA, the genetic code it carries isn’t ready to be translated to make a protein. First, it needs to be edited by a machine made of RNAs and proteins, called a spliceosome. It’s like an editor that proofreads the first draft of an mRNA. Like scissors and glue, it chops out unwanted sections of mRNA, called introns, and sticks the remaining pieces back together again. This is called splicing.

A single mRNA copied from one gene can code for lots of different proteins if it’s spliced in different ways. This explains how humans are able to produce about 100,000 proteins from only about 20,000 genes.