IB Biology Topic D1.2.6: Roles of Mrna, Ribosomes and Trna in Translation Notes

Translation depends on three cooperating molecules: mRNA carries the codons, ribosomes read them, and tRNA delivers the matching amino acids

1. During translation the cell builds a polypeptide by following the sequence of codons carried on mRNA.
2. Three molecules do the work: mRNA holds the instructions, ribosomes are the site of assembly, and tRNA brings the amino acids.

mRNA carries the coded message from DNA to the ribosome

1. Messenger RNA (mRNA) is a single-stranded copy of a gene that carries genetic instructions from the DNA to the ribosome.
2. Its sequence is made of nucleotides carrying one of four bases: adenine (A), uracil (U), cytosine (C) and guanine (G).
3. Codons are groups of three bases, and each codon specifies one amino acid or a start or stop signal.
   1. The start codon AUG marks where translation begins.
   2. The stop codons UAA, UAG and UGA mark where translation ends.
4. mRNA also carries a ribosome binding site where the small subunit attaches to begin translation.

The ribosome has two subunits and three tRNA binding sites where the polypeptide is assembled

1. Ribosomes are the molecular machines that read the mRNA and join amino acids into a polypeptide.
2. Every ribosome is built from a small subunit, which binds and positions the mRNA, and a large subunit, which joins the amino acids together.

1. Between the two subunits the ribosome has three tRNA binding sites (A, P and E).
   1. A site (aminoacyl): holds the incoming tRNA carrying the next amino acid.
   2. P site (peptidyl): holds the tRNA attached to the growing polypeptide chain.
   3. E site (exit): releases the empty tRNA once its amino acid has been added.
2. Two tRNAs occupy neighbouring sites at once, one in the P site and one in the A site, so a peptide bond can form between their amino acids.

tRNA is the adapter that matches each codon to its correct amino acid

1. Transfer RNA (tRNA) is the adapter molecule that links each mRNA codon to the amino acid it specifies.
2. Each tRNA carries an anticodon, a set of three bases complementary to one specific mRNA codon.
3. Each tRNA also has an amino acid attachment site at the end where its matching amino acid is held.
4. The enzyme aminoacyl-tRNA synthetase charges each tRNA by attaching its correct amino acid.
5. At the ribosome the anticodon pairs with the codon by complementary base pairing, so the amino acid added always matches the coded instruction.