Transfer RNA (tRNA) is a molecule synthesized from the process of transcribing DNA into segments located in specific regions of chromosomes. It is involved with protein synthesis in the mechanism of gene translation. This molecule is responsible for driving amino acids to form the polypeptide chains that make up proteins. While it is true that a cell has extensive regulatory systems to prevent the attachment of an incorrect amino acid to a tRNA, it is not true that the attachment of an incorrect amino acid is impossible, although rare, can happen.
The aminoacyl-tRNA synthetase is responsible for attaching the correct amino acid to the tRNA molecule. It does this with high fidelity, however once in 10^4 to 10^5 it can attach an incorrect amino acid.
Threonyl-tRNA synthetase has to be able to discriminate between threonine, it's correct amino acid substrate from similar amino acids such as serine (has a hydroxyl group like threonine) and valine (has same shape as threonine). The active site of threonyl-tRNA excludes the valine because it's methyl group side chain cannot interact with the aspartate residue that hydrogen bonds with the hydroxyl group side chain of threonine.
However, the serine residue cannot be discriminated in this way because of the similar hydroxyl group and can be incorrectly incorporated at a rate of 10^-2 to 10^-3 to that of threonine. The threonyl-tRNA synthetase has an editing site which is able to hydrolyze incorrectly incorporating serine residue thus increasing the fidelity to one out 10^4 to 10^5 mistakes.
Without crossing over, each chromosome would be either maternal or paternal, greatly reducing the number of possible genetic combinations, which would greatly reduce the amount of genetic variation between related individuals and within a species
