Transcription, Translation, and Mutations

We have been learning how the information in DNA is used to produce proteins. We spent lots of time learning about traits and phenotypes and all sorts of other fun stuff about DNA and genetics. Now, we are tying the ideas together.

Here is a big problem the cell faces. DNA is contained in the nucleus of the cell. Protein production occurs on the ribosome in the cytoplasm of the cell. Hmmm...

Enter RNA.

The first step is called TRANSCRIPTION. During transcription, DNA is copied into mRNA. This process is similar to DNA replication, when all the bases pair up, but RNA contains uracil, not thymine. So, if we had a strand of DNA that read:

• TAC GGG CAT AAA, the complimentary DNA strand would be:
• ATG CCC GTA TTT, BUT RNA does not contain T, it has been replaced by U, so the RNA strand would be:
• AUG CCC GUA UUU

The next step is called TRANSLATION. During translation, the coded message from the mRNA is translated into a chain of amino acids. The mRNA strand travels from the nucleus to the cytoplasm where it attaches to a ribosome. The mRNA strand is read 3 bases at a time, called a codon. Another type of RNA, tRNA carries amino acids to the ribosome and matches up with certain codons. The tRNA drops off the amino acid, which is added to the chain, and then leaves to go find more amino acids. This process continues until a complete protein is made. You can use a codon chart to figure out what amino acids certain codons code for. In the example mRNA strand above, the amino acid sequence would be:

• Met-Pro-Val-Phe

Sometimes mistakes occur and these mistakes can result in a change in the DNA or RNA sequence. These changes are called mutations. Mutations may or may not result in any change in the protein, it depends on the change. If a mutation does cause a change in a protein, that can lead to a different phenotype, which may be helpful or harmful to the organism. It often depends on the mutation and the environment.

There are 3 types of mutations that we discussed in class: substitution, deletion, and addition. The names pretty much explain what happens, but in a substitution, one base takes the place of another. In deletion, a base is removed and in addition an extra base is added to the chain.

These can result in a point mutation, in which only one amino acid is incorrect. For example (using the example above), if the first A were to be a U, then the amino acid chain would start with Leu, not Met, but that would be the only change. Changes can also result in a frameshift mutation, this is where the reading frame is shifted and generally leads to a nonsense amino acid chain. Imagine reading the sentence THE FAT CAT ATE THE PIE. If the F from FAT was removed, but the sentence was still read 3 letters at a time, it would read THE ATC ATA TET HEP IE. Nowhere near what we were going for. Framshift mutations can lead to a nonsense protein, protein production ending too soon, or many other problems. Again, it may or may not be harmful depending on several factors.