With several students missing class today because of testing and a field trip, we practiced converting amino acid sequences to DNA sequences and then reviewed the DNA mutations introduced yesterday. Students modeled frameshift mutations by inserting or deleting individual DNA bases into their initial DNA sequences and decoding the new amino acid sequences encoded for by the mutated DNA.
Several students observed the introduction of premature stop codons into their amino acid sequences. We then looked at the structure of chromosomes, zooming in on a segment of chromosome 22 and then focusing further on a single gene. Students sketched out the gene and then learned the following vocabulary terms:
- Regulatory DNA sequences: DNA bases that control gene expression. Not transcribed into mRNA.
- mRNA: introns and exons transcribed from a DNA segment (a gene)
- Introns: stretches of DNA transcribed into mRNA that are excised (removed) before the mRNA is translated to protein. Also called “junk DNA” – but shouldn’t be!
- Exons: segments of DNA transcribed into mRNA that are stitched together after intron excision for translation into protein
The white board pictures below show how precursor mRNA is processed to remove introns and stitch together exons to form the mature mRNA.
Previously, scientists believed one gene coded for one protein. As our understanding of introns and exons improved, and as scientists developed improved methods for sequencing mRNA, scientists realized that one gene coding for one precursor mRNA could actually make many different (yet related) proteins. For example, one mature mRNA might include all of the exons (after all introns are removed), while another mature mRNA might be missing one or more exons. Called alternative splicing, this process allows a gene to code for more than one protein and helps explain the vast diversity of proteins found in the different cells that make up the human body. Remember, all cells share the same DNA, but only some of that DNA is expressed by certain cells, and that expressed DNA (mRNA) can be spliced in various ways resulting in many different proteins.
As we transition toward our study of chromosomes, students will research disorders with a known genetic cause (start here). Students will also have access to a variety of posters showing the molecular basis of health conditions they may also wish to study. One they have identified a disorder, students will research genes known to associate with the condition. Finally, they identify the number of the chromosome on which their gene resides. The chromosome number they identify will be their assigned chromosome for the Chromosome Project. Students will have a substitute teacher tomorrow who will instruct them to use the Chromebooks to visit the class website for details about the Project and instructions for how to begin.