A fundamental principle called the Central Dogma describes how genetic information flows from the DNA in our genetic material via RNA to proteins which in turn carryout all of the cellular functions necessary for life. Initially scientists believed that nucleic acids DNA and RNA served solely as carriers of the genetic information, whereas proteins in the form of enzymes catalyze the chemical processes of life. However, in 1960, Francis Crick and Sydney Brenner of England and François Jacob and Jaques Monod of France simultaneously proposed the role RNA must play in order for cells to make proteins using information from DNA. They were awarded the Nobel Prize in Chemistry for their groundbreaking work in 1965. The chemist's perspective on the properties of genetic information, and thus the basis of human inheritance, was further influenced by the discoveries of Nobel Laureates Sidney Altman and Thomas Cech which revealed that ribonucleic acid or RNA can also function as an enzyme.
Sidney Altman and Thomas Cech independently studied how the genetic code was transferred from DNA to RNA. They knew, however, that part of the genetic information is not required and must be removed from RNA before the cell can utilize the RNA molecule. While searching for the catalysts of RNA maturation, Altman and Cech discovered that these enzymes were composed of catalytic RNA, and not of protein as would have been expected at the time. The discovery of catalytic RNA enabled this pair of Nobel laureates to provide evidence that RNA molecules can possess all the properties required of the original biomolecule since RNA has both the ability to simultaneously function as an enzyme and genetic material. The basis for the Central Dogma of the Chemistry of Life was born with this discovery and in 1989 Thomas Cech and Sidney Altman won the Nobel Prize in Chemistry for their influential work.
Additional information about the discoveries upon which the Central Dogma is based can be found in a great read for both instructor and student: Scientific American (1986) Vol 255, 76-84. To access a great visual representation of this process visit the Learn.Genetics site and study the Central Dogma interactive graphic.
After this lesson students should be able to:
1. Describe DNA's function as the basic hereditary material controlling cellular activity via control of the cell's enzyme system.
2. Describe transcription and translation.
3. Describe the functions of the promoter and the terminator.
4. Contrast prokaryotic and eukaryotic genetic "traffic signals".
1. Photocopies of DNA/RNA genetic-code triplet sheet templates
2. Tape or glue sticks
3. Construction paper/copy paper
4. Protein Modeling Teacher Notes
During this modeling activity, students act out transcription and translation by moving around the classroom with assigned roles as the DNA, mRNA, tRNA, proteins, etc.. Some instructors and classrooms are find with this form of activity extremely effective in creating an picture of an abstract concept however this type of "strategic chaos" that can be extremely difficult for many students as well. In such an instance, the activity also works great as an instructor demonstration or guided student inquiry.
Procedure for Modeling Protein Synthesis:
1. Set the scene for modeling. For example, have anchor charts prepared, template pieces photocopied and cut out, any additional visuals, models, or manipulatives in a "central" location so that the modeling activity moves along at an effective pace with few interruptions.
2. Distribute the DNA sequence card pairs and desired regulatory elements and the matching mRNA codes.
3. Students with the DNA card pairs line up in the classroom area designated "nucleus". A student with the promoter card should line up first.
4. Students move based on the cues gathered from the instructor script. Students should also be encouraged to record the modeling experience on the following Gene to Protein Record Sheet.
At this point in the lesson I pause and survey my students by asking several of the questions listed below. Based on the types of responses students provide, we may extend our learning with an additional Modeling DNA and Protein Synthesis Lab activity.
Students can respond to the following inquiries individually or with a partner in writing and the inquiries can be made during a whole group discussion:
1. DNA is double stranded. One strand is the coding strand, and the other is the noncoding strand. The noncoding strand is used as the template to make the mRNA. What is the relationship between the base sequences of the coding and the noncoding strands?
2. What is the relationship between the base sequence of the coding strand and the base sequence of mRNA?
3. What is the relationship between the base sequence of the noncoding strand and the base sequence of the mRNA?
4. Would there be a problem if the RNA polymerase transcribed the wrong strand of DNA and the cell tried to make a protein?
5. A frameshift mutation is caused by the insertion or deletion of one or two DNA bases. What would be the effect on the amino acid sequence of a protein if one extra base were inserted into the gene near the beginning? Use an example to show what you mean.
6. Suppose an individual has a nutrient deficiency due to poor diet and is missing a particular amino acid. How would transcription and translation be affected?
7. Fill in the missing boxes in the data table by writing the DNA sequence of the coding strand and the correct mRNA codons, tRNA anticodons, and amino acids.
8. A new and exciting branch of biotechnology is called protein engineering. To engineer proteins, molecular biologists work backward through the protein synthesis process. They first determine the exact sequence of the polypeptide they want and then create a DNA sequence to produce it. Use the rules of transcription and translation to "engineer" the peptide sequence below. Fill in the two DNA strands. Use the tRNA-amino acid sequences determined in Question 7.