Using a DNA model like the one created in the DNA Models lesson, students take on the role of various parts of the cell in order to model the process of protein synthesis. Each student receives a card describing, step by step, what s/he should be doing. In a class of 30:
Can explain how DNA codes for a sequence of amino acids.
Can explain the differences between DNA and RNA.
Can describe the process of transcription and translation.
The process of turning DNA into protein is called the “central dogma of molecular biology” because it is the foundation of all modern genetics, biotech and pharmacology. There are 6 major players in the process.
There are 2 major steps in the protein synthesis process. The first is the synthesis of messenger RNA in a process known as transcription. This process is similar to DNA replication, except that only a tiny portion of one strand is copied and it is copied into a single-stranded RNA molecule, not a double stranded DNA molecule.
To start transcription, RNA polymerase binds to a specific DNA sequence known as a promotor. Promotors sequences are very diverse, however, generally are found in the stretch of DNA in front of the gene and contain a place for RNA polymerase to bind as well as a transcriptional start sequence that indicates where transcription should begin. They range in length from less than a hundred base pairs to several thousand base pairs. Many promotor sequences contain the sequence TATAAA, known as a TATA box by biologists. This TATAAA sequence is used in this activity to indicate where the RNA polymerase should bind and begin transcription.
Once, the RNA polymerase binds to the promotor, it follows along the DNA, unzipping the base pairs, reading one of the two DNA strands, matching an RNA nucleotide to each DNA nucleotide, and assembling a messenger RNA molecule. The RNA polymerase continues moving along the DNA until it reaches a specific terminator sequence, at which point it releases the messenger RNA and disassembles. Messenger RNA molecules may extend over 2 million bases in length. At this point, the messenger RNA travels out of the nucleus to the ribosome where proteins are actually made.
This second step of the protein synthesis process is known as translation. First, a ribosome assembles around the messenger RNA molecule. Translation always begins at the messenger RNA sequence AUG. The messenger RNA then feeds its way through the ribosome like a tape. As it proceeds, each codon on the messenger RNA is matched to a transfer RNA. The ribosome forms bonds between the amino acids carried by the transfer RNAs and the empty transfer RNA molecules detach and float away. Gradually, the amino acid chain grows longer and longer until a stop sequence (UAG, UAA, or UGA) is reached. At that point, the protein is released.
From here, the protein may go through many stages of further processing. Depending on the sequence of amino acids, some parts of the protein like water and some curl away from it. Thus, the protein will fold itself up to protect the water-hating parts of the protein from the surrounding cytosol. In addition, proteins may be cut, spliced, joined together, packaged and reshaped into a final functional protein.
Some basic introduction to the protein synthesis process (see Secret Codes lesson).
- RNA polymerase finds the TATAAA promotor sequence on the DNA molecule.
- RNA polymerase unzips the DNA nucleotide after the promotor and finds a matching RNA nucleotide.
- RNA polymerase unzips the next DNA nucleotide and finds a matching RNA nucleotide.
- RNA polymerase joins the RNA nucleotides together.
- RNA polymerase continues unzipping, finding nucleotides, and joining them together until the end of the DNA molecule.
- DNA zips itself back up again.
- The newly assembled messenger RNA floats out of the nucleus to the ribosome.
- The ribosome finds the AUG start sequence on the messenger RNA.
- The ribosome finds a matching transfer RNA and lines it up alongside the messenger RNA strand.
- The ribosome finds a matching transfer RNA to the next 3 nucleotides.
- The ribosome removes the amino acid from the first transfer RNA and attaches it to the amino acid that just arrived.
- The ribosome continues finding transfer RNA molecules and joining amino acids until it reaches a stop codon (UAG, UAA, or UGA).
- The empty transfer RNA molecules leave the ribosome.
This activity was put together from the bright ideas of several great teachers: Lori Lambertson of the Exploratorium Teacher Institute and Jim Youngblom of CSU Stanislaus.
Chemistry of Living Systems (Life Sciences)
6. Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept:
c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA.
4. Genes are a set of instructions encoded in the DNA sequence of each organism that specify the sequence of amino acids in proteins characteristic of that organism. As a basis for understanding this concept:
a. Students know the general pathway by which ribosomes synthesize proteins, using tRNAs to translate genetic information in mRNA.
b. Students know how to apply the genetic coding rules to predict the sequence of amino acids from a sequence of codons in RNA.
e. Students know proteins can differ from one another in the number and sequence of amino acids.