The sequence of protein synthesis from a gene on a chromosome to a correctly-folded protein appears in the Triple or Separate Sciences content on GCSE specifications.
In this learning sequence, students are expected to know that a template made of RNA is complementary to the gene sequence. This template passes through the ribosome where it is translated to form a protein with the correct amino acid sequence.
Questions I want students to be able to answer by the end of this learning sequence:
- What is present in the nucleus of a cell?
- What is the structure of a gene on a chromosome?
- What does complementary mean?
- How do the DNA bases pair?
- What is a codon?
- What is mRNA?
- What is transcription?
- What is tRNA/carrier molecule?
- What is translation?
- What sub-unit are proteins made of?
- What is the function of the ribosome?
- What is the type of bond between amino acids in a protein?
I start off with a recap of the functions of the nucleus and the ribosome in a cell. This is part of the key pre-requisite knowledge that needs to be activated prior to new information being taught.
Then I discuss the structure of the ribosome to show it has two subunits. I do this now to make the later structure and function of the ribosome less cognitively demanding.
I follow this with a description of the basic structure of a chromosome. This is to introduce the idea of non-coding DNA that can play an important role in other processes such as protein synthesis.
Next, I move on to what the term complementary means. For this, a recap of the DNA base pairs is important (A pairs with T; G pairs with C).
Here, I add the fact that information from a DNA sequence needs to be transcribed into a separate template and then this needs to be translated to form a protein. The following (in bold) is what I would say to students:
The entire genome cannot leave the nucleus for the information within a gene to be translated into an amino acid sequence. If it did, it would be prone to damage.
Instead, a transcript of the gene sequence is created using a slightly different molecule to DNA. This molecule is called RNA. As this information is a transcript that then needs to be translated to an amino acid sequence, we call this transcript ‘messenger RNA’ or mRNA.
So if we look at the same gene sequence as earlier, a complementary RNA sequence is generated as below:
Note that one of the bases is different in RNA compared to DNA. T (thymine) is replaced with U (uracil). Therefore, in RNA, A would pair with U.
Next, I introduce the idea of codons, which is a group of 3 bases. I separate groups of 3 bases by drawing lines as in the diagram below.
Codons are a group of 3 bases, which can code for an amino acid. Different codons can code for the same or different amino acids.
The amino acids are in the sequence coded for by the gene sequence. A peptide bond forms between amino acids resulting in a chain of amino acids in the right sequence (blue line between amino acids in the diagram).
Finally, the amino acid sequence folds into a particular shape that results in the completed, functional protein.
It is important to stress here that the functionality of the protein depends on the right amino acid sequence but also on the shape and fold of the protein.
Next, before embarking on the entire process taking place in a cell, I discuss the structure and function of tRNA (or the carrier molecule). In the diagram below, the purple box is a tRNA molecule, which has three bases on the bottom that are complementary to the codon on the mRNA. Also attached to this tRNA molecule is the corresponding amino acid (AA1). I make the point that the tRNA ‘carries’ the right amino acid to the mRNA template at the ribosome, leaving it behind so the amino acid can form peptide bonds with the next amino acid in the sequence, and so on. This becomes clearer when looking at the whole picture.
Once these basic steps and all the new tier 3 vocabulary are introduced and embedded, then I can discuss the entire process of protein synthesis that takes place in the cell.
The diagram below is a review of the fact that the DNA sequence in the chromosome is transcribed into an mRNA sequence, which then leaves the nucleus to enter the cytoplasm.
At this point, if following certain specifications, the enzymes involved in these processes should also be taught.
These are helicase, which unzips the DNA double strand so one strand is available to be transcribed into an mRNA sequence. The second enzyme is RNA polymerase, which binds to the non-coding DNA at the start of a gene and then copies the DNA sequence into a complementary mRNA sequence.
The mRNA sequence leaves the nucleus to enter the cytoplasm. Here it binds to the ribosome to be translated into an amino acid sequence.
At each point, it is important that students are reminded of and quizzed on the terms codon, complementary, mRNA, and ribosome.
The diagram below should be constructed in stages. First, I show a tRNA molecule with three bases on the bottom and the corresponding amino acid attached at the top. Then, I remind students how the three bases on the tRNA are complementary to the codon on the mRNA sequence.
The amino acids that arrive at the ribosome form peptide bonds, which then results in a complete amino acid sequence. Remind students that once the amino acid sequence is complete, it folds into a particular shape and orientation to form the complete, functional protein.