1. Ribosomes and Protein Synthesis
Read the lesson title aloud to students.

2. Learning Objectives
Identify the genetic code and explain how it is read.
Summarize the process of translation.
Describe the central dogma of molecular biology.
Click to show each of the learning objectives.
Make sure that students understand that at the end of the presentation, they should be able to explain how the genetic code is read, summarize the process of translation, and be able to describe the central dogma of molecular biology.

3. The Genetic Code
RNA has four bases: adenine, cytosine, guanine, and uracil. These bases form a “language”: A, C, G, and U.
Ask: What is the genetic code and how is it read?
Answer: The four bases of RNA form the “language” called the genetic code.
The genetic code is read three “letters” at a time, so that each “word” is three bases long and corresponds to a single amino acid.
Remind students that the four bases of RNA are adenine, cytosine, guanine, and uracil.
Click to highlight these in the legend.

4. The Genetic Code: Codons
The genetic code is read in three-letter groupings called codons.
A codon is a group of three nucleotide bases in messenger RNA that specifies a particular amino acid.
AUG
AAC
UCU
Tell students: A codon consists of three consecutive bases that specify a single amino acid to be added to the polypeptide chain. All living organisms read the genetic code in this way, three bases at a time.
Ask: What are the three-letter groups of codons shown here?
Answer: AUG, AAC, and UCU
Click to highlight each group as students provide the answers.

5. Genetic Code Table
There are 64 possible three-base codons in the genetic code.
Explain to students that because there are four different bases in RNA, there are 64 possible three-base codons (4 × 4 × 4 = 64) in the genetic code.
Reading them is simplified by the use of a genetic code table like that shown.
Tell students: Most amino acids can be specified by more than one codon.
For example, six different codons—UUA, UUG, CUU, CUC, CUA, and CUG—specify leucine.
Click to highlight the codons that specify leucine.
Tell students: Only one codon—UGG—specifies the amino acid tryptophan.
Click to highlight the UGG codon.
Point out that although the code table shown here applies to most multicellular organisms, including animals and plants, there are slight differences in the code as used by some microorganisms, as well as mitochondria and chloroplasts.

6. Reading Codons
Start at the middle of the circle with the first letter of the codon and move outward.
Step students through reading, or decoding, a codon. Use CAC as your example.
Ask volunteers to decode CAC.
Have a student find the first letter in the set of bases at the center of the circle.
Click to highlight the correct letter.
Then, have a volunteer find the second letter of the codon, A, in the “C” quarter of the next ring.
Have a volunteer find the third letter, C, in the next ring, in the “C-A” grouping.
Then, have the class read the name of the amino acid in that sector—in this case, histidine.
Click to highlight the amino acid name.
Practice using the genetic code “decoder” by having volunteers provide codons for other volunteers to identify the associated amino acid.
Then, reverse the process: Select amino acids and ask for other volunteers to provide the codon or codons that represent them.
Ask: How many amino acids does each codon represent?
Answer: one
Ask: How many codons can code for a single amino acid?
Answer: from one to six
CAC = Histidine

7. Start and Stop Codons
The methionine codon AUG serves as the “start” codon for protein synthesis. There are three “stop” codons.
UAA, UAG, and UGA are “stop” codons
Continue the analogy of the genetic code being a language and point out that languages need punctuation marks.
In the genetic code, punctuation marks are “start” and “stop” codons.
The methionine codon AUG serves as the initiation, or start, codon for protein synthesis.
Click to highlight the start codon and summary label.
Explain that following the start codon, mRNA is read, three bases at a time, until it reaches one of three different stop codons, which end translation.
Ask for volunteers to point out the three different stop codons.
Click to highlight the first stop codon (UAA).
Click again to highlight each base for the second codon (UAG).
Click to show third base option (UGA) and summary label.
AUG = methionine = “start” codon

8. Translation Transcribed mRNA directs the translation process.
Translation is the process that produces proteins by decoding the sequence of mRNA codons.
Explain to students that translation occurs after transcription.
Transcribed mRNA directs the translation process.
Click to highlight mRNA.
Remind students that in a eukaryotic cell, transcription goes on in the cell’s nucleus. Translation is carried out by ribosomes after the transcribed mRNA enters the cell’s cytoplasm.

9. Translation: Transfer RNA
Translation starts when a ribosome attaches to an mRNA molecule. Then, tRNA molecules, carrying amino acids with them, bind to mRNA codons.
Tell students: Translation begins when a ribosome attaches to an mRNA molecule in the cytoplasm.
Explain that translation initiates at AUG, the start codon.
Click to highlight AUG.
Tell students: tRNA molecules carry in the amino acids coded for by the codon.
Since AUG always codes for methionine, the first amino acid brought in for every round of translation is methionine.
Click to highlight methionine.
As each codon passes through the ribosome, more tRNAs bring the proper amino acids into the ribosome.
Click to highlight amino acid being brought in.
Each transfer RNA has an anticodon whose bases are complementary to the bases of a codon on the mRNA strand. The tRNA attaches its anticodon to the appropriate codon of the mRNA.
Click to highlight the anticodon.
Ask: What is the anticodon for methionine codon, the start codon?
Answer: UAC
Ask: What amino acid does the mRNA codon UUC bring to the polypeptide chain?
Answer: phenylalanine
Ask: If an mRNA codon has the bases CUA, what bases will the corresponding transfer RNA anticodon have?
Answer: GAU
anticodon

10. Translation: The Polypeptide Assembly
The ribosome helps form a peptide bond. It breaks the bond holding the first tRNA molecule to its amino acid.
Explain to students that the ribosome helps form a peptide bond between the first and second amino acids—methionine and phenylalanine.
Click to highlight.
Point out that, at the same time, the bond holding the first tRNA molecule to its amino acid is broken.
That tRNA then moves into a third binding site, from which it exits the ribosome.
Tell students that the ribosome then moves to the third codon, where tRNA brings in the amino acid specified by the third codon.
Explain that the ribosome moves along the mRNA from right to left, binding new tRNA molecules and amino acids.
Address misconceptions:
Tell students: Keep in mind the overall picture and relationship between transcription and translation.
Ask: What is the product of transcription?
Answer: mRNA
Ask: What is the product of translation?
Answer: a protein
Ask: Where do the amino acids come from that make up the protein?
Answer: They are available in the cell and are picked up by the tRNA molecules.
The tRNA molecules then bring them into ribosome to be used to build the protein.

11. Translation: Completing the Polypeptide
The ribosome reaches a stop codon, releasing the newly synthesized polypeptide and the mRNA molecule, completing the process of translation.
Tell students: The translation process continues until the ribosome reaches one of the three stop codons.
Ask a volunteer to identify the stop codon in this illustration.
Click to highlight the answer (UGA).
Explain that when the ribosome reaches the stop codon, it releases both the newly synthesized polypeptide and the mRNA molecule, completing the process of translation.
Click to highlight the polypeptide and mRNA molecule.
Ask: What would happen if the stop codon mistakenly had been made into a regular codon?
Answer: Translation would not stop; another amino acid would be attached and the protein product would have an error in its structure.

12. Roles of RNA in Translation
All three major forms of RNA—mRNA, tRNA, and rRNA—are involved in the process of translation.
Remind students that all three major forms of RNA are involved in the process of translation.
Click to reveal messenger RNA.
Tell students: The mRNA molecule carries the coded message that directs the process of translation.
Click to reveal transfer RNA.
Tell students: The tRNA molecules deliver the amino acids, enabling the ribosome to translate the mRNA’s message into protein form.
Click to reveal ribosomal RNA.
Tell students: The ribosomal RNA molecules hold ribosomal proteins in place and may even carry out the chemical reaction that joins amino acids together.
Point out to students that these components, which translate the genetic code for the purpose of synthesizing new protein molecules, are common to all organisms.

13. The Molecular Basis of Heredity
The central dogma of molecular biology is that information is transferred from DNA to RNA to protein.
Ask: What is the central dogma of molecular biology?
Answer: The central dogma of molecular biology is that information is transferred from DNA to RNA to protein.
Click to reveal this answer.
Write the following on the board: DNA → RNA → Protein
Tell students this represents the central dogma of molecular biology.
Call on several volunteers to express the central dogma in their own words. Then, lead a discussion about its implications and limitations.
Ask: What does the central dogma imply about the role of RNA?
Answer: It’s the step between DNA and proteins.
Point out that the central dogma does have limitations. For example, it doesn’t represent the other roles of RNA. There are also exceptions to this dogma, including viruses that transfer information in the opposite direction, from RNA to DNA.

14. Gene Expression
When a gene (segment) of DNA code is used to build a protein, scientists say that gene has been expressed.
Have students consider the illustration of gene expression in this slide―the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells.
Point out to students that this illustration is a simplification and have them review the other figures used in this presentation if they are confused on that point.
Tell students: The protein made at the end of transcription and translation is the ultimate use or expression of the code in the DNA segment used in transcription. Scientists will say that region of DNA has been expressed. This means its code was used to build a protein.
Summarize gene expression in the following manner:
DNA carries information for specifying the traits of an organism.
The cell uses the sequence of bases in DNA as a template for making mRNA.
The codons of mRNA specify the sequence of amino acids in a protein.
Proteins, in turn, play a key role in producing an organism’s traits.
Explain that many RNA molecules are not translated into proteins but still play important roles in gene expression.
Relate the diagram to the central dogma of molecular biology.
Ask a volunteer to point out the DNA in the illustration.
Click to highlight the DNA.
Ask a volunteer to point out the RNA in the illustration.
Click to highlight the RNA.
Then, ask a volunteer to point out the protein in the illustration.
Click to highlight the protein.
Share with students that one of the most interesting discoveries of molecular biology is the near-universal nature of the genetic code. Although some organisms show slight variations in the amino acids assigned to particular codons, the code is always read three bases at a time and in the same direction.
Despite their enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level, the molecular biology of the gene.