Anatomy 8-19-26

Anatomy and Physiology, Grades 11 and 12

Protein Synthesis

Module 1  |  40 Minutes  |  Mr. Gibbs

At a Glance

• 0:00 to 0:03  Bridge from the organelle review

• 0:03 to 0:10  DNA, RNA, and the central dogma

• 0:10 to 0:18  Transcription

• 0:18 to 0:30  Translation

• 0:30 to 0:38  Activity, Transcribe and Translate

• 0:38 to 0:40  Close

Materials: whiteboard, Transcribe and Translate handout, one per student. Keep the board diagram from nucleus to ribosome up for the whole period.

1. Bridge from the Organelle Review

3 minutes.

• Cold call: what does the nucleolus assemble? What is the difference between free and bound ribosomes? What does rough ER do that smooth ER does not?

• Then set up the period: last class you learned that the nucleus controls the cell by determining which proteins get made. Today is the mechanism. Every organelle we named in the manufacturing group exists to serve this one process.

• Say why an anatomy course cares: proteins are the working parts of every tissue in this book. Enzymes, receptors, collagen, actin and myosin, hemoglobin, antibodies, channels. If you understand how one gets built, you understand where every structure in the course comes from.

2. DNA, RNA, and the Central Dogma

7 minutes. Refresher pace. Ask more than you tell.

• Put the flow on the board and leave it: DNA to RNA to protein. Transcription is the first arrow. Translation is the second.

• DNA structure, fast: double helix, two antiparallel strands, nucleotides of sugar, phosphate, and base. Adenine pairs with thymine, cytosine with guanine. A gene is a segment coding for one polypeptide.

• Why two strands matter here: only one is read. Name it the template strand, and note the other is the coding strand, whose sequence matches the mRNA except for uracil.

• RNA versus DNA, three differences: ribose rather than deoxyribose, uracil rather than thymine, and single stranded rather than double.

• Three RNAs and their jobs: messenger RNA carries the message out. Transfer RNA delivers amino acids. Ribosomal RNA plus protein builds the ribosome itself.

• The problem the whole system solves, state it as a question: DNA cannot leave the nucleus, and ribosomes are outside it. So how does the instruction get to the machine? A copy is made and sent. That copy is mRNA.

3. Transcription

8 minutes. Location: nucleus.

The three stages

• Initiation: RNA polymerase binds a promoter region on the DNA and unwinds the double helix locally.

• Elongation: the polymerase reads the template strand and assembles a complementary mRNA strand. Adenine on the template calls for uracil, thymine calls for adenine, cytosine calls for guanine, guanine calls for cytosine.

• Write the pairing rule on the board and box it: this is the single most common place students make an error, because they forget uracil replaces thymine only in the RNA.

• Termination: the polymerase reaches a termination sequence and releases the new transcript.

RNA processing, and do not skip it

• The transcript is not ready to use: a cap is added to one end and a poly A tail to the other. Both protect the molecule and help the ribosome recognize it.

• Splicing: introns are cut out and exons are joined together. Tell them the working definition plainly. Exons are expressed, introns are the intervening sequences that are removed.

• Why this matters for anatomy, one line: the same gene can be spliced more than one way, so one gene can yield more than one protein. That is part of how roughly 20,000 genes produce a far larger number of proteins.

• Then the mRNA exits through a nuclear pore into the cytoplasm.

4. Translation

12 minutes. Location: ribosome, free in cytosol or bound to rough ER.

The code itself

• A codon is three bases. Ask why three. With four bases, pairs give only 16 combinations, which is not enough for 20 amino acids. Triplets give 64.

• So the code is redundant: 64 codons for 20 amino acids means most amino acids have several codons. Note that redundancy is protective, since many single base changes produce no change in the protein at all.

• AUG is the start codon and also codes for methionine. UAA, UAG, and UGA are stop codons and code for no amino acid.

The players

• tRNA: an anticodon at one end and a specific amino acid at the other. The anticodon is complementary to the mRNA codon. Aminoacyl tRNA synthetases are the enzymes that attach the correct amino acid to the correct tRNA.

• The ribosome: two subunits that clamp around the mRNA, with three sites. A site accepts the incoming tRNA, P site holds the growing chain, E site is where the spent tRNA exits.

The three stages

• Initiation: the small subunit binds the mRNA and scans to the start codon, the initiator tRNA carrying methionine pairs with AUG, and the large subunit joins.

• Elongation, walk one full cycle slowly on the board: a charged tRNA enters the A site and pairs with the next codon. A peptide bond forms between its amino acid and the chain in the P site. The ribosome translocates one codon down, moving that tRNA to the P site and ejecting the empty one from the E site. Repeat.

• Termination: a stop codon enters the A site, no tRNA matches it, a release factor binds, and the polypeptide is freed.

Walking one elongation cycle out loud, slowly, is worth more than describing all three stages quickly. Students who can narrate one cycle can reconstruct the rest.

After translation, and this closes the loop with last class

• The chain folds into its functional shape. Sequence determines shape, shape determines function. This is the course principle at the molecular scale.

• Destination determines the route: proteins made on free ribosomes stay in the cytosol. Proteins made on rough ER move to the Golgi for modification and sorting, then out. Point back at the organelles they reviewed last class and note that the manufacturing group exists entirely to serve this pathway.

Mutations, three minutes

• Silent: a base change that still codes for the same amino acid. No effect. This is redundancy doing its job.

• Missense: a different amino acid is inserted. Effect ranges from none to severe depending on where it sits.

• Nonsense: a codon becomes a stop codon and the protein is truncated. Usually severe.

• Frameshift: an insertion or deletion that is not a multiple of three shifts every codon downstream. Almost always catastrophic.

• The clinical example to name: sickle cell disease is a single base substitution changing one glutamic acid to valine in the beta globin chain. One amino acid out of 146, and the hemoglobin polymerizes and deforms the entire cell. Ask what that says about the relationship between sequence and function.

5. Activity, Transcribe and Translate

8 minutes. Individual, at desks. Handout with codon chart.

• Walk the first codon together on the board before releasing them: template TAC becomes mRNA AUG, which is the start codon, methionine. Then let them run.

• The sequence of tasks: transcribe the template strand to mRNA, split it into codons, write the tRNA anticodons, then read the amino acid sequence off the chart.

• Watch for the two predictable errors: writing thymine in the mRNA, and reading the chart in the wrong order. The chart is read first base down the left, second base across the top, third base down the right.

• Part 2 is the mutation: they change one specified base in the template and redo the sequence. The result is a stop codon in the fourth position, so the protein truncates after three amino acids.

• If a student finishes early: ask them to classify the mutation using the four types from the lecture. It is a nonsense mutation.

6. Close

2 minutes.

• Ask the closing question: the mutation changed a single base out of twenty one. What happened to the protein, and what would that mean for a tissue built from it?

• Tie it back: sequence to shape to function. Every structure in the remaining fifteen modules is built by this process.

• Collect handouts. Name the reading and the due date. Dismiss by procedure.

Objectives

• Students will distinguish transcription from translation by location, product, and machinery.

• Students will apply complementary base pairing to produce an mRNA sequence from a DNA template strand.

• Students will use a codon chart to determine an amino acid sequence and identify start and stop codons.

• Students will explain why the genetic code is redundant and what protection that redundancy provides.

• Students will classify a mutation as silent, missense, nonsense, or frameshift and predict its effect on the protein.