SBI 4UNOVEMBER 19 T H , 2012

Translation:From RNA to Protein

Agenda

Take up homeworkLesson on transcription

Open your Notebooks, and respond to the following questions

Describe the process of initiationWhat is the difference between the coding

strand and the template strand?What is the function of the spliceosomes?

Recall

mRNA has left the nucleus (after the transcription process)

The Ribosome

mRNA in cytoplasm – ready for translation Ribosomes attach to mRNA – 5’ cap 2 subunits: 60S & 40S Subunits clamp mRNA in 5’ to 3’ direction – growing polypeptide chain One nucleotide at a time – each codon codes for 1 amino acid

The Role of Transfer RNA (tRNA)

tRNA delivers amino acids to polypeptide building site

2 “arms” – 1 arm, anticodon recognizes codon of the mRNA . 2nd arm carries corresponding amino acid

Recognition by tRNA of mRNA is facilitated by the complementarity of base pairing – UAU is AUA

Every tRNA carries 1 amino acid only

The Role of Transfer RNA (tRNA)

Aminoacyl-tRNA – tRNA molecule containing its corresponding amino acid attached to its acceptor site

Elongation of the Polypeptide Chain

Start codon: AUG – ensures that the correct reading frame is used by the ribosome

AUG – methionineRibosome has 2 sites for tRNA: Acceptor (A)

site and Peptide (P) site

Elongation of the Polypeptide Chain

Peptide site: when peptide bonds are formed between adjoining amino acids on a growing polypeptide chain

Acceptor site: site in the ribosome where tRNA brings in an amino acid

Steps in the Elongation of the Polypeptide Chain

Methionine – P siteNext tRNA carrying

amino acid enters A siteRibosome shifts over

one codonThird amino acid enters

A site and second moves over to P site

All 3 amino acids bonded by a peptide bond

Termination of Protein Synthesis

Stop codon3 stop codons: UGA, UAG, UAANo corresponding tRNAs Release factor: protein that recognizes that

the ribosome has stalled and aids in the release of the polypeptide chain from the ribosome

2 subunits fall of mRNA and translation is done

Debate Groups

Genetically Modified FoodsFor:Against:CloningFor:Against:Gene TherapyFor:Against:

Debate Groups

Genetic ScreeningFor:Against:

Remember …

Please hand in a typed copy of your debate presentation. Each group will hand in one copy

Hand in your references with the typed copy. References should be in APA format

Structure of Debate

Opening Statements, by each group. Affirmative speaks first.

Like an introduction to a formal paper, introduce yourself/team and the topic you will be debating. Tell what you are going to argue during the debate (like a thesis statement) and tell why your position should win.

Oral Arguments Like the body of a formal paper, present the evidence that proves

your position is correct. This argument should be very factual and persuasive. Opinions not backed by facts could be used to show the weakness of your case, so be careful!

Rebuttal Arguments Presentment of counter evidence that shows: false, inaccurate,

misrepresented, or weak points in the opposition’s argument.

Structure of Debate, cont’d

Cross Examinations As each team finishes its rebuttal, the other team

has time to ask questions about the evidence presented in order to prove the invalidity of the argument.

 Closing Arguments Like the closing paragraph of a formal paper,

summarize the key points you presented. Conclude with a persuasive argument that will win the debate for you even if you are losing based on facts!

Homework

HIV Case Study: pg 249Page: 254, q: 1, 2, 4, 6

Homework: HIV Case Study

1. No HIV cannot attach itself to a muscle or skin cell. HIV’s antigens are not complementary to the binding sites (receptors) of skin cells and muscle cells

3. Reverse transcriptase uses RNA as a template and builds a complementary DNA strand, thereby transcribing RNA into DNA. Reverse transcriptase is an appropriate name for the enzyme, given that conventional transcription of genetic material is in the direction of DNA to RNA. In this case, RNA is transcribed into DNA, meriting it the label of reverse transcription

4. If a helper T cell divides and viral DNA has been incorporated into the cell’s genome, it will also be replicated and be present in both of the daughter cells

5. HIV can stay dormant for many years before symptoms are exhibited in its carrier, therefore an individual can be infected with HIV but not necessarily show any symptoms

6. People who are infected with HIV usually die of another infection because HIV attacks helper T cells, which are part of a human’s immune system. Helper T cells act as guards against invading pathogens. Since HIV destroys helper T cells, the body cannot launch an immune response to secondary infections such as pneumonia.

8. Antibiotics and other drugs are specifically designed to target a specific virus. The drugs must recognize the virus to be effective. Drugs recognize viruses by the protein particles that they carry on their outer membrane. If a virus mutates, it is possible that the protein found on its outer membrane will be affected and change shape. If protein conformation changes, the drug may no longer be able to recognize the virus and therefore will become inneffective

Translation Answers

1. (a) The A and P sites are found in a ribosome that is translating an mRNA sequence into protein. The A (acceptor) site is where tRNA molecules bring in the appropriate amino acid. The P (peptide) site is where peptide bonds are formed between adjoining amino acids on a growing polypeptide chain

(b) A codon is a triplet of ribonucleotides on mRNA that encodes a single amino acid. An anticodon is a triplet of ribonucleotides on tRNA that recognizes and pairs with a codon on the mRNA

(c) The start codon signals to the ribosome to start synthesizing the polypeptide chain. The start codon is always AUG and codes for methionine. The stop codon signals to the ribosome to stop the process of translation. The stop codons are UGA, UAG, and UAA

2. The three types of RNA are mRNA, tRNA and mRNA. Messenger RNA represents the product of transcription of a gene. It encodes the sequence of triplet codons that will be read by the ribosomes to build proteins. It also encodes the start and stop codons that ribosomes use to initiate and terminate translation. Transfer RNA delivers amino acids to the ribosome, which are used to build proteins during the process of translation. Each amino acid has a corresponding transfer RNA. Ribosomal RNA binds with ribosomal proteins to form ribosomes.

4. An error in the third base of a codon in mRNA may not necessarily result in an error during the process of translation because more than one codon encodes a particular amino acid. The codons differ by the third nucleotide. For example: proline can be encoded by the codons CCU, CCC, CCA, and CCG. If a mistake is made in the third nucleotide of the codon, it is negligible. It does not matter what the nucleotide is – the two first nucleotides CC, will always code for proline. The possibility of flexibility in the third nucleotide of a codon is termed the wobble hypothesis

6. The genetic code must be used. AUG is always the start codon and encodes for the amino acid methionine:

5’-GGC-CCA-UAG-AUG-CCA-CCG-GGA-AAA-GAC-UGA-GCC-CCG-3’

Met-Pro-Pro-Gly-Lys-Asp-Stop