Here are the questions related to PROTEIN SYNTHESIS for Lv3 Biology external exam. Only some questions are answered. 
Q1. The information carried by DNA controls protein synthesis. Protein synthesis includes transcription and translation. Discuss the reasons why both transcription and translation are necessary for protein synthesis.
ANSWER
Transcription: mRNA is synthesized using DNA (or a section of DNA/ allele) as a template. Must occur in the nucleus because DNA is confined there – in this way, DNA is protected. mRNA is needed so that the instructions carried by DNA (genetic code) can be delivered to the ribosome and expressed as a specific protein.
Translation: mRNA carries the code (sequence of bases) from DNA to the ribosome, where amino acids are joined to produce a protein and polypeptide. mRNA attaches to a ribosome in the cytoplasm, tRNA molecules bring amino acids (their order determined by the anticodon of the tRNA which matches the codon of the mRNA. Amino acids are joined on the ribosome (peptide bonds). tRNAs ensure that the genetic code is accurately translated into a sequence of amino acids in the polypeptide. Translation ensures that the instructions on mRNA are accurately interpreted, and protein beings and ends at the correct points.
DNA carries the genetic code. It is kept safe in the nucleus (protected by the nuclear envelope and histone proteins). However, to be expressed (genes used to make proteins), DNA must unwind. mRNA is made as an expendable, temporary carrier of genetic information. It is used to make the polypeptides. mRNA has a relatively short life expectancy and more mRNA can be manufactured when needed. Transcription must precede translation, but both are needed if a protein is to be produced.
Q2. Deoxyribonucleic acid is found in almost all cells, and carries the genetic code that controls many aspects of cellular structure and function.
(a) Explain how the DNA molecule carries genetic information.
(b) Compare and contrast process of transcription and translation.
(c) The proteins that some genes code for are constantly expressed, eg those involved in cellular respiration. Other proteins are produced only as needed, ie the expression of these genes is controlled. Explain the role of enhancers and Eukaryote gene expression.
(d) Discuss reasons for the role of inducers and repressors in Prokaryote gene expression.
Q3. Fully describe the role of tRNA in the synthesis of proteins in cells.
Q4. The relationship between DNA and a protein can be shown by the following diagram.
DNA in nucleus –> mRNA –> Protein in cytoplasm
Explain why there is an intermediary (mRNA) between the DNA and the protein it encodes.
Q5. The gene that causes the Leukemia formed by the exchange of chromosome was the target for genetic drugs to cure the patients. These are short sequences of nucleotides that act by binding to DNA or mRNA molecules by preventing the synthesis of disease-causing proteins. There are two main types of genetic drug.
- Triplex drugs, which are made from DNA nucleotides and bind to DNA forming a triple helix.
- Antisense drugs that are made of RNA nucleotides and blok the function of mRNA that has been made.
(a) Describe a difference between a nucleotide of DNA and RNA.
(b) Describe a function of mRNA that will be inhibited by Antisense drugs.
(c) One of the problems in using Triplex and Antisense drugs has been to stop them being broken down by extracellular nuclease enzymes before they get to their target cell. Scientists can do this by altering the chemical arrangement of the nucleotides. Discuss why the scientists are trying to alter only two of the three chemical groups of the nucleotides of these drugs in order to make them as effective as possible.
Q6. Briefly name and describe the two processes that are part of protein synthesis. Explain why there are different types of nucleic acids involved.
ANSWER
Process 1: Transcription.
DNA used as template to produce mRNA (code or sequence of nucleotides in DNA copied or determines sequence of bases in mRNA). mRNA a temporary molecule and carries ‘message’ from DNA (safely stored in nucleus) to cytoplasm where gene expressed / protein sythesized.
Process 2: Translation.
‘Message’ or sequence of bases on mRNA determines order of amino acids in polypeptide being synthesized on ribosome. tRNA transfers / carries amino acids from solution in cytoplasm to ribosome. rRNA is structural component of ribosome and provides physical site for protein synthesis. All RNA molecules have common molecular structure but different shapes reflect different in function.
Q7. The genetic code is often described as a ‘triplet’ code. Describe what this means and explain why a triplet code meets the criteria needed for a genetic code.
ANSWER
Triplet code means 3 consecutive bases on DNA code for 1 amino acid (ie 3 base pairs in DNA transcribed into 3-base codon in mRNA; this is translated into one amino acid in polypeptide chain). The genetic code requires sufficient mRNA codons to code for 20 amino acids + Stop codon. Because there are 4 different bases (A,C,G and U/A) in each nucleic acid, this gives 64 = 4 x 4 x 4 different codons (more than enough). A duplet code would only give 14 = 4 x 4 codes (insufficient).
Q8. Compare and contrast the nature of a DNA molecule in a living cell and mRNA molecule in that same cell.
ANSWER
DNA do not exposed to rigours of cell cytoplasm. Is kept secure in nucleus, coiled and protected by histone proteins, mRNA temporary molecule, broken down once sufficient polypeptide made. Reasons include:
(1) DNA too big to pass through nuclear pores to cytoplasm and engage in translation.
(2) mRNA ‘intermediate’ made so introns can be removed from the primary transcript.
(3) Many copies of mRNA can be made, but only 1 copy of DNA.
(4) Original DNA can be kept safe from mutagenic agents.
(5) mRNA needs to be constantly broken down (by ribonucleases) to exert control over protein production.
*Both similarities (compare) and differences (contrast) need to be fully discussed for both molecules.
Comparison: Both carry codes for at least only polypeptide and carry errors.
Differences:
(1) DNA is well protected by nuclear envelope and histone proteins. DNA is a long lived molecules (the life of the cell). It is permanent.
(2) mRNA is unprotected and maintained in cell as long as polynucleotide is required. Therefore it is temporary. mRNA actively broken down by ribonuclease enzymes.
Q9. DNA is often referred to as the ‘blue print of life’. Describe the characteristics of DNA that allow it to perform these 3 functions and in each case explain why it is necessary for the genetic blueprint molecule to do so.
(a) Carries a large amount of genetic information.
(b) can be accurately replicated to produce copes that can be inherited.
(c) genetic information can be corrected if mistakes occur.
ANSWER
DNA is very long, but can be coiled and compacted into a tertiary structure/ chromosome. It carries large number of triplets which are units of the code with an infinite number of possible combinations. Large numbers of genes in each chromosome thus possible. The large variety of living things and need for constant improvement and evolution requires coding system sequence of bases in each new strand complementary to original and that both DNA copies identical. Genetic continuity is necessary – ensures each daughter cell has exact copy of all genetic material and thus able to make same proteins. Inaccurate DNA copies make faulty proteins that disrupt functioning of cell.
DNA has 2 strands. Small errors in replication can thus be recognized and corrected. Mutations are usually harmful and disadvantageous, thus reducing survivorship of organisms carrying them. Changes to DNA not likely to lead successful variations, current DNA has survived natural selection/evolution.