Higher Biology Mrna Essay Outline

Previous IB Exam Essay Questions: Unit 4

Use these model essay question responses to prepare for essay questions on your in class tests, as well as the IB Examination, Paper 2. These questions have appeared on recent IB examinations, exactly as shown below. Following each question is the markscheme answer which was used to evaluate student answers on the examination paper.
1. Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA.5 marks

Award 1 for each pair of statements in a table, and 1 for any statement below the table.

U=Unique sequences; H= Highly repetitive sequences

  • U: occur once in genome; H: occur many times;
  • U: long base sequences; H: short sequences/5–300 bases;
  • U: (may be) genes; H: not genes;
  • U: (may be) translated/coding sequences; H: never translated;
  • U: small differences between individuals; H: can vary greatly;
  • U: exons (are unique sequences); H: introns (may be repetitive);
  • U: smaller proportion of genome; H: higher proportion of genome;
  • satellite DNA is repetitive;
  • repetitive sequences are used for profiling;
  • prokaryotes do not (usually) contain repetitive sequences;

2. Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands. 5 marks

Award 1 for each of these structures clearly drawn and labelled.

  • four nucleotides shown in diagram with one nucleotide clearly labelled;
  • base, phosphate and deoxyribose (shown as pentagon) connected between the
  • correct carbons and labelled at least once;
  • backbone labelled as covalent bond between nucleotides correctly shown as 3 to 5 bond;
  • two base pairs linked by hydrogen bonds drawn as dotted lines and labelled;
  • two H bonds between A and T and three H bonds between C and G;
  • adenine to thymine and cytosine to guanine; do not accept initials of bases
  • antiparallel orientation shown;

3. Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together.8 marks

  • subunits are nucleotides
  • one base, one deoxyribose and one phosphate in each nucleotide
  • description/ diagram showing base linked to deoxyribose C1 and phosphate to C5
  • four different bases - adenine, cytosine, guanine and thymine
  • nucleotides linked up with sugar-phosphate bonds
  • covalent/ phosphodiester bonds
  • two strands (of nucleotides) linked together
  • base to base
  • A to T and G to C
  • hydrogen bonds between bases
  • antiparallel strands
  • double helix drawn or described

4. Outline the structure of the nucleosomes in eukaryotic chromosomes.4 marks

  • contain histones
  • eight histone molecules form a cluster in a nucleosome
  • DNA strand is wound around the histones
  • wound around twice in each nucleosome
  • (another) histone molecule holds the nucleosome(s) together

5. State a role for each of four different named enzymes in DNA replication.6 marks

Award 1 mark for any two of the following up to 2 marks maximum.

  • helicase
  • DNA polymerase / DNA polymerase III
  • RNA primase
  • DNA polymerase I
  • (DNA) ligase
Award 1 mark for one function for each of the named enzymes.

helicase

  • splits/ breaks hydrogen bonds/ uncoils DNA/ unwinds DNA
(DNA) polymerase III
  • adds nucleotides (in 5' to 3' direction) extending existing strand
(RNA) primase
  • synthesizes a short RNA primer (which is later removed) on DNA
(DNA) polymerase I
  • replaces RNA primer with DNA
(DNA) ligase
  • joins Okazaki fragments/ fragments on lagging strand/ makes sugar-phosphate bonds between fragments

6. Explain the process of DNA replication.8 marks

Accept any of the points above shown on an annotated diagram.

  • occurs during (S phase of ) interphase/in preparation for mitosis/cell division;
  • DNA replication is semi-conservative;
  • unwinding of double helix / separation of strands by helicase (at replication origin);
  • hydrogen bonds between two strands are broken;
  • each strand of parent DNA used as template for synthesis;
  • synthesis continuous on leading strand but not continuous on lagging strand;
  • leading to formation of Okazaki fragments (on lagging strand);
  • synthesis occurs in 5 3direction;
  • RNA primer synthesized on parent DNA using RNA primase;
  • DNA polymerase III adds the nucleotides (to the 3 end)
  • added according to complementary base pairing;
  • adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.)
  • DNA polymerase I removes the RNA primers and replaces them with DNA;
  • DNA ligase joins Okazaki fragments;
  • as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates
  • broken off releasing energy to form bond;

7. Explain how the process of DNA replication depends on the structure of DNA.9 marks

  • DNA molecule is double (stranded)
  • hydrogen bonds linking the two strands are weak/ can be broken
  • DNA can split into two strands
  • split by helicase
  • helicase moves progressively down the molecules
  • backbones are linked by covalent/ strong bonds
  • strands do not therefore break/ base sequence conserved
  • reference to semi-conservative replication
  • base pairing/ sequences are complementary
  • A=T and C=G
  • the two original strands therefore carry the same information
  • the two new strands have the same base sequence as the two original ones
  • the strands have polarity
  • base/ nucleotides added in 5` to 3` direction
  • the two strands have opposite polarity
  • discontinuous segments/ Okazaki fragments added to one strand
  • DNA ligase needed to connect the segments

8. Describe the genetic code.6 marks

  • composed of mRNA base triplets
  • called codons
  • 64 different codons
  • each codes for the addition of an amino acid to a growing polypeptide chain
  • the genetic code is degenerate
  • meaning more than one codon can code for a partiuclar amino acid
  • the genetic code is universal
  • meaning it is the same in almost all organisms
  • (AUG is the) start codon
  • some (nonsense) codons code for the end of translation

9. Discuss the relationship between genes and polypeptides.5 marks

  • originally assumed one gene codes for one polypeptide
    • (one) gene is transcribed into (one) mRNA
    • mRNA is translated by a ribosome to synthesize a polypeptide
  • many exceptions to one gene --> one polypeptide found
    • many more proteins made than there are genes
    • some genes do not code for polypeptides
    • some genes code for tRNA/rRNA
    • some genes regulate gene expression
    • genetic information transcribed by eukaryotes is edited before it is translated
    • polypeptides may be altered before they become fully functional proteins

10. Explain briefly the advantages and disadvantages of the universality of the genetic code to humans.4 marks

  • genetic material can be transferred between species/ between humans
  • one species could use a useful gene from another species
  • transgenic crop plants/ livestock can be produced
  • bacteria/ yeasts can be genetically engineered to make a useful product
  • viruses can invade cells and take over their genetic apparatus
  • viruses cause disease

11. Compare the processes of DNA replication and transcription.9 marks

  • both involve unwinding the helix
  • both involve spearating the two strands
  • both involve breaking hydrogen bonds between bases
  • both involve complementary base pairing
  • both involve C pairing with G
  • both work in a 5` --> 3` direction
  • both involve linking/ polymerization of nucleotides
  • replication with DNA nucleotides and transcritpion with RNA nucleotides
  • details of ribose/ deoxyribose difference
  • adenine pairing with uracil instead of thymine
  • only one strand copied not both
  • no ligase/ no Okazaki fragments with transcription
  • DNA or RNA polymerase
  • both require a start signal
  • but this signal is different for each
  • transcripiton has only one starting point
  • but replication has multiple starting points
  • replication gives two DNA molecules whilst transcription gives mRNA

12. Distinguish between RNA and DNA.3 marks

  • DNA is double-stranded while RNA is single-stranded;
  • DNA contains deoxyribose while RNA contains ribose;
  • the base thymine found in DNA is replaced by uracil in RNA;
  • one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);

13. Describe the roles of mRNA, tRNA and ribosomes in translation.6 marks

  • mRNA with genetic code/ codons
  • tRNA with anticodon
  • tRNA with amino acid attached
  • ribosome with two sub-units
  • mRNA held by ribosome
  • start codon
  • two tRNA molecules attached with mRNA on ribosome
  • peptide bond between amino acids on tRNA
  • polypeptide forms
  • continues until a stop codon is reached
  • polypeptide is released

14. Outline the structure of tRNA.5 marks

Accept any of the points above if clearly explained using a suitably labelled diagram

  • tRNA is composed of one chain of (RNA) nucleotides
  • tRNA has a position/end/site attaching an amino acid (reject tRNA contains an amino acid)
  • at the 3' terminal / consisting of CCA/ACC
  • tRNA has an anticodon
  • anticodon of three bases which are not base paired / single stranded / forming part of a loop
  • tRNA has double stranded sections formed by base pairing
  • double stranded sections can be helical
  • tRNA has (three) loops (somethimes with an extra small loop)
  • tRNA has a distinctive three dimensional / clover leaf shape

15. Outline the structure of a ribosome.4 marks

  • small subunit and large subunit;
  • mRNA binding site on small subunit;
  • three tRNA binding sites / A, P and E tRNA binding sites;
  • protein and RNA composition (in both subunits);

16. Explain the process of translation.9 marks

  • translation involves initiation, elongation/translocation and termination;
  • mRNA binds to the small sub-unit of the ribosome;
  • ribosome slides along mRNA to the start codon;
  • anticodon of tRNA pairs with codon on mRNA:
  • complementary base pairing (between codon and anticodon);
  • (anticodon of) tRNA with methionine pairs with start codon / AUG is the start codon;
  • second tRNA pairs with next codon;
  • peptide bond forms between amino acids;
  • ribosome moves along the mRNA by one codon;
  • movement in 5 to 3 direction;
  • tRNA that has lost its amino acid detaches;
  • another tRNA pairs with the next codon/moves into A site;
  • tRNA activating enzymes;
  • link amino acids to specific tRNA;
  • stop codon (eventually) reached;

17. Compare DNA transcription with translation.4 marks

  • both in 5` to 3` direction
  • both require ATP
  • DNA is transcribed and mRNA is translated
  • transcription produces RNA and translation produces polypeptides/ protein
  • RNA polymerase for transcription and ribosomes for translation/ ribosomes in translation only
  • transcription in the nucleus (of eukaryotes) and translation in the cytoplasm/ at ER
  • tRNA needed for translation but not transcription

Cells express their genes by converting the genetic message into protein. This process of protein synthesis occurs in two stages - transcription and translation.

Transcription

When a gene is to be expressed, the base sequence of DNA is copied or transcribed into mRNA (messenger RNA). This process takes place in the nucleus and occurs in a series of stages.

  1. RNA polymerase moves along the DNA unwinding the strand.
  2. Hydrogen bonds between base pairs break which allows the unzipping of the double helix.
  3. As RNA polymerase breaks the bonds, it synthesises a primary transcript of mRNA using RNA nucleotides which form hydrogen bonds with the exposed DNA strand by complementary base pairing.
  4. The primary transcript of mRNA is processed to produce a mature transcript of mRNA.
  5. The mature mRNA transcript is now ready to leave the nucleus and travel to the ribosome.

Translation

  1. The mRNA molecule travels through the cytoplasm and attaches to the ribosome.
  2. tRNA molecules transport specific amino acids to the ribosome.
  3. Each mRNA codon codes for a specific amino acid.
  4. The first codon of an mRNA molecule is a start codon. This signals the beginning of translation.
  5. The anti-codons and codons match up and form complementary base pairs.
  6. Peptide bonds form between the adjacent amino acids to form the polypeptide (protein).
  7. Used tRNA molecules exit the ribosome and collect another specific amino acid.
  8. The last codon of an mRNA molecule is a stop codon which signals the end of translation.

After translation, the polypeptide chain can be changed by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups to the protein.

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