Some regions of DNA do not code for the production of proteins. What are these regions of DNA used as?

A. They have no known function and are recycled to provide nucleotides

B. Gene regulation and coding for production of enzymes used in translation

C. Telomeres and coding for production of tRNA

D. Introns and coding for production of structural proteins

C

The diagram shows the three-dimensional structure of tRNA.

[Source: Structure reproduced with the kind permission of N.R. Voss.]

What can attach to the region marked X?

A. mRNA

B. An amino acid

C. An anticodon

D. The P site of the ribosome

B

The diagram shows the arrangement of introns and exons in a segment of pre-mRNA. Pre-mRNA is immature mRNA containing introns and exons.

Which is a mature mRNA molecule derived from the pre-mRNA by splicing?

[Source: © International Baccalaureate Organization 2019]

A

The graph shows results of an experiment by Hershey and Chase in 1952 in which bacteria were infected with a mixture of virus particles labelled with either ³²P or ³⁵S. A suspension of the infected bacteria was agitated with a blender, and samples collected from the suspension were centrifuged to record the percentage of isotope remaining on the outside of the cells.

[Source: Republished with permission of ROCKEFELLER UNIVERSITY PRESS, from Independent functions of protein and nucleic acid in growth of bacteriophage. Hershey, A.D. and Chase, M., 1952. (Journal of General Physiology, 36(1), p.47). Society of General Physiologists, Rockefeller Institute for Medical Research, Rockefeller Institute; permission conveyed through Copyright Clearance Center, Inc.]

What do curves X and Y represent?

B

The statistics show that the candidates found this question difficult, and it did not discriminate well. The graph in the stem of the question was taken from the original Hersey and Chase 1952 publication. The G2 forms suggest two reasons why the candidates found this question difficult.

First, they may not have remembered what is meant by supernatant and sediment. However, these terms are important in understanding the Hershey and Chase experiment and should have been known.

Secondly, 'the percentage of isotope remaining on the outside of the cells' was possibly misleading. This may have led candidates to think that some of the isotopes were still attached to the outside of the bacterial cell when the intended meaning was the amount of isotope apart from the bacterial cells which would therefore be in the supernatant.

As the question is only considering the percentage of the isotope in the supernatant the 35S from the protein shell of the bacteriophage would be found in the highest percentage as it does not enter the bacterial cell as shown by line X. As some of the 32P from the phage genetic material would have entered the bacterial cell, a lower percentage would be found in the supernatant which corresponds to line Y. This gives correct answer B.

Scientists have heated a solution containing the protein albumin and measured its relative alpha helix content, shown on the graph.

What does the zone labelled X indicate?

A. Rapid increase in beta pleated sheets

B. Rapid formation of hydrogen bonds

C. Rapid increase in denatured protein molecules

D. Rapid decrease in peptide bonds

C

The diagram shows the structure of a nucleosome.

[Source: Zephyris. Nucleosome 1KX5 colour coded. Available at https://en.wikipedia.org/wiki/Nucleosome#/media/File:Nucleosome_1KX5_colour_coded.png 
This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license (https://creativecommons.org/licenses/by-sa/3.0/deed.en).]

What is the structure labelled T?

A. 5' end of RNA

B. 5' end of uncoiled DNA

C. N-terminal tail of one DNA strand

D. N-terminal tail of one histone

D

This question received some criticism in the comments on the G2 forms. It was suggested that this question was too difficult for IB level students, the diagram was not clear and the term ‘N terminal tail’ was out of the scope of the IB guide. The question related to 7.1 in the biology guide “Utilization of molecular visualization software to analyse the association between protein and DNA within a nucleosome”. Despite criticism, the question did discriminate well, possibly because the more able students were able to eliminate the incorrect responses by looking at the diagram leaving response D as the correct response. It is understood that questions on multiple choice tests would not normally be expected to be answered by this means.

Rosalind Franklin produced X-ray diffraction images of DNA.

[Source: © King’s College, London. KCL Department of Biophysics KDBP/1/1.]

Which previously unknown aspect of DNA was deduced from these images?

A. Chromosomes are made of DNA.

B. Deoxyribose sugar is attached to a phosphate group.

C. The base A pairs with T and C with G.

D. The DNA molecule is a double helix.

D

What is the primary function of the free ribosomes shown in the electron micrograph?

[Source: J Gordon Betts, et al. Anatomy and Physiology. Houston Texas: OpenStax, 2013. https://openstax.org/books/
anatomy-and-physiology/pages/3-2-the-cytoplasm-and-cellular-organelles. Image file available at https://commons.
wikimedia.org/wiki/File:0313_Endoplasmic_Reticulum.jpg#/media/File:0313_Endoplasmic_Reticulum_b_labeled.
png. This file is licensed under the Creative Commons Attribution 3.0 Unported license. https://creativecommons.org/
licenses/by/3.0/deed.en.]

A. Synthesize proteins to be used within the cell

B. Synthesize proteins for use in lysosomes

C. Carry amino acids to mRNA for protein synthesis

D. Synthesize proteins for secretion

A

This DNA sequence was used to synthesize a polypeptide.

DNA (sense strand): 3′ T A C T G A 5′

DNA (template strand): 5′ A T G A C T 3′

Which are the bases of the tRNA (anticodons)?

A. T A C   T G A

B. U A C   U G A

C. A U G   A C U

D. A T G   A C T

C

The discrimination index for this question was low, and as only 44 % answered correctly, the explanation was not that it was too easy. The question asked for the bases of the anticodons used to produce a polypeptide. They will be complementary to the DNA sense strand, because it will have the same base sequence (apart from T/U) as the codons of the mRNA that is translated into the polypeptide. B was almost as popular as the correct answer, but gives the codons, rather than the anticodons.

The mRNA codon UAC codes for the amino acid tyrosine. Which tRNA carries tyrosine?

B

The G2 forms raised two concerns with this question.

Firstly, the use of the term codon to describe a DNA sequence of 3 bases when this term is normally used for sequences of bases on mRNA. It is normal to use the term triplet referring to bases on a strand of DNA though codon is an accepted but infrequently used alternative.

Secondly, the question does not make it clear if it is referring to the sense or antisense strand, both of which would give different responses.

Both answers C and D were accepted as correct, and the question will be changed before the paper is published.

How can environmental factors affect the expression of genes?

A. By promoting the replication of nucleosomes

B. By inactivating epinephrine

C. By making specific changes to the base sequence of genes

D. By causing the pattern of DNA methylation to be changed

D

Current understanding of DNA is an example of the accumulated discoveries of many scientists. Which scientists are credited with establishing that DNA is the genetic material?

A.  Franklin and Wilkins

B.  Watson and Crick

C.  Meselson and Stahl

D.  Hershey and Chase

D

The number of protein-coding genes in the human genome is estimated to be about 20 000, which is much less than the size of the proteome. What is one reason for this?

A. Exons are removed from RNA before translation.

B. There are more types of amino acids than nucleotides.

C. mRNA can be spliced after transcription.

D. Base substitutions occur during transcription.

C

In which image are polysomes visible? (The images do not have the same magnification)

C

During modification in eukaryotes, mRNA is spliced. What is splicing of mRNA?

A. Separation of mRNA from DNA during transcription

B. The removal of non-coding RNA sections in prokaryotic cells

C. Linking together exons

D. Replacement of primers with RNA bases

C

The diagram shows how pre-mRNA is processed into mature mRNA. Which structures are indicated by the letters W and X?

D

Gene expression in cells is regulated at various stages. During which stage does splicing of RNA occur?

A. During replication

B. During transcription

C. Post transcription

D. Translation

C

What is the difference between the DNA of adult identical (monozygotic) twins?

A. Order of genes

B. Sequence of nucleotides

C. Methylation pattern

D. Ratio of complementary base pairs

C

What are polysomes?

A. Strings of amino acids

B. Packages of eight histones with DNA

C. Many ribosomes joined to one mRNA

D. Complexes of tRNA with amino acids

C

How do R group interactions contribute to protein structure?

I. Determining the sequence of amino acids in the primary structure

II. Stabilizing beta pleated sheets in the secondary structure

III. Stabilizing further foldings of a polypeptide into a tertiary structure

A. I only

B. II and III only

C. III only

D. I, II and III

C

In the diagram of a ribosome, which letter indicates the P site?

[Source: iStock.com/ttsz]

C

What does eukaryotic DNA have that is missing from prokaryotic DNA?

A. Uracil

B. Promoter DNA

C. Introns

D. Coding sequences

C

What applies to DNA base sequences?

I. Some genes do not code for proteins.

II. Promoters are transcribed along with the gene.

III. Introns are only found within genes coding for proteins.

A. I only

B. II only

C. II and III only

D. I, II and III

B

From Topic 7.1 candidates should know that introns are regions of DNA that do not code for proteins. This confused some candidates that failed to realize that introns are found within genes coding for proteins. In order to make this question more fair, the mark scheme was adapted and answers A and B were both accepted as correct.

Which regions of DNA code for the production of specific proteins?

A. Telomeres

B. Genes for ribosomal RNA

C. Exons

D. Regulators of gene expression

C

The image shows a replication fork.

Which row identifies X, Y and Z in the diagram?

B

Which statement applies to tRNA?

A. There is at least one type of tRNA that combines with each known amino acid.

B. One type of tRNA can combine with all of the known amino acids.

C. tRNA carries out its main role within the nucleus.

D. tRNA is produced by the process of translation.

A

Which cell component synthesizes actin and myosin?

A. Free ribosomes

B. Rough endoplasmic reticulum

C. Smooth endoplasmic reticulum

D. Nuclear membrane

A

What is a feature of transcription in the single-celled organism shown in the electron micrograph?

[Source: George E. Palade Electron Microscopy Slide Collection Harvey Cushing/John Hay Whitney Medical Library Yale University Library.]

A. mRNA splicing

B. Removal of introns

C. Codon-anticodon binding

D. Synthesis of RNA in a 5′ to 3′ direction

D

The diagram shows the structure of E. coli ribonuclease HI, a bacterial protein consisting of one polypeptide chain.

[Source: RCSB PDB. 1JL1 D10A E. coli ribonuclease HI. PDB DOI: 10.2210/pdb1JL1/pdb Mol* (Goedken, E.R., Marqusee, S. Native-state energetics of a thermostabilized variant of ribonuclease HI. (2001) J Mol Biol 314:
863–871 DOI:10.1006/jmbi.2001.5184) [image online] Available at: https://www.rcsb.org/structure/1jl1 
[Accessed 25 November 2019]. This file is licensed under the Creative Commons CC0 1.0 Universal (https://creativecommons.org/publicdomain/zero/1.0/).]

Which level(s) of protein structure is/are shown?

A. Alpha helix only

B. Quaternary only

C. Primary and secondary

D. Secondary and tertiary

D

This question did not discriminate well, as many students answered that both primary and secondary structures are shown in the diagram. The diagram clearly shows secondary and tertiary structure. The stem of the question indicates the protein had only one polypeptide ruling out quaternary structure. The sequence of amino acids is not shown in the diagram making D the only correct response.

The image shows a polysome in a bacterial cell.

[Source: © International Baccalaureate Organization 2019]

How many genes have been transcribed?

A. 1

B. 2

C. 3

D. 4 

A

The tense given in this question was misleading – ideally, the question should have read “How many genes have been transcribed?” The answer to the question actually asked was zero, but as this was not offered the next best answer was A – one gene has been transcribed and in the diagram the mRNA product of this transcription is being repeatedly translated.

Which letter (A–D) indicates where a new nucleotide would attach?

B

Which are two proteins that assist in the unwinding and separation of DNA strands during replication?

A. Helicase and DNA polymerase III

B. DNA gyrase and DNA polymerase I

C. Helicase and DNA primase

D. Single-strand binding protein and DNA gyrase

D

For reasons that are hard to explain, this question had one of the highest ever discrimination indices at 0.72, with about half of candidates answering correctly.

A nucleotide containing dideoxyribose is shown.

What is the reason for the use of dideoxyribonucleotides to terminate sequences in base sequencing?

A. Nucleotides cannot form 5′ to 3′ linkages with dideoxyribonucleotides.

B. Nucleotides cannot form base pairs with dideoxyribonucleotides.

C. Dideoxyribonucleotides cannot form hydrogen bonds with deoxyribose.

D. Dideoxyribonucleotides do not have all four nitrogenous bases.

A

This question discriminated well. In Topic 7.1 it states that candidates should be able to understand the use of nucleotides containing dideoxyribonucleic acid to stop DNA replication in preparation of samples for base sequencing. Some candidates believed dideoxynucleotides failed to form base pairs with nucleotides, showing lack of understanding of the sequencing process.

Where does a tRNA-activating enzyme attach the appropriate amino acid to the tRNA molecule?

C

What are the functions of DNA primase and DNA polymerase I in DNA replication?

D

In Topic 7.1 it says that the proteins and enzymes involved in DNA replication should include helicase, DNA gyrase, single strand binding proteins, DNA primase and DNA polymerases I and III. There was a complaint in the G2s mentioning that DNA primase is really RNA primase. This is a misunderstanding, as it is really an RNA polymerase. This question showed the highest discrimination index.

Hershey and Chase used a bacteriophage (a virus that infects bacteria) to investigate the chemical nature of genes. The diagram shows a bacteriophage.

[Source: Graham Knott and Christel Genoud, ‘Commentary: is EM dead?’, Journal of Cell Science (2013),
126: 4545–4552, reproduced with permission. http://jcs.biologists.org/content/126/20/4545.figures-only
doi: 10.1242/jcs.124123 http://www.biologists.com/journal-of-cell-science]

The sulphur in the protein and the phosphorus in the DNA of the bacteriophage were radioactively labelled. The data obtained after bacterial infection and centrifugation are shown in the table.

What did Hershey and Chase conclude from their experiment?

A. DNA was mainly outside the bacterial cells.

B. Viruses infect bacterial cells with proteins.

C. Viral DNA was found within the bacterial cells.

D. Neither protein nor DNA were chemicals making up genes in viruses.

C

This was a good question, it discriminated very well and was not too difficult.

In which process(es) do nucleosomes play a role in eukaryotes?

I. tRNA activation

II. Transcription regulation

III. DNA supercoiling

A. I only

B. II only

C. II and III only

D. I, II and III

C

This image is taken from a visualization of a eukaryotic ribosome. The arrows show the direction of movement of mRNA. Which letter represents a tRNA exiting from the E site?

A

What is a function of DNA polymerase I?

A. Adds nucleotides in a 5’ to 3’ direction to elongate the chain

B. Uncoils the DNA double helix and splits it into two template strands

C. Removes RNA primer and replaces it with DNA

D. Produces sugar-phosphate bonds to link Okazaki fragments

C

What does post-transcriptional modification of eukaryotic mRNA include?

I.   Introns are removed from mRNA.
II.  Exons are joined together to form mature mRNA.
III. A 5' cap and 3' poly-A tail are added to mRNA.

A. I only
B. I and III only
C. II and III only
D. I, II and III

D

Promoters are non-coding regions in DNA. What is the role of a promoter?

A. It starts translation.

B. It starts mRNA splicing.

C. It is a binding site for DNA polymerase during DNA replication.

D. It is a binding site for RNA polymerase during transcription.

D

What contribution did the X-ray diffraction conducted by Rosalind Franklin make to our understanding of DNA?

A. It identified DNA as the genetic material.

B. It indicated the helical shape of the molecule.

C. It showed that the DNA of eukaryotes was associated with histone proteins.

D. It showed that replication was a semi-conservative process.

B

Variations in the types of antibodies are produced by mRNA splicing. What is an advantage of this process?

A. Reduces the size of mRNA required for the translation of antibodies
B. Increases the number of different antibodies that can be synthesized
C. Ensures that one gene codes for one antibody
D. Speeds up transcription of antibodies

B

Which types of interactions are found in a part of a protein with secondary but not tertiary structure?

I. Hydrogen bonds

II. Disulphide bridges

III. Ionic bonds

A. I only

B. I and II only

C. II and III only

D. I, II and III

A