IB Biology HL D1.2 Protein Synthesis Questions

Practise IB Biology HL D1.2 with sequence and expression questions covering transcription, translation, mRNA processing, genetic-code tables and mutation consequences.

Exam points

explain promoter-controlled transcription by RNA polymerase, including template selection and 5′→3′ mRNA synthesis
derive mRNA from DNA using A–U/C–G pairing and verify template strand and sequence direction
interpret mRNA-expression tables, graphs or blots across genes, tissues or treatments
identify promoters, introns, telomeres and rRNA/tRNA genes as non-coding DNA with regulatory or structural roles
explain pre-mRNA splicing, intron removal and 5′ cap/poly-A tail formation before export
explain how alternative exon combinations from one pre-mRNA produce multiple mRNAs and protein variants
trace ribosome decoding of mRNA codons via tRNA, peptide bonds and polypeptide elongation
use 5′→3′ direction, AUG and A/P/E ribosome sites to explain translation initiation
explain the triplet, degenerate and near-universal genetic code, including start and stop signals
convert template DNA to mRNA and use genetic-code tables to derive amino-acid sequences
trace a beta-haemoglobin base substitution to Glu→Val, altered structure and sickle-cell effects
explain precursor-polypeptide cleavage using proinsulin, pepsinogen and trypsinogen activation

Covered syllabus points

D1.2.1—Transcription D1.2.2—Hydrogen bonding in transcription D1.2.3—DNA template stability D1.2.4—Transcription for gene expression D1.2.5—Translation D1.2.6—Roles in translation D1.2.7—Complementary base pairing D1.2.8—Genetic code features D1.2.9—Using genetic code table D1.2.10—Elongation of polypeptide D1.2.11—Mutations changing protein structure D1.2.12 (HL)—Directionality D1.2.13 (HL)—Initiation of transcription at promoter D1.2.14 (HL)—Non-coding sequences in DNA D1.2.15 (HL)—Post-transcriptional modification D1.2.16 (HL)—Alternative splicing D1.2.17 (HL)—Translation initiation D1.2.18 (HL)—Polypeptide modification D1.2.19 (HL)—Amino acid recycling by proteasomes

[Maximum number: 5]

Auxin can be used to promote the development of roots from stem and leafy cuttings in some plants. In a study into the distribution of auxin in the development of these roots, scientists measured the amount of auxin in different leaves of a shoot tip of Petunia hybrida.

The figure indicates the numbering of leaves on the shoot, from L1 as the youngest and smallest to L6 as the largest and oldest leaf. The developmental stage of L5 and L6 was very similar, so L5 was not analysed. The stem base is the lowest part of the cutting where roots may form.

The graph shows the auxin concentration in the different leaves.

(a)

The scientists wanted to know whether the accumulation of auxin over time in the stem base of the controls affected expression of the GH3 gene, known to have a role in growth regulation in different plants. The technique that was used to quantify the level of transcription of the GH3 gene was Northern blotting. In this procedure the darkness and thickness of the band is an indicator of the level of transcription of a particular gene. The image shows the result of the Northern blot from 2 hours to 24 hours after cutting.

[ 5 ]

(i)

State the name of the molecule which is produced by transcription.

[ 1 ]

(ii)

Compare the pattern of GH3 transcription with the pattern of auxin concentration in the stem base control cuttings. You may use the table provided to help you to record the patterns before you compare them. (Please note: a simple comparison in the table will not gain marks)

[ 2 ]

(iii)

The scientists concluded that auxin activates the transcription of the GH 3 gene. Using the information on the auxin concentration in the stem base in the graph on page 4 and the Northern blot, evaluate whether this conclusion is supported.

[ 2 ]

[Maximum number: 1]

The sequence of bases on a short section of the antisense strand of a gene undergoing transcription is shown:

5^{\prime} \text { CATG } 3^{\prime}

What is the sequence of bases on the resulting mRNA?

$3^{\prime}$ CATG $5^{\prime}$

5' GUAC 3'

$3^{\prime}$ GUAC $5^{\prime}$

$3^{\prime} \mathrm{GTAC}$ 5'

[Maximum number: 4]

Proteins play essential roles in all forms of life.

Outline how mRNA is translated in the cytoplasm of a cell.

[Maximum number: 1]

The graph shows the auxin concentration in the different leaves.

State the name of the molecule which is produced by transcription.

[Maximum number: 2]

Outline the functions of the two binding sites on the tRNA.

[Maximum number: 2]

The structure of a transfer RNA (tRNA) molecule is shown in the diagram. The attached amino acid, which is aspartic acid, is indicated by the arrow.

Explain how living organisms ensure that the amino acid linked to this tRNA molecule is always aspartic acid.

[Maximum number: 4]

There is a relationship between form and function in the diversity of molecules and of systems.

Describe the relationship between codons and polypeptide structure.

[Maximum number: 1]

Which sequence of mRNA bases and amino acids could be produced by transcription and translation of the DNA molecule shown?

3' AAAGTGGCACGTATATTT 5'
5' TTTCACCGTGCATATAAA $3^{\prime}$

Sequence of mRNA bases

Sequence of amino acids

UUU-GAG-GCU-CGA-UAU-UUU

Phe-Glu-Ala-Arg-Tyr-Phe

AAA-CUC-CGA-GCU-AUA-UUU

Lys-Leu-Arg-Ala-Ile-Phe

UUU-CAC-CGU-GCA-UAU-AAA

Phe-His-Arg-Ala-Tyr-Lys

AAA-GUG-GCA-CGU-AUA-UUU

Lys-Val-Ser-Arg-Ile-Phe

[Maximum number: 1]

The image shows a polysome in a bacterial cell.

How many genes have been transcribed?

[Maximum number: 8]

Explain the cause of sickle cell anemia and how this disease affects humans.

a. caused by a single nucleotide/base substitution mutation/GAG to GTG
b. «mutation of» a gene of $\beta$ -globin/a subunit of hemoglobin
c. mRNA copies the mutation of DNA and substitutes an amino acid in hemoglobin «subunit»
d. glutamic acid is substituted by valine
e. sickle cell anemia involves distorted hemoglobin protein/HbS
f. «distorted HbS causes» distortion/sickling/shape change of red blood cells
g. «distorted/sickled red blood cells» block capillaries/blood flow
h. HbS/sickled red blood cells cannot carry enough oxygen «for the body»/leads to fatigue
i. low oxygen concentration seriously affects structure of HbS
j. homozygous «HbS/HbS» state causes severe anemia/death at low oxygen concentrations
k. heterozygous state has less anemia/minor effects/less effect of structure of hemoglobin OR heterozygous state only affected at high altitude/extreme exercise/low levels of oxygen
I. «heterozygous state» provides protection against malaria parasite/selective advantage in malaria areas

[Maximum number: 1]

The electron micrograph shows part of a cell in the pituitary gland of a rat. Ribosomes appear as dark granules. Some of the ribosomes are arranged in a linear array and some are in circles. The diagram shows how the ribosomes in a circular array are connected.

Using the diagram, identify, with a reason, whether X or Y is the start codon.

Y is the start codon because translation goes from 5' to 3'
OR
Y is the start codon because it is in the $5^{\prime}$ end of the mRNA
OR
Y is the start codon because it is the first to enter the ribosomes;

[Maximum number: 2]

The diagram shows the tandem repeats on two different chromosomes for a couple and their children. The number of tandem repeats of the sequence AGAT is shown on chromosome 5 (Chr 5) and the sequence of AGAA on chromosome 18 (Chr 18). One child is adopted.

Explain the function of a promoter in DNA.

[Maximum number: 1]

Some regions of DNA act as telomeres or produce tRNA. State one other function of DNA sequences that do not code for protein.

[Maximum number: 1]

What happens to mRNA after transcription in eukaryotic cells?

Binding to the large subunit of the ribosome

Removal of introns

Addition of exons

Attachment of an amino acid

[Maximum number: 1]

The number of protein-coding genes in the human genome is estimated to be about 20000 , which is much less than the size of the proteome. What is one reason for this?

Exons are removed from RNA before translation.

There are more types of amino acids than nucleotides.

mRNA can be spliced after transcription.

Base substitutions occur during transcription.

[Maximum number: 3]

The structure of a transfer RNA (tRNA) molecule is shown in the diagram. The attached amino acid, which is aspartic acid, is indicated by the arrow.

During translation, three binding sites for tRNA molecules are used. Outline how each of these binding sites is used.

[Maximum number: 1]

Insulin production involves processing pre-proinsulin to proinsulin and then processing proinsulin to insulin. In a model cell, the second processing step is blocked while synthesis of pre-proinsulin and its conversion to proinsulin continue normally.

Which product is most directly expected to accumulate?

Proinsulin

Pre-proinsulin

Mature insulin

Unspliced insulin pre-mRNA

All question bank results loaded

a	mRNA attaches to a ribosome OR translation is initiated by assembly of ribosome, mRNA and tRNA;
b	translation happens by a repeated cycle of events OR ribosome moves along mRNA;
c	tRNA carrying an amino acid binds to ribosome / to A/P site (of ribosome);
d	anticodon on tRNA binds codon on mRNA;
e	complementary base pairing links anticodon to codon/ensures the amino acid corresponds to the codon OR base sequence determines amino acid sequence due to complementary base pairing;
f	peptide bond links amino acid (on tRNA) to the peptide chain already assembled OR peptide bond links amino acids (attached to tRNAs) on the P and A sites;
g	polypeptide released from ribosome when stop codon is reached;

a	is a non-coding/non-transcribed section of DNA/bases;
b	transcription starts/is initiated at the promoter;
c	region for binding (to DNA) of RNA polymerase OR region for RNA polymerase to separate the DNA strands;
d	located upstream of the gene/coding sequence;
e	helps to control/regulate gene expression;

Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17