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'

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question 2