6.4.3 One gene giving more than one protein Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 6

The 20 questions

1. Alternative splicing of pre-mRNA means that:

   • The DNA sequence of the gene is altered by mutation each time it is transcribed into RNA
   • Different combinations of exons are joined together
   • Ribosomes read the mRNA in both directions, producing two proteins from the same reading frame
   • Two different genes are fused into one mRNA molecule, which is then translated into a single protein

2. Which molecule is changed to give rise to different proteins from a single gene?

   • Messenger RNA, through post-transcriptional processing such as alternative splicing
   • Transfer RNA, which is changed in its anticodon sequence before each translation step
   • Genomic DNA, which is rearranged in every cell so that a different gene is expressed
   • Ribosomal RNA, which is altered by the addition of different amino acids to its tail

3. The human genome contains a number of protein-coding genes that is:

   • Much larger than the number of different proteins the body produces, due to repeats
   • Equal to the total number of nucleotides in the genome divided by three exactly
   • Smaller than the number of different proteins that the body produces
   • Exactly the same as the number of different proteins the body produces in total

4. Which region of pre-mRNA is cut out during splicing and does not code for protein?

   • Promoters, which are removed so that the gene can be transcribed a second time
   • Exons, which are removed from the pre-mRNA and then translated into several different proteins
   • Codons, which are removed from the mRNA so that the reading frame shifts to a new position
   • Introns, which are removed so that only the exons remain in the mature mRNA

5. Which statement describes exons?

   • Segments of DNA that code for ribosomal RNA and are not transcribed into mRNA at all
   • Coding regions of pre-mRNA that are retained in the mature mRNA and may be joined in different combinations
   • Non-coding regions of pre-mRNA that are always removed completely from the mature mRNA
   • Proteins that bind to introns and cause them to be transported out of the nucleus

6. Why might a single gene produce a membrane-bound protein in one cell and a secreted protein in another?

   • The DNA is replicated in one cell type, which doubles the gene and so produces two proteins
   • The gene itself is mutated in one cell type, so it codes for a different protein in that cell
   • Different splicing patterns in each cell type can include or exclude exons that encode membrane anchoring regions
   • The ribosomes in one cell type translate the mRNA backwards, producing a different amino acid order

7. A gene has five exons. A particular splicing pattern includes exons 1, 2 and 4 only. How many exons are in the mature mRNA?

   • 4 exons, since exon 3 is the only one removed and the others are all retained
   • 2 exons, since splicing always removes two exons from the gene in any cell type
   • 3 exons, since only exons 1, 2 and 4 are retained
   • 5 exons, since every exon is always retained in the mature mRNA regardless of splicing

8. Which process happens after transcription but before translation in eukaryotic cells?

   • Phosphorylation of the amino acids, which adds phosphate groups to the completed polypeptide
   • DNA replication, which copies the gene so that two identical copies are present in the nucleus
   • Protein folding, which produces the tertiary structure of the protein in the endoplasmic reticulum
   • RNA splicing, which removes introns from the pre-mRNA and joins the exons together

9. Why is alternative splicing considered an important source of protein diversity?

   • Because it increases the number of genes in each cell, so more genetic information is present
   • Because it allows ribosomes to produce proteins from non-coding regions of the mRNA
   • Because it makes the DNA in each cell different from the DNA of the other cells in the body
   • Because a single gene can produce several mRNAs and therefore several related but distinct proteins

10. Which statement about post-transcriptional changes is correct?

    • They alter the DNA sequence of the gene before transcription begins in the nucleus
    • They alter the genetic code so that a single codon specifies more than one amino acid
    • They alter the mRNA after it has been made from the DNA template, without changing the gene sequence
    • They alter the sequence of the amino acids after the protein has been released from the ribosome

11. A researcher finds two different protein isoforms from the same gene. What is the most likely explanation?

    • Alternative splicing of the pre-mRNA produces two different mature mRNAs, each translated into one protein
    • The ribosomes have made a random mistake in translation, so both proteins are errors
    • The gene has been duplicated during meiosis, so there are now two copies of its protein
    • The gene has been transferred from another species, so it now codes for two proteins

12. Which of these is a true statement about the relationship between genes and proteins?

    • The relationship is many to many, since genes and proteins are unrelated in eukaryotic cells
    • The relationship is one to one, since every gene always produces exactly one protein in all cells
    • The relationship is not one to one
    • The relationship is one to many, since each protein is always coded for by more than one gene

13. Which statement evaluates the claim that every gene in the human genome gives rise to exactly one protein?

    • The claim is accurate, since the one-gene one-protein rule has never been challenged in any cell
    • The claim is inaccurate only for bacteria, since human genes always produce exactly one protein each
    • The claim is accurate, since splicing has no effect on the proteins that cells eventually make
    • The claim is inaccurate, since alternative splicing means many genes produce several protein variants

14. Which molecule carries the instructions that are translated after splicing?

    • Mature messenger RNA, which is exported from the nucleus to the cytoplasm
    • Ribosomal RNA, which is exported to the cytoplasm and carries the amino acid sequence
    • Transfer RNA, which is spliced and then carries the complete protein sequence to the ribosome
    • Pre-mRNA, which still contains introns and is translated directly in the nucleus

15. Which feature of the eukaryotic gene structure makes alternative splicing possible?

    • The absence of introns in the gene, which means that every exon must always be retained
    • The circular shape of the gene, which allows it to be read in multiple directions
    • The presence of plasmids within the gene, which allow it to replicate as independent units
    • The presence of introns and exons within the gene

16. Which statement describes how the immunoglobulin heavy chain gene is used in B cells?

    • The gene is copied into a new chromosome for each antibody class, which then encodes all antibodies
    • The gene is deleted from the B cell when it becomes a plasma cell, so no antibody is made
    • Alternative RNA processing gives membrane-bound and secreted forms of the same antibody chain
    • The gene produces only one antibody protein, which is then modified by antigen binding in the blood

17. Which result would most strongly support the idea that alternative splicing is a regulated process rather than a random error?

    • Individual cells show a random mixture of splice variants that changes from one hour to the next
    • Splice variants appear only in cells that have been damaged by radiation, and not in normal tissue
    • All tissues produce identical splice variants, which suggests that splicing is unrelated to tissue type
    • Different tissues consistently produce different splice variants of the same gene at predictable levels

18. Which of the following cannot be a direct effect of alternative splicing?

    • Producing a protein with a different C-terminal region from the same gene
    • Producing a shorter protein that lacks one functional domain present in the longer form
    • Producing a protein that is secreted rather than retained in the membrane
    • Changing the sequence of the gene's introns in the chromosome of the cell

19. A gene produces a 300-amino-acid protein in liver cells and a 180-amino-acid protein in brain cells. Which explanation is most consistent with this?

    • The liver cells use a different genetic code, in which fewer codons are needed for each amino acid
    • Different exons are included in the mRNA in each tissue
    • The gene is different in each tissue because the DNA is mutated in brain cells after birth
    • The brain cells translate the same mRNA twice, which removes 120 amino acids from the chain

20. Post-transcriptional changes to mRNA include capping and polyadenylation as well as splicing. What is the main effect of capping?

    • It removes all the exons from the mRNA so that only introns remain for translation
    • It adds an amino acid to the start of the protein so that the chain can be folded correctly
    • It joins two mRNAs together so that one ribosome can translate both simultaneously
    • It protects the mRNA from degradation and helps ribosomes recognise its start for translation

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