15. Gene Expression

Mutations

Imagine discovering a loss-of-function mutation in a eukaryotic gene. You determine the gene’s nucleotide sequence from the start site for transcription to the termination point of transcription and find no differences from the wild-type sequence. Explain where you think the mutation might be and how the mutation might be acting.

 No. Nonsense mutations are single base changes that convert an amino-acid-encoding codon into a stop codon. 

 No. The question does not describe an inversion. 

 No. Silent mutations have no phenotypic effects. 

Consider how an insertion or a deletion would affect triplets.

A point mutation in which a single base pair is inserted or deleted from DNA is called a(n) __________.

 the DNA to break up into thousands of short segments 

 No. The DNA itself will probably not break apart. 

 incorrect pairing between mRNA codons and amino acids 

 No. Specific base pairing would not be affected. 

 no bad effects, as long as the stop codons are not also inserted into tRNA 

 No. tRNAs contain anticodons, not codons. 

 manufactured proteins to be short and defective 

Consider how the resulting polypeptides would be affected.

A virus infects a cell and randomly inserts many short segments of DNA containing a stop codon throughout an organism's chromosomes. This will probably cause __________.

Which of the following describes mutations? Select True or False for each statement.
T/F Point mutations can occur in any DNA sequence.
T/F Frameshift mutations can occur in any DNA sequence.
T/F Neutral mutations depend on the degeneracy of the genetic code.
T/F Deleterious mutations occur only in protein-coding sequences of DNA.

 No. UV radiation acts directly on the DNA. 

 No. Although UV radiation can damage nucleic acids and may therefore hinder translation, it is unlikely to stop translation altogether. 

 deactivates the enzymes needed for DNA replication 

Consider that exposure to UV radiation increases skin cancer risk.

Ultraviolet (UV) radiation is damaging to cells because it __________.

Animation: Mutation Types

Which of the following mutations would be most likely to have a harmful effect on an organism?
a. a deletion of three nucleotides near the middle of a gene
b. a single nucleotide deletion in the middle of an intron
c. a single nucleotide deletion near the end of the coding sequence
d. a single nucleotide insertion downstream of, and close to, the start of the coding sequence

Which of the following mutations, occurring just after the start codon in the mRNA is likely to have the most serious effects on the polypeptide product?

 No. This would change the reading frame. 

 No. Insertion of three bases into a gene results in the addition of one amino acid to the resulting protein, but leaving the rest of the amino acid sequence intact. The effect of the additional amino acid will vary but will certainly be less dramatic than a frameshift mutation resulting from insertion or deletion of a single base. 

Consider which of these would have the least impact on the triplet code.

Which type of mutation is least likely to affect protein function?

Causes an incorrect amino acid to be inserted into a polypeptide chain.

Causes synthesis of a polypeptide chain to be terminated prematurely.

Only affects the mRNA code but does not affect the amino acid sequence of the polypeptide.

 No. This would change the reading frame, likely introducing many amino acid changes and stop codons into the coding sequence. 

 No. This would change an amino acid in the encoded protein, potentially altering or even destroying its function. 

 No. This would change the reading frame and likely introduce many amino acid changes and stop codons into the coding sequence. 

Consider the nature of the genetic code.

A particular mutation had no effect on the polypeptide encoded by a gene. This mutation probably involves __________.

Explain what’s wrong with this statement: All point mutations change the genotype and the phenotype.

Types of Mutations

The carnivorous plant Nepenthes bicalcarata (“fanged pitcher plant”) has a unique relationship with a species of ant—Camponotus schmitzi (“diving ant”). The diving ants are not digested by the pitcher plants, but instead live on the plants and consume nectar. Diving ants also dive into the digestive juices in the pitcher, swim to the bottom, and capture and consume trapped insects, leaving uneaten body parts and ant feces behind. What nutritional impact do the ants have on fanged pitcher plants? Do the pitcher plants derive any nutritional benefit from this relationship?

Nitrogen is a key nutrient often obtained by carnivorous plants from the insects they digest. Are the results presented here what would be expected if nitrogen is a limiting nutrient? Explain.

If a base-pair change occurs in DNA, this
a. is a mutation.
b. would be a mutation only if it falls in a protein-coding part of a gene.
c. would be a mutation only if it falls in a transcribed part of the genome.
d. is not a mutation, because only one base pair has been altered.

Skin color is often one of the first traits people notice in each other. Studies in zebrafish uncovered a mutation that altered a transport protein and resulted in light-colored fish. This discovery led to the finding that the same gene in humans has a strong influence on skin pigmentation in many populations. The zebrafish mutation that reduced coloration created a null allele of the transport protein gene. Which of the following types of mutation would be most likely to create this null allele?
a. a missense mutation
b. a frameshift mutation
c. a neutral mutation
d. a silent mutation

HHMI BioInteractive: Sickle-Cell Anemia

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General Biology

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Jason Amores Sumpter

1. Introduction to Biology

2. Chemistry

3. Water

4. Biomolecules

5. Cell Components

6. The Membrane

7. Energy and Metabolism

8. Respiration

9. Photosynthesis

10. Cell Signaling

11. Cell Division

12. Meiosis

13. Mendelian Genetics

14. DNA Synthesis

16. Regulation of Expression

17. Viruses

18. Biotechnology

19. Genomics

20. Development

21. Evolution by Natural Selection

22. Evolution of Populations

23. Speciation

24. History of Life on Earth

25. Phylogeny

26. Prokaryotes

27. Protists

28. Plants

29. Fungi

30. Overview of Animals

31. Invertebrates

32. Vertebrates

33. Plant Anatomy

34. Vascular Plant Transport

35. Soil

36. Plant Reproduction

37. Plant Sensation and Response

38. Animal Form and Function

39. Digestive System

40. Circulatory System

41. Immune System

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