Biology 100/101
Lecture 15: Mutations
Text readings in Life by Ricki Lewis:
Chapter 14, Chromosomes, pp. 297-304
Chapter 15, DNA Structure and Replication, pp. 319-322
Chapter 16, Gene Function, pp. 338-345
Review questions:
Pg. 305; question 11
Pg. 347; questions 15 and 16
"To think about":
Pg. 347; questions 2, 5, 9, and 10
Answers to many of these questions can be found on the "Answers to End-of-Chapter Questions" page at the text website.
For feedback, post possible answers and ideas in the folder "Text 'Review' and 'To Think About' Questions" in the Biology Chat Section of Web Crossing.
Web resources:
Objectives:
After studying this material you should be able to:
- Define the term mutation and describe how mutations can occur.
- Explain the causal relationship between mutation and inherited conditions involving protein abnormalities like sickle cell disease, cystic fibrosis, and lactose intolerance.
- Distinguish between germinal and somatic mutations and describe the consequences of each for a person's children.
- Describe the types of mutations that can occur in a gene and the
effect, if any, they have on the protein that is produced when the gene
is expressed.
What are mutations and what causes them?
A mutation is any physical change in the genetic material (such as a gene or a chromosome). A mutation is not necessarily bad (it may even be good).
Chromosomal Mutations
- Changes in chromosome structure (Access Excellence)
- Deletions
- Duplications
- Inversions
- Translocations
- Gene Mutations Changes to the Sequence of Nucleotides
(A,C,G,&T) of the DNA in a Chromosome
Causes of Mutation:
-
Spontaneous mutations
- DNA sequences are altered for no apparent reason, usually due to a DNA replication error.
- Spontaneous mutations are more rapid in those organisms with very short generation times than in those with long generations.
- Chromosome replication is 99.999% accurate. Errors in the DNA replication process happen only about once in 100,000 bases.
- However, the human genome has about 6 billion bases. That means each replication cycle (the S phase in each cell cycle) will have 6,000 errors associated with it!
- DNA repair is also possible. Indeed, it is the rule.
- Cells contain several complex systems to fix damage before, during, and after replication. As a result, DNA replication and repair disorders are pretty rare (see text, pp. 319-322).
- Induced Mutations
- exposure to chemical mutagens
- exposure to ionizing radiation, including ultraviolet rays or radioactivity
- Note: UV light is a mutagen that you should think about before hitting the beaches or the tanning salons to get your suntan.
Somatic Mutations and Germinal Mutations
- Somatic (Gk. soma, body) Mutations - mutations in the body cells of an organism, include any type of cell EXCEPT the cell lines destined to produce sperms or eggs by meiosis.
- Somatic mutations early in development may affect the entire development process of the organism.
- Somatic mutations may result in unusual growth, e.g. cancer.
- Somatic mutations result in the cells of an individual not being entirely genetically uniform.
- Germinal (Gk. germinare, to sprout) Mutations in cells destined to produce gametes may occur earlier during development to produce the sex organs or even during meiosis.
- Germinal mutations result in genetically altered gametes and may be passed on to the individual's offspring.
- Germinal mutations may not affect the individuals in which they occur, but may result in genetic disorders in their offspring.
Mutations in genes and their effect, if any, on the resultant protein
There are many ways that mutations can occur and affect gene expression! To understand them, you need to refamialiarize yourself with the use of the genetic code (See Lewis text, p. 333).
- Point mutations - changes in single DNA nucleotides.
| Transcribed Strand | ACGTTACG --> | ACGTTGCG |
| Non-transcribed Strand | TGCAATGC --> | TGCAACGC |
- A missense mutation substitutes a different amino acid for the original one.
- A nonsense mutation results in a stop codon being inserted someplace brfore the end of the gene.
- How will this affect the amino acid content of the potein?
- Silent mutations are point mutations that do not change the amino acid sequence of the protein.
- How can there be a change in the DNA WITHOUT a change in the protein?
- Change in numbers of nucleotides - additions or deletions of one or more nucleotides.
- Frameshift mutations result in "garbage" genes. The entire amino acid sequence in the code after the change is devastated.
- Frameshifts may occur because of either additions or deletions of bases in the gene.
- How many bases must be added or deleted to cause a frameshift?
- How many bases may be added or deleted and cause the addition or deletion of one or more amino acids WITHOUT causing a frameshift?
- Larger deletions may just remove a single amino acid, or an entire chunk of chromosome.
- Some genes have repeated 3 base sequences, and the number of these may increase in each generation. These expanding genes are responsible for increasingly severe cases of muscular dystrophy (CTG repeats), Huntington disease (CAG repeats) and Fragile X syndrome (CGG repeats).
How will these expanding genes affect the amino acid make up of the protein?
Practice making your own Mutations
- Relationship of DNA, mRNA, and amino acid sequences from the University of Virginia
- Alter the DNA base sequences in the example above, and predict the resulting changes in the mRNA and Amino acid sequences. Can you illustrate all the types of mutations listed above?
Word Analogies for types of Mutations
Table 16.7 (text, p. 343) uses a sentence of three-letter words as an analogy to demonstrate the effects of mutations on gene sequence.
Wild type
"Normal Gene" |
THE ONE BIG FLY HAD ONE RED EYE |
|---|
| Missense |
THQ ONE BIG FLY HAD ONE RED EYE |
| Nonsense |
THE ONE BIG |
| Frameshift |
THE ONE QBI GFL YHA DON ERE DEY |
| Deletion |
THE ONE BIG HAD ONE RED EYE |
| Duplication |
THE ONE BIG FLY FLY HAD ONE RED EYE |
| Insertion |
THE ONE BIG WET FLY HAD ONE RED EYE |
| Expanding |
(P) THE ONE BIG FLY HAD ONE RED EYE |
| Expanding |
(F1) THE ONE BIG FLY FLY FLY HAD ONE RED EYE |
| Expanding |
(F2) THE ONE BIG FLY FLY FLY FLY FLY FLY HAD ONE RED EYE |
Some Mutation Examples
Mutations can be not-so-bad and even good.
- Mutations may occur in non-coding regions of DNA.
- The amount of DNA you have is much greater than that accounted for by your genes. Even with 50,000 proteins and a daily production rate of billions of protein molecules, the vast majority of your DNA is not involved in protein coding.
- Within an allele, as much as 95% of the DNA is non-coding. Introns get clipped out before protein synthesis starts.
- Mutations in non-coding regions usually do nothing to the phenotype of the individual.
- Within the coding regions of alleles, some types of mutations have no effect on the resulting proteins.
- Mutations do much more than cause genetic disorders or cancers. They do not occur only in humans.
- Mutations are a way to introduce new alleles into a population. This means, mutations increase the genetic variability of a population.
- Genetic variability is essential to the process of evolution and the formation of new species. Without mutations in the genetic material of some earlier primate species, we would not be here.
Take me away.