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Announcements
Objectives
Web Resources
What are Mutations?
Types of Mutations
Effects on Proteins
Word Analogies
Lecture Activity
Causes of Mutations
Somatic/Germinal Mutations
Not All Bad
Lecture
Syllabus
IB 100/101 Home
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Announcements
Text Readings in
Lewis et al. |
Testing Your Knowledge |
Thinking Scientifically |
| Chapter 13, Gene Function, pp. 255-260 |
Page 265, Questions 8-9 |
Page 266, Question 2 |
Answers to many of these questions can be found at the Text On-Line Learning Center
You may also ask questions and see answers to your classmates'
questions in Web Crossing in the "Talk to Jim and Ed" discussion.
Objectives:
After studying this material you should be able to:
- Define the term mutation.
- 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.
- Describe how a mutation might occur by distinguishing between
spontaneous and induced mutations.
- Distinguish between somatic and germinal mutations and describe the
consequences of each for a person's child.
- Explain why mutations are not all harmful.
- Understand these terms:
| induced mutation
| spontaneous mutation
| germinal mutation
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| somatic mutation
| missense mutation
| nonsense mutation
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| frameshift mutation
| mutagen
| silent mutation
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Web Resources:
What are Mutations?
A mutation
is any physical change in the genetic material (such as a gene or a
chromosome). When a gene contains a mutation, the protein encoded by
that gene will be abnormal. Some protein changes are insignificant,
others are disabling.
More than 4,000 diseases are thought to stem from mutated genes
inherited from our parents.
A mutation may or may not affect the phenotype.
A mutation is not necessarily bad. It may even be good.
General Types of Mutations
Chromosomal Mutations
Point Mutations
- Changes made by substituting a single base with another or by adding
or deleting one or more nucleotides.
Genetic Mutations and their Effects on Proteins
A
Review of Protein Synthesis, by Access Excellence.
Gene
Expression via Protein Synthesis, from Access Excellence. For a
cell to make protein, the information from a gene is copied, base by
base, from DNA into new strands of messenger RNA (mRNA). Then mRNA
travels out of the nucleus into the cytoplasm, to cell organelles called
ribosomes. There, mRNA directs the assembly of amino acids that fold
into a completed protein molecule.
How
are genes linked to disease? When a gene contains a mutation, the
protein encoded by that gene may well be abnormal.
There are many ways that mutations can occur and affect gene
expression. To understand them, you need to familiarize yourself with
the use of the
Genetic Code. The same Code is found in Table 13.2 in your text.
THE GENETIC CODE CHART USES THE CODONS IN THE mRNA!!!!!!
Point Mutations: Changes in single DNA
nucleotides.
- A missense mutation substitutes a different amino acid for
the original one.
- A nonsense mutation results in a stop codon being inserted
someplace before the end of the gene.
- Silent mutations are point mutations that do not change the
amino acid sequence of the protein. These are most likely to have no
effect. Redundancy of the Genetic Code reduces the chance that point
mutations do not alter the specified amino acids.
The mRNA codons GAA and GAG code for the amino
acid Glutamic Acid (Glu).
The mRNA codons GCU, GCC, GCA, and GCG all code
for the amino acid Alanine (Ala).
The mRNA codons GGU, GGC, GGA, and GGG all code
for the amino acid Glycine (Gly).
- Frameshift Mutations: Additions or deletions of one or more
nucleotides.
- May result in "garbage" genes, as the entire amino acid sequence in
the code after the change is devastated.
- Large deletions may remove a single amino acid, or an entire chunk
of chromosome. The most common mutation that causes severe cystic
fibrosis deletes only a single codon.
- Real examples of missense, nonsense, and frameshift mutations:
Hemoglobin
mutants and Hemoglobin
molecule
A note of caution. These examples show the NON-template DNA sequence
rather than the template DNA sequence as in our previous examples. This
is a standard used by DNA scientists. To get the mRNA codons, just
change the Ts to Us.
- Some genes have repeated base sequences, and the number of these may
increase 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).
Fragile X Syndrome:
6-50 CGG repeats in an unaffected
individual
50-200 CGG repeats in a carrier
>200 CGG repeats in
an affected individual
Word Analogies for Types of Mutations
Table 13.4 (text, p. 260) uses a sentence of three-letter
words as an analogy to demonstrate the effects of mutations on gene
sequence.
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