Transcript Mutations Can Change the Meaning of Genes

Mutations Can Change the Meaning
of Genes
CH 11 Section 6
How Mutations Affect Genes
Mutation: any change in the nucleotide
sequence of DNA
 Types of Mutations:
 Base substitutions: replacement of one
nucleotide w/ another. May or may not affect
protein
 Base deletions & Base insertions:
 May be more harmful b/c all subsequent
codons will be altered

There are two general categories of gene mutation: base substitution and
base insertion (or deletion). The effect on the resulting polypeptide is shown
here, following substitution (a) and deletion (b).
What Causes Mutations?
Errors during DNA replication
 Errors during chromosome crossovers
 Mutagens: physical or chemical agents that
cause mutations

X-rays
 Ultraviolet light

Examples

The sickle-shaped cells clog tiny blood
vessels, dangerously blocking the normal
flow of blood. The molecular basis for the
disease lies in the difference of only one DNA
nucleotide out of a 438-base sequence.
Reviewing Concepts - Multiple Choice

1. The work of several scientists helped
to show that the hereditary material is
a. DNA.
b. proteins.
c. ribosomes.
d. codons.

2. The backbone of nucleic acid polymers
is composed of
a. nitrogenous bases and phosphates.
b. polypeptides.
c. sugars and phosphates.
d. nucleotides.

3. Excluding the stop sequence, how
many nucleotides are necessary to code
for a polypeptide that is 100 amino acids
long?
a. 33
b. 66
c. 100
d. 300

4. Which of the following occurs first
during the process of transcription?
a. Introns are removed and exons are
joined together.
b. Two DNA strands start to separate.
c. DNA polymerases join together
complementary base pairs.
d. tRNA translates codons.

6. Which of the following does not directly
participate in translation?
a. ribosomes
b. tRNA
c. mRNA
d. DNA

7. A geneticist found that a certain
mutagen had no effect on the polypeptide
coded by a particular gene. This mutagen
most likely caused
a. a silent mutation.
b. loss of one nucleotide.
c. addition of one nucleotide.
d. deletion of a gene.