1. Module 7

1.14. Page 2

Lesson 4

Module 7—Molecular Genetics: DNA, RNA, and Protein Synthesis

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mutation: a permanent change in the genetic code (DNA) of a cell

 

somatic cell mutation: mutation that occurs in a body cell; passed on to daughter cells, but not to the next generation of individual organisms

 

germ line mutation: mutation that occurs on a gamete and can be passed to the next generation

 

point mutation: permanent change in the genetic material of a cell that affects one or just a few nucleotides

 

chromosomal mutation: mutation that involves the deletion, insertion, or crossing over of chromosomes

 

substitution: a type of point mutation in which one nucleotide is switched for another nucleotide in a DNA sequence

 

insertion: a type of point mutation in which one nucleotide is added to the DNA sequence, causing a frameshift mutation

 

deletion: a type of point mutation in which one nucleotide is removed from a DNA sequence, causing a frameshift mutation

The structure of DNA is not permanent. It is actually constantly changing. Enzymes like DNA polymerase repair many changes immediately, but many changes are missed and not fixed. If a change in the genetic code is permanent, it is called a mutation. If a mutation occurs in a body cell (somatic cell mutation), the mutation will be passed on to daughter cells when the cell divides.

 

Somatic mutations usually affect only the individual organism. When a mutation occurs in the DNA of a gamete cell (called a germ line mutation), this mutation can be passed on to the next generation of organisms. Germ line mutations are one way in which genetic variation in a population can occur. You will examine two types of mutation that can occur: point mutations and chromosomal mutations.

 

Point mutations occur when one of the following occurs:

  • substitution of one nucleotide for another
  • insertion of one or more nucleotides
  • deletion of one or more nucleotides

Substitution

 

The effect of a substitution will depend on the actual nucleotide substituted and the subsequent effect on the protein. As you have seen in “Table 18.3” on page 637, the same amino acid may have several slightly different codes. This feature is called redundancy. A change in one of the nucleotides does not necessarily result in a different amino acid being used in the protein change; however, sometimes a different amino acid does result. Study the following examples noting the effects of a substitution mutation.

 

Example A

 

Example A is the normal mRNA nucleotide sequence and coding amino acid sequence.

 

UGC  AUA  AAU GGC ← mRNA

cys – iso – asp – gly ← amino acid sequence

 

Example B

 

In Example B, a C nucleotide was substituted for the A nucleotide in the second codon triplet. Both the original triplet, AUA, and the mutated triplet, AUC, code for the same amino acid, isoleucine. This type of mutation does not affect the amino acid chain or the protein produced.

 

UGC  AUC  AAU GGC

cys – iso – asp – gly

 

Example C

 

In Example C, the substitution of U for A in the second triplet causes a change in the amino acid in the sequence. This type of mutation can cause the protein to be less effective or to not function appropriately for a process. This is the type of mutation that causes the harmful sickle cell anemia disease. This type of mutation could also produce new types of proteins that might meet different needs.

 

UGC  UUA  AAU GGC

cys –  leu –  asp –  gly

 

Example D

 

In Example D, the substitution does not allow the protein to function at all. In Example D, the amino acid sequence is terminated, and the protein will be cut short and will be non-functioning.

 

UGA  AUA  AAU GGC

Stop*

 

frameshift mutation: permanent change in the genetic material of a cell caused by the insertion or deletion of one or two nucleotides, so that the entire reading frame of the gene is altered

Example E

 

Insertion and deletion mutations can cause a frameshift mutation. These mutations cause the entire reading frame of the gene to be altered or to shift. The result is a series of new codons for different amino acids and leftover nucleotides. Study Example E to observe the consequences of insertion. Deletion will have similar consequences in coding amino acids.

 

UGC  AUA  AAU GGC      NORMAL original

cys –  iso –  asp –  gly

 

UGC  AUGA  AAU GGC    mRNA after insertion of one nucleotide

UGC AUG AAA UGG C     frameshift caused by insertion, resulting in a new amino acid sequence

 

cys – met – lys - trp

 

Chromosomal Mutations

 

Chromosomal mutations are another type of mutation. As you discovered in Module 5, these mutations occur when chromosomes cross over and recombine genetic material. Other chromosomal mutations can occur if part of the chromosome is lost or duplicated during DNA replication.

 

Recall that non-disjunction can also result in major differences. In non-disjunction, chromosomes are not segregated correctly. The resultant cells can have too many (trisomy), or not enough (monosomy) chromosomes.

 

Read the section “Types of Mutations” on pages 643 to 644 of your textbook to review these different types of mutations. Choose how you will summarize this information for your course folder.

 

Self-Check

 

To confirm your understanding, complete the following questions. If you had trouble with these questions, go to the next page and watch the video in the Watch and Listen section now and review the questions again. The video may explain mutations in a different way that could help your understanding. If you still have questions, consult with your teacher.

 

SC 1. What feature of the genetic code helps to protect a cell from the effects of nucleotide substitution?

 

SC 2. What is a frameshift mutation?

 

SC 3. Why is a mutation caused by an insertion or a deletion more likely to have serious consequences for a cell than one caused by a substitution?

 

SC 4. One mutation results in the replacement of a G nucleotide with a T nucleotide in the sense strand of a DNA molecule. Under what circumstances will this substitution produce each of the following situations?

  1. no change in the production of a protein
  2. a new and different protein is produced that doesn’t function

SC 5. Explain the difference between a germ line mutation and a somatic cell mutation. Which type of mutation contributes more to the variations among organisms?

 

Check your work.
Self-Check Answers

 

SC 1. The redundancy of the genetic code protects a cell from the effects of substitution mutations. A change in the coding sequence of a gene does not always result in a change to the polypeptide product of a gene.

 

SC 2. The insertion or deletion of one or two nucleotides results in a frameshift mutation. A frameshift mutation causes the entire reading frame of the gene to be altered. A shift in the reading frame usually results in a nonsense mutation.

 

SC 3. A frameshift mutation occurs when a gene is altered by the insertion or deletion of one or two nucleotides, and it is usually more serious than a substitution mutation. A frameshift mutation causes the entire reading frame of the gene to be altered. The resulting shift in the reading frame usually causes a mutation that results in a nonfunctional protein. On the other hand, a substitution mutation of a single nucleotide may lead to no effect because of the redundancy of codes, to a slightly altered but still functional polypeptide, or to a mutation that can be harmful but generally less so than frameshift mutations.

 

SC 4.   

  1. No change in the protein results when the substitution still results in coding of the same amino acid.

  2. The substituted nucleotide resulted in a different amino acid occurring in the sequence producing the protein. A different protein was produced that may be less effective or may not be effective at all.

SC 5. A germ line mutation occurs in the gamete and is passed to the next generation of individuals. A somatic mutation occurs in body cells and cannot be passed to the next generation of individuals. The mutation can only be passed along that cell line.