6.3 DNA Mutations. SBI4U Ms. Ho-Lau

Transcription

1 6.3 DNA Mutations SBI4U Ms. Ho-Lau

2 DNA Mutations Gene expression can be affected by errors that occur during DNA replication. Some errors are repaired, but others can become mutations (changes in the nucleotide sequence of a cell s DNA) All cells undergo spontaneous mutation Mutagens: substances or events that increase the rate of mutation

3 DNA Mutations in reproductive cells can affect the next generation in somatic cells can affect daughter cells can be neutral or harmful in rarer cases, they are beneficial important in terms of species change and adaptation

4 DNA Mutations 2 categories: single-gene mutations chromosome mutations

5 Single-gene Mutations involve changes in the nucleotide sequence of one gene categorized according to how they affect the amino acid sequence: 1. Silent mutations 2. Missense mutations 3. Nonsense mutation categorized according to how they affect the nucleotide sequence: 1. Point mutations 2. Frameshift mutations

6 1. Silent Mutations one nucleotide is changed But does not change the amino acid sequence of the protein

7 2. Missense Mutations one nucleotide is changed changes the amino acid sequence of the protein usually harmful Occasionally, produce a protein that helps with survival

8 3. Nonsense Mutations Introduced a premature stop codon Shorten the mrna Shorten the protein Usually harmful

9 Single-gene Mutations involve changes in the nucleotide sequence of one gene categorized according to how they affect the amino acid sequence: 1. Silent mutations 2. Missense mutations 3. Nonsense mutation categorized according to how they affect the nucleotide sequence: 1. Point mutations 2. Frameshift mutations

10 1. Point Mutations result from a change in a single base pair within a DNA sequence can involve substitution, addition, or deletion of one nucleotide A substitution may not affect the protein due to the redundancy built into the genetic code.

11 2. Frameshift Mutations result from the insertion or deletion of nucleotides not divisible by three causes a change in the reading frame

12 DNA Mutations 2 categories: single-gene mutations chromosome mutations

13 Chromosome Mutations Changes in chromosomes and many genes 1. Deletion: a series of nucleotides are deleted from the chromosome 2. Duplication: a series of nucleotides are duplicated on the same chromosome 3. Inversion: a chromosome segment is broken and re-inserted in the opposite direction 4. Reciprocal Translocation: a group of nucleotides from one chromosome are exchanged with the nucleotides of a different chromosome

14 Chromosome Mutations

15 Causes of Mutations Mutations may be spontaneous or induced.

16 Causes of Spontaneous Mutations the result of normal molecular interactions incorrect base pairing by DNA polymerase during replication transposition, in which specific DNA sequences (called transposons) move within and between chromosomes

17 Causes of Induced Mutations 1. physical mutagens an event or substance such as high-energy radiation (X rays and UV rays) that physically change the structure of DNA example, UV radiation distorts the DNA molecule at adjacent C and T bases, causing replication interference.

18 Causes of Induced Mutations 2. chemical mutagens molecules that can enter the nucleus of a cell and induce mutations by reacting with DNA Example: nitrites and chemicals in cigarette smoke Some mutagens have a structure similar to DNA nucleotides and can incorporate into a DNA strand, causing incorrect base pairing during replication.

19 DNA Repair DNA polymerases and mismatch repair mechanisms help to repair replication errors Two additional types of DNA repair mechanisms are: specific repair mechanisms: tailored to fix certain types of damage (eg. photorepair) non-specific repair mechanisms: can correct different forms of damage (eg. excision repair)

20 Specific Repair Photorepair is a specific mechanism to repair damage to DNA caused by exposure to UV radiation. A photolyase enzyme recognizes the damage, binds to the dimer, and uses visible light to cleave the dimer.

21 Non-specific Repair Excision repair is a non-specific mechanism of DNA repair because it can fix a variety of damage.