DNA replication occurs with high accuracy, making only about 1 error per billion base pairs. Errors are corrected by proofreading enzymes. Mistakes that escape correction are called mutations, which can be harmful, neutral, or beneficial by allowing for genetic variation. Mutations include point mutations that change a single base pair and frameshift mutations that delete or insert bases, altering all subsequent codons and usually causing greater effects. Mutations accumulate over time and can lead to cancer, aging, or beneficial species adaptations.

1. DNA Replication:
Accuracy and Repair

2. Chargaff’s Rule
 The complementary nature of the two
chains/strands of DNA is the foundation for
accuracy in DNA replication.
A ---- T
C ---- G
T ---- A
G ---- C

3.  DNA replication actually occurs simultaneously at
different sites along DNA.
 This makes the process faster.
 Eukaryotic Prokaryotic

4.  DNA replication occurs with a high degree of
accuracy.
 ~ 1 error per 10,000 base pairs
 These errors are proofread and corrected by
enzymes so that the error rate is actually
 ~ 1 error per 1 billion base pairs

5. Mistakes in DNA Replication
 Mistakes in DNA replication are called
 MUTATIONS
 Mutations Clip
 Mutations may be harmful, neutral, or
beneficial.
 ??? How can a mutation be beneficial???
 Mutations allow for variation, which may lead to
better ability to survive and reproduce.

6. DNA Mutations
 A mutation simply means there is a change in
a base (A, T, C, or G), or the addition or
deletion of a base.

7. Types of mutations
 Point mutations
 Aka: substitutions
example:
G-C G-C
A-T T-A
A-T A-T
C-G C-G
This change affects only one base pair.
T

8. Frame-shift mutations
1. Deletion
original DNA mutated DNA
C-G C-G
T-A T-A
A-T A-T
C-G
C-G
A-T

9. Frame-shift mutations
2. Insertion
original DNA mutated DNA
C-G C-G
T-A T-A
A-T A-T
A-T
C-G A-T
C-G
T-A

10. Effect of DNA Mutations
 Which do you think would cause the greatest
change in the resulting protein?
 Point mutations
 Change one codon, not necessarily changing the
amino acid (therefore protein)
 WHY?
 Frameshift mutations
 Causes a change in every codon after that!

11. Causes of mutations
 Chemicals (toxins)
 ie: Carcinogens
 Radiation
 ie: ultraviolet (UV), x-rays, nuclear
 Viruses
 All of these are called MUTAGENS.

12. Ultimately…
 Mutations accumulate over time…
 may cause cancer
 may be related to aging
 may lead to beneficial adaptations in species.
 Mutations can be linked with genetic
disorders that are passed on from one
generation to the next.
 Ex: Tay Sach’s (a point mutation)