DNA Replication, Repair, and Recombination: Mechanisms and Disorders

DNA Replication, Repair, and Recombination

Introduction to DNA Damage and Repair

For genetic information to be accurately passed on to daughter cells, DNA replication must be highly faithful. However, errors and damage can occur, necessitating robust repair mechanisms. Mutations may arise spontaneously or be induced by environmental agents, and their correction is essential for cellular health and genetic stability.

• Mutation: Any change in the DNA sequence; can be spontaneous or induced.

• DNA repair: Cellular processes that correct damage or errors in DNA to maintain genome integrity.

• Environmental agents: Factors such as chemicals or radiation that can cause DNA damage.

Spontaneous Mutations During Replication

Some mutations occur naturally during DNA replication due to transient errors or chemical instability of DNA bas es.

• Spontaneous mispairing of bases: Occurs due to transient formation of rare base tautomers, leading to nonstandard base pairing.

• Strand slippage: Especially in regions of repetitive DNA, can result in insertion or deletion mutations.

• Spontaneous damage to individual bases: Includes depurination and deamination events.

Tautomeric Shifts and Base Pairing Errors

DNA nucleotides can exist in rare tautomeric forms, which can pair incorrectly during replication, leading to mutations.

• Tautomer: Rare, alternate resonance structure of a nucleotide base.

• Tautomeric shift: Temporary change in base structure that allows nonstandard pairing (e.g., G-T or A-C).

• Result: Incorporation of incorrect base into the new DNA strand, potentially causing a permanent mutation after subsequent replication cycles.

• Example: If a guanine shifts to its rare tautomeric form, it may pair with thymine instead of cytosine.

Replication Errors in Repetitive DNA: Trinucleotide Repeats

Regions of repetitive DNA, such as trinucleotide repeats, are prone to strand slippage during replication, leading to repeat expansion disorders.

• Trinucleotide repeats: Short DNA sequences repeated multiple times in tandem (e.g., CAG, CGG).

• Strand slippage: DNA polymerase may replicate a segment twice, increasing repeat number.

• Trinucleotide repeat disorders: Diseases caused by expansion of these repeats, such as Huntington's disease.

• Example: Huntington's disease is caused by expansion of CAG repeats in the HTT gene.

Mechanism of Strand Slippage

• During replication, the growing daughter strand may detach and reattach incorrectly, forming a hairpin loop.

• This can result in the addition of extra repeat units in the daughter strand.

Table: Human Trinucleotide Repeat Disorders

Disease	Repeat Sequence	Normal Repeat Number	Disease Repeat Number	Primary Symptom
Fragile X syndrome	CGG	6–50	200–2000	Mental retardation
Friedreich ataxia	GAA	6–29	200–900	Loss of motor coordination
Huntington's disease	CAG	10–34	40–200	Uncontrolled movement, dementia
Myotonic dystrophy (type I)	CTG	5–37	80–1000	Muscle weakness
Spinal bulbar muscular atrophy	CAG	14–32	40–65	Muscle wasting
Spinocerebellar ataxia	CAG	4–44	45–140	Loss of motor coordination

Additional info: Trinucleotide repeat expansion is a dynamic mutation, meaning the number of repeats can increase in successive generations, leading to genetic anticipation (earlier onset and increased severity in offspring).