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SCAR (Sequence Characterized Amplified Region) Markers


SCAR (Sequence Characterized Amplified Region) Markers 

Introduction


SCAR markers are PCR-based DNA markers derived from RAPD, AFLP, or other random markers.
Developed by Paran and Michelmore in 1993 to convert dominant, less reproducible markers into specific, reproducible, co-dominant markers.
SCAR markers are locus-specific, reproducible, and sequence-characterized, making them ideal for marker-assisted selection (MAS).
Principle
SCAR markers are designed based on known DNA sequences obtained from cloned RAPD/AFLP fragments.
Specific primers (18–24 bp) are synthesized to amplify a single, defined locus.
The PCR amplification of this region generates a distinct band, which is highly reproducible and can distinguish homozygotes from heterozygotes if designed as co-dominant.


Key idea:


Random marker (e.g., RAPD) → Cloning & sequencing → Design specific primers → PCR → SCAR marker
Materials Required
Genomic DNA from the organism
Specific primers (18–24 bp) designed from sequenced fragments
PCR reagents: Taq polymerase, dNTPs, buffer, Mg²⁺
Thermal cycler
Agarose or polyacrylamide gel for PCR product separation
DNA staining dye (ethidium bromide, SYBR Green)


Procedure


Step 1: Identification of a Random Marker
Use RAPD, AFLP, or other markers to find polymorphic fragments linked to a trait or locus of interest.

Step 2: Cloning and Sequencing

Excise the polymorphic fragment from the gel
Clone it into a plasmid vector
Sequence the DNA fragment

Step 3: Primer Design

Design long, specific primers (18–24 nucleotides) flanking the sequenced fragment.
Primers are locus-specific, ensuring specific amplification.

Step 4: PCR Amplification


Set up PCR with genomic DNA, specific primers, Taq polymerase, buffer, and dNTPs
PCR cycles: Standard denaturation, annealing (50–60°C), and extension (72°C)


Step 5: Gel Electrophoresis and Detection


Run PCR products on agarose or polyacrylamide gel
Visualize bands with ethidium bromide or other DNA dye
Presence/absence of expected fragment indicates SCAR marker polymorphism


Diagram in Words


RAPD/AFLP marker → Polymorphic fragment identified
      ↓
Fragment cloning → DNA sequencing
      ↓
Design SCAR primers (18–24 bp)
      ↓
PCR with genomic DNA
      ↓
Specific band detection on gel → Locus-specific marker.



Characteristics of SCAR Markers


Co-dominant or dominant
Locus-specific and reproducible
Sequence-characterized
Based on PCR, no need for restriction digestion or hybridization
Highly reliable for marker-assisted breeding


Merits

Highly reproducible compared to RAPD and AFLP
Locus-specific: Amplifies a single defined DNA fragment
Can be co-dominant, allowing identification of heterozygotes
Useful for MAS, gene mapping, variety identification
PCR-based, fast, and relatively simple


Limitations


Requires initial identification of polymorphic fragment (from RAPD/AFLP)
Sequencing and primer design are labor-intensive
Costlier than simple RAPD markers
Limited by availability of polymorphic sequences linked to trait
Cannot detect random genome-wide variation like RAPD or AFLP


Applications


Marker-Assisted Selection (MAS) in plant and animal breeding
Variety and cultivar identification
Genetic mapping of traits or disease resistance genes
Detection of disease resistance genes in crops
Verification of hybrids and backcross progeny
Can be applied in forensic studies where high specificity is needed



Applications


Genome-wide survey
MAS, gene mapping, hybrid verification
Conclusion
SCAR markers convert random, less reproducible markers into locus-specific, reliable markers.
They are ideal for molecular breeding programs, especially in marker-assisted selection and trait mapping.
Main limitation is the need for sequencing and primer design, but their accuracy and reproducibility make them highly valuable in genetics and breeding.


SCAR Markers – 50 MCQs with Answers




1. SCAR stands for:
A. Sequence Characterized Amplified Region
B. Single Copy Amplified Region
C. Short Conserved Amplified Repeat
D. Specific Cloned Amplified Region
Answer: A
2. SCAR markers are derived from:
A. RFLP fragments
B. RAPD or AFLP fragments
C. SSR sequences
D. Protein markers
Answer: B
3. SCAR markers are:
A. Random markers
B. Locus-specific markers
C. Morphological markers
D. Biochemical markers
Answer: B
4. SCAR markers are usually:
A. Dominant
B. Co-dominant
C. Both dominant and co-dominant
D. Multi-allelic
Answer: C
5. SCAR markers require:
A. No prior DNA sequence info
B. DNA sequence information for primer design
Answer: B
Principle and Procedure
6. SCAR marker principle is based on:
A. Random amplification
B. PCR amplification using specific primers
C. Protein digestion
D. RNA hybridization
Answer: B
7. SCAR primers are typically:
A. 10 nucleotides
B. 18–24 nucleotides
C. 30 nucleotides
D. 50 nucleotides
Answer: B
8. SCAR markers are converted from:
A. Co-dominant markers
B. Dominant markers like RAPD
C. SSR markers
D. RFLP only
Answer: B
9. First step in SCAR marker development is:
A. Design primers
B. Identify polymorphic fragment using RAPD or AFLP
C. PCR amplification
D. Gel electrophoresis
Answer: B
10. SCAR marker PCR is used to:
A. Amplify a specific locus
B. Digest DNA
C. Hybridize RNA
D. Detect proteins
Answer: A
Technical Features
11. SCAR markers are:
A. Highly reproducible
B. Random and unreliable
Answer: A
12. SCAR markers are useful because they are:
A. Locus-specific
B. Genome-wide
Answer: A
13. SCAR markers can distinguish:
A. Homozygotes only
B. Heterozygotes only
C. Both homozygotes and heterozygotes
D. Only dominant alleles
Answer: C
14. SCAR markers require:
A. Restriction digestion
B. DNA cloning and sequencing
C. Southern blotting
D. RNA extraction
Answer: B
15. SCAR markers are:
A. Less reliable than RAPD
B. More reliable and reproducible than RAPD
Answer: B
Advantages
16. SCAR markers are advantageous because:
A. PCR-based
B. Locus-specific
C. Highly reproducible
D. All of the above
Answer: D
17. SCAR markers are used for:
A. Marker-assisted selection
B. Variety identification
C. Hybrid verification
D. All of the above
Answer: D
18. SCAR markers can be:
A. Co-dominant or dominant
B. Only co-dominant
C. Only dominant
Answer: A
19. SCAR markers are better than RAPD because:
A. They are locus-specific
B. Highly reproducible
C. Can identify heterozygotes
D. All of the above
Answer: D
20. SCAR markers are PCR-based, meaning:
A. They require gel electrophoresis only
B. They amplify a specific DNA fragment
C. They detect proteins
D. They digest DNA
Answer: B
Limitations
21. Main limitation of SCAR markers:
A. Random amplification
B. Requires prior sequence info
C. Low reproducibility
D. Cannot detect polymorphism
Answer: B
22. SCAR markers are labor-intensive because:
A. Need cloning and sequencing
B. Require RNA extraction
C. Need protein digestion
D. Only PCR is needed
Answer: A
23. SCAR markers are costlier than:
A. RAPD
B. AFLP
C. SSR
D. Morphological markers
Answer: A
24. SCAR markers cannot detect:
A. Genome-wide variation
B. Trait-specific loci
Answer: A
25. SCAR markers are mainly used for:
A. Random DNA profiling
B. Locus-specific analysis
Answer: B
Applications
26. SCAR markers are widely used for:
A. Marker-assisted selection (MAS)
B. Disease resistance gene identification
C. Hybrid verification
D. All of the above
Answer: D
27. SCAR markers are useful in:
A. Plant breeding
B. Animal breeding
C. Forensic studies
D. All of the above
Answer: D
28. SCAR markers can verify:
A. Varieties and cultivars
B. Phenotypes only
Answer: A
29. SCAR markers are ideal when:
A. High specificity is required
B. Random genome survey is needed
Answer: A
30. SCAR markers are derived from:
A. Cloned polymorphic RAPD fragments
B. Protein markers
C. RNA sequences
D. Random DNA only
Answer: A
Technical Knowledge
31. SCAR primers amplify:
A. Single, defined DNA locus
B. Random fragments
Answer: A
32. SCAR markers are more reproducible than:
A. RAPD
B. AFLP
C. SSR
Answer: A
33. SCAR markers can be:
A. Dominant
B. Co-dominant
C. Either
Answer: C
34. SCAR markers reduce:
A. Randomness of RAPD
B. DNA quality
C. PCR efficiency
Answer: A
35. SCAR markers are developed by:
A. Sequencing RAPD/AFLP fragment → Designing specific primers
B. Random PCR only
C. Protein extraction
D. Southern blotting only
Answer: A
Comparison with RAPD
36. SCAR markers vs RAPD – Reproducibility:
A. Less reproducible
B. More reproducible
Answer: B
37. SCAR markers vs RAPD – Locus specificity:
A. RAPD is locus-specific
B. SCAR is locus-specific
Answer: B
38. SCAR markers vs RAPD – Heterozygote detection:
A. RAPD can detect
B. SCAR can detect (if co-dominant)
Answer: B
39. SCAR markers vs RAPD – PCR primer length:
A. RAPD: 10-mer, SCAR: 18–24-mer
B. Both 10-mer
Answer: A
40. SCAR markers vs RAPD – Use in MAS:
A. Only SCAR suitable
B. Only RAPD suitable
C. Both suitable
Answer: A
Advanced Applications
41. SCAR markers are used in:
A. Detection of disease resistance genes in crops
B. Hybrid verification
C. Variety identification
D. All of the above
Answer: D
42. SCAR markers are PCR-based and:
A. Require restriction digestion
B. Do not require restriction digestion
Answer: B
43. SCAR markers can distinguish:
A. Morphological differences
B. Allelic differences at a locus
Answer: B
44. SCAR markers are:
A. Reproducible and trait-specific
B. Random and unreliable
Answer: A
45. SCAR marker development involves:
A. Cloning → Sequencing → Primer design
B. Direct PCR
Answer: A
Miscellaneous
46. SCAR markers are widely used in:
A. Plant breeding
B. Animal breeding
C. Forensic analysis
D. All of the above
Answer: D
47. SCAR markers are highly suitable for:
A. Trait-linked marker identification
B. Random genome-wide studies
Answer: A
48. SCAR markers can be:
A. Dominant
B. Co-dominant
C. Either
Answer: C
49. SCAR markers are converted from:
A. RAPD or AFLP polymorphic fragments
B. Protein markers
C. SSR
Answer: A
50. SCAR markers are preferred over RAPD because:
A. Highly reproducible
B. Locus-specific
C. Can detect heterozygotes
D. All of the above
Answer: D






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