SSR (Simple Sequence Repeat) Markers –
Detailed Notes
Introduction
SSR markers, also called microsatellites, are short tandem repeats (1–6 bp) of DNA sequences found throughout the genome.
Examples: (A)n, (CA)n, (GATA)n, where n is the number of repeat units.
SSRs are highly polymorphic, co-dominant, and locus-specific, widely used in genetic mapping, variety identification, population genetics, and marker-assisted selection (MAS).
SSRs are similar to STRs; in plants and animals, the term SSR is more commonly used in molecular breeding, while STR is used more in forensics and human genetics.
Structure of SSR
Repeat motif: 1–6 bp
Number of repeats: Variable among individuals → basis of polymorphism
Flanking regions: Conserved sequences used to design specific PCR primers
SSR loci are generally abundant in non-coding regions, though some occur in genes.
Principle
SSR markers exploit variation in the number of repeat units at a specific locus.
PCR amplification using primers flanking the SSR locus produces fragments of different lengths, reflecting the number of repeats.
These length differences are detected through gel electrophoresis or capillary electrophoresis, allowing genotyping and polymorphism analysis.
Key idea:
Conserved flanking sequences → PCR amplification → Fragment size variation → Allele determination → Genetic analysis
Materials Required
Genomic DNA
SSR-specific primers (flanking repeats)
PCR reagents: Taq polymerase, dNTPs, buffer, Mg²⁺
Thermal cycler
Electrophoresis system: Polyacrylamide gel or capillary electrophoresis
DNA stains: Ethidium bromide, SYBR Green, or fluorescent labeling.
Procedure
Step 1: DNA Isolation
Extract high-quality DNA from blood, leaf, seed, tissue, or other biological samples.
Step 2: Primer Design
Identify SSR loci in genomic sequences.
Design primers flanking the repeat motif for locus-specific amplification.
Step 3: PCR Amplification
Set up PCR with DNA, primers, Taq polymerase, buffer, and dNTPs.
PCR cycles:
Denaturation: 94–95°C
Annealing: 50–60°C (primer-dependent)
Extension: 72°C
25–35 cycles typical for good amplification.
Step 4: Fragment Analysis
Separate PCR products based on size differences:
Polyacrylamide gel electrophoresis (high resolution)
Capillary electrophoresis (fluorescent detection)
Determine allele size corresponding to the number of repeat
Step 5: Data Analysis
Genotype individuals based on allelic patterns
Calculate heterozygosity, polymorphism information content (PIC), or genetic diversity
Apply results in breeding, mapping, or diversity studies
Characteristics of SSR Markers
Highly polymorphic → variation in repeat number
Co-dominant marker → distinguish homozygotes and heterozygotes
PCR-based, requires small DNA amounts
Locus-specific and reproducible
Abundant in non-coding regions, occasionally in genes
Merits
High polymorphism, suitable for genotyping and diversity studies
Co-dominant, can detect heterozygosity
PCR-based, works with small or degraded DNA
Locus-specific and reproducible
Useful for genetic mapping, MAS, population genetics, and variety identification.
Limitations
Requires sequence information for primer design
PCR inhibitors in samples may affect amplification
High mutation rate in repeats may complicate evolutionary studies
Equipment for high-resolution electrophoresis can be expensive
Development of SSR markers can be labor-intensive and costly
Applications
Genetic mapping and linkage analysis in plants and animals
Marker-assisted selection (MAS) for traits like disease resistance, yield, or stress tolerance
Variety and cultivar identification in crop breeding
Population genetics: genetic diversity, gene flow, inbreeding analysis
Conservation biology: individual identification in endangered species
Can be used in human and animal genetics for trait or disease association studies
Conclusion
SSR markers are powerful, co-dominant, and highly polymorphic markers widely used in plant and animal breeding, genetic mapping, and population genetics.
They provide high-resolution, reproducible, locus-specific genotyping, but require prior sequence knowledge and specialized equipment for analysis.
SSR (Simple Sequence Repeat) MCQs
What does SSR stand for in molecular biology?
a) Single Sequence Repeat
b) Simple Sequence Repeat ✅
c) Short Strand RNA
d) Single Strand Replication
SSR markers are also known as:
a) RFLP
b) Microsatellites ✅
c) AFLP
d) RAPD
What type of sequences do SSRs consist of?
a) Repeated DNA motifs of 1–6 bp ✅
b) Long stretches of 100–200 bp
c) Protein-coding regions
d) RNA sequences
SSR markers are highly polymorphic due to:
a) Point mutations
b) Variation in repeat number ✅
c) Large insertions
d) Gene duplications
Which genome regions commonly contain SSRs?
a) Exons
b) Introns and non-coding regions ✅
c) Ribosomal RNA genes
d) Only mitochondrial DNA
Which technique is commonly used to detect SSRs?
a) PCR ✅
b) Southern blot
c) ELISA
d) Western blot
What is the main advantage of SSR markers over RAPD?
a) Dominant markers
b) Co-dominant and highly reproducible ✅
c) Random amplification
d) Require radioactive labeling
SSR primers are designed to:
a) Amplify the repeat region ✅
b) Bind to RNA
c) Cut DNA
d) Sequence proteins
SSR markers are co-dominant. This means:
a) Can distinguish homozygous and heterozygous genotypes ✅
b) Only dominant alleles can be detected
c) Detects only coding regions
d) Cannot be used for mapping
SSR markers are especially useful for:
a) Protein analysis
b) Gene mapping and diversity studies ✅
c) RNA splicing analysis
d) Chromosome staining
A dinucleotide repeat in SSR means:
a) 2 bp motif repeated ✅
b) 3 bp motif repeated
c) 4 bp motif repeated
d) 6 bp motif repeated
Which of these is a trinucleotide SSR motif?
a) (CA)n
b) (CAG)n ✅
c) (AT)n
d) (G)n
SSR markers require which kind of DNA template?
a) RNA
b) High-quality genomic DNA ✅
c) Protein extracts
d) Plasmid only
Which of the following is NOT an advantage of SSR markers?
a) High polymorphism
b) Co-dominant inheritance
c) Low reproducibility ✅
d) Genome-wide distribution
SSR markers are inherited in a:
a) Maternal fashion
b) Mendelian fashion ✅
c) Random fashion
d) Cytoplasmic fashion
SSR length variations are caused by:
a) Point mutation
b) Slipped-strand mispairing ✅
c) Large chromosomal rearrangements
d) RNA editing
Which type of repeats is most abundant in eukaryotic genomes?
a) Mononucleotide
b) Dinucleotide ✅
c) Trinucleotide
d) Tetranucleotide
SSR markers can be used for:
a) Marker-assisted selection ✅
b) RNA transcription
c) Protein folding
d) Metabolic profiling
SSR detection by PCR requires primers flanking:
a) Only coding sequences
b) The repeat region ✅
c) Random DNA sequences
d) Mitochondrial DNA
SSR analysis is often visualized by:
a) Gel electrophoresis ✅
b) ELISA
c) Mass spectrometry
d) Flow cytometry
The number of SSR alleles per locus is usually:
a) Low
b) High ✅
c) Always 2
d) None
SSR markers are particularly useful in:
a) Cloning genes
b) Population genetics studies ✅
c) Protein sequencing
d) RNA interference
Which factor affects SSR polymorphism?
a) Repeat motif length ✅
b) Primer length
c) DNA extraction method
d) RNA concentration
A tetranucleotide SSR has:
a) 2 bp repeats
b) 4 bp repeats ✅
c) 3 bp repeats
d) 1 bp repeat
SSR markers can detect heterozygosity because:
a) They are dominant
b) They are co-dominant ✅
c) They mutate slowly
d) They are in coding regions
Which type of SSR is most stable?
a) Mononucleotide
b) Dinucleotide
c) Trinucleotide ✅
d) Hexanucleotide
SSR markers are expensive because:
a) Require radioactive labeling
b) Need sequence information for primer design ✅
c) Cannot be amplified
d) Only work in bacteria
SSR marker length is measured in:
a) Base pairs (bp) ✅
b) Amino acids
c) Kilodaltons
d) Nucleotides per microliter
SSR markers are ideal for:
a) Detecting large chromosomal changes
b) Fine mapping of traits ✅
c) Protein analysis
d) RNA splicing
Which of the following is a limitation of SSR markers?
a) Low polymorphism
b) Requires prior sequence information ✅
c) Cannot detect heterozygotes
d) Only used in bacteria
SSR markers can be used to study:
a) Gene expression
b) Genetic diversity ✅
c) Protein folding
d) RNA transcription
Which of these plants is widely studied using SSRs?
a) Arabidopsis ✅
b) E. coli
c) S. cerevisiae
d) Drosophila
SSR markers are sometimes labeled with:
a) Fluorescent dyes ✅
b) Enzymes only
c) Radioactive proteins
d) Antibodies
The main source of SSR polymorphism is:
a) Environmental factors
b) Variation in repeat number ✅
c) Epigenetic changes
d) RNA interference
SSR markers can be used in animals for:
a) Paternity testing ✅
b) Protein analysis
c) Metabolic profiling
d) RNA transcription
The first step in SSR analysis is:
a) RNA extraction
b) DNA isolation ✅
c) Protein purification
d) Southern blotting
SSR markers are more informative than RAPD because:
a) They are dominant
b) They are co-dominant and reproducible ✅
c) They require no primers
d) They detect fewer loci
Microsatellites are distributed in:
a) Exons
b) Introns and intergenic regions ✅
c) Only promoters
d) Only ribosomal DNA
SSR markers help in constructing:
a) Protein structure
b) Genetic linkage maps ✅
c) RNA secondary structure
d) Chromosome staining
SSR primers are usually:
a) 10–25 nucleotides long ✅
b) 50–100 nucleotides
c) 1–5 nucleotides
d) 100–200 nucleotides
Which type of repeat is least polymorphic?
a) Mononucleotide ✅
b) Dinucleotide
c) Trinucleotide
d) Tetranucleotide
SSR markers are also useful in:
a) Forensic identification ✅
b) RNA sequencing
c) Enzyme kinetics
d) Protein crystallography
Which of the following is an example of a dinucleotide repeat?
a) (CA)n ✅
b) (CAG)n
c) (GATA)n
d) (ATG)n
SSR polymorphism can be analyzed using:
a) Polyacrylamide gel electrophoresis ✅
b) Western blot
c) ELISA
d) Flow cytometry
SSR markers are considered:
a) Low-throughput
b) Highly reliable and reproducible ✅
c) Non-informative
d) Dominant
Which factor increases SSR mutation rate?
a) Longer repeats ✅
b) Short repeats
c) GC content only
d) Protein coding region
SSR-based maps are useful for:
a) Metabolic engineering
b) Quantitative trait locus (QTL) mapping ✅
c) RNA splicing
d) Protein folding
SSR detection can be automated using:
a) Fluorescent capillary electrophoresis ✅
b) ELISA
c) Northern blot
d) Spectrophotometry
SSR markers are less affected by:
a) Environmental conditions ✅
b) DNA quality
c) PCR conditions
d) Repeat length
SSR markers can be used in:
a) Plants, animals, and microbes ✅
b) Only plants
c) Only animals
d) Only bacteria
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