Satellite DNA

Satellite DNA is tandemly repeated sequences of 1-500bp and make up about 6% of the human genome. 

Microsatellites: defined as 1-13 bp ,but usually 1-4bp, repeated less than 150 times 

Minisatellites: 14-100bp repeated in tandem arrays of 1-5kb around the genome

Satellite DNA: 100-500bp repeats. Important in mammalian centromeres 

There are differences in satellite DNA between individuals. This occurs due to errors in replication and crossing over. Most of the time sequenes are inherited normally, but these sequences are more likely to have mutations and change in length. 

Replication slippage: dissocaition of DNA during polymerase replication. Nascent DNA strand can rehybridise with another repeat in the array and misalign. Replication continues creating a daughter strand that is longer or shorter than the template. 

Unequal crossing over during meiosis: crossover between misaligned repeats on sister chormatids. One gamete ends up with more copies and one with less copies of the repeat. 

Uses polymorphisms in minisatellite length between individuals to identify them: paternity testing, forensics, genetic mapping 

Process for simple sequence length polymorphisms: 

• Extract DNA 
• Digest with a convenient restriction enzyme 
• Seperate fragments on an agarose gel
• Southern blot (apply radioactive probe using satellite sequence) 
• Observe characteristic bands for each individual (two alleles for each locus in a diploid) 
• Do this for a number of minisatellite sequences

More than two bands are usually present for one minisatellite sequence as the sequence is present at other places in the individuals genome. 

Nowadays PCR is used and primers anneal to the consevred sequence at either end of the tandem array. Visualise the products on a agarose gel.