 When you are in a merry-go-round, you feel this outward force called the centrifugal force. If you mix sand with water and make it rotate just like you rotate in a merry-go-round, the sand particles also feel this centrifugal force. If you keep rotating this mixture of water and sand, the sand particles will eventually settle at the bottom. This principle is used in biology to separate out cells or even biomolecules from a water suspension. For the rotation there is this device called a centrifuge and centrifuges can actually rotate in large speeds so that even smaller particles like biomolecules such as proteins or DNA which are much smaller than sand particles or us in a merry-go-round even they can settle down with such high speeds and this process of taking an acute solution or suspension and rotating it in a centrifuge is called centrifugation. There are different types of centrifugation depending on what we need to do and one of the types is called density gradient centrifugation. In this type of centrifugation what we do is we take a substance and make a concentration gradient of it along this solution in the tube that is being rotated so the substances that we usually make a density gradient of are say sucrose or cesium chloride. So the concentration of this substance increases towards the bottom of the tube so when you take genomic DNA and centrifugate using the density gradient technique what you see is you get a band like this. The DNA settles right here not at the bottom even though DNA is heavier than water it still settles here because there is a density gradient usually the substance used is cesium chloride so there is this gradient and the DNA will go and settle at that point where the density of the solution matches the density of the DNA so since the density of the solution because of cesium chloride is increasing as you go downwards there will come a point where the density of the DNA matches the density of the solution that is where the DNA goes and settles and forms this band. Now you would expect only one band right for DNA but in fact we see a few minor bands and those are called satellite bands or satellite DNA so what is the satellite DNA so when you isolate the genomic DNA from a cell it has a fixed density for that particular cell or for that particular species and that depends on the nitrogen base content so what is the proportion of ATGC adenine thymine guanine and cytosine the density of a particular genomic DNA depends on that that's why you see the main band this band this is the genomic DNA band which has a particular density there are some DNA sequences in our chromosomes which have tandem repeats you can take a look at our video on DNA polymorphism in which we talk about tandem repeats where a sequence of DNA is repeated over and over again in a DNA stretch for example if you say something have something like this and this is repeated again and again this is called a tandem repeat and these stretches of DNA usually have a different density why because they are often more AT rich or more GC rich as compared to the genomic DNA now whenever there is a stretch of DNA that is more GC rich it has higher density I'm not going to the details of the structures but if you take a look at the structures you will see that guanine is the biggest nitrogen base hence it has a higher molecular weight and hence it has a higher density so when you look at the satellite DNA bands when genomic DNA is centrifuged you might see some above the genomic DNA band and some below the genomic DNA band so naturally the ones above the genomic DNA bands will have lower density right so that will be AT rich and the satellite DNA bands that are below the genomic DNA band or the main band are the GC rich sequences which have a higher density satellite DNA can be of different types based on the length of the sequence that is being repeated over and over again the two major types of satellite DNA that we see very often are mini satellites and micro satellites mini as the name suggests is bigger than micro mini satellites have a base pair length of around 10 to 100 and micro satellites they have a base pair of length of less than 10 base pairs usually both the types of satellite DNA have a high degree of polymorphism as you can find in our video on polymorphism this means that there is a high probability of variations in these DNA sequences why because these sequences are not found in the coding regions of DNA they are found in places like telomeres or centromeres since they're not found in the coding regions the probability of them accumulating in subsequent generations is high because they are not really affecting the proteins that we produce from our coding DNA sequences and because of their high degree of polymorphism they result in a lot of variations that we see around us satellite DNA has applications and various fields a popular one is DNA fingerprinting which we will look at in a different video