DNA molecules may be the digital storage medium of the future. I was looking at how much data you could fit if you would synthesize a DNA molecule the size of a human genome.
So DNA base molecules are represented by A, C, T, G and they are always found in pairs (so A connected to T and C connected to G). One such pair is called a basepair. The human genome contains ~3 billion of those basepairs (i.e. 6 billion bases). Now you mostly read about assigning two bits to every base, so A = 00, C = 01, G=10, T = 11. Then you could say a basepair stores 4 bits e.g. AT = 0011. BUT there are only 4 possible basepairs: AT and CG and the reverse, so TA and GC. This AT,TA,GC,CG system could then again be represented by only 2 bits for every pair.
Assuming the latter, a human genome contains 2 bits/basepair*3 billion basepairs = 6 billion bits. Divide by 8 billion to convert to GB, so a human genome could store 0.75 GB.
Now in case of digital DNA storage a part of the molecules needs to be used for error correction, indexing, etc. To account for this ‘overhead’ you can conservatively assume that only 50% of the total storage can be used for the data storage, which would result in 0.375GB.
Now in the whitepaper of the ‘DNA data storage alliance’ backed by Twist, Microsoft, etc. (https://dnastoragealliance.org/dev/wp-content/uploads/2021/06/DNA-Data-Storage-Alliance-An-Introduction-to-DNA-Data-Storage.pdf Page 20) they come to a different result: they assume 2 bits per base, but as 50% is needed for overhead, they say every base stores 1 bit, so 2 bits per basepair AFTER accounting for overhead (above I concluded 2 bits per basepair before accounting for overhead). The total storage then becomes 2 bits/basepair * 3 billion basepairs = 6 billion bits. Divide by 8 billion and we get 0.75GB can be stored in the human genome after we account for overhead. According to this line of thought the full human genome would have a size of 1.5GB (?). General consensus on the internet seems to be that the full human genome stores 0.75GB.