Eli5 what makes everyones dna unique and how can we see that under the microscopen?


I was wondering how each dna off someone is unique to another and how we can see those changes.

In: 5

I don’t know how to answer the first question. Everyone’s DNA is just different. It uses the same 4 molecules, just in a different order.

For your second question, we don’t see molecules with a microscope. Molecules are way too small for that. We instead use one of various chemical processes to project the sequence onto a visible spectrum. The most common ways involve 4 spectro-chemicals that each bind only to one of the four nucleotides, then using spectroscopic analysis to determine the order of the spectro-chemicals, then translating that into a sequence of nucleotides. I’m simplifying a lot here though, so take this with a grain of salt.


DNA between humans is about 50% similar to that of plants and animals in general. We’re 80% identical to cows and about 90% similar to felines!

Humans are probably 99.9%+ similar to each other with a genome length of ~3.2 billion bases (each of those locations is a nucleobase either A, T, G or C). Each cell’s DNA is spaced into 46 chormosomes (23 pairs – each being a large blocks of DNA).

There is a technology called gene sequencing, where the order of each of these blocks can be read out (1st position is A T, G or C, – 2nd position is? etc). There are MANY ways that one can do this, but more modern versions use lasers and electrical currents to quickly record which base is present at either position.

Older methods (sanger sequencing) would take a piece of DNA and chemically break it into parts, separate them, and you could use the output of the experiment to effectively identify the sequence like putting together a puzzle.

>what makes everyones dna unique

First, identical twins/triplets have identical DNA, because they are clones from a split zygote.

Humans have 23 pairs of **chromosomes**, or sections of DNA that make up the human genome. 22 of those pairs are functionally the same, and one pair (the sex chromosomes) have significant differences.

During the process that creates sex cells (sperm in males, eggs in females), these chromosomes are unpaired, so the sex cell only has half the chromosomes – one of each of the first 22, and then one of the sex chromosomes (for an egg, this will be a X chromosome, for a sperm cell this will be either an X or a Y chromosome). This process randomly selects the set of 23 chromosomes in the sex cell from the parent. When the sex cell combines with another sex cell, it also gets a random set of chromosomes from that parent. This shuffles the chromosomes and generates genetic variability.

So for chromosome 1, the mother might have 1a and 1b chromosomes, and the father has 1c and 1d. Their children will have one of the following 1a1c, 1b1c, 1a1d, 1b1d. For 23 chromosomes, that is a lot of combinations. There are 8,324,608 possible combinations of 23 chromosome pairs.

But there is more variation in the DNA strands themselves – each chromosome codes DNA for thousands of proteins (genes). During DNA replication or separation, they might tangle or break. The molecules that fix DNA might mix up the broken ends, and rebuild the chromosomes into two chromosomes that are now different. The replication molecule might make a mistake, or it might be affected by chemicals in the environment or by radiation. These are mutations. Sometimes those mutations are dangerous or non-viable, but sometimes they introduce variation that can be useful, detrimental or neutral. This process of mutation also introduces changes into human DNA, and can affect minor things like skin, hair or eye colour, or major things that cause genetic diseases.

For some genes, one chromosome will override the same gene on the other chromosome (dominant genes), even if they are different. In other cases, the gene on one chromosome may not work, but because the other chromosome has a good copy, there isn’t a problem. But if both chromosomes don’t work, a genetic disease may result – a disease like cystic fibrosis is one where both parents are healthy but carry a recessive faulty gene. When they have children, they have a 1 in 4 chance of having a child with two recessive genes and the disease of cystic fibrosis.

All these mechanisms work together and introduce variability into our DNA, and that level of variability is so great that the likelihood of finding two people with the same DNA (apart from identical twins/triplets) is so great that we say that everybodies DNA is different.

To compare DNA, scientists first use a chemical process to unravel the DNA, and then clone it (a polymerase reaction). Then they slice it into segments using a chemicals that slice DNA at specific places. The fragments are tagged with fluorescent chemicals and washed into a gel using an electric field that isolates specific fragments at specific places after a fixed time. Comparing sufficient known points (loci) allows comparison of two DNA sequences.

Modern DNA sequencers can read whole DNA strands base by base, allowing full genome sequencing and comparison.

We can’t see the structure of someone’s DNA under a microscope. We have to take a sample of cells and run a process on them. This is called sequencing a genome. In the end, we get a list of billions of letters (ACGT), and the order of those letters represents the structure of the DNA.

Now, all humans’ DNA is 99.9% similar. Meaning we share almost all of it. That .1% is still millions of letters in the DNA sequence, but it’s a very small portion of the total genome.

We share 99% of our DNA with chimpanzees, and about 50% of our DNA is shared with bananas.