I was wondering how each dna off someone is unique to another and how we can see those changes.
In: 5
>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.
Latest Answers