Since Francis Crick and James Watson discovered the structure of DNA in 1953, DNA has dominated scientific and medical research.
Criminal cases are based on the discovery of DNA evidence,
Instead, it releases innocent people from prison.
It is used as a tool for archaeology, anthropology and ecology to track humans and species that have long died or are very active.
But how is this material obtained?
What is its limit?
In order to understand the meaning of DNA evidence, here is the basic knowledge of DNA retrieval. What is DNA?
Dna is a long double.
Stranded molecules in the shape of a twisted rope ladder.
There are four bases on the backbone of DNA.
D. , D-
This pair up, the steps to form a ladder.
The sequences of these pairs form a genetic code that allows the protein to replicate information and transfer it to other cells.
DNA can be called the blueprint of life.
In addition to some viruses, all organisms contain DNA, and all plants and animals store their DNA in the nucleus of the cell.
For humans, DNA found in the nucleus comes from genetic material transmitted by both parents and exists in almost every tissue of the body.
Except for identical twins, each person's DNA is unique to them.
This unique feature makes DNA evidence more and more popular in forensic science, whether it is used to identify blood, follicles, skin, semen, or samples of sweat and other tissues used in crime scenes and missing persons investigations.
However, scientists can only identify a person by comparing DNA with other sources of genetic material.
How do scientists separate and analyze it?
When scientists take small biological samples from things like blood cells, they have to clear all proteins and lipids around the DNA in the cells.
Although there are dozens of different methods, most use some combinations of enzymes and detergents to break down cells and bind to lipids instead of DNA, as well as acids that remove impurities.
Use a centrifuge or other separation machine to separate the genetic material.
But over time, the DNA splits.
The older the sample, the more likely the sequence obtained after separation is to be incomplete or smaller in volume.
Scientists use a kind of enzyme chain reaction (PCR)amplification.
In essence, DNA is heated until its two chains are separated to form a template, and then the primer material can form the pattern itself.
The primer copies the pattern at a lower temperature, and once the temperature rises, the primer combines with the template to effectively double the amount of DNA samples.
This process can be repeated 30 or more times, turning small samples of dozens of cells into large enough samples to process and analyze.
Can DNA be obtained from other sources?
Blood, skin and other DNA sources found in the nucleus are not the only substances that leave traces of DNA.
The bone also contains mitochondrial DNA inside, providing crime investigators with a way to identify bodies from the coldest cases, and also for archaeologists and archaeologists to investigate the past centuries.
Mitochondrial DNA is found outside the nucleus of the mitochondrial, where cells generate energy.
Because there are thousands of neurons in the cell, mitochondrial DNA is richer than nuclear DNA.
Mitochondrial DNA may come from dried hair or bone cells lacking in the nucleus, or from smaller, older pieces of evidence.
However, unlike nuclear DNA, the vast majority of mitochondrial DNA is inherited from the mother, so it does not provide a unique fingerprint for everyone.
Siblings, mothers, and grandmothers may share mitochondrial DNA, so the test cannot distinguish these family members.
How far can scientists trace DNA?
The oldest DNA sample widely accepted came from 400,000-year-
Old plants found in Siberian ice.
The oldest DNA found in humans is 100,000 years old.
The teeth of the Neanderthals found in the Scladina Cave in Meuse basin, Belgium.
Most archaeological sites will not last that long.
After the death of the organism, bacteria, oxidation, hydrolysis and even radiation will slowly break down the genetic material.
"We don't have a good understanding of the rules of DNA preservation yet," said Dr.
Yang Dongya, professor of archaeology at Simon Fraser University.
"Each burial environment is unique and each set of conditions has a different effect on the sample.
"In general, high temperatures and humidity are bad for DNA survival because the soil is either too acidic or too alkaline. The best-
The preserved samples come from frozen sources in the Arctic, where bacteria are less active.
How accurate is the evidence?
As with all the evidence, if the DNA is not contaminated, then its sample is more reliable.
The contamination may be caused by the above environmental conditions, or it may simply be due to the exposure of the sample to other DNA.
For example, bone samples may provide a great deal of information about the composition of human genes centuries ago.
But if countless people hold the bone in their hands before it finally enters the lab for an examination, some of these people's skin DNA is likely to rub on the sample and appear in the test, could destroy the result.
In any DNA separation process, many methods of removing foreign DNA are used, but the cleaner the sample starts, the more likely the final sample is to be clean.
While considered a precise process, the constant change in technology can lead to a second
Speculation about past tests
In the UK, the Association of Chief Police Officers said in February 2007 that it would re-examine about 200 cases involving DNA testing because of concerns that due to the different ways in which the forensic laboratory tests DNA, the critical evidence may be left out of the sample between 2000 and 2005. DNA evidence —
Is it used to put a person at the crime scene, or is it used to determine if the bone found came from a primitive person or another primitive person --
Usually only as good as the evidence around, Yang said.
"DNA itself cannot provide an answer," Yang said . "
"You have to put it in context and make sure you're explaining other information around you.