DNA
Meaning of DNA: Two meters of it are packed into a cell too small to see, arranged so any short stretch can still be found and read.
Definition of DNA
DNA is the molecule inside cells that stores genetic information and passes it from one cell generation to the next and from parents to offspring. It is a double helix of two strands linked by paired chemical bases, and the order of those bases along a strand is the code that cells read.
Where is DNA kept in a cell?
| Location | What is there | How it is inherited |
|---|---|---|
| Nucleus | The great majority of DNA, packaged into chromosomes | Half from each parent |
| Mitochondria | A small separate circular genome inside the cell's energy compartments | From the mother alone |
Nuclear DNA is wound around protein spools and coiled repeatedly, which is how a length measured in meters fits inside a nucleus measured in millionths of one. How tightly a region is wound also affects whether it can be read, so packaging is part of how cells control which instructions are in use. Mitochondrial DNA is much smaller, present in many copies per cell, and inherited down the maternal line, which is why conditions caused by faults in it follow an inheritance pattern unlike any other.
How does a sequence become a trait?
A gene is a stretch of DNA carrying the instructions for one product, usually a protein. The cell copies the gene into RNA, and that copy is translated into a chain of amino acids that folds into the finished protein. Proteins do nearly all the work of the body, as enzymes, structural material, signals and receptors, so a change in the sequence can change a protein's shape or stop it being made at all. Much of the genome is not genes but sequences that control when and where genes are switched on, which is why cells with identical DNA become liver, muscle or nerve.
Why is DNA copied and how do errors arise?
Before a cell divides, the two strands separate and each serves as a template for a new partner, producing two identical molecules. The process is extremely accurate and is proofread, but not perfectly, and DNA is also damaged continually by radiation, chemicals and ordinary cell chemistry. Repair systems correct most of this. What escapes becomes a permanent change in the sequence: harmless in most cases, occasionally the cause of a disease, and in reproductive cells passed to the next generation. Genetic testing reads specific stretches, or the whole sequence, to find such changes.
Also known as
Used in a sentence
A sample of the patient's DNA was sent for genetic analysis.
Good to know
Most DNA sits in the cell nucleus and is inherited from both parents, while the small separate genome in mitochondria passes down the maternal line only, so conditions caused by faults in it follow a different inheritance pattern.
Word origin
Where “DNA” comes from
An abbreviation of deoxyribonucleic acid, itself naming the molecule's chemistry: an acid, containing the sugar ribose lacking one oxygen atom, first isolated from cell nuclei.