What Is DNA and What Does It Do?
What Is DNA and What Does It Do?
DNA is the molecule that stores most of the inherited information needed to build and run the human body. It helps cells make proteins, control their activities, copy themselves, and pass information to new cells. Most of your DNA came from your biological parents, but cells use different parts of it depending on the jobs they perform.
What Is DNA?
DNA stands for deoxyribonucleic acid. It is a long chemical molecule that carries genetic information in humans and almost all other living organisms.
DNA is often called an instruction manual. This comparison is helpful, but it is not exact. DNA does not contain one simple set of commands that every cell follows in the same way. Instead, it contains many instructions and control signals that cells use at different times.
For example, a muscle cell and a nerve cell usually contain the same basic DNA. However, each cell uses different parts of that DNA. This allows the two cells to have different shapes and perform different jobs.
DNA at a Glance
| Feature | Simple explanation |
|---|---|
| Full name | Deoxyribonucleic acid |
| Main job | Stores and passes on genetic information |
| Main location | Inside the cell nucleus |
| Basic units | Nucleotides |
| Four bases | Adenine, thymine, cytosine, and guanine |
| Shape | A two-stranded double helix |
| Main uses | Helps cells make proteins, divide, and control their activities |
Where Is DNA Found in the Body?
Most DNA is stored inside the nuclei of the body’s cells. A nucleus—plural: nuclei—is a membrane-bound compartment that protects and organizes DNA.
Most cells that have a nucleus contain the same basic set of DNA. There are some exceptions. Mature red blood cells lose their nuclei as they develop, so they do not contain nuclear DNA.
What Is Nuclear DNA?
Nuclear DNA is the large collection of DNA stored in the nucleus. It is arranged into chromosomes.
Most body cells contain two sets of nuclear chromosomes. One set came from the biological mother through the egg, and the other came from the biological father through the sperm.
This inherited DNA influences many features, including blood type, natural hair color, and part of a person’s height. However, most human traits are shaped by several genes working together. Nutrition, health, surroundings, and other factors may also affect how a trait develops.
What Is Mitochondrial DNA?
A much smaller amount of DNA is found inside mitochondria. These are structures that help cells release usable energy from food.
This DNA is called mitochondrial DNA, or mtDNA. It is separate from the chromosomes in the nucleus and contains a small number of genes. Human mitochondrial DNA is normally inherited from the biological mother. (MedlinePlus Genetics)
What Is DNA Made Of?
DNA is built from millions of smaller chemical units called nucleotides. These units join together to form long strands.
What Is a Nucleotide?
Each DNA nucleotide contains three parts:
- A sugar called deoxyribose
- A phosphate group
- One chemical base
The sugar and phosphate parts join to form the outside of each DNA strand. The bases point inward and carry the genetic information.
What Are the Four DNA Bases?
DNA contains four bases:
- Adenine, or A
- Thymine, or T
- Cytosine, or C
- Guanine, or G
The order of these bases is called the DNA sequence. The sequence carries information in much the same way that the order of letters changes the meaning of a word.
For example, changing the order of letters can turn one word into another. In DNA, changing the order of bases may change the information stored in that part of the molecule.
How Do DNA Bases Pair?
The bases on the two DNA strands usually pair in a fixed way:
- Adenine pairs with thymine: A–T
- Cytosine pairs with guanine: C–G
These matching pairs are called base pairs. Their predictable pairing helps DNA make accurate copies of itself.
Why Is DNA Shaped Like a Twisted Ladder?
DNA usually consists of two strands wound around each other. This shape is called a double helix.
The double helix looks like a ladder that has been gently twisted:
- The sugar-phosphate backbones form the two sides.
- The paired bases form the rungs.
- A pairs with T, while C pairs with G.
The two strands fit together because of these matching base pairs. When the strands separate, each one can guide the building of a new matching strand.
The double helix is not the only reason DNA fits inside a tiny cell. DNA is also wrapped around proteins called histones. It is then folded and packed into more compact structures.
This packing protects the DNA and helps control which parts a cell can use. (National Human Genome Research Institute)
How Does DNA Store and Use Information?
DNA stores information in the order of its A, T, C, and G bases. Some DNA sequences contain instructions for making proteins. Other sequences help control when and where those instructions are used.
A cell does not use every DNA sequence at once. It turns different genes on or off according to its type and current needs.
For example, cells in the pancreas use genes needed to make certain digestive enzymes and hormones. Muscle cells use many genes involved in movement and energy use. This selective use of DNA is called gene regulation.
How Do Cells Copy a DNA Message?
DNA normally stays protected inside the nucleus. When a cell needs to use a protein-making instruction, it first makes a working RNA copy of the needed DNA section.
This step is called transcription. The RNA copy is usually called messenger RNA, or mRNA.
The mRNA can leave the nucleus and carry the information to a ribosome. A ribosome is a small cell structure that helps build proteins.
How Does a Cell Make a Protein?
At the ribosome, the mRNA sequence is read in groups of three bases. Each three-base group is called a codon.
Most codons represent a particular amino acid. Amino acids are the smaller units from which proteins are built.
The ribosome joins the amino acids in the order given by the mRNA. The amino-acid chain then folds into a protein. Some proteins also need further changes before they can perform their jobs.
Proteins help form body structures and carry out many cell activities. They include enzymes, receptors, antibodies, hormones, and transport proteins.
The overall movement of information can be shown simply as:
DNA → RNA → Protein
Not every gene produces a protein. Some genes provide instructions for functional RNA molecules that perform other jobs in the cell. (MedlinePlus)
How Do Genes, Chromosomes, and the Genome Fit Together?
DNA, genes, chromosomes, and the genome are closely connected, but they do not mean the same thing.
What Is a Gene?
A gene is a region of DNA that has a biological function. Many genes provide information for making proteins. Others produce useful RNA molecules or help control the activity of other genes.
Humans have about 20,000 protein-coding genes. A single gene may sometimes help produce more than one form of a protein.
Genes also vary greatly in size. Some contain only a few hundred base pairs, while others contain more than a million.
What Is a Chromosome?
A chromosome is one long DNA molecule packaged with proteins. The proteins help fold, support, and organize the DNA.
Most human body cells normally contain 46 nuclear chromosomes. These are arranged into 23 pairs.
A person usually receives one chromosome from each pair through the egg and the other through the sperm. Egg and sperm cells themselves normally contain only 23 chromosomes.
What Is the Genome?
The genome is the complete collection of genetic material in an organism.
The human nuclear genome includes:
- Protein-coding genes
- Genes that produce functional RNA
- DNA that controls gene activity
- Other noncoding DNA
- DNA found between genes
One complete set of human chromosomes contains about 3 billion base pairs. Because most body cells contain two chromosome sets, they hold about twice that amount of nuclear DNA.
If the nuclear DNA from one typical human body cell were stretched out, it would measure about 6 feet, or roughly 1.8 meters. Cells fit this DNA into a microscopic nucleus by wrapping and folding it tightly. (National Human Genome Research Institute)
How Is DNA Copied and Passed On?
Before most cells divide, they must copy their DNA. This process is called DNA replication.
Replication begins when the two DNA strands separate. Each original strand then acts as a pattern for building a new strand.
Because the bases pair in a fixed way, the cell can add the correct matching nucleotide:
- A is matched with T.
- T is matched with A.
- C is matched with G.
- G is matched with C.
Enzymes carry out the copying and join the new nucleotides together. They also check for many errors.
At the end of replication, two DNA molecules have formed. Each contains one original strand and one newly built strand.
The copied chromosomes can then be shared between the two new cells. This allows growing tissues and repaired tissues to keep the genetic information they need.
DNA is also passed from one generation to the next through egg and sperm cells. This is why children share inherited features with their biological parents.
Can DNA Change?
A lasting change in a DNA sequence is called a genetic variant. The word mutation is also used, although variant is often preferred because a DNA change is not always harmful.
Variants may occur in several ways:
- They may be inherited from a biological parent.
- They may arise while an egg or sperm cell is being formed.
- They may occur when DNA is copied during cell division.
- They may result from DNA damage caused by ultraviolet light, certain chemicals, or other sources.
Cells have repair systems that correct much of this damage. However, some changes remain.
Many DNA variants have little or no noticeable effect. Some help create normal differences among people. Others affect how a gene works and may contribute to disease.
A variant that occurs in an egg or sperm cell may be passed to a child. A variant that develops later in an ordinary body cell usually remains only in that cell and the cells produced from it. (MedlinePlus)
Written by Chris Morais, MSc, MPhil, PhD — Making complex biology simple
Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
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