DNA

Deoxyribonucleic acid, or DNA, is a special kind of molecule that carries the instructions for how all living things grow, develop, and function. It looks like a twisted ladder, called a double helix, and is made up of smaller parts called nucleotides. These nucleotides contain important chemicals called bases, which come in four types: cytosine (C), guanine (G), adenine (A), and thymine (T). The order, or sequence, of these bases acts like a code that tells cells how to build and maintain an organism.

The structure of the DNA double helix (type B-DNA). The atoms in the structure are colour-coded by element and the detailed structures of two base pairs are shown in the bottom right.

DNA is found in almost every cell of a living thing. In animals, plants, fungi, and other complex life forms, DNA is stored inside a part of the cell called the nucleus. Before a cell divides to make new cells, it makes a copy of its DNA so each new cell gets the same instructions. This copying process is very important for making sure all cells have the right information to work properly. Even tiny living things like bacteria have DNA, but they keep theirs in a different part of the cell.

DNA works together with another molecule called RNA. RNA helps turn the instructions in DNA into actual proteins, which are the tiny machines that carry out most of the work in cells. Without DNA, life as we know it wouldn’t exist, because it provides the essential blueprint for building and running every living organism.

Chromosome

(10⁷ - 10¹⁰ bp)

DNA

Gene

(10³ - 10⁶ bp )

Function

A chromosome and its packaged long strand of DNA unraveled. The DNA's base pairs encode genes, which provide functions. A human DNA can have up to 500 million base pairs with thousands of genes.

Properties

DNA is a long molecule made from smaller parts called nucleotides. It often twists into a shape like a spiral staircase, called a double helix. The two strands of this helix stick together with special bonds, forming pairs of matching parts.

DNA carries important instructions for building and running living things. Its structure helps it store and share these instructions accurately. The strands can separate and come back together, which is key for cells to copy and use the instructions inside them.


Single branch	Multiple branches

Chemical modifications and altered DNA packaging

A covalent adduct between a metabolically activated form of benzo[a]pyrene, the major mutagen in tobacco smoke, and DNA

The way DNA is packed affects which genes are active. Parts of DNA that are tightly packed usually have fewer active genes. This packing can be changed by adding small pieces to the DNA building blocks or by changing the proteins that help DNA stay organized.

DNA can also be harmed by things in the environment or by normal cell activities. These harms can change the DNA’s instructions. For example, certain types of light can cause parts of DNA to stick together. Even though cells try to fix these problems, some damage can build up over time.


cytosine	5-methylcytosine	thymine

Biological functions

DNA is a special kind of molecule that carries instructions for how living things grow, work, and make more of their kind. It looks like a twisted ladder, called a double helix, and is made of smaller parts linked together. In animals and plants, DNA is found inside special parts of the cell called the nucleus, but tiny cells called prokaryotes keep their DNA in a different spot.

DNA’s instructions are stored in pieces called genes. When a cell needs to use a gene, it makes a copy of the instructions into a molecule called RNA. This RNA then helps build proteins, which are tiny workers inside cells that do many important jobs. DNA also makes copies of itself when cells divide, so each new cell gets the same instructions. This helps living things grow and stay healthy.

Interactions with proteins

All the functions of DNA depend on its interactions with proteins. These interactions can be general, where proteins bind to DNA without needing a specific sequence, or specific, where proteins attach to particular DNA sequences. Enzymes, such as polymerases, are important because they copy DNA during processes like transcription and DNA replication.

Interaction of DNA (in orange) with histones (in blue). These proteins' basic amino acids bind to the acidic phosphate groups on DNA.

Structural proteins help organize DNA into compact structures. In cells called eukaryotes, DNA wraps around proteins called histones to form structures named nucleosomes. These interactions help control how tightly DNA is packed and whether genes can be read or not. Other proteins help bend and arrange DNA into larger structures inside cells. Some proteins bind only to single-stranded DNA, protecting it until it is needed.

Specific proteins called transcription factors bind to certain DNA sequences to control whether genes are turned on or off. They do this by helping or stopping the process that makes RNA from DNA. Because these factors can affect many genes, they help cells respond to changes in their environment and control development.

Genetic recombination

Further information: Genetic recombination

A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.

DNA usually stays separate from other DNA segments, with each chromosome in human cells occupying its own area in the nucleus. One important time when chromosomes do interact is during sexual reproduction, in a process called chromosomal crossover. This is when two DNA helices break, swap a piece, and then rejoin, allowing chromosomes to exchange genetic information.

This exchange creates new combinations of genes, which helps in natural selection and can lead to the development of new proteins. It also plays a role in repairing DNA, especially when the DNA gets broken. The most common type of this exchange happens between chromosomes that have very similar sequences.

Evolution

Further information: Origin of DNA and RNA world hypothesis

DNA carries the instructions that help all living things grow, function, and reproduce. Scientists are still learning how long DNA has been part of life on Earth. Some think early life might have used RNA, a molecule similar to DNA, before DNA took over. RNA could both store genetic information and help with chemical reactions in cells.

Researchers have found that some of the building blocks of DNA might have formed in outer space. They have created some of these molecules in labs by mimicking space conditions. This helps us understand how life on Earth might have begun. Scientists have also studied DNA from very old organisms, like the woolly mammoth, to learn about how genomes change over time.

Uses in technology

Further information: Molecular biology, Nucleic acid methods, and Genetic engineering

Scientists have learned how to take DNA from living things and change it in labs. This helps them make new medicines and improve crops. By mixing pieces of DNA from different sources, they create what is called recombinant DNA. This modified DNA can be put into cells, where it helps produce useful products.

DNA can also help solve crimes. Tiny bits of DNA from blood, skin, or hair found at a crime scene can be compared to DNA from suspects to see if they match. This method, called DNA profiling, was invented in 1984 and has helped solve many mysteries. It is also used to confirm the identity of victims in accidents or wars and to prove parentage in paternity tests.

Further information: Deoxyribozyme

Scientists discovered that certain DNA sequences can act like tiny enzymes, speeding up chemical reactions. These are called deoxyribozymes. They can be designed to detect specific substances, such as metals, and have potential uses in medicine.

The DNA structure at left (schematic shown) will self-assemble into the structure visualized by atomic force microscopy at right. DNA nanotechnology is the field that seeks to design nanoscale structures using the molecular recognition properties of DNA molecules.

Further information: Bioinformatics

Bioinformatics is the study of how to store and analyze biological data, including DNA sequences. Computers help scientists find patterns in DNA, compare sequences from different species, and predict how genes work. This has led to big advances in understanding life and evolution.

DNA can be used to build tiny structures. Scientists use its natural ability to match up with itself to create shapes and even tiny machines. These structures can organize other molecules, which could lead to new technologies in medicine and materials science.

Further information: Phylogenetics and Genetic genealogy

By looking at DNA, scientists can trace the history of living things. Small changes in DNA over time act like a timeline, showing how species are related and how populations have moved and changed.

DNA has the potential to store vast amounts of information in a very small space. While it is not yet practical for everyday use because of cost and speed issues, research continues in this exciting area.

History

Further information: History of molecular biology

DNA was first isolated by the Swiss physician Friedrich Miescher in 1869. He found a substance in old surgical bandages and called it "nuclein" because it was in the nuclei of cells. Later scientists discovered that DNA carries genetic information, helping to explain how traits are passed from parents to children.

In the 1950s, scientists Francis Crick and James Watson worked together to uncover the structure of DNA. They found that DNA looks like a twisted ladder, called a double helix. This discovery helped explain how genetic information is stored and copied in living things.