Deoxyribonucleic acid (DNA) is often referred to as the building blocks of life. It contains the genetic instructions used in the development and functioning of all known living organisms and many viruses. DNA is a nucleic acid that carries this genetic information.
What is the structure of DNA?
DNA has a double helix structure that looks like a twisted ladder. The sides of the ladder are made up of alternating sugar and phosphate molecules. The rungs of the ladder consist of pairs of four nucleotide bases – adenine (A), thymine (T), cytosine (C) and guanine (G). A always pairs with T, and C always pairs with G. This pairing provides the “blueprint” for genetic instructions. The sequence of the base pairs encodes the information in DNA.
The two strands that make up the double helix run in opposite directions. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar molecule is one of the four bases. The two strands are held together by hydrogen bonds between the bases. Adenine forms a double bond with thymine, and cytosine forms a triple bond with guanine.
What does DNA look like under a microscope?
Under an electron microscope, DNA appears as a long, twisted ladder that has been wound into a tight coil. The diameter of the DNA double helix is about 2 nanometers wide. If the DNA from one human cell was stretched out and joined end to end, it would extend up to 2 meters in length!
DNA is packed very efficiently inside cells. With the aid of various proteins, DNA is wrapped around organizing proteins called histones to form structures called nucleosomes. This allows several feet of DNA to fit into the microscopic nucleus of each cell.
How is DNA organized in the cell?
DNA is packaged into structures called chromosomes within the nucleus of each cell. Humans have 46 chromosomes arranged into 23 pairs – one set inherited from each parent. Each chromosome contains a single continuous piece of DNA that carries many genes.
Genes are distinct sequences of DNA that provide the code for making specific proteins. Different genes contain the instructions for making the array of proteins that give each type of cell its unique properties. The entirety of an organism’s DNA is called its genome.
Key structural features of DNA
Let’s recap some of the key structural features of DNA:
- Double helix structure composed of two complementary strands.
- Strands made of alternating sugar and phosphate groups forming a backbone.
- Nitrogenous bases – A, T, C, G – attached to the sugar molecules.
- Base pairing through hydrogen bonds – A with T, C with G.
- Base sequence encodes genetic instructions.
- Tightly coiled and packaged into chromosomes within the cell nucleus.
What are the different forms of DNA?
DNA exists in three main structural forms – A, B and Z. The most common form is B-DNA, which exists as a right-handed double helix under normal physiological conditions. The main forms are:
- B-DNA: Right-handed double helix; common form under physiological conditions.
- A-DNA: Shorter, wider right-handed double helix; form under dehydration.
- Z-DNA: Left-handed double helix; form under high salt concentrations.
The different forms are influenced by the base sequence, hydration, salt concentration and presence of metal ions. However, the basic double helix structure is maintained in all DNA forms.
How were Watson and Crick able to determine the structure of DNA?
In 1953, James Watson and Francis Crick determined the double helix structure of DNA using insights from previous research combined with model building. Key discoveries that aided their work include:
- X-ray diffraction images of DNA taken by Rosalind Franklin provided evidence of a helical structure.
- Erwin Chargaff showed that the amounts of A = T and C = G in DNA, implying base pairing.
- Linus Pauling demonstrated that proteins fold due to hydrogen bonding between molecules.
Armed with these clues, Watson and Crick were able to correctly deduce the double helix model of DNA with the bases on the interior. They utilized metal plates to build models of the double helix until they found a configuration that matched the X-ray patterns.
How can we visualize DNA sequences?
Researchers use a variety of techniques to visualize DNA sequences:
- Gel electrophoresis – Separates DNA fragments based on size via electric current.
- DNA sequencing – Determines exact base pair sequence.
- Fluorescence microscopy – Uses fluorescent labels to image DNA within cells.
- Atomic force microscopy – Uses a nanoscale scanning tip to image surface topography of DNA.
- X-ray crystallography – Reveals structure via diffraction patterns from DNA crystals.
These techniques allow researchers to “see” DNA structures and sequences in order to study their properties and unravel their genetic code.
What are some examples of different DNA conformations?
Here are some examples of conformations that DNA can adopt:
| Conformation | Description | Image |
|---|---|---|
| B-DNA | Most common right-handed double helix form. | |
| A-DNA | Shorter, wider right-handed double helix. | |
| Z-DNA | Left-handed double helical structure. |
The flexibility of DNA allows it to bend, twist, and loop into various shapes as it is packed inside the cell’s nucleus.
What are some common DNA motifs and shapes?
Some common DNA structural motifs and shapes include:
- Major and minor grooves – The major and minor indentations that run along the double helix surface.
- Cruciform – Formed when inverted repeated sequences cause intrastrand base pairing.
- Triple-stranded DNA – When a third strand binds within the major groove.
- G-quadruplexes – Formed from stacking of G-tetrads, planar arrangements of 4 guanines.
- Holliday junctions – Branched DNA structures that form during recombination.
- Hairpins/loops – Formed when DNA folds back and base pairs with itself.
These shapes arise from DNA’s ability to base pair, stack, and form other non-covalent interactions. They are important for DNA metabolic processes.
How can DNA exist as single-stranded or double-stranded?
DNA is normally found as a double-stranded helix. However, it can exist in a single-stranded form during processes such as DNA replication and transcription. Some key points:
- Double-stranded DNA forms when complementary bases on opposing strands pair up.
- Heat or chemical agents can be used to melt or denature DNA into single strands.
- Single-stranded binding proteins can stabilize single-stranded DNA.
- Polymerases use single-stranded DNA as a template for assembling a complementary strand.
- Sections of single-stranded DNA form temporarily during DNA metabolic processes.
The ability to separate the two strands allows critical cellular processes like copying and reading genetic information to take place. The complementary base pairing then enables the double helix to re-form.
How can the structure of DNA be altered or damaged?
The DNA double helix is a relatively stable structure, but there are chemical and environmental factors that can alter or damage its shape:
- Mutagenic chemicals can modify bases to distort the double helix.
- Ionizing radiation can create strand breaks.
- Errors during replication can lead to mismatched bases.
- Adducts can form when chemicals bond to bases.
- Pyrimidine dimers arise when UV light causes thymine bases to bond.
- Free radicals from normal cell metabolism can alter bases.
DNA repair mechanisms exist to fix these insults and restore the proper structure. But unrepaired DNA damage can lead to mutations or cell death.
What are some ways we can model DNA structures?
There are several methods for modeling DNA structures:
- Physical models – Assembling representations using wire, wood, plastic or metal.
- Computer simulations – Using molecular dynamics modeling and visualization software.
- Mathematical models – Describing shapes using parameters like twist angle and curvature.
- X-ray diffraction – Analyzing scattering patterns from DNA crystals.
- Atomic force microscopy – Using a nanoscale tip to image DNA shape.
These methods help researchers deduce 3D conformations, thermodynamic stability, and flexibility of DNA under different conditions.
Conclusion
In summary, DNA has a elegant double helical structure that provides an efficient system for encoding, storing, and transmitting genetic blueprints. The sequence of nucleotide bases in its twisting ladder carries the heritable information that governs all living organisms. DNA’s conformational flexibility also allows it to organize, regulate, and express those genes. The ability to visualize DNA structures continues to provide essential insights into genetics and life’s operations at the molecular scale.
