DNA Structure and Functions

In the early 1950s, Rosalind Franklin and Maurice Wilkins used X-ray crystallography to discover the DNA structure. Using Franklin’s data and their studies, James Watson and Francis Crick established the double-helical structure of DNA in 1953.

According to Watson and Crick, the basic building block of DNA is the nucleotide, which consists of three parts: a sugar molecule called deoxyribose, a phosphate group, and a nitrogenous base.

1. Deoxyribose: It is a five-carbon sugar molecule that forms the backbone of the DNA strands. The deoxyribose sugar molecules are connected through a phosphodiester bond, creating a sugar-phosphate backbone.
2. Phosphate Group: It is attached to the 5’ carbon of the deoxyribose sugar. The phosphate group is responsible for linking adjacent nucleotides together through phosphodiester bonds. The phosphate group gives the DNA a negative charge.
3. Nitrogenous Bases: In a cell, there are four types of nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases project from the sugar-phosphate backbone into the interior of the helix and are involved in complementary base pairing.

The two DNA strands (polymers) wound around each other in a right-handed double helix that looks like a twisted ladder or a spiral-shaped staircase. Each helix turn or complete double helix rotation measures approximately 34 Angstroms (Å) in length.

The 5’ carbon of one nucleotide is connected to the 3’ carbon of another nucleotide by phosphodiester bonds. There is a covalent bonding between the oxygen of the 3’ carbon of the first nucleotide and the phosphorous in the phosphate group of the 5’ carbon of the second nucleotide. Thus, these are called 3’-5’ phosphodiester bonds. During the polymerization of DNA, when a new nucleotide is added, a water molecule is lost through dehydration synthesis.

The two strands run in opposite directions, known as antiparallel orientation. One strand has a 5’ end with a free phosphate group and a 3’ end with a free hydroxyl group (-OH). The other strand is oriented in the opposite direction having its 5’ end facing the 3’ end of the first strand and vice versa.

Characteristics of a DNA Strand

Some critical features of a DNA strand are:

• Complementary Base Pairing: The nitrogenous bases in DNA form pairs through noncovalent hydrogen bonding, ensuring specificity in base pairing. Adenine (A) always pairs with thymine (T) through two hydrogen bonds, while cytosine (C) always pairs with guanine (G) through three hydrogen bonds. This complementary base pairing is the foundation of DNA’s ability to replicate accurately and transmit genetic information. According to Erwin Chargaff, the ratios of adenine (A) to thymine (T) and guanine (G) to cytosine (C) are equal.
• Base Stacking: The flat, planar structure of the nitrogenous bases allows them to stack on top of each other within the double helix. This base-stacking interaction stabilizes the DNA molecule and contributes to its structure and shape.
• Major and Minor Grooves: The base pairing along the helix forms grooves on the surface of the DNA double helix. These grooves are known as the major groove and the minor groove. They are critical in interactions with proteins that bind to specific DNA sequences and regulate cellular processes.

DNA is necessary because it contains the instructions for an organism to grow and reproduce. Here are the essential functions of DNA:

Store and Transmit Genetic Information (Inheritance)

The primary function of DNA is to store and transmit genetic information from a parent cell to a daughter cell. DNA contains the instructions that determine an organism’s characteristics, such as its physical traits, behavior, and metabolic processes. These genetic instructions are passed from parents to offspring during reproduction, ensuring species continuity and trait inheritance.

Helps to Synthesize Proteins

One of the most crucial functions of DNA is its role in protein synthesis. Proteins are essential molecules that perform various functions in cells and organisms. DNA serves as a template for synthesizing proteins through a two-step process: transcription and translation.

During transcription, a segment of DNA is transcribed into a molecule called messenger RNA (mRNA) by an enzyme called RNA polymerase. The mRNA is further converted to proteins, a process called translation.

Helps to Copy DNA

Before a cell divides, it must duplicate its genetic material to ensure that each daughter cell receives an identical copy of the DNA. Copying parent cell DNA, called DNA replication, helps maintain its exact copies generation after generation.

Bringing Genetic Diversity within the Population

While DNA replication is remarkably accurate, errors can occasionally occur, leading to changes in the DNA sequence known as mutations. These mutations are the raw material for genetic diversity and evolution. Some modifications can be advantageous, offering new adaptations that confer survival benefits in specific environments. Others may be neutral or even harmful. Over time, the accumulation of beneficial mutations can drive the evolution of species, allowing them to adapt to changing conditions.

Regulating Gene Functions

Not all genes in a cell are active all the time. Instead, they are precisely regulated to respond to specific cues and signals. DNA plays a central role in gene regulation. Regulatory proteins and chemical modifications to DNA determine whether a gene is turned on or off. This regulation allows cells to differentiate into various types during development and adapt their functions to environmental changes, ensuring the coordinated operation of complex cellular processes.