Hey there, dear readers! Today, we’ll dive deep into the field of genetics by beginning with its fundamental building block: DNA.
DNA, or deoxyribonucleic acid, is a molecule that contains the genetic instructions utilised by all known living organism in their growth, development, functioning, and reproduction. Yet, before we get into its interesting structure and function, let’s go back in time to see how it was discovered.
The Quest for DNA’s Structure
The concept of heredity, which refers to the transmission of traits from parents to offspring, has existed for as long as people have existed. Farmers and livestock breeders have long understood that offspring usually resemble their parents. However, despite intensive research, the mechanisms of heredity have remained a mystery for millennia, until recent times.
The Beginnings: Early Understanding of Heredity
People used to hold a wide range of beliefs about heredity. The majority of these were founded on observation and hypothesis rather than an understanding of how genetics worked. Here are some examples:
- Blending inheritance: It was once widely held that offspring possessed a ‘blend’ or’mix’ of their parents’ characteristics. If one parent is short and the other is tall, the children will be of average height. However, this theory was unable to explain how traits that had been lost in a previous generation could suddenly reappear in succeeding generations.
- Preformationism held that all organisms formed at the same time and that each embryo or egg contained a complete, miniature organism (known as a “homunculus”) that grew larger as the organism developed. This theory is unable to explain how traits inherited from both parents may manifest in offspring.
- According to the Lamarckism theory, an organism can change in response to its environment during its lifetime, and these changes are then passed on to the organism’s progeny. Jean-Baptiste Lamarck proposed this idea. Although not entirely accurate, the example of the giraffe’s neck growing across generations in order to reach higher vegetation is commonly used to demonstrate this.
- The pangenesis theory, proposed by Charles Darwin, states that every part of the organism produces particles known as “gemmules.” These are then accumulated in sperm and eggs and passed on to the next generation. This hypothesis attempted to explain how changes that an organism acquires over the course of its life can be passed down to its descendants.
The Father of Genetics: Gregor Mendel
The first major breakthrough in our understanding of heredity occurred around the middle of the nineteenth century, thanks to and unusual source—a monastery in what is now the Czech Republic. Between 1856 and 1863, an Augustinian friar named Gregor Mendel, was responsible for a comprehensive set of studies on pea plants.
Mendel painstakingly crossed plants with distinct characteristics. He then observed the progeny’s characteristics over many generations. He was able to deduce two fundamental principles of inheritance from this single piece of evidence: the law of segregation and the law of independent assortment. However, the modern era of Mendelian genetics did not begin until 1900, when Gregor Mendel’s work was discovered.
The Chromosome Theory of Inheritance
At the turn of the twentieth century, scientists began to associate the physical manifestation of heredity (traits) with their location within the cell. The chromosome theory of inheritance was proposed independently by Walter Sutton and Theodor Boveri in 1902. They proposed that the behaviour of chromosomes during meiosis could explain Mendel’s laws.
The Discovery of DNA
It was recognised prior to the 1950s that chromosomes carried genetic information. However, the exact molecule responsible for this was unknown. Oswald Avery, Colin MacLeod, and Maclyn McCarty conducted a series of tests in 1944 to demonstrate that DNA is the substance responsible for bacterial transformation. This established that DNA serves as genetic material. In 1952, Alfred Hershey and Martha Chase conducted studies using bacteriophages, which are viruses that infect bacteria. These investigations corroborated the previous conclusion.
The Double Helix: Watson, Crick, Franklin, and Wilkins
The discovery of the structure of DNA should be credited to a group of researchers who collaborated to discover the now-famous double helix. The double helix concept, devised by James Watson and Francis Crick in the 1950s, sparked the revolution in genetics and molecular biology. Their discovery was a huge accomplishment, but it was built on the work of other scientists who contributed significantly to the scientific understanding of DNA.
Rosalind Franklin, who specialised in X-ray crystallography, was one such researcher. “Photograph 51” is a photograph depicting a blurry X in the centre of a field of black and white splotches. This X-ray diffraction image of DNA was crucial to understanding how the double helix structure of DNA is organised.
The recognition given to Watson and Crick for their contributions to the discovery of the structure of DNA obscured the important role that Franklin played in the discovery. Furthermore, Franklin died in 1958 as a result of ovarian cancer. She passed away four years before Watson, Crick, and Maurice Wilkins were awarded the Nobel Prize in Physiology or Medicine for their work on DNA.
Further Information
Developing the Chromosome Theory
Studies by Oswald Avery, Colin MacLeod and Maclyn McCarty
DNA: The Blueprint of Life
Let us now turn our attention to the main character of our story: DNA. DNA is found in the nucleus of every cell in our bodies. It contains the instructions, or genes, for making all of our proteins.
The building blocks of DNA
The structure of DNA is what makes it so suitable for storing data. DNA molecules are polymers, which are large molecules formed by repeatedly linking together smaller molecules known as monomers. These monomers are known as nucleotides in the case of DNA.
The DNA “alphabet” is made up of four “letters,” or nucleotide monomers. All nucleotides are composed of three parts: a sugar, a phosphate group, and a nucleobase. Each nucleotide has the same sugar and phosphate groups, but the nucleobase can be one of four types: adenine (A), thymine (T), guanine (G), or cytosine (C). Adenine and guanine are also called purines, and thymine and cytosine pyrimidines. The pyrimidines have a one ring structure. The purines have two rings fused together.
The Structure of DNA
The manner in which these nucleotides connect is critical to DNA’s function. The nucleotide monomers in a DNA polymer are linked together by strong covalent bonds known as phosphodiester bonds. The bonds connect the 3′ carbon of one nucleotide to the 5′ carbon of another nucleotide. This results in a nucleotide chain known as a single strand of DNA.
However, DNA does not exist as a single strand. Instead, two of these chains come together to form a double helix, which is a spiralling ladder-like structure. The two strands of DNA are antiparallel, which means that the 5′ end of one is parallel to the 3′ end of the other. The rungs of this “ladder” are made up of hydrogen-bonded pairs of nucleobases from each strand. Adenine (A) always pairs with thymine (T), while cytosine (C) always pairs with guanine (G). This is referred to as complementary base pairing.
What are 5′ and 3′?
The numbering convention used for DNA strands is based on the carbon atoms in the sugar component of the nucleotides. Deoxyribose—the sugar molecule in DNA—has a five-carbon structure. The carbons in the sugar are labeled from 1′ to 5′.
The 3′ end of a DNA strand refers to the end where the 3′ carbon of the deoxyribose sugar is located. On the other hand, the 5′ end of a DNA strand refers to the end where the 5′ carbon of the deoxyribose sugar is located.
Complementary base pairing is required for DNA to play a role in heredity. During cell division, the DNA double helix unwinds, and each strand serves as a template for a new partner strand. This ensures that each new cell receives an exact copy of the DNA.
To summarise, DNA is a remarkable molecule with a structure that is perfectly suited to its function as the blueprint for life. Many dedicated scientists collaborated to solve its mysteries. This process has served as a testament to the importance of collaboration in furthering our understanding of nature.
Self-check questions
- Why is DNA referred to as the ‘information storage molecule’?
- Can you explain the difference between a DNA polymer and a DNA monomer?
- What are the three components that make up a nucleotide?
- How are the two strands of DNA held together in the double helix structure?
- Can you explain what complementary base pairing is?
Main Concepts
* DNA is the biological information storage molecule that contains instructions for making proteins.
* DNA is a polymer made up of smaller units called nucleotides.
* There are four types of nucleotides: A, C, T, G, each composed of a sugar, a phosphate group, and a nitrogenous base.
* Nucleotides in a DNA polymer are connected by phosphodiester bonds.
* DNA forms a double helix structure, with two DNA strands running antiparallel and held together by hydrogen bonds.
* The principle of complementary base pairing (A-T and C-G) is crucial in DNA replication and transcription (we’ll get into these in future posts).
Further Information
Khan Academy | Molecular structure of DNA