In this tutorial, you will learn about the fundamentals of DNA, including its structure and composition, the different forms of DNA (B-DNA, A-DNA, and Z-DNA), and the concept of DNA supercoiling. You will also explore the significance of the DNA double helix, understand the process of nucleic acid hydrolysis, and examine genomic DNA and its applications in genetic analysis.
Contents:
- Introduction to DNA
- What is B-DNA?
- What is A-DNA?
- What is Z-DNA?
- DNA Supercoiling
- DNA Double Helix
- Nucleic Acid Hydrolysis
- Genomic DNA (gDNA)
- Components of DNA Structure
Introduction to DNA
Deoxyribonucleic acid (DNA) is the primary genetic material in most living organisms, serving as the blueprint for development, functioning, and reproduction. It is composed of nucleotides, which consist of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The arrangement of these components forms the structural integrity of DNA.
What is B-DNA?
B-DNA is the most common form of DNA found under physiological conditions. Its discovery was made possible through X-ray diffraction techniques used by Watson and Crick. Key features of B-DNA include:
- Helical Structure: B-DNA has a right-handed helical structure that is elongated and narrow.
- Grooves: It possesses a narrow minor groove and a wider major groove, facilitating protein binding and interaction.
- Strands Orientation: The two strands run antiparallel to each other, and hydrogen bonds connect the complementary nitrogenous bases.
- Base Pairing: The glycosidic bonds in B-DNA are not diametrically opposite, allowing for a stable helical arrangement.
What is A-DNA?
A-DNA is less common and typically forms under conditions of low humidity (below 75%). Its structural differences from B-DNA include:
- Helical Structure: A-DNA has a right-handed helix but is wider and more compact than B-DNA.
- Asymmetry: The strands of A-DNA are anti-parallel and asymmetrical, with glycosidic bonds that are diametrically opposite.
- Hydrogen Bonding: A-DNA also features hydrogen bonds between bases, similar to B-DNA, but its structural conformation allows for different interactions with proteins.
What is Z-DNA?
Z-DNA is a left-handed helical form of DNA characterized by a zigzag appearance. Its distinctive features include:
- Zigzag Structure: The backbone of Z-DNA gives it a unique zigzag form, making it visually and structurally different from both A-DNA and B-DNA.
- Helix Width: Z-DNA is narrower (1.8 nm) than A-DNA and B-DNA, contributing to its less stable nature.
- Stability: Z-DNA is less stable than its counterparts, making it difficult to observe and limiting its biological applications.
DNA Supercoiling
DNA supercoiling refers to the over or under-winding of the DNA helix, which plays a significant role in the compact packaging of DNA within cells. Key aspects include:
- DNA supercoiling involves anchoring of two different ends coming together and joining. This process gives new properties to newly formed supercoiled DNA.
- After joining, the two ends get intertwined like links of a chain. Linking number is a property that describes joining of one strand with other.
- Topoisomerase enzyme which are nearly found in all cells, regulate this whole process of DNA supercoiling.
- Type I and type II are two different types of topoisomerases which play a significant role in DNA supercoiling.
DNA Double Helix
DNA double helix is a important structure that is formed from secondary structure of DNA.
- DNA double helix structure is the base for tertiary structure formation. Tertiary structure of DNA relies on double helical structure. Double helical structure was developed by Watson and Crick for the purpose of secondary structure.
- Strands were first coupled with each other. The coiling of coupled strand leads to formation of DNA double helix. This DNA double helix has a deeper and wider major groove. It also consists of narrower and shallower minor groove.
Nucleic Acid Hydrolysis
Nucleic acid hydrolysis is the process of breaking down nucleic acids into their constituent parts—sugars, inorganic phosphates, and nitrogenous bases. This process is crucial for various biological functions and can be summarized as follows:
- Sugar can be ribose or deoxyribose while bases are heterocyclic in nature. There are four different types of heterocyclic bases.
- Adenine and guanine and purine bases while thymine and cytosine are pyrimidine bases of DNA. In case of RNA thymine is replaced by uracil.
- Pyrimidine bases are monocyclic while purine bases are bicyclic. Nucleic acids are acidic in nature due to the presence of phosphoric acid moiety.
- Nucleic acid hydrolysis leads to formation of RNA and DNA, depending whether the sugar is ribose or deoxyribose respectively. Both DNA and RNA play important roles in many biological pathways.
Genomic DNA (gDNA)
gDNA stands for genomic DNA. It is the chromosomal DNA of any species.
- These genomic DNA is of no particular size. Size of genomic DNA changes from organism to organism. It ranges from few kilo-base pairs in viruses to 149 billion base pairs in a flower named Paris japonica.
- There are a variety of genomic DNA types found all over the world. Calf thymus DNA, bacteria genomic DNA and salmon sperm are few examples of the that accounts for the variety of genomic DNA.
- Genomic DNA has a wider application in southern blotting, SNP analysis, polymerase chain reaction, molecular diagnosis asssays etc.
Components of DNA Structure
Deoxyribose nucleic acid is the most important and widely found genetic material in the molecular world. DNA as the original genetic material was proven by Hershey and Chase in their experiment.
- This figure depicts about the various components of DNA structure. B is Nitrogenous base and C is Deoxyribose sugar which signifies that Oxygen is absent at second position.
- There are 4 different types of DNA bases i.e., adenine, thymine, cytosine, and guanine. Adenine pairs with thymine and guanine pairs with cytosine.
- Deoxyribose sugar is another important component. Sugar forms the DNA backbone. DNA backbone leads to stability of DNA molecule. Stable DNA molecule helps in effective processing without getting denatured.
Key Points to Remember
Here is the list of key points we need to remember about “DNA Structure”.
- Deoxyribonucleic acid (DNA) is composed of nucleotides, which include a deoxyribose sugar, a phosphate group, and nitrogenous bases (adenine, thymine, cytosine, guanine).
- B-DNA is the most common form, A-DNA forms under low humidity, and Z-DNA has a left-handed zigzag structure, with each type having distinct structural characteristics.
- DNA supercoiling refers to the over- or under-winding of the DNA helix, facilitated by topoisomerases, which help regulate supercoiling and maintain DNA structure during processes like replication and transcription.
- The DNA double helix consists of two strands that are coiled around each other, with major and minor grooves that play crucial roles in protein binding and genetic functions.
- Nucleic acid hydrolysis breaks down nucleic acids into sugars, phosphates, and nitrogenous bases, essential for various biological functions and the formation of RNA and DNA.
- Genomic DNA varies widely in size across organisms and has numerous applications in genetic analysis, including southern blotting and polymerase chain reaction (PCR).
