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Different Types of DNA Conformations (A-DNA, B-DNA and Z-DNA: A Comparison Table)
DNA, the genetic information carrier molecule of the cell, is a long polymer of nucleotides and can adopt different types of structural conformations. The various types of conformations that the DNA can adopt depend on different factors such as:
1. Hydration level 2. Salt concentration 3. DNA sequence 4. Quantity and direction of super-coiling 5. Presence of chemically modified bases 6. Different types of metal ions and its concentrations 7. Presence of polyamines in solution.
The most common types of structural conformations of DNA are named as:
Among these three types, the most abundant type of DNA is B-DNA, commonly known as Watson-Crick Model of DNA double helix. The present post describes the structural features of A, B and Z forms of DNA in a comparative manner. We will also discuss the similarities and differences between A-DNA, B-DNA and Z-DNA.
A-DNA is a rare type of structural conformation that a DNA can adopt under dehydrating conditions. A-DNA is a double stranded helical structure almost similar to B-DNA but with a shorter and more compact structural organization. A-DNA was discovered by Rosalind Franklin and the credit for the naming of A-DNA and B-DNA was also accounted to her. Important structural features of A-DNA are given below:
Ø A-DNA is formed from B-DNA under dehydrating condition.
Ø A-DNA is much wider and flatter than B-DNA.
Ø Similar to B-DNA, the A-DNA is also a right handed helix.
Adaptation definition:‘Any feature of an organism or its part which enables it to exist under conditions of its habitat is called adaptation.’ The adaptations are mainly to withstand the adverse conditions of the environment and to use the maximum benefit of the environment.
What are Parasites?
A parasite is an organism which lives in or on another organism (called host) and benefits by deriving shelter and nutrients from them. The parasitism is a type of negative ecological / biological interaction in nature where one organism gets benefited (the parasite) and the other is harmed (the host). The parasites may be microbes such as bacteria, virus and Mycoplasma, or animals such as liver fluke, worms, nematodes, some insects and plants such as Loranthus, Cuscuta etc. All types of parasites show peculiar adaptations to survive in or on the host system and to get maximum benefit from them.
Receptacular vs Appendicular Theory (A Comparative Approach)
Based on the position of ovary there are three types of flowers- (1) Hypogynous, (2) Perigynous, and (3) Epigynous.
(1). Hypogynous Flower: The most primitive type with convex shaped Thalamus (torus). The ovary is superior and all other floral parts (calyx, corolla and androecium) arise from the base of the ovary.
(2). Perigynous Flower: An intermediate or transient type between Hypogynous and Epigynous flowers. The thalamus is more or less cup shaped and the ovary is half inferior, located at the centre of the thalamus cup. All other floral parts arise from the rim of the thalamus cup. Sometimes the thalamus cup forms a long tube like structure called hypanthium.
(3). Epigynous Flower: The most advanced type of flower. The ovary is inferior and all other floral parts arise from the above portion of the ovary.
(image source: cc wikipedia)
Ø The origin and evolution of inferior ovary is a well debated question in the phylogenetics of Angiosperms.
Ø Comparative morphological, anatomical and paleo-botanical studies suggest that the inferior ovary has evolved many times among different groups of Angiosperms in different ways and in different times in the remote past.
Ø In order to explain the formation of inferior ovary, two theories have developed by the evolutionary biologists.
Ø The two theories are:
(1). Appendicular Theory
(2). Receptacular (axial) Theory
Ø Both these theories are proposed on close attention to the organization of the course of vascular bundles supply to the ovules in the flower.
Ø The two theories also considered that the hypogynous flower is the most primitive one. Moreover, the epigynous condition evolved from a hypogynous condition through the transient perigynous state.
(1). Appendicular Theory
Ø Proposed by Eames in 1961.
Ø According to this theory,extensive fusion (both connation and adnation) of the outer lower portion floral whorls to one another and to the ovary wall has occurred. This result in the formation of an inferior ovary (epigynous condition).