DNA’s B Form, A Form and Z Form
In a DNA molecule, the two strands are not parallel, but intertwined with each other. Each strand looks like a helix. The two strands form a “double helix” structure, which was first discovered by James D. Watson and Francis Crick in 1953. In this structure, also known as the B form, the helix makes a turn every 3.4 nm, and the distance between two neighboring base pairs is 0.34 nm. Hence, there are about 10 pairs per turn. The intertwined strands make two grooves of different widths, referred to as the major groove and the minor groove, which may facilitate binding with specific proteins.
The normal right-handed “double helix” structure of DNA, also known as the B form.
In a solution with higher salt concentrations or with alcohol added, the DNA structure may change to an A form, which is still right-handed, but every 2.3 nm makes a turn and there are 11 base pairs per turn.
Another DNA structure is called the Z form, because its bases seem to zigzag. Z DNA is left-handed. One turn spans 4.6 nm, comprising 12 base pairs. The DNA molecule with alternating G-C sequences in alcohol or high salt solution tends to have such structure.
Comparison between B form and Z form.
Double helix DNA
DNA Structure
Watson and Crick Model of DNA
The following are the features of the DNA molecule as described by Watson and Crick in 1953.
2 chains
purine opposite a pyrimidine
chains held together by H-bonds
Guanine is paired with cytosine by three H-bonds
Adenine is paired with thymine by two H-bonds
anti-parallel orientation of the two chains
5′—————>3′
3′<—————5'
the molecule is stabilized by:
large # of H-bonds
hydrophobic bonding between the stacked bases
Components of DNA
DNA is composed of two chains of repeating nucleotides. Each nucleotide consists of three components. These components are:
Phosphate Group
2-deoxyribose sugar
A nitrogen containing base
cytosine
adenine
guanine
thymine
Types of DNA
The DNA molecule that Watson and Crick described was in the B form. It is now known that DNA can exist in several other forms. The primary difference between the forms is the direction that the helix spirals.
A, B, C = right-handed helix
Z = left-handed helix (found in vitro under high salt)
B is the major form that is found in the cell. Z-DNA was initially found only under high salt conditions, but the cellular environment is actually a low-salt environment. The question then is whether type Z exist under cellular conditions. Several features have been discovered that can stablize Z-DNA under in a low salt environment.
Alternating purine/pyrimidine tracts
poly GC or poly AT stretchesvvv
5-methyl-cytosine
Because both of these conditions can exists in the cell, it is suggested that stretches of Z-DNA may actually exists in the cell along with other stretches of B-DNA.
In addition to the direction the molecule turns, several other differences exists between the various forms of DNA. The following table summarizes the features of the different forms of DNA.
Form Direction Bases/
360o Turn Helix
Diameter
A Right 11.0 23A
B Right 10.0 19A
C Right 9.3 19A
Z Left 12.0 18A
source: http://www.ndsu.edu/pubweb/~mcclean/plsc731/dna/dna4.htm
DNA
What is DNA?
We all know that elephants only give birth to little elephants, giraffes to giraffes, dogs to dogs and so on for every type of living creature. But why is this so?
The answer lies in a molecule called deoxyribonucleic acid (DNA), which contains the biological instructions that make each species unique. DNA, along with the instructions it contains, is passed from adult organisms to their offspring during reproduction.
What is DNA made of?
DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group and one of four types of nitrogen bases. To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating.
The four types of nitrogen bases found in nucleotides are: adenine (A), , thymine (T), guanine (G) and cytosine (C). The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. For example, the sequence ATCGTT might instruct for blue eyes, while ATCGCT might instruct for brown.
Each DNA sequence that contains instructions to make a protein is known as a gene. The size of a gene may vary greatly, ranging from about 1,000 bases to 1 million bases in humans.
The complete DNA instruction book, or genome, for a human contains about 3 billion bases and about 20,000 genes on 23 pairs of chromosomes
What does DNA do?
DNA contains the instructions needed for an organism to develop, survive and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies
How are DNA sequences used to make proteins?
DNA’s instructions are used to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA.
Next, the information contained in the mRNA molecule is translated into the “language” of amino acids, which are the building blocks of proteins. This language tells the cell’s protein-making machinery the precise order in which to link the amino acids to produce a specific protein. This is a major task because there are 20 types of amino acids, which can be placed in many different orders to form a wide variety of proteins.
Who discovered DNA?
The German biochemist Frederich Miescher first observed DNA in the late 1800s. But nearly a century passed from that discovery until researchers unraveled the structure of the DNA molecule and realized its central importance to biology.
For many years, scientists debated which molecule carried life’s biological instructions. Most thought that DNA was too simple a molecule to play such a critical role. Instead, they argued that proteins were more likely to carry out this vital function because of their greater complexity and wider variety of forms.
The importance of DNA became clear in 1953 thanks to the work of James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin. By studying X-ray diffraction patterns and building models, the scientists figured out the double helix structure of DNA – a structure that enables it to carry biological information from one generation to the next.
What is the DNA double helix?
Scientist use the term “double helix” to describe DNA’s winding, two-stranded chemical structure. This shape – which looks much like a twisted ladder – gives DNA the power to pass along biological instructions with great precision.
To understand DNA’s double helix from a chemical standpoint, picture the sides of the ladder as strands of alternating sugar and phosphate groups – strands that run in opposite directions. Each “rung” of the ladder is made up of two nitrogen bases, paired together by hydrogen bonds. Because of the highly specific nature of this type of chemical pairing, base A always pairs with base T, and likewise C with G. So, if you know the sequence of the bases on one strand of a DNA double helix, it is a simple matter to figure out the sequence of bases on the other strand.
DNA’s unique structure enables the molecule to copy itself during cell division. When a cell prepares to divide, the DNA helix splits down the middle and becomes two single strands. These single strands serve as templates for building two new, double-stranded DNA molecules – each a replica of the original DNA molecule. In this process, an A base is added wherever there is a T, a C where there is a G, and so on until all of the bases once again have partners.
In addition, when proteins are being made, the double helix unwinds to allow a single strand of DNA to serve as a template. This template strand is then transcribed into mRNA, which is a molecule that conveys vital instructions to the cell’s protein-making machinery.
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