describe the structure of a Dna molecule?

Answer 1

The building blocks are nucleotides.

The nucleotides are joined in such a way that they form a double helix that is like a twisted rope ladder.

The steps of the ladder are made by the paired nitrogen bases of opposing nucleotides. A-T and C-G

The sides of the ladder are alternating sugars and phosphates.

Answer 2

In physiological conditions, the double stranded DNA molecule is in the B-form helical duplex, that is, two complementary strands of DNA that H-bond together and spiral in a right-handed fashion with the base pairing perpendicular to the helical axis. Hydrophobic base stacking, however, is the most important factor in keeping the strands together, not the H-bonds (which are primarily for genetic info storage). There are major and minor grooves present on the B-form DNA molecule. In the minor grooves, water is able to bind and help stabilize the B-form helix, forming what is called the "spine of hydration." Each strand of DNA consists nucleotides linked together via a sugar phosphate backbone. The linkage is between 5' C on one sugar ring and the 3' C on another sugar ring. The phosphates would disrupt and break up the DNA structure were it not for the ions/charged molecules present in physiological solution (eg structure would break up if DNA placed into pure water).

Answer 3

A double helix

Answer 4

DNA structure
DNA structure was first described by James Watson and Francis Crick in 1953.

Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and function of living organisms. All living things contain DNA, with the exception of some viruses with RNA genomes. The main role of DNA in the cell is the long term storage of information. It is often compared to a blueprint, since it contains the instructions to construct other components of the cell, such as proteins and RNA molecules. The DNA segments that carry genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the expression of genetic information.

In eukaryotes such as animals and plants, DNA is stored inside the cell nucleus, while in prokaryotes such as bacteria, the DNA is in the cell's cytoplasm. Unlike enzymes, DNA does not act directly on other molecules; rather, various enzymes act on DNA and copy its information into either more DNA, in DNA replication, or transcribe it into protein. In chromosomes, chromatin proteins such as histones compact and organize DNA, as well as helping control its interactions with other proteins in the nucleus.

DNA is a long polymer of simple units called nucleotides, which are held together by a backbone made of sugars and phosphate groups. This backbone carries four types of molecules called bases and it is the sequence of these four bases that encodes information. The major function of DNA is to encode the sequence of amino acid residues in proteins, using the genetic code. To read the genetic code, cells make a copy of a stretch of DNA in the nucleic acid RNA. These RNA copies can then be used to direct protein synthesis, but they can also be used directly as parts of ribosomes or spliceosomes.



The structure of DNA is illustrated by a right handed double helix, with about 10 nucleotide pairs per helical turn. Each spiral strand, composed of a sugar phosphate backbone and attached bases, is connected to a complementary strand by hydrogen bonding (non- covalent) between paired bases, adenine (A) with thymine (T) and guanine (G) with cytosine (C).

Adenine and thymine are connected by two hydrogen bonds (non-covalent) while guanine and cytosine are connected by three.

This is what they already knew from the work of many scientists, about the DNA molecule:

DNA is made up of subunits which scientists called nucleotides.
Each nucleotide is made up of a sugar, a phosphate and a base.
There are 4 different bases in a DNA molecule:
adenine (a purine)
cytosine (a pyrimidine)
guanine (a purine)
thymine (a pyrimidine)
The number of purine bases equals the number of pyrimidine bases
The number of adenine bases equals the number of thymine bases A=T
The number of guanine bases equals the number of cytosine bases G=C
The basic structure of the DNA molecule is helical, with the bases being stacked on top of each other
Working with nucleotide models made of wire, Watson and Crick attempted to put together the puzzle of DNA structure in such a way that their model would account for the variety of facts that they knew described the molecule. Once satisfied with their model, they published their hypothesis, entitled "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" in the British journal Nature (April 25, 1953. volume 171:737-738.) It is interesting to note that this paper has been cited over 800 times since its first appearance!

Here are their words:

"...This (DNA) structure has two helical chains each coiled round the same axis...Both chains follow right handed helices...the two chains run in opposite directions. ..The bases are on the inside of the helix and the phosphates on the outside..."

"The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases... The (bases) are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side...One of the pair must be a purine and the other a pyrimidine for bonding to occur. ...Only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine)."

"...in other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined."

and

"...It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

And with these words, the way was made clear for tremendous strides in our understanding of the structure of DNA and, as a result our ability to work with and manipulate the information-rich DNA molecule.