Molecular Biology: 2.6 DNA & RNA Structure Flashcards
Components of a nucleotide and state how they are structured together
Each nucleotide is comprised of three principal components:
5-carbon pentose sugar (pentagon) Phosphate group (circle) Nitrogenous base (rectangle)
Both the phosphate group and nitrogenous base are attached to the central pentose sugar
The nitrogenous base is attached to the 1’– carbon atom
The phosphate base is attached to the 5’– carbon atom
Define DNA
DNA (deoxyribonucleic acid) is a more stable double stranded form that stores the genetic blueprint for cells
Define RNA
RNA (ribonucleic acid) is a more versatile single stranded form that transfers the genetic information for decoding
State differences between DNA and RNA
DNA and RNA are both polymers of nucleotides, however differ in a few key structural aspects:
Pentose Sugar- DNA has a deoxyribose sugar (lacks one oxygen atom) whereas RNA has ribose sugar
Composition of nitrogenous bases- DNA and RNA has the same 3 bases except for DNA has Thymine (T) instead of Uracil (U) which is present in RNA
Number of strands present- DNA is double stranded whereas RNA is single stranded
Explain how nucleic acids are formed from the linking together of nucleotides?
Nucleic acids are composed of nucleotide monomers which are linked into a single strand via condensation reactions
The phosphate group of one nucleotide attaches to the sugar of another nucleotide (at the 3’– hydroxyl (-OH) group)
This results in a phosphodiester bond forming between the two nucleotides (and water is produced as a by-product)
Successive condensation reactions result in the formation of long polynucleotide strands
How are the two polynucleotide chains of DNA held together?
wo polynucleotide chains of DNA are held together via hydrogen bonding between complementary nitrogenous bases
Adenine (A) pairs with Thymine (T) via two hydrogen bonds
Guanine (G) pairs with Cytosine (C) via three hydrogen bonds
How are the two strands of DNA oriented and why?
In order for the bases to be facing each other and thus able to pair, the strands must be running in opposite directions
The two strands of DNA are described as being antiparallel. One strand is oriented in the direction 5’ to 3’
and the other is oriented in the direction 3’ to 5’.
The two strands are wound together to form a double helix.
When was the correct model of DNA structure proposed and by whom and how?
The structural organisation of the DNA molecule was correctly proposed in 1953 by James Watson and Francis Crick
These British scientists constructed models to quickly visualise and assess the viability of potential structures was
based on using the evidence collected about DNA structure
What did the first DNA model consist of by the British scientists?
As Watson and Crick’s model building was based on trial and error, a number of early models possessed faults:
The first model generated was a triple helix
Early models had bases on the outside and sugar-phosphate residues in the centre, and magnesium
holding the two strands together with ionic bonds to the phosphate groups on each strand.
Nitrogenous bases were not initially configured correctly and hence did not demonstrate complementarity
What successes and faults were pointed out by other scientists about the first model of DNA proposed?
The model consisted DNA organised into a helical structure which was proposed by Franklin from X-ray crystallography.
Fault included:
it was rejected when Franklin pointed out that there would not be enough magnesium available to form the cross links between the strands.
Another deficiency of this first model was that is that it did not take account of Chargaff’s finding that DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T)
Finally, after much trial and error what did the new and corrected model of DNA consisted of?
Using trial and error, Watson and Crick were able to assemble a DNA model that demonstrated the following:
DNA strands are antiparallel and form a double helix
DNA strands pair via complementary base pairing (A = T ; C Ξ G)
Outer edges of bases remain exposed (allows access to replicative and transcriptional proteins)
What was the consequence of the correct model of DNA proposed?
The structure immediately suggested a mechanism for copying DNA. It also led quickly to the realisation that the genetic code must consist of triplets of bases
