nucleotides
Nucleic acids are the genetic material of the cell and are composed of recurring monomeric units
Each nucleotide is comprised of three principal components
- 5-carbon pentose sugar (pentagon)
- Phosphate group (circle)
- Nitrogenous base (rectangle)
what are the phosphate group and nitrogenous base attached to?
Both the phosphate group and nitrogenous base are attached to the central pentose sugar
where is the nitrogenous base attached?
The nitrogenous base is attached to the 1’– carbon atom (right point)
where is the phosphate base attached?
The phosphate base is attached to the 5’– carbon atom (left point)
There are two types of nucleic acids present in cells
DNA and RNA
DNA
DNA (deoxyribonucleic acid) is a more stable double stranded form that stores the genetic blueprint for cells
RNA
RNA (ribonucleic acid) is a more versatile single stranded form that transfers the genetic information for decoding
how do DNA and RNA differ?
DNA and RNA are both polymers of nucleotides, however differ in a few key structural aspects:
- Number of strands present
- Composition of nitrogenous bases
- Type of pentose sugar
DNA vs. RNA pentose sugar
DNA: deoxyribose
RNA: ribose
DNA vs. RNA base composition
DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
RNA: Adenine (A), Guanine (G), Cytosine (C), Uracil (U)
DNA vs. RNA number of strands
DNA: double stranded (forms double helix)
RNA: single stranded
Nucleic acids are composed of…
nucleotide monomers which are linked into a single strand via condensation reactions
nucleic acid structure and bonds
- 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
Two polynucleotide chains of DNA are held together via…
hydrogen bonding between complementary nitrogenous bases
hydrogen bonding
- Adenine (A) pairs with Thymine (T) via two hydrogen bonds
- Guanine (G) pairs with Cytosine (C) via three hydrogen bonds
The two strands of DNA are described as being…
antiparallel
As the antiparallel chains lengthen, the atoms will organise themselves into the most stable energy configuration
This atomic arrangement results in the double-stranded DNA forming a double helix (~10 – 15 bases per twist)
The structural organisation of the DNA molecule was correctly proposed in and by?
in 1953 by James Watson and Francis Crick
Watson and Crick’s key experimental discoveries:
- DNA is composed of nucleotides made up of a sugar, phosphate and base – Phoebus Levene, 1919
- DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) – Erwin Chargaff, 1950
- DNA is organised into a helical structure – Rosalind Franklin, 1953 (data shared without permission)
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)
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
- Nitrogenous bases were not initially configured correctly and hence did not demonstrate complementarity
The final construction of a correct DNA molecule owed heavily to the X-ray crystallography data generated by Franklin
- This data confirmed the arrangement of the DNA strands into a helical structure
- The data was shared without Franklin’s knowledge or permission and contributed profoundly to the final design
- Hence, Franklin is now recognised as a key contributor to the elucidation of DNA structure
RNA functions
to transfer genetic instructions from the nucleus to the cytoplasm, where the information is decoded
