The different bases are categorised into two different groups. The purines, which have two rings of carbon and nitrogen atoms, and the pyrimidines, which have one ring of carbon and nitrogen atoms. Therefore the purines are the larger molecules. Purines include Adenine and Guanine, whereas Thymine, Cytosine and Uracil are pyrimidines. Below is a diagram illustrating the 3 components of nucleotides.
Polymerisation of nucleotides
We know that DNA and RNA are polynucleotides, meaning that they are made up of many nucleotide monomers linked together. The nucleotides are linked together via a condensation reaction between the hydroxyl group molecule in the phosphate and the hydroxyl group in 3-carbon of the pentose sugar. This reaction takes place in the nucleus during cell division, more precisely, during the interphase stage, where the cell is fully-grown and begins to function.
We can see from the diagram that the nitrogen containing bases are the only components of the nucleotide not to be involved in the reaction, therefore we can see that the bases project of the side of the sugar-phosphate back bone created after condensation reaction. This back bone is referred to as a polynucleotides strand. The strand is strong and stable, as covalent bonding, between hydroxyl groups from the pentose and the phosphate group, holds the structure together. This is called a phosphodiester bond.
This reaction applies to both RNA and DNA. However DNA consists of a double helix, therefore, two polynucleotides strands. In this case the two polynucleotide strands face each other side by side, and are held together by hydrogen bonds between the nitrogen containing bases (see diagram below).
In DNA, between the two polynucleotide strands, there is just enough space for one small molecule (base; pyrimidine) and on large molecule (purine), so therefore the base pairings in double polynucleotide chains is always purine–pyrimidine. The nitrogen bases bond as follows in DNA:
A – T
G – C
Or in case of RNA:
A – U* Notice how in RNA, the Uracil replaces th Thymine molecule.)
G – C
This is called complimentary base pairing hence meaning that whatever the sequence of nitrogen containing bases in one strand, the sequence of the other strand must be complimentary to it. However these hydrogen bonds between base pairings can easily be broken, this occurs when the DNA is replicated, or when DNA is being manufactured.
Differences between DNA and RNA
Amongst numerous differences between the different nucleic acids, the main difference is that DNA has a deoxyribose (pentose) sugar on its nucleotide structure, therefore it is a deoxyribonucleic acid. However RNA, has a ribose sugar on its nucleotide structure, therefore it is ribonucleic acid. Ribose has OH on 2-carbon, unlike deoxyribose, which lacks an O (oxygen) in 2-carbon, and is only left with a H (hydrogen). Moreover, DNA is double stranded polynucleotide, unlike RNA, hence RNA is not paired, therefore it will have a greater variety of nucleic bases (refer to different types of RNA)., whereas DNA will only have 4. The polynucleotide strand of RNA is shorter than that of the DNA, as DNA has to carry a large amount of genetic instructions, which need to be transcribed.. Finally, RNA has a pyrimidine in its strand; uracil, whereas DNA does not, instead it contains Thymine.
The different types of RNA and their roles
There are three main types of RNA. These are tRNA, mRNA and rRNA. All the different types of RNA mentioned are all transcribed from DNA, in a process called Transcription. The roles of each of these RNA types are explored below.
- mRNA: (messenger ribonucleic acid) this is single stranded RNA manufactured on the DNA, and it contains instructions how to construct a protein (protein synthesis). mRNA travels from the cell of a nucleus to the ribosomes, where protein synthesis takes place.
- Transfer RNA or tRNA for short, translates the language of nucleotides into the language of amino acids. It carries amino acids and places them in a protein that is being produced according to the instructions of mRNA. This assembly process is called translation.
- Ribosomal RNA (rRNA are also single stranded RNA polynucleotides. Together with a protein, rRNA forms ribosomes, which are the site of protein synthesis and mRNA translation.
Classic Scientific Work
Francis Crick, and James Watson had introduced the structure of the nucleic acids in 1953. The two scientists at Cavendish Campus in Cambridge University had worked on the structure of DNA, after they were inspired by the work of Rosalind Franklin, and had decided to discover the structure of DNA. When they did, 9 years after their discovery they were awarded the Nobel Prize, this was shared with their colleague Maurice Wilkins, however not with Rosalind Franklin, as she had passed away before the Nobel Prize, and the prize could not be awarded to the deceased.
Just by looking at the model they had created, Watson and Crick could see from the structure of the DNA, that it could be copied or replicated perfectly. They were so certain that the structure of the DNA was correct as before they experimented they knew that, there was four things that the experiment had to find out; that the sugar-phosphate backbone was on the outside, and the bases were on the inside, that the strands went opposite ways, anti parallel, that the molecule was a double helix and also that there was specific base pairings. The picture shows Watson and Crick demonstrating how the DNA molecule is structured.
Further classical genetics works have taken place regarding the structure of the DNA. For instance when Watson and Crick stated that the DNA molecule would copy itself perfectly, time and time again, although now we know that its is true, at the time, there was no proof. So an experiment was carried out by Mehelson and Stahl in 1958 to prove that semi conservative replication took place. This meant that one strand of the structure of DNA would be replaced by a new strand. Semi conservative replication was only one of the theories, which existed, however it was the most likely, the other two theories of replication of the DNA structure were conservative and dipersive replications.
