DNA, RNA and Protein Synthesis 5.1

DNA

DNA (deoxyribose nucleic acid) is found in the cells of all organisms. It is the molecule that contains all the genes that code for the characteristics and cell functions of an organism. The sequence and arrangement of your DNA determines everything about you.

DNA is a polynucleotide. It is made up of many repeating units called nucleotides, but these nucleotides are not all the same. A DNA nucleotide consists of a deoxyribose sugar, a phosphate group and a nitrogenous base, These three components are very important in determining the physical structure of DNA.

The above picture shows us a mononucleotide. But how do these bond to form the polynucleotide DNA?

The phosphate from one nucleotide forms a covalent bond with the ribose of another in a condensation reaction, forming a phosphodiester bond.

These bonds form between many nucleotides to make one long DNA strand. But DNA is a double helix--two strands spiralling around one another. Base pairing shows how two strands come together to make the double helix.

Base pairing

Nucleotides come in four main flavours--Adenine (A) Guanine (G) Cytosin (C) and Thymine (T). They can be categorised into two types:

Purines (two nitrogenous organic rings)

• Adenine
• Guanine

Pyrimidines (one nitrogenous organic rings)

• Cytosine
• Thymine

Nucleotides always bond in the same way-- adenine always bonds with thymine and cytosine always bonds with guanine. This is because cytosine and guanine can form three hydrogen bonds between one another and adenine and thymine can form two between eachother--it is the most chemically stable combination.

So, two long strands of DNA run along side eachother (antiparallel to one another) bound down the middle by hydrogen bonds and the difference in size between the purines and pyrimidines causes DNA's helical shape. One strand, the Sense Strand, contains all the nucleotide sequences that determine your characteristics. The other strand is the AntiSense Strand and is mostly important for DNA replication. Sections of these nucleotide sequences are known as genes and will be looked at later on.

DNA Replication

During cell division, DNA replicates. During mitosis, two genetically identical daughter cells are produced from one parent cell. This means that each daughter cell must have identical DNA to its parent cell, thus one molecule of DNA must split into two identical molecules of DNA.

To do this, an enzyme called DNA polymerase moves along the length of the DNA molecule breaking hydrogen bonds and splitting the two DNA strands. As this is going on, DNA nucleotides from the cytoplasm attach via the exposed bases onto either strand, forming two identical strands. The complementary base pairing rule ensures that the right nucleotides are added to each side of the molecule. 

So, each of the two new molecules has one strand of the original DNA and one strand of new DNA. This is semi-conservative replication.

Evidence for Semi-Conservative Replication

In 1953, using X-ray diffusion techniques devoloped by Rosalind Franklin, Watson and Crick pieced together evidence to suggest a possible structure for DNA and also suggested that DNA replicated…