DNA structure and cis-platin

In a nutshell

Deoxyribonucleic acid (DNA) is a polynucleotide found in living cells. It is a chain of nucleotides and is formed via condensation polymerisation. It also adopts a double helix structure due to hydrogen bonding and complementary base pairing. DNA carries the genetic information of living organisms. Cis-platin is an anti-cancer drug which can bind to DNA and prevent it from replicating, terminating the reproduction of tumour cells.

Nucleotides

A nucleotide is a molecule that contains a nitrogen-containing base, a phosphate group and a pentose sugar. A schematic structure is shown below.

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin

The nitrogen-containing bases can either be purines or pyrimidines. Purines, like adenine (A) and guanine (G) have two nitrogen-containing rings. Pyrimidines, like thymine (T) and cytosine (C) have one nitrogen-containing ring.

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin — 1. Adenine; 2. Guanine; 3. Cytosine; 4. Thymine.

DNA structure

A DNA nucleotide consists of a phosphate group, a pentose sugar called $2$ -deoxyribose and a base. The base can be adenine (A), thymine (T), cytosine (C) or guanine (G).

You can find below a schematic structure of a DNA nucleotide, followed by the specific structures of each DNA nucleotide.

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin — Thymine nucleotide

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin — Adenine nucleotide

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin — Guanine nucleotide

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin — Cytosine nucleotide

DNA consists of two polynucleotides. A polynucleotide is a polymer of nucleotides. Adjacent nucleotides are joined via a condensation reaction between the phosphate group of one nucleotide and the sugar of another nucleotide. This gives rise to a sugar-phosphate backbone. The bond between nucleotides is called a phosphodiester bond. A sugar from another nucleotide may react at point 1. and a phosphate group may react at point 2. below.

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin

Hydrogen bonds form between the bases of two DNA polynucleotides. Adenine always pairs with thymine ( $A-T$ ) using two hydrogen bonds. Cytosine always pairs with guanine $(C-G)$ using three hydrogen bonds. This is known as complementary base pairing.

Note: Due to complementary base pairing, there will always be equal amounts of adenine and thymine in a molecule of DNA. The same applies to cytosine and guanine.

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin — 1. Hydrogen bonding

The polynucleotides of DNA must form a spiral in order for the bases to align at the correct angle, at the perfect distance and correctly base pair. This leads to DNA to adopt a double helix structure.

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin

Cisplatin: anti-cancer drug

Cis-platin is a complex used in chemotherapy as an anti-cancer drug.

Chemistry; Amino acids, proteins and DNA; KS5 Year 12; DNA structure and cis-platin

The complex has a central platinum ( $II$ ) ion, two chloride ligands and two ammonia ligands. The complex adopts a square planar shape.

Tumours are abnormal cell growths. The uncontrollable division of cells which spreads to other parts of the body is called cancer. Cells undergo DNA replication in order to divide. Cis-platin binds to DNA preventing DNA from being copied, helping to hamper the reproduction of tumour cells.

procedure

1.	Guanine replaces a chloride ion via ligand substitution. A co-ordinate bond is formed between a nitrogen atom on guanine in DNA and the platinum ( $II$ ) ion in cis-platin.
2.	Another nitrogen from a neighbouring guanine replaces the second chloride ion, also via ligand substation.
3.	The binding of cis-platin to DNA causes a kink in the DNA structure.
4.	DNA is unable to unwind and undergo DNA synthesis. Therefore, the cells are no longer able to divide.

It's great to have cis-platin as an anti-cancer drug but it comes with disadvantages.

Cis-platin binds to DNA in normal cells too. This causes a great problem for cells which divide, hence replicate their DNA, frequently, such as erythrocytes and hair cells. As a consequence, patients taking cis-platin as a drug treatment experience hair loss and their immunity is compromised. These side effects can be reduced by administering low doses or by delivering cis-platin to the site of the tumour, decreasing the chances of it affecting healthy cells.

Although there are unpleasant side effects, it is continued to be used in chemotherapy as the positive long-term effects outweigh the bleak short-term side effects.