Deoxyribonucleic acid (DNA)

What decides whether a rose is red or white, whether a pea plant grows tall or stays short, or whether your eyes are black or brown? The instructions for every one of these qualities are written inside each living cell, in a chemical called deoxyribonucleic acid. It is usually shortened to DNA. DNA is a long, thread-like structure found inside the cell, and it carries genetic information.

Where DNA sits depends on the kind of cell. Prokaryotic cells, such as bacteria, have no true nucleus, so their DNA lies in the cytoplasm. Eukaryotic cells, such as the cells of plants, animals and humans, keep their DNA in the chromosomes inside the nucleus. A virus is not a cell at all, yet some viruses also carry DNA. In such a virus the DNA is covered by a capsid, which is a protein coat.

A long necklace is made by threading small beads one after another. DNA is built in the same way, from small units joined in a chain. Each unit is made from three kinds of part: a nitrogen base, a deoxyribose sugar, and a phosphate ion. What the unit is called depends on how many of these parts it has.

A single DNA molecule has thousands upon thousands of nucleotides joined in a row, and the order in which they sit is what matters most. The letters of the alphabet do the same job in words: the same letters in a different order give "listen" and "silent", two different words. In the same way, the sequence of nucleotides is different from one individual to another. That is why individuals of the same kind also differ in their qualities.

DNA is not a single chain of nucleotides. It has two strands, twisted around each other like a spiral ladder. The two side rails of the ladder are made of sugar and phosphate joined turn by turn. The steps of the ladder are pairs of nitrogen bases: a base on one strand joins a base on the other strand in the middle.

The two strands run in opposite directions. When one strand runs upward, the other runs downward, just as the traffic in the two lanes of a two-way road moves in opposite directions. This is why the strands of DNA are called antiparallel.

Four types of nitrogen base are found in DNA:

• adenine
• guanine
• cytosine
• thymine

These bases do not join at random. Adenine joins only with thymine, by making a double bond (two hydrogen bonds). Guanine joins only with cytosine, by making a triple bond (three hydrogen bonds). In short form each base is written by its first letter, A, G, C and T, and the double and triple bonds are shown by two and three lines:

\[ \text{A} = \text{T} \]

\[ \text{G} \equiv \text{C} \]

Example: a short part of one DNA strand has its bases in the order A T G C C A. Which bases sit opposite them on the other strand, and how many bonds hold these six pairs together?

Step 1: The given strand is:

\[ \text{A T G C C A} \]

Step 2: Writing the partner of each base, putting T opposite A, A opposite T, C opposite G and G opposite C, gives the partner strand:

\[ \text{T A C G G T} \]

Step 3: Counting the pairs, there are 3 pairs of A with T (at the first, second and sixth places) and 3 pairs of G with C (at the third, fourth and fifth places). Since each A and T pair has 2 bonds and each G and C pair has 3 bonds, the total number of bonds is:

\[ 3 \times 2 + 3 \times 3 = 6 + 9 = 15 \]

So the partner strand is T A C G G T, and the six pairs are held together by 15 bonds in all.

DNA carries the hereditary information, or characters, of an organism. But a whole DNA molecule does not stand for one single quality. A small segment of it carries the information for one particular character. For example, one segment of DNA decides whether a pea plant grows tall or dwarf, and another segment decides the colour of its flowers. Such a segment is called a gene. A chromosome carries a very large number of genes arranged one after another along it.

Arranged from the largest to the smallest, these structures fit inside one another like this:

• Chromosome: a structure in the nucleus, made of DNA.
• DNA: the long thread-like molecule inside the chromosome.
• Gene: a small segment of DNA that codes one character.
• Nucleotide: the smallest structural unit from which DNA is built.

When a cell divides, the hereditary information in the DNA of the mother cell is passed to the daughter cells. In this way every new cell receives the same instructions. This is what helps characters pass from one generation to the next. It is the reason children look like their parents: they have received their parents' DNA.

To put the information in DNA to use, RNA is formed from DNA. This process is called transcription. The RNA formed in this way helps the cell to synthesize proteins. So the order is: DNA forms RNA, and RNA helps make protein.

RNA is also made of nucleotides joined together, so it is called a polynucleotide. Each of its nucleotides is formed by the combination of a nitrogen base, ribose sugar and phosphate. Notice the sugar: DNA has deoxyribose sugar, while RNA has ribose sugar. The two names come from these sugars.

RNA has only one strand. It is generally found in the cytoplasm, and some amount is also found in the chromosomes. Some viruses have no DNA. In them the RNA is enclosed within the capsid, and that RNA serves as their genetic material.

RNA also has four types of nitrogen base: adenine, guanine, cytosine and uracil. Here uracil takes the place of thymine. Adenine is linked with uracil by a double bond, and guanine is linked with cytosine by a triple bond:

\[ \text{A} = \text{U} \]

\[ \text{G} \equiv \text{C} \]

There are three types of RNA. The main function of RNA is to synthesize proteins, and the three types each play a different part in that work:

• Messenger RNA (m-RNA): carries the message from DNA to the place where proteins are made.
• Transfer RNA (t-RNA): brings the building materials needed to make the protein.
• Ribosomal RNA (r-RNA): forms part of the ribosomes, where proteins are put together.

DNA and RNA are both made of nucleotides, but they differ clearly in their strands, sugar, bases, location and function. The table below puts these differences side by side.

An easy way to hold the difference in mind: DNA is a strong two-stranded ladder that keeps the information safe, and RNA is a light single-stranded copy that takes the information to where the work is done. One thing is the same in both: guanine pairs with cytosine by a triple bond.