DNA — The Code of Life

The structure of DNA

Deoxyribonucleic acid (DNA) is the molecule that carries the genetic instructions for building and running every living organism. DNA is shaped like a twisted ladder — a double helix — made of two strands winding around each other.

Each strand is a chain of repeating units called nucleotides. Every nucleotide has three parts:

• A deoxyribose sugar (a 5-carbon sugar)
• A phosphate group
• A nitrogenous base — either adenine (A), thymine (T), guanine (G), or cytosine (C)

The sugar and phosphate groups alternate to form the "backbone" (the sides of the ladder), while the nitrogenous bases point inward and form the "rungs".

Complementary base pairing

The two strands of the double helix are held together by hydrogen bonds between the bases, and the bases only pair up in specific combinations — this is called complementary base pairing:

Because of this strict pairing rule, if you know the sequence of bases on one strand, you can always work out the sequence on the other — the strands are complementary, not identical.

Packaging DNA — chromosomes, genes and alleles

A DNA molecule is extremely long, so it is tightly coiled and packaged around proteins to form a chromosome. A section of DNA that codes for a particular characteristic (such as eye colour) is called a gene. Different versions of the same gene are called alleles — for example, an allele for brown eyes and an allele for blue eyes are both versions of the eye-colour gene, found at the same position (locus) on homologous chromosomes.

DNA replication

Before a cell divides, its DNA must be copied so that each new cell receives a complete set of instructions. This process is called DNA replication, and it is semi-conservative:

• The double helix "unzips" as the hydrogen bonds between bases break, separating the two strands.
• Each original strand acts as a template.
• Free nucleotides pair up with the exposed bases on each template strand, following the A-T and G-C rule.
• Two new double helices are formed, each containing one original (old) strand and one newly-made strand — hence "semi-conservative".

Accurate replication is essential — it ensures that when a cell divides (during mitosis or meiosis), every daughter cell receives an identical, complete copy of the genetic information.

From gene to protein: the genetic code

Genes don't build the body directly — they carry the instructions for making proteins, which do the actual work in cells (as enzymes, structural components, hormones, and more). This happens in two main steps:

• Transcription — in the nucleus, the DNA code for a gene is copied into a molecule of messenger RNA (mRNA), which then leaves the nucleus.
• Translation — at a ribosome, the mRNA sequence is "read" three bases at a time. Each group of three bases is called a codon, and each codon specifies one particular amino acid. Amino acids are joined together in the order specified by the codons, building up a chain that folds into a functional protein.

Mitochondrial DNA and stem cells

Most of a cell's DNA is found in the nucleus, but mitochondria also contain their own small circular DNA molecule. Mitochondrial DNA is passed down almost exclusively from mother to offspring (sperm mitochondria are not passed on at fertilisation), which makes it a useful tool for tracing matrilineal ancestry — the maternal line of a family, generations back.

Stem cells are undifferentiated cells that have the potential to develop into many different specialised cell/tissue types in the body. Because of this ability, they are studied for their potential in medical treatments — for example, replacing damaged tissue or treating certain blood disorders.