Unravelling the DNA mystery

DNA is an acronym for Deoxyribo nucleic Acid. This, along with RNA, Ribonucleic Acid, is contained in every living organism. The tiniest mold, the most aggressive bacteria, and you and I contain DNA. DNA comprises all the information that makes you, you, and me, me.

DNA is a complex chemical that comes in two forms. One is in a structure in the cytoplasm called mitochondrial DNA. The other, which is the subject of this article, is found in the nucleus of cells of all eukaryotes (higher organisms with a nucleus in their cells) and found loosely in the cytoplasm of prokaryotes (bacteria with no nucleus in their cell).

DNA is made of two sugar-phosphate spines, with four base pairs of nucleic acids (nucleotides) joining the spine in a shape known as a double-helix. The double helix is kind of like a spiral staircase or a twisted ladder.

The rungs of the ladder are the adjoined base-pairs, and the handles of the ladder are the sugar-phosphate spines. This structure was discovered in 1953 by James Watson and Francis Crick, with assistance from Rosalind Franklin.

Although the sugar-phosphate spines are important, it is the four base pairs that determine who, and even what you are. The four bases are Thymine, Adenine, Guanine and Cytosine, known as T, A, G and C or "TAGCi" (pronounced ``Taxi!'') to help my pupils remember. Importantly, T always bonds with A, and G always bonds with C. It is estimated that the DNA in your cells is meters long, which means that with your approximately one trillion cells, hypothetically your DNA, end to end, would easily stretch to the moon and back! As it is, it is contained in coils upon coils within coils inside the nucleus.

Genes: Transcription and translation

DNA replication or DNA synthesis is the process of copying a double-stranded DNA molecule. — PHOTO COURTESY OF MARIANA RUIZ

Lengths of DNA in a specific base-pair order are known as genes. Genes are the things that give us our characteristics. These characteristics, such as blue eyes, the ability to roll your tongue and other features, are known as your phenotype, which is a physical expression of your genotype: your genetic code.

The Human Genome Project is still a work in progress and the aim is to map the human genome; that is to know every gene in the human body, and what they code for.

The actual number of genes in the human genome is still unknown, but it is estimated to be between 20,000-30,000.

The primary role of genes is to make proteins. Each gene has a specific code that can be transcribed into RNA. In RNA, Thymine (T) is replaced by a base called Uracil (U), therefore A now only bonds with U. This specific strand of RNA travels out of the nucleus to other parts of the cell where it is translated into Amino Acids, which are three base-pairs long, also known as a codon.

There are 20 different kinds of amino acid, and when they are linked together in chains called polypeptide chains, often hundreds of amino acid links long, they form particular proteins. These proteins are essential to the workings of the body, as they make hormones, enzymes, antibodies and on a large scale, muscles. So essentially, genes are just instructions for making proteins, and proteins do pretty much everything.

Chromosomes

When your cells divide, as they are doing right now in order to make new cells to replace old ones, your DNA coils to form structures known as chromosomes. Humans have 46 chromosomes in 23 pairs. Chromosomes only form during cell division as they come together in order to replicate themselves to make sure that the newly formed cells have the same 23 pairs of chromosomes.

During this replication, the DNA unzips along the middle of the ladder leaving two separate rungs with bases attached. (See diagram below.) Then, the free bases of T, A, G and C are attracted to the rungs, making the DNA reform and double. The reason it is copied correctly is because A and T attract only each other, and G and C only attract each other. This ensures that every cell in your body (excluding cells without a nucleus, such as red-blood cells) has all of your genetic information.

One particular pair of chromosomes you might be familiar with are chromosomes 23. These are also known as the sex chromosomes and determine whether you are male or female. Females have an XX combination whereas males have an XY combination, with the Y chromosome being a tiny partner in the pair.

When your cells split to form sex cells, sperm and egg, only half of the pairs, therefore 23 chromosomes, are carried in the nucleus. This means that when the sperm meets the egg during fertilization, the 23 from the sperm and the 23 from the egg pair up to make the complete set of 46 chromosomes.

From this one new cell, the cell with the DNA replicates to form two cells, then four, then eight, 16, 32 and so on until you are complete. And as the sperm carries either an X or a Y chromosome in its nucleus, the father's sperm essentially determines the sex of the child.

As all of your cells have the same unique DNA and due to this accurate replication, if a person's cells, such as skin, blood, semen or hair, are left at a crime scene, a simple swab of cheek cells can match your DNA to any of the other cells. Therefore, science is outwitting criminals as a result of the infallible and undeniable evidence written in your cells.

David Canavan has an MSc in Behavioral Ecology and teaches science, math and ICT at Garden International School. David is fascinated by science and loves animals, especially the dangerous kind; the more dangerous the better. You may contact David at davidc@gardenbangkok.com .