22 Jan, 2018

Genetics 101

The recipe book for life

Every living organism, from the simplest bacteria to the emotional sack of cells we call humans, carries an instruction manual in every cell. This, of course, is DNA, which is a long string of the letters A, T, G, and C which encodes the information fundamental to human life.

In humans, the string of DNA is about 6 billion letters long. How these 6 billion letters guide us from the moment that sperm meets egg to through the rest of our life has driven hundreds of years of research.

A,T,G, and C are the four nucleotides. A pairs with T and G pairs with C. Genetic information is stored in the precise sequence of these nucleotides. Change one letter and your cell may start acting very differently.

Image showing a strand of dna with nucleotides A, T, G and C in sequence

Genes become proteins

Proteins are the machinery that every cell in your body (skin, eyes, heart, neurons, etc) uses to get the job done. Every protein made in our bodies is encoded in the DNA, specifically by genes. You can think of DNA like a recipe book — some of the text is devoted to describing the ingredients (called ‘genes’, which turns into protein) and some of the text is devoted to describing the ‘process’ by which the ingredients are combined (called ‘regulatory’).

Only about 2% of your genome codes genes that turn into proteins

The other 98% is a mix of sequence that is involved in regulation, as well as some evolutionary ‘junk’ that has accumulated over time. Deciphering this ‘recipe book’ to understand what genes and regulatory regions do and what happens when their sequence is altered by a mutation is the focus of an immense amount of research.

DNA is inherited

The genome does not fall far from the tree. You get about half of your genome from your mom, and the other half from your dad. This fundamental feature of sexual reproduction contributes to genetic diseases that run in families, shared behaviours/traits with family members, and allows us to track our ancestry to find distant relatives.

DNA sequencing is changing the world

Gregor Mendel, an Austrian monk, is considered the father of genetics. His experiments on pea plants focused on a number of different traits including flower colour, stem length, and characteristics of the seeds. His work was published in 1866, and when it was rediscovered almost 30 years later, it would define the basic principles of inheritance.

Diagram showing Mendel's experiment: 1 tall plant x 1 short plant results in all tall plants in the next generation. When this generation is self fertilised, the next generation has roughly 3 tall plants to one short. 'Mendel's actual numbers: 787 tall : 277 short (2.84:1)'

In the first generation, the tall feature (the dominant trait) takes over the short feature (the recessive trait). In the second generation, the recessive trait reappears in a minority of the plants. Credit Khan Academy.

Fast forward 100 years later, the structure of DNA (the double helix) was discovered in Cambridge, UK by James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin. In 1977, Fred Sanger, also in Cambridge in the UK, invented the first widely adopted genome sequencing technology, which would be dubbed the ‘Sanger Sequencing’ method, allowing the genome to be systematically read for the first time. Our ability to read and manipulate DNA has skyrocketed in the last twenty years. An international effort to sequence the first human genome (called the Human Genome Project), began in 1990 and ended in 2001, costing upwards of $1 Billion.

In the ~15 years since the Human Genome Project was completed, the cost of genome sequencing has dropped to less than $1,000 and shows no signs of slowing down. In the last year alone, more than 1 million people have been involved in a genome-sequencing project. Still, we have only just begun to understand how our genome impacts our health and behaviours.

Photo credit: Wooozxh - Unsplash
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