If you ask biologists how they define The Human Genome, they will definitely say, “It is the Book of Life”. It is true, indeed. If you write down every base (A, T, G, C) sequence of our genome in a book, you will end up with one billion words, equivalent to 800 Bibles in volume. Really, it is a gigantic document with 3.2 billion base pairs, equipping more than 20,000 protein coding genes, wrapped up as 23 chromosomes, all fits inside the nucleus of a tiny cell in our body. But, how these lifeless molecule sequences became a blueprint to conscious life? What does a chromosome teach us about humans? Probably no one could explain this better than Matt Ridley did in his book Genome: The Autobiography of a Species in 23 Chapters. Although a 20-year-old book, it is one of the greatest books written on human genome. Ridley picks up a prominent gene on each chromosome and frames a chapter on it connecting with a human characteristic, which is a brilliant way to narrate the story of genome.
(As I’m interested in discussing about each chromosome i.e each chapter of the book, I’ve divided this article into two parts, Part-1 and Part-2)
What is life? How life originated on Earth? Ridley starts the book with these big questions in chapter one. Life, according to biology is a characteristic which has the ability to replicate and to create order. It consists of the interplay of two kinds of chemicals DNA(contains information) and proteins (performs functions). But, neither can exist without the other. There is another chemical substance that links both. This substance can act as both replicator (like DNA) and catalyst (like protein). It is RNA, the messenger chemical. Probably they were the first life forms on Earth. Ridley further discusses about the last universal common ancestor (Luca) and the striking similarity of self evolving synthetic RNAs with 5S gene on chromosome 1.
Ridley talks about the ancestry and the origin of our species in this chapter. He points out that even though we (Homo sapiens) have one pair lesser chromosomes than apes, our genes are 98% identical to them. He further stresses that this 2% difference in genes lead to anatomical and behavioural differences with our closest relative species. In fact, Chromosome 2 was formed from the fusion of two medium-sized ape chromosomes.
Ridley recounts the history of scientific research on heredity and solving DNA structure, right from Gregor Mendel to Watson and Crick. Read my previous article The Greatest Puzzle Ever Solved: The DNA to know more about this enthralling history of genetic research.
On chromosome 4, there lies the most famous of all the disease genes. A complete lack of this gene causes Wolf-Hirschhorn syndrome. Mutated versions of the gene causes Huntington’s chorea, cerebellar ataxia and other incurable neurological diseases. This gene contains only one ‘word’ C-A-G and the number of repeats of this ‘word’ can decide our destiny. Ridley repeats that our fate lies in our genes due to the chance of these dreadful mutations.
Ridley details the scientific research on finding asthma gene. For instance, a mutation in ADRB2 gene on chromosome 5 was thought to be the cause of asthma. But, there lies another fifteen genes on this chromosome which can cause this disease. Ridley introduces here the concept of pleiotropy (multiple effects of multiple genes) and pluralism to analyze the case of asthma.
Can we inherit intelligence from our parents? Are there any genes to increase our IQ? Ridley explores these questions in this chapter detailing with the scientific quest for ‘intelligence gene’. So far, we didn’t find one. Although, IGF2R gene on chromosome 6 was thought to be the gene that may influence IQ, but no direct relation found. In fact, there is no accepted definition for intelligence. Mostly, it is a combination of analytic, creative and practical skills. Neither there is an accurate method to measure intelligence (IQ tests ignore many aspects of intelligence). Ridley finally concludes, ‘Genes may create appetite but not aptitude. Intelligence, by far, is a matter of training’.
This chapter talks about the influence of genes on our behaviour, for example ‘our language instinct’. Ridley presents various studies to show the genetic root for language. For instance, mutation of a specific gene on chromosome 7, lowers linguistic ability by impairing grammatical complexity as well as speech and hearing capabilities.
Sex chromosomes (X and Y)
This is the only unique pair in the whole set of 23 pairs of chromosomes in terms of length (Y chromosome is shorter than X).They determine the sex of the body (males have X-Y pair, while females have X-X pair). As we know, every female egg carries an X chromosome, where as the sperm cell carries either X or Y. And the gene SRY on Y chromosome plays a prominent role in bringing masculine characteristics to the body. Ridley further mentions the concept of sexual antagonism to explain the conflicting genes and dissimilar length of both chromosomes. He further discusses the studies of Dr. Hamer, who observed a version of genetic marker Xq28 (on chromosome X ) was quite commonly shared by gay people. However, there is no gene that directly determines our sexual orientation by birth.
97% of our genome does not contain true genes at all. Most of the non-coding DNA sequences are pseudo-genes, minisatellites, transposons and telomeres. As the chromosome 8 possess very few working genes, this chapter is dedicated to the discussion of non-coding DNA. Retrotransposons, for example, just copies a genetic sequence through reverse transcriptase and stitches it back to genome, thus increasing the size of genome. These are called long interpreted nuclear elements (LINE), mostly inactive, 21% of our genome contains this “selfish” replicated junk. Sometimes these may be stitched back in the middle of working genes, messing them up, causing mutations.
On chromosome 9, there lies a gene that determines our blood groups (ABO). Ridley looks at the genetic differences between the blood groups, which are very minimal (just a few letters of difference in the sequence). Studies on diseases linked to blood groups show, for example, people with AB group are more resistant to cholera than people with O group. It seems like, in evolutionary history, whenever a blood group was more vulnerable to a particular disease, natural selection produced more resistant (new) blood group type through mutations. The chapter further discusses the genetic variability with respect to infectious diseases; most of the times leaving an imprint on our genes.
CYP17 gene on chromosome 10 makes enzymes to convert cholesterol into crucial hormones such as testosterone, progesterone, cortisol, aldosterone and estradiol, all called as steroids. Cortisol, for example, suppresses the immune system by reducing the activity of WBC through switching genes. Ridley discusses here how our conscious reaction to external events and psychological behaviour alters the expression of genes. For instance, when we are under stress (external factor), our brain stimulates the release of cortisol, which lessens the level of good cholesterol (HDL) in blood, thereby increasing the risk of coronary heart disease.
This chapter focuses on the influence of genes on our personalities. D4DR gene on chromosome 11 makes proteins for dopamine receptor. Dopamine is thought to be brain’s motivation chemical. Higher the level of it, greater adventurous the personality. Similarly, another brain chemical, Serotonin, found to be associated with impulsive behaviour when its levels are low. At this point, we may think that our behaviour is solely determined by chemicals, but there is no clear-cut evidence for this. Numerous studies conclude that mood, mind, personality and behaviour are indeed socially determined. The dominant chemical in our brain results from our behaviour itself, not vice versa.
Now think about these,
How cells in an embryo self organise themselves to form organs? • How memory is stored in our brain? • Why death is inevitable to humans and all life forms?
Chromosomes 12-22 can answer these questions. They are even more interesting. Check out the next part here The Epic Story of Human Chromosomes (Part – 2)
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