Ever wondered how each cell in your body has the same genes as each other?Posted: January 29, 2013
Remember in my last post I mentioned that our DNA is like a language that allows us to form words?
Well, just like how individual words don’t make sense unless put into context (can you imagine trying to make sense of the word ‘the’ if there was no context?), we have to think of DNA in terms of genes to give it context.
Genes are units of heredity, made up of DNA codons – this means that the traits that your parents have passed down to you, like blonde hair, brown eyes, your height and whether you have dangly earlobes – are all handed down via genes. This also means you can have lots of different combinations of genes (also known as genotypes), which, when expressed, means lots of different kinds of people (also known as phenotypes) – kind of like how you can make almost any kind of sculpture you want with LEGOs.
Notice how I said “almost any kind”? That’s because genes are linked together on structures known as chromosomes. Chromosomes help keep genes neat and tidy in your cells, so that they don’t float around and get in the way of proteins that are trying to do their work, and most importantly, so that all 2 m (about 6 ft) of your DNA can fit into each of your cells, which are only about 10 µm in width – about 200000 times smaller.
Genes that are stored on the same chromosome are said to be linked, and the closer they are to each other on the chromosome, the more closely they are linked. Because genes are stored on chromosomes in a fixed order, linked genes tend to be inherited together, kind of like how if you tried to tear out a picture of Iron Man from a magazine, you would also be tearing out the article on the Hobbit that was behind it. This decreases the number of possible combinations of genes, because it places restrictions on how the genes can be re-arranged, but the number of combinations is still a pretty big number.
In fact, because genes are almost always found in the same locations – meaning that gene a is always found at location a on chromosome a – scientists have come up with a system of naming the locations of genes, also known as a locus. (Hey look, I said ‘almost’ again. I won’t be covering why here though, because that would make this blog post too long. Look out for a post on genetic mutation for an explanation on that.)
Here’s where it gets a teensiest bit confusing – genes also come in different flavours, called alleles, and different flavours of genes share the same locus – kind of like how you can use differently-coloured LEGO bricks for the house you are building out of LEGO pieces.
For instance, the reason why people have A, B, AB and O blood types is because there are three different ‘colours/flavours’ of the gene that determines blood type in humans. These three alleles are called IA, IB, and IO. All humans (and many other organisms) are diploid, meaning that they have two sets of chromosomes – one from their mother, and one from their father. Depending on their parents, people may inherit IAIA, IAIO, IBIB, IBIO, IOIO or IAIB. (Remember – combinations of genes are known as genotypes!) IA and IB are co-dominant to each other, and both are dominant to IO. This means that the people with that carry the combinations of IAIA and IAIO will have blood type A; likewise for people with blood type B, and IBIB and IBIO. Only people with IOIO combination will have blood type O; on the other hand, because of co-dominance, people with the IAIB combination will have blood type AB. Differing blood types is a kind of phenotype, so we can say that some people have a phenotype of type A blood, while other people have a phenotype of type AB blood.
Because there are at least 20,000 genes, and if each gene has at least two different alleles, the chances that someone shares a genetic profile with you (unless you have an identical twin) is 1 in 10 to the power of several thousands. Yes, that is a rather mind-boggling set of zeros. However, our technology thus far only allows us to narrow the chance that the DNA of two people (who both don’t have identical twins) will match down to 1 in 100 billion.
But if everyone starts out with two sets of chromosomes (one from each parent) – how did our parents each give us only one set?
Because, aside from regular cells, there is a special subset of cells known as gametes – or more simply, sex cells. (Geddit? Sex cells? Hurhur.)
Anyway, these gametes carry only one set of chromosomes instead of the usual two, by a process known as meiosis.
First, we have a regular old stem cell with two sets of chromosomes.
Via meiosis, the two sets of chromosomes are first duplicated, resulting in four sets of chromosomes (1). Then the cell splits into two, and then into two again, resulting in four different daughter cells; the four sets of chromosomes are dragged apart, so that each cell only ends up with one set of chromosomes. (2) These, then, are four different gametes, or sex cells.
When two gametes combine, the resulting cell that then forms has two sets of chromosomes, and eventually grows up to become another person; inside this person, more sex cells are being produced, which will potentially combine with other sex cells to produce more people, and so on and so forth.
What does all this have to do with why our cells all have (almost) the same DNA, you ask? Because ordinary cells go through a similar process called mitosis, where the two sets of chromosomes are again duplicated, resulting in two copies of two sets of chromosomes. The cell splits into only two daughter cells this time, and each daughter cell has two sets of chromosomes. Both sets of chromosomes are mostly identical copies of one another, so each daughter cell has (almost) the same DNA as its parent, as well as its sister cell. (Oh hey, again with the ‘almost’ and ‘mostly’. Just look out for the post on genetic mutation.)
So that’s why you can take the cheek cells of a person, test the DNA inside them, and say to that person, “There is, like, a one in 100 billion chance (barring laboratory errors) that the DNA in your cheek cells matches the DNA found in the blood on the knife. Man, you are soooo guilty.” (P.S. Ask me about human chimeras sometime.)
Genes – how traits are inherited by the next generation – e.g. hair colour, eye colour, whether you’re balding or not; made up of DNA
Alleles – different flavours of genes; e.g. blonde hair, black hair, brown hair, red hair. Not blue or green hair, though, sadly enough
Chromosomes – an organization of genes, like a chapter in a book (if the book is an instruction manual on how to build a human)
Locus – where genes are found on a chromosome
Linked genes – genes that are generally found on the same chromosome (or the same chapter, if you prefer), and are hence usually inherited (or handed down) together
Chromatid – the really, really singular version of ‘chromosome’
Genotype – the combination of genes in anybody; can be thought of as a kind of DNA profile
Phenotype – the person that results when the combination of genes is expressed altogether; two people may have different genotypes but share a phenotype, e.g. John may have genotype IBIB and Beth may have genotype IBIO but they both have phenotype blood type B
Diploid – having two sets of chromosomes (Also, haploid – having one set of chromosomes)
Meiosis – a cell-splitting procedure by which a cell goes from having two pairs of two sets of chromosomes (2×2 = 4) to dividing into four cells with one set of chromosomes each; used for producing sex cells (hurhur)
Mitosis – a cell-splitting procedure by which a cell goes from having two pairs of two sets of chromosomes to dividing into two cells with two sets of chromosomes each; regular old cell division; also, kind of why you can match a person to the DNA left behind at a crime scene