Author: Silke Schmidt / Source: Science News for Students
Genes are the blueprints for building the chemical machinery that keeps cells alive.
That’s true for humans and all other forms of life. But did you know that with 20,000 genes, people have almost 11,000 fewer genes than water fleas? If the number of genes doesn’t predict complexity, what does?The answer is that our genetic material contains much more than the units we call genes. Just as important are the switches that turn a gene on and off. And how cells read and interpret genetic instructions is far more complex in people than in those water fleas.
Genes and the switches that control them are made of DNA. That’s a long molecule resembling a spiral ladder. Its shape is known as a double helix. A total of three billion rungs connect the two outer strands — the upright supports — of this ladder. We call the rungs base pairs for the two chemicals (pair) from which they are made. Scientists refer to each chemical by its initial: A (adenine), C (cytosine), G (guanine) and T (thymine). A always pairs with T; C always pairs with G.
In human cells, the double-stranded DNA doesn’t exist as one gigantic molecule. It’s split into smaller chunks called chromosomes (KROH-moh-soams). These are packaged into 23 pairs per cell. That makes 46 chromosomes in total. Together, the 20,000 genes on our 46 chromosomes are referred to as the human genome.
The role of DNA is similar to the role of the alphabet. It has the potential to carry information, but only if the letters are combined in ways that make meaningful words. Stringing words together makes instructions, as in a recipe. So genes are instructions for the cell. Like instructions, genes have a “start.” Their string of base pairs must follow in a specific order until they reach some defined “end.”
If genes are like a basic recipe, alleles (Ah-LEE-uhls) are versions of that recipe. For instance, the alleles of the “eye color” gene give directions for making eyes blue, green, brown and so on. We inherit one allele, or gene version, from each of our parents. That means most of our cells contain two alleles, one per chromosome.
But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene (instead of two), packaged into 23 chromosomes. Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person.
By combining two sets of 23 chromosomes — one set from the egg, one set from the sperm cell — that new person ends up with the usual two alleles and 46 chromosomes. And her unique combination of alleles will never arise in the exact same way again. It’s what makes each of us unique.But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of…
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