Gene vs Chromosome: What Is the Difference between a Gene and a Chromosome?

Let’s start by putting some facts out there. Humans have an estimated 19,000–20,000 and 23 chromosomes.

The majority of us have heard the terms gene and chromosome. To some of us, the terms are very familiar and yet to others not so much. The difference between genes and chromosomes is very simple if you know what each is. What about the x chromosome, mutations, alleles, and non-coding genes? If you are unsure, you are probably thinking that genes and chromosomes are similar enough. So, what’s the fuss?

For laughs and giggles, let’s dive into the differences of the smallest parts of our bodies that are hugely different. But, let’s zoom out first, all the way—before we get down the finer details.

Let’s first talk about the human building blocks, the cells (taken from the Latin word cells that means “small room”). The human body has trillions of cells that are little factories that provide structure for it. The cell has several compartments, each of which is responsible for its own task. One of the compartments(also called an organelle), is the nucleus that contains all of the human genetic material, or the DNA (short for deoxyribonucleic acid). The DNA molecular structure is composed of two intertwined strands or helices that look just like a winding staircase. Instead of stairs, the two helices are held together by molecular bonds. Each strand has a so-called backbone that is made up of sugars and phosphates. This backbone binds to one of four nitrogenous bases, adenine, guanine, thymine and cytosine. These nitrogenous bases bind to other nitrogenous bases (via hydrogen bonds) that are part of another strand with the same sugar and phosphate backbone. This binding happens in a specific fashion—guanine always pairs with cytosine and vice versa, while thymine always binds to adenine. The two strands twist around one another. They can be untwisted and separated as part of a complex and elegant replication process.

Furthermore, replication heavily relies on the order of the nitrogenous bases as they make up the genetic code. This alphabet that consists of four letters, namely adenine (A), guanine (G), thymine (T) and cytosine (C). Three of these nitrogenous bases form what are called codons that are letter combinations. Each codon codes for a specific amino acid and a sequence of amino acids gives rise to a protein. To back up a little, a gene is a sequence of nucleotide along the DNA strand. So, in other words, genes are stored within the DNA strand, and these genes then give rise to proteins.

human genome in a sub-compartment

But let’s get back to the twisting of the double helix. It has a very special role. By now you are probably wondering just how it is humanly possible to store the entire human genome in a sub-compartment of the human cell, the nucleus. One word: twisting. And then two more words: tight twisting. This is also referred to as super coiling, which is extremely tight twisting. So, you are probably guessing correctly at the moment, chromosomes are DNA strands that are twisted really really tightly to the point that they are super coiled. This is the only way that all the DNA information will fit into a nucleus. In other words, it all begins at the stage of A,T,C,G levels that make up the DNA strand. This strand then wraps around other proteins, also called his tone proteins, and form a complex called the nucleo some. The sequence of nucleo somes then coils further into something called a solenoid, which is just before the entire structure coils some more into the chromosome structure.

Each nucleus of each human cell contains 46 chromosomes or a set of 22 autosomes (non-sex hormones)and one sex chromosome, known as the x chromosome. So, all together, 23 chromosome pairs. One pair is inherited from males (the father) and the other from females(the mother). By virtue of each chromosome being a super coiled really long strand of DNA that contains the genetic material, or genes, chromosomes are really tightly packed strands of genes. And this is exactly where they are different. Genes are composed of DNA sequences while chromosomes are entire DNA strands that are packed tightly together so they can fit into a cell. So, the entire chromosome is lined with genes. In other words, one chromosome contains several genes, while that cannot be said the other way around. Just by virtue of that, genes and chromosomes are not the same. Another perhaps easier way of thinking about is an apartment complex. A building and an apartment within the building are not the same things. In that scenario, genes are represented by the apartments, while the chromosomes are the high-rises that encompass all of the apartments. Just like that, genes and chromosomes are not the same.

But there is another layer of information. One chromosome has two sister chromatids. Each sister chromatid is inherited by the mother and the father. That is the reason why each chromosome is X-shaped. The two sister chromatids are joined together at the center.

So, by now, you have a better idea of the differences between chromosomes and genes. Furthermore, you also understand that the two are not the same and that there are vast differences between such small things within the deepest compartments of our bodies.

It is incredible how the body ensured that so much information is packaged in such a small space. We have so many genes and only 23 chromosomes—no problem, let’s just super coil the whole genome and package it into a nucleus. Also, this super coiled information, the most precious pieces that contain all of the instructions to make us who we are, is packaged within a nucleus that is protected by a double layer (membrane) within a cell that has yet another double layer (membrane) that is extremely selective about who can come in and who cannot. But isn’t that so incredibly elegant?

Laura Day
 

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