Lesson Explainer: The Human Genome Biology • First Year of Secondary School

In this explainer, we will learn how to describe the components of the human genome and outline why scientists want to study the human genome.

Have you ever wondered what makes us, us? What determines if we will have brown or red hair, if we will have blue or green eyes, or even if we will have dimples when we smile?

The answer to this, in short, is our genome. Our genome is all of the genetic material within our cells. In humans, genetic material takes the form of DNA, with long molecules of DNA coiling and condensing into structures called chromosomes. We can see the relationship between DNA and chromosomes in our body cells in Figure 1.

In the nucleus of a typical human body cell, also called a somatic cell, there are 23 pairs of chromosomes (making up 46 in total). Located on the chromosomes are hundreds, if not thousands, of genes. Genes are sections of DNA that code for a functional unit, typically a protein. Genes come in different versions called alleles.

The 22 pairs of non-sex chromosomes in your body (somatic) cells are numbered according to their size. Non-sex chromosomes can be referred to using their chromosomal pair number. Figure 2 shows a karyotype, which is an image produced to help us visualize the chromosomes within a cell. You can see in Figure 2 that chromosome 1 is much larger than chromosome 22. The smaller the number used to name the chromosome pair, the larger the chromosome. For example, chromosome 1 (meaning the first pair of chromosomes) is the largest of all non-sex chromosome pairs. It is estimated to carry just over 2‎000 genes. However, the only exception to this general rule is chromosome 22. It was believed chromosome 22 was the smallest of the non-sex chromosomes, and that is why it was named 22, until it was discovered that, in fact, chromosome 21, which is estimated to carry between 200 and 300 genes, is a little bit smaller than chromosome 22, which is estimated to carry between 474 and 496 genes. The 23rd pair of chromosomes represents the sex chromosomes, whose combination determines the person’s biological sex: XX for female and XY for male.

A homologous chromosome pair is a pair of chromosomes that have similar lengths and have the same genes at the same location. For the homologous pairs of chromosomes in your body cells, one chromosome in each pair is inherited from your biological mother and the other is inherited from your biological father. This is because the sex cells that combine during fertilization (sperm and egg cell) contain only half the number of chromosomes that a typical body cell has. A gamete does not contain 23 pairs of chromosomes (so 46 in total), but rather one chromosome from each of the pairs. When these sex cells, or gametes, combine, they then have the correct number of chromosomes, 46. This process is briefly outlined in Figure 3.

Inheriting one set of chromosomes from your biological mother and one set from your biological father means that you will likely end up looking like a mix of both of them.

Let’s have a look at some of the scientific research and discoveries that have furthered our understanding of the human genome.

In the 1950s, multiple groups of scientists started working to propose a model explaining the structure of DNA molecules. At King’s College London, Rosalind Franklin was using a technique called X-ray crystallography to study the structure of DNA within the nuclei of cells. Franklin and one of her students managed to produce an X-ray diffraction image, called Photo 51, which provided crucial evidence that DNA forms a double helix.

Without her consent or knowledge, Franklin’s image of DNA was shared with Francis Crick and James Watson at the University of Cambridge. They used this image to finalize their proposed model of DNA taking the form of a double helix in 1953.

One of the reasons why determining the shape of DNA was so important is the discoveries and subsequent research that this led to. Before the 3D shape of DNA was confirmed, it was already known that each molecule of DNA is made up of repeating units called nucleotides. These nucleotides contain a sugar-phosphate backbone, with bonds joining adjacent nucleotides, and the information is contained within their nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases, along with the basic structure of a double-stranded DNA molecule, are shown in Figure 4. The particular sequence of these bases and how they are read to create proteins in organisms are known as the genetic code. After his contribution to the discovery of the DNA structure, Francis Crick and other scientists provided evidence that a sequence of bases is read in groups of three during the process of creating proteins.

Once scientists understood the shape of DNA and how we can read DNA sequences to determine what amino acids, and therefore proteins, will be produced, this allowed them to start sequencing genomes. Genome sequencing is the process by which the order of DNA bases in a genome is determined.

Figure 5 provides a timeline of some of the most important discoveries in our journey to understand the human genome, including major sequencing events.

One of the largest and most important biological research efforts in human history is the Human Genome Project.

The Human Genome Project was an international collaboration between many groups of scientists aimed at determining the sequence of all the DNA bases in the human genome and accurately map all of the human genes present. This was a huge task, as the average human genome contains approximately 3 billion base pairs. This is a huge number; if you recited the human genome at a rate of one base per second, for 24 hours a day, it would take you around 100 years to say it out loud.

Sequencing the human genome has incredibly valuable applications in the field of medicine. By identifying and mapping genes, scientists can locate genes that cause disease or make a person more susceptible to a particular disease. It is also possible to identify genes, or their versions (alleles), that react negatively to a particular medicine or reduce its effectiveness. This means that medicine can potentially be targeted for different individuals.

Over time, our knowledge of DNA, genes, chromosomes, and genomes has increased. Scientists have discovered more and more genes in our genome, and we now believe that humans have around 21‎ ‎000 protein-coding genes located across 23 pairs of chromosomes.

The discoveries of the human genome also have valuable applications in other areas of science. For instance, sequencing the genomes of other organisms has allowed us to compare the genomes of different species and provided valuable evidence for how organisms have evolved over time.

Let’s recap some of the key points that we have covered in this explainer.