Lesson Explainer: Genes and Chromosomes Science

In this explainer, we will learn how to describe the structure of chromosomes and explain what a gene is.

Have you ever had a cut or scrape that broke the skin and required a bandage?

Being an active human, you may have had a cut or scrape that broke the skin and required a bandage. While the bandage may not seem like an extraordinary place to find scientific evidence, it was a surgical bandage that helped researchers identify the chemical compound that includes all the instructions necessary for building living organisms.

Deoxyribonucleic acid, more commonly called DNA, was first identified on a used surgical bandage in the late 1800s by the Swiss scientist Friedrich Miescher. The DNA shown in the image below was extracted from human blood and is likely very similar to the DNA that Miescher isolated from surgical bandages in the 1800s. Yet, while Miescher was the scientist who first discovered DNA, the central importance of this discovery was not realized until the 1950s.

DNA is the molecule that carries the genetic instructions for life and is found in the cells of most living organisms. These genetic instructions contained within the DNA determine your unique characteristics that were passed down to you from your biological parents, like your eye and hair color.

The central importance of DNA became clear in 1953 after its structure was determined by scientists.

In the early 1950s, two teams of university researchers were locked in a heated race to unlock the importance of the DNA’s structure. The first team was at Cambridge University and comprised a graduate student named Francis Crick and a research fellow named James Watson. The second team was at King’s College London and comprised a senior researcher named Dr. Maurice Wilkins and Rosalind Franklin, a rising star in the field of X-ray modeling of DNA.

The two teams had varying approaches. The Cambridge team, comprising Crick and Watson, used physical models to narrow down their choices and create an accurate model of the DNA molecule. In contrast, the King’s College London’s team used an experimental approach and X-ray imaging of DNA.

In 1951, Watson attended a lecture by Franklin discussing her work to date. She had found that DNA can exist in two forms, depending on the relative humidity in the surrounding air. This helped her deduce that the phosphate part of the molecule was on the outside. Watson returned to Cambridge with a rather muddy recollection of the facts that Franklin presented, despite being clearly critical of her lecture style and personal appearance. Using this information, Watson and Crick attempted to create a model, but this model failed. This is why the head of their unit asked them to stop their research on DNA, but the subject just kept coming up.

Franklin, who was working mostly alone, found during her X-ray diffraction experiments that the wet form of DNA (at higher humidity) exhibits all the characteristics of a helix. In January 1953, Maurice Wilkins showed Franklin’s results to Watson, apparently without her knowledge or consent.

Using a special imaging technique for biological molecules (called X-ray crystallography), Dr. Franklin and her student, Raymond Gosling, were able to capture the first images of the DNA structure. This famous photo was named “Photograph 51.”

While Dr. Franklin had her doubts regarding the helical structure of DNA, she was waiting to gather more evidence before publishing her findings. In her notes, she also noted that she suspected Photograph 51 to be an image of a triple helix. Dr. Franklin’s choice to withhold publishing her findings was said to frustrate her lab mate, Dr. Maurice Wilkins.

One day, when Dr. Maurice Wilkins was engaged in a conversation with the team of researchers at Cambridge University, he was prompted to show Photograph 51 to Drs. Francis Crick and James Watson. Together, these three scientists, using the data gathered by Dr. Franklin, were able to correctly determine the three-dimensional structure of DNA to be that of a double helix.

So, Watson and Crick are not the discoverers of the helical structure of DNA, but rather the first scientists to accurately describe the complex structure of DNA.

An illustration of the double helix structure is shown in Figure 2. DNA is made of two strands twisted around each other, creating a shape that resembles a twisted ladder. The two strands of DNA that make up the double helix are complementary and fit together like coordinated pieces. The many connections between the two strands are made of two complementary segments called chemical bases, or nucleotides.

The two strands in the DNA are largely the same except for their chemical bases, which are one of four possible options: adenine (A), cytosine (C), guanine (G), and thymine (T). These bases are complementary, meaning that each base has a specific pair. The order, or sequence, of the base pairs determines the genetic code for each living organism, from humans to hibiscus plants.

James Watson and Francis Crick also determined that this double helix shape allows the genetic information in one cell to be split, copied, and passed on to future offspring. This feature of the DNA structure means that the genes found in our biological parents’ DNA are also likely to be found within our DNA.

Small sections of the DNA sequence, as shown in Figure 3, are called genes. Genes can have various lengths. The complete set of your DNA (called the genome) has approximately 20‎ ‎687 genes. When these small sections of DNA sequence are translated, they inform what characteristics, like eye or hair color, you will have that make you a unique individual.

Genes make up only about of the genetic information in our DNA. While scientists are still working to understand the function of the remaining of the genetic information in our DNA, the current data suggest that this genetic information helps control genes.

Most genes are found in pairs. The two sets of genes that make up our DNA come from our biological parents. One set of genes is from your biological mother, and the other one is from your biological father. Different forms of the same gene are called alleles. For instance, the gene for hair color may have two different alleles: one that gives red hair and one that gives brown hair.

Among the two alleles, one is dominant and the other is recessive. When an allele is dominant, this means that the trait carried by that allele will probably be seen in that person’s appearance. For instance, the allele for brown hair is dominant to the allele for red hair. So, if a person has both the red hair allele and the brown hair allele, they will have brown hair.

In eukaryotes, such as human cells, DNA is found in the nuclei of the cells, as shown in Figure 4. The nucleus, whose name originated from a Latin word meaning the “kernel of a nut,” is the cell organelle used to store hereditary genetic material. Similar to a kernel, the nucleus is the central part of the eukaryotic cell.

While the nucleus is considered the largest organelle in animal cells, it is still very small, measured on a micrometre  scale. This means that all the DNA in one cell, which is a very long molecule (reaching up to 2 metres in length in every human cell), must fit into a space that is about a tenth of a human hair.

To fit into the nucleus, DNA is stored as long threadlike structures called chromosomes, as shown in Figure 4. Typically, chromosomes are so small that they are nearly impossible to visualize unaided, that is, unless the cell is dividing. Chromosomes become tightly packed when the cell is getting ready to divide, making it possible to see them with the help of a microscope.

Chromosomes vary in number and shape among living organisms. In most animals and plants, chromosomes are linear and are tightly coiled around proteins and arranged in pairs within the nucleus of the cell. The micrograph below is an actual image of human chromosomes, where both the linear shape and arrangement in pairs can be seen. This is in contrast to the circular shape of chromosomes found in most bacteria.

You may notice in the micrograph above that the chromosomes have an X-shaped structure. This is because this micrograph was taken after DNA replication. Chromosomes generally assume this X-shaped structure at particular moments of the cell life, when replicated chromosomes are highly condensed. The condensation of chromosomes is when they are reorganized from long, thin strands into compact, short chromosomes for division and replication.

In most human cells, there are 46 individual chromosomes, which are arranged into 23 pairs. In contrast, a fruit fly has 4 pairs of chromosomes (so 8 chromosomes in total), whereas a rice plant has 12 pairs of chromosomes and a dog has 39 pairs of chromosomes.

Example 3: Understanding the Importance of the Structure of Chromosomes

Inside the nucleus, DNA is wound and coiled into long strands. What are these strands called?

1. Helices
2. Genes
3. Chromosomes
4. Nucleosomes
5. Ribosomes

Answer

DNA is found in the nucleus in every cell. The nucleus, whose name is derived from the Latin word meaning the “kernel of a nut,” is the double-membrane cell organelle used to store hereditary genetic material. While the nucleus is the largest organelle in the animal cell, it is still very small, measured on a micrometre  scale. This means that the long strands of DNA, which measure about 2 metres in length per cell, must find a way to fit into the micrometre-sized nucleus. DNA is stored as long threadlike structures called chromosomes.

Chromosomes vary in number and shape among living organisms. In most animals and plants, chromosomes are linear and are tightly coiled and arranged in pairs within the nucleus of the cell.

Therefore, inside the nucleus, DNA is wound and coiled into long strands called chromosomes.

The genetic information contained in the DNA is very important that any change in the DNA sequence or the structure of the chromosome can have a large impact on your health. Any unexpected change in DNA, such as a change in the sequence of DNA or the shape of a chromosome, is called a mutation.

Mutations can occur naturally or due to environmental events that damage the structure of DNA. Most mutations are changes in the DNA sequence called gene mutations, as shown in Figure 6. Gene mutations can occur either as a change to a single base of the DNA sequence or as a change to a section of DNA.

Gene mutations may occur as a result of environmental events or may be inherited from our biological parents. Environmental events can be anything, ranging from smoking to UV radiation or even prolonged exposure to air pollution. When a mutation is inherited or passed down from our biological parents, this means that this mutation was also in the DNA sequence of our parents.

Although some mutations may not have any impact on health, others can have a huge impact on our ability to function and our overall health. Sometimes many changes to a person’s DNA have no effect while some mutations can cause changes in their characteristics or even the function of a gene. Not all mutations are harmful or bad news. In fact, some mutations can offer an advantage or desired ability to the organism. This is why mutations are also a driving force of evolution and contribute to genetic variation within a species. In this manner, mutations are the “engines” of evolutionary progress and adaptation.

During evolution, mutations help increase genetic variation. If a mutation results in a trait that is advantageous and can help the organism survive and create offspring, it is more likely that this mutation will be passed down to the next generation. This process is known as natural selection and was first theorized by Charles Darwin.

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