The Design of Our Genes

Our body is divided into several main systems – circulatory, digestive, endocrine, immune and more – and in each system, there are so many functions taking place, often at the same time. The marvels of our body can fill up volumes of medical textbooks. Through it all, what controls our beautifully complex body functions?

If you think that it is your brain, well, that is not always true. Some instinctive or reflex actions (such as breathing) are controlled by the nerves in our spinal cord, without involving the brain. So what is telling our body what to do, when to do it, how to do it and when to stop?

The answer lies in our very cells. Believe it or not, a structure in most of our cells, called the nucleus, carries an entire library of information needed for our body to function properly. This library is passed down from our parents, and we will pass down half of the information contained in the library to our children. This library is our genome.  

What’s in the library?

Our genome consists of about 20,000 to 25,000 information segments called genes. The actual number will change as we learn more about these genes, and what is more important here is what genes do. These genes are carried in molecular structures called deoxyribonucleic acid, more famously known as DNA.

“Wait, everything boils down to some … tiny molecules?” you may be wondering by now.

While it may seem implausible that such significant importance hinges on something as very microscopic as DNA, our body has a complex system based on a simple premise that works very well.

Different genes direct the production of different proteins that would act as ‘workers’ to carry out specific body functions. What happens is as follow – you can refer to Diagram 1 for a visual representation of what happens in your cell.

Diagram 1: How our cells create proteins from the information in our DNA.

Step 1: Let’s Get to Work!

Let’s say that our body needs more red blood cells to help us carry more oxygen from the lungs to various parts of our body. Our body recognises that we need more red blood cells, and our kidneys, getting this message, release a hormone to tell the relevant part of the body that make red blood cells (in this case, our bone marrow) to get to work.

Step 2: Let’s Work on the DNA.

In each nucleus within the bone marrow cells, work begins. It is like a factory in motion. Specific molecules, whose job is to read the DNA, quickly scan the long strands of DNA to identify the gene that contains information on how to make the necessary framework for red blood cells. (They are really good at their job, and can locate the correct gene very quickly!)

Step 3: From DNA to mRNA.

Special worker molecules in the cell then use the information in the gene to create a strand-like molecule called the ribonucleic acid (RNA). The RNA contains a modified version of the information found in the gene. Another group of worker molecules then work on the RNA, producing a more refined version called the mRNA (‘m’ stands for ‘messenger’).

Step 4: We’re Almost There!

Next, a special group of molecules called ribosomes work on the mRNA, using the information in that strand to create the protein parts that would be needed to form red blood cells. Once enough red blood cells are made, the body would then signal the bone marrow cells to take a short break. Until next time!

This same mechanism works for every other important body function. It is an incredible system – deep, complex and yet so simple in its basic design. It is perhaps unsurprising that even Bill Gates was moved to say, “DNA is like a computer program but far, far more advanced than any software ever created.”

What can go wrong?

In very rare cases, mistakes may arise in the above process. Information in the DNA or mRNA may be incorrectly read and the resulting abnormal protein may have function differently, possibly resulting in breakdown of the body process that it is involved in.

In other cases, factors from the environment and our diet may change the information present in the gene. This can also give rise to the formation of abnormal proteins.

These changes – called acquired mutations – usually happen in certain types of cells rather than all the cells in the body. As long as these mutations do not occur in the sperm or egg cells, they would not be passed down from parent to child.

As we learn more about our genes, research progresses into the possibility of modifying problematic genes to remove gene-related problems from our lives. Recently, the new technology called CRISPR is showing promise in ‘turning off’ genes that are responsible for making us sick. It may also be used on the plants and animals that we eat! While this technology is still being worked on, it opens up a whole world of possibilities … and also spurs heated debates on the ethics of what some people perceive as humans trying to play God.

Meet the chromosomes

Our genome is a library, and our DNA comprises many, many books that contain valuable information. Like all libraries, there is some kind of shelving system that allows the genes to be arranged and – in the case of passing them on to our children – safely and easily transported. Our cells do this by packing our DNA into special structures called chromosomes.

A single DNA strand is very, very, very long as it contains many, many information – 6 feet long (almost 2 m) if fully unwound! Therefore, in order to fit it inside a nucleus (which is about 0.0002 cm), the DNA strand is very tightly wrapped around special proteins to produce a chromosome.

We human beings have 23 pairs of chromosomes. The pairs are mostly similar in males and females, barring the pair we call the sex chromosomes. Males have one X and one Y sex chromosome, while females have two X chromosomes. Whether we inherit one X and one Y chromosome or two X chromosomes from our parents determines our sex during conception.

Passing our genes to our children

A new life is formed when a sperm fuses with an egg. Biology makes sure that each sperm or egg cell contains only 23 chromosomes. That way, when the egg and the sperm cell fuses, the baby that is conceived has 46 chromosomes (23 pairs of chromosomes), rather than 92!

What can go wrong?

The chromosomes may become abnormal, such as becoming damaged, losing certain segments, having certain segments containing mutations and such. These changes usually arise when cells divide, such as when the body produces sperm or egg cells. When the sperm or egg cell containing abnormal chromosomes is involved in conceiving new life, the child will end up with every cell in his or her body containing these abnormal chromosomes.

Also, when sperm or egg cells are made, the coupled chromosomes are separated during the cell division process, so that the resulting two sperm or egg cells contain only 23 chromosomes. Sometimes, the separation does not occur properly, and one sperm or egg cell may end up with 46 chromosomes while the other one will have zero chromosome. When an abnormal sperm or egg cell is involved in conceiving new life, every cell in his or her body will have an abnormal number of chromosomes.

A common example of this is when the 21st chromosome pair fails to fully separate during the sex cell division process, and the resulting sperm or egg cell has two copies of this chromosome instead of the usual one. When it is involved in creating new life, the offspring has trisomy 21 (three copies of the 21st chromosome, instead of two like normal people) – which gives rise to the condition we all know as Down Syndrome.

As you can see, it often takes genetic or chromosomal abnormality in a single sex cell in a parent to cause hereditary conditions in every generation down the family tree. Genetic screening helps to detect these abnormalities and allows us to prepare ourselves and make a decision that is most appropriate for our circumstances.

References:

Genetics Home Reference. Available at www.ghr.nlm.nih.gov

Specter, M. (2015). The gene hackers. Annals of Science, The New Yorker, Nov 16. Available at www.newyorker.com

[1] http://ghr.nlm.nih.gov/handbook/howgeneswork/makingprotein

[2] http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/genemutation

[3] http://www.newyorker.com/magazine/2015/11/16/the-gene-hackers

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