DNA and the Building Blocks of Life

Your body has about 37 trillion cells. At the center of each cell lies a nucleus, which contains the information the cell needs to do its job. That information exists in a code — deoxyribonucleic acid (DNA) — the blueprint for life. That code is stored in dense, coiled structures called chromosomes.

A single strand of DNA within a cell is about 2 meters long. Chromosomes allow the entire genetic code to fit in a much more compact space. They ensure when cells divide, each new cell gets a complete copy of the genetic code. They are also how humans pass on genetic information from one generation to the next.

Genes are segments of DNA on chromosomes that code for specific proteins. These proteins carry out various functions throughout your body and determine various traits, such as your eye color or whether you can produce a specific hormone.

This is known as gene expression — translating the DNA code within a gene into a function. Environmental factors that occur during our lifetime can also affect our genes and how they’re expressed.

Scientists assign the letters A, T, C, and G to “bases,” which are part of the structure of DNA. Strings of these letters make up your complex genetic code.

If you think of yourself as a book of genetic information, DNA would be the words, made up of letters.

Your genes would be sentences, made up of several words that come together to form a meaningful point or action.

Chromosomes are chapters that neatly organize those sentences.

The whole book is called the genome.

Humans typically have 46 chromosomes arranged in 23 pairs in their somatic cells. Almost all cells are somatic. Nonsomatic cells are germ cells (aka sex cells), which in humans are sperm and egg cells.

Somatic cells are diploid, meaning they have two sets of 23 chromosomes — one set from each biological parent.

Germ cells are haploid, meaning they contain only one set of 23 chromosomes. That’s the set a person will pass on to any biological children they may have.

So 23 of your 46 chromosomes come from a parent egg cell, and the other 23 come from a parent sperm cell.

Scientists number the first 22 of these pairs 1 through 22. These are known as autosomal chromosomes or autosomes.

The final pair are sex chromosomes, which scientists label X and Y. The Y chromosome is associated with the development of male sex characteristics.

As mentioned in the previous section, scientists refer to the first 22 pairs of chromosomes as autosomal chromosomes.

Autosomal chromosomes are homologous, meaning they code for similar traits. For example, the two copies of chromosome 1 will have copies of the same genes — one from each parent. Sometimes one of these copies may be more dominant, meaning it will express more than the other.

The final pair of chromosomes are the sex chromosomes, X and Y. You typically get one sex chromosome from a parent egg cell (always an X) and the other from a parent sperm cell (an X or a Y).

People assigned female at birth typically have two X chromosomes (XX), while people assigned male at birth typically have one X and one Y chromosome (XY). However, other combinations of sex chromosomes and other factors can also determine sex at birth.

The X and Y chromosomes are not homologous. The Y chromosome is much shorter than the X chromosome and has fewer genes.

People with only one X chromosome may only have one copy of a gene and may inherit certain conditions linked to that gene. This is known as X-linked inheritance.

Genetics plays an integral role in almost every human trait and health condition. It’s the blueprint for how your cells function and how you respond to the world around you. Some key ways genetics can contribute to your health include:

• Inherited genetic conditions: Some people inherit genetic conditions, meaning they have them from birth. Some conditions, like familial Alzheimer’s disease, don’t develop until later in life, even though the genetic mutation is present at birth.
• Risk of conditions: Conditions such as type 1 diabetes, heart disease, and some cancers aren’t directly inherited, but your genetics can increase your risk of developing them. Sometimes your genes can also be protective against some diseases, such as people with sickle cell trait having a lower risk of malaria.
• Response to health conditions: Your genetics can influence how much a condition affects you or how your body responds to an infection like COVID-19.
• Response to medications: Your genes can influence how your body metabolizes and responds to medications you take. The study of this phenomenon is known as pharmacogenomics.
• Alcohol and tobacco use: Genetics may influence alcohol and tobacco use, which can have profound effects on your overall health.

Furthermore, your chromosomal DNA can change slightly throughout your lifetime due to your behaviors and environment. Scientists refer to this as epigenetics and recognize it as a natural part of aging.

Most traits and health conditions are polygenic, meaning they result from the complex interaction between multiple genes. Environmental factors also often play a pivotal role.

But in some cases, mutations in genes on specific chromosomes are enough to cause a significant health condition.

Monogenic disorders are inherited conditions that result from a mutation in a specific gene. Notable examples include:

Trisomies result from errors in cell division that cause someone to have an extra chromosome. So instead of having two copies of a chromosome, they have three.

Some trisomies are fatal before or very early in life. However, some common trisomies that people live with include:

Sex chromosome aneuploidies occur when someone does not have the typical number of sex chromosomes. Examples include:

Your chromosomes are how your cells store your DNA. They also help ensure the accurate copying of DNA during cell replication.

This DNA is grouped into segments called genes. Your genes determine your traits, which can range from the smallest details of your physical appearance to whether you’re susceptible to a certain virus. As such, genetics has a significant influence on our health.

But our genes can change throughout life due to our environment or actions, such as exercising or smoking. While the genes we inherit are integral to who we are, they do not tell the whole story. There’s much we can still write ourselves.