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This is your DNA. Your DNA is basically the genetic instruction for your body. The genetic information of an organism is stored and inherited as nucleic acids; the DNA is such an acid. DNA is actually short for Deoxyribonucleic acid. 

It is assembled as a double helix structure that can be thought of as a twisted rope ladder, made up of three components: sugar, phosphate and a base pair. The backbone of the ladder is made of sugar and phosphate and the rungs consist of base pairs.  

There are four bases called adenine (A), thymine (T), guanine (G) and cytosine (C), which only occur paired as A+T and G+C because of their complementary shape. These bases are held together by weak hydrogen bonds.  

The three components of the ladder, so a base-, a sugar- and a phosphate part, together make up what is called a nucleotide. Nucleotides are the building blocks of the DNA so to say. 

So now you know how the DNA is built, but where is it located? It doesn’t just randomly swim around in your body; it is found in the chromosomes. 

Chromosomes are located in the nucleus of almost every cell of your body and are made up of long strings of DNA, tightly wrapped around proteins that provide structural support to the chromosome, called histones. Each chromosome has two short arms called p arms and two long arms called q arms. These arms are held together in the center by the centromere. The tips of the chromosomes are called telomeres.  

In this tightly wrapped up form, chromosomes keep your DNA safe and thereby make it possible for you to perform daily activities. The most important mechanism is that DNA is accurately copied during cell divisions, which are necessary for the organism to function properly. 

In total, human body cells contain two sets of 23 chromosomes, one from each parent, resulting in 46 chromosomes in total.  

You probably want to find out now if DNA is found somewhere else as well? Then let’s move on to the mitochondria! 

Chromosomes are not the only structures that carry DNA. Every cell of your body contains organelles called mitochondria. The mitochondrion is the powerhouse of the cell. It produces ATP (short for Adenosine triphosphate), a molecule that provides energy for the cell. Mitochondria have their own DNA, the mitochondrial DNA (mtDNA), which is located in the cell fluid, called cytoplasm, independent from the DNA in the cell nucleus.

Now that you know the basics, let’s zoom out and move on to the bigger picture.

Everything you just learned is part of the genome. The genome is defined as the totality of our genes, meaning the total genetic material of a cell. The human genome consists of DNA which is stored on 46 Chromosomes in the cell nucleus. All genetic information the human body needs to grow and develop is stored in the genome. Nearly every cell in the body has a complete copy of the genome.  

Congratulations! You now know that DNA carries genetic instructions and is compressed to X-like structures called chromosomes. You have 23 chromosomes of your mother and 23 chromosomes of your father, which make up a total of 46. Almost every cell in your body carries all 46 chromosomes in the nucleus + mitochondrial DNA, which provides energy to each cell. All together the totality of your genetic material is referred to as the genome. 

Since you now have a high-level understanding of how genetics works, you are ready for the deep dive! Up next, we will show you the full picture of what goes on in our bodies that makes us unique. This brings us back to the start, the smallest components of the DNA, the nucleotides. 

You have already heard about the nucleotides, do you remember? Nucleotides are the basic building blocks of nucleic acids (DNA and RNA) which are assembled in a way that can be thought of as a twisted rope ladder. They are molecules consisting of a sugar, phosphate and one of the bases adenine (A), guanine (G), cytosine (C) and thymine (T) in DNA or uracil (U) in RNA. Let’s dive deeper into these bases, also referred to as genetic letters! 

All the information on your DNA is encoded by sequences of four different nucleotide bases: Adenine (A), cytosine (C), guanine (G), and thymine (T). These are called the genetic letters. As mentioned before in section DNA, the four bases only occur paired as A+T and G+C, because their shapes complement each other. 

Different combinations of these letters can be thought of genetic codes. These codes are specific instructions for the body – most importantly the creation of proteins. Many proteins together make up tissue which then produces organs, muscles and bones. So, proteins are crucial for the development and health maintenance of the body.

But what or who decides how the letters are combined in your body? Well, for that we have to dive a little bit deeper, so let’s talk about genes!

Chances are high that you have already heard about genes. But what exactly is a gene?  A gene is a segment of a specific length on the DNA. Length denotes the amount of subsequent bases – the length can vary from a few hundred DNA bases to more than 2 million bases. Most genes contain information on how to build proteins, so they can be seen as a construction manual for your body. Humans have between 20.000 and 25.000 genes. 

If you are thinking that this still does not quite explain what makes you as a person unique, you are right! For that we need to talk about alleles. 

Simply put, an allele is the variation of a genetic letter, a sequence of letters or a whole gene you have at a specific place on your DNA. As you have two copies of each chromosome from each of your parents, you have two alleles for each specific location. Your individual combination of alleles is your genotype. It determines the traits that are expressed. 

You have two alleles for each specific position on your DNA and their combination is important for how they are expressed. Thus, it is important to know if they are the same or not. If you inherited the same allele from both parents, you are homozygous. If they are different, then you are heterozygous for this specific location. 

You now have all the knowledge up your toolbelt to learn about how variations in your DNA have an influence on your traits. You are ready to learn about SNPs (SNP /snɪp/; plural SNPs /snɪps/) – single nucleotide polymorphisms. 

First of all, let us explain this seemingly complicated word: single nucleotide polymorphism 

Single = one 

Nucleotide = you already know what this is! Go back to Nucleotides for a refresher. 

Poly= multiple 

Morphism = form, shape 

So, a SNP is a specific position at an exact location in the DNA where genetic variations are known to occur among humans. Those positions have been found to be the most common type of genetic variant. This means that many people have different genetic letters at those positions compared to other parts of the DNA where almost all people have the same genetic letter. For example, a SNP may replace cytosine (C) with thymine (T) in a certain segment of DNA.  

Most SNPs have no effect on health or development. But others directly influence the expression of traits or the risk of getting certain diseases. This is mostly the case when a SNP changes the instruction code of a protein. 

Remember? Proteins are the building blocks of our bodies – the DNA codes for proteins that are then responsible for our internal as well as our external constitution. Let’s look at an example... 

Let’s say your mum has blue eyes, which we’ll refer to as B from now on and you dad has green eyes, which is going to be b for us now. B and b in this case are the alleles, which we heard about before already. For these two alleles there are three possible pairings: BB, Bb, bb  

You can refer to these pairings in two different ways: genotype and phenotype.  

The genotype indicates the exact pairing of alleles. So, either BB, Bb or bb. It has nothing to do with the expression of the trait, meaning the eye color in this case. The genotype describes the genetic information of an organism, hence your body. You cannot see the genotype from the outside, as you can’t see the DNA directly. 

In contrast to the genotype, the phenotype describes the physical, biochemical and physiological appearance of an organism. It can be seen as the product of the genotype combined with environmental factors. Some examples would be the eye or hair color of an individual as well as physiological characteristics such as allergies or blood groups.   

For the phenotype we have to differentiate between dominant and recessive alleles. Dominant alleles express features in someone even if only one parent passed it on (heterozygous individuals). Recessive alleles on the other hand only show a trait in homozygous individuals. This means that a certain recessive feature will only be expressed in someone who has inherited it from both parents. 

We are almost at the end! Only one more chapter and you know the foundations of genetics!  

Everything we just talked about leads back to one topic – you, and why you are unique. That’s where the word heredity comes into play. Heredity is the passing on of traits from parents to their children before they are born.  

An example for inherited characteristics is eye color. The color of your eyes is passed down to you by genes, alleles and all the other mechanisms you just learned about. The study of heredity is called genetics – and this brings us back to the start – you now understanding the fundamentals of genetics!