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A Definitive Guide to DNA

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Denis Law
A Definitive Guide to DNA

DNA testing has become a mainstream phenomenon today as the technology becomes more advanced and significantly more accessible for regular people. That means that today, you can learn more about your ancestry and your family’s history than ever before. However, behind the easy-looking tests and quick results is a wealth of science, and some concepts you may not fully understand before you take a test. 
Once you do see your results, you may be confused about what they mean, as well as some of the terms you may uncover when you review them. DNA testing and genetics can be complex and daunting, but they’re surprisingly easy to comprehend. Read on below to get a comprehensive introduction into DNA and how it can show you your ancestry. 

What is DNA?

DNA (short for “deoxyribonucleic acid”) is the hereditary material our cells contain that tells them how to construct and maintain our bodies. Almost all the cells in your body contain DNA, and it contains a sequence of nucleotides (small groupings of sugars, phosphates, and nitrogen-containing bases) that provide instructions on how to build proteins along with other vital functions. 
Your DNA stores information strands that are made up of chained nucleotides, which have identical pairs and are formed into a double helix structure (much like a twisted ladder). This double structure is made up of DNA combined from both your mother and your father (as you inherit an equal number of strands from each). 
When they’re packed into tight molecules, these DNA strands are known as chromosomes, of which you have 23 from each parent. 22 of these are known as autosomes, and they are responsible for a variety of functions tied to DNA replication. The remaining chromosome defines your biological sex, either with two X chromosomes (for women) or with an X and a Y (for men). 
These strands are organized into sequences of four chemical bases: adenine (“A”), cytosine (“C”), guanine (“G”), and thymine (“T”). Your DNA contains over 3 billion of these chemical bases, and you share 99% of them with everyone else on earth. The other 1% is what makes you unique, with variations of existing genotypes called alleles making up the difference. 
Your DNA also stores information about you and your ancestry, as it is equally inherited from your parents, who inherited their DNA from their parents, and so on. In this manner, you can trace back your family’s lineage several generations, and in some cases even thousands of years in the past. 

The DNA Double Helix

Although we describe it as a long instruction set, DNA is not simply written in long lines, but rather structured as a double helix. This is a result of how DNA nucleotides are shaped, and how they come together to create the chains of DNA required to function properly. Every nucleotide contains within it a phosphate group, a sugar known as deoxyribose, and a nitrogen-containing base (the A, C, G, and T mentioned above). We inherit an identical number of these from each of our parents, and they’re combined into our own unique genetic code as we grow and develop. 
To pair with each other, our nucleotides form chains that connect a phosphate group to a sugar, creating one side of the helix. To pair these single strands, each of the bases finds a match on the other end and connect like a puzzle to create the ladder structure observed in DNA. Once they’re paired and formed into helices, DNA strands are spooled and compacted into chromosomes which store the full instruction set.

What is the Function and Structure of DNA?

The function of DNA is to store and execute the information required for life to flourish. Every living organism on earth has DNA, and in all cases, their genotypes are responsible for providing the information necessary for organisms to develop, grow, function, and reproduce. Put another way, your DNA is the instruction manual for life. 
DNA’s structure is also closely tied to its function. Because of the use of A, C, T, and G bases, DNA connects with other bases into a helix. This is partly because of the massive amount of information that exists within each DNA strand. Once a helix is completed, it is further condensed and coiled into a chromosome. To start creating life and the structures necessary for it to exist, DNA will replicate itself, unspooling from its helix and creating copies that will eventually create new coils.

Where is DNA found?

Broadly speaking, DNA is found inside of every single living organism on the planet. Both plants and animals have it, and in all cases, DNA is usually stored in the same place. When it comes to humans, our DNA is stored in tightly coiled chromosomes (of which you have 46) and then inside of each of our cells. Most of our DNA is found inside of each cell’s nucleus, although a portion of it is stored in the exterior section of cells known as the mitochondria (this is what we refer to as mitochondrial DNA).

What is DNA Made Of?

As we’ve mentioned above, DNA is made up of groups of smaller molecules that combine into long strands and eventually into full genomes that describe everything about our anatomies. Each double helix is made up of nucleotides, small groups of three distinct components chained together into the twisted ladder. Each nucleotide holds a single line of our genetic code and combines with others to create a full set of instructions.

DNA’s Structure

DNA’s structure originates out of necessity. We have roughly 3 billion nucleotides in our genetic code, and to properly store it inside of a cell, it must be raveled and coiled into structures that keep it whole but compact. Once a DNA helix is formed, it is condensed into a chromosome that fits inside of a cell nucleus.

DNA Nucleotides

Nucleotides are the building blocks of the DNA double helix. Each nucleotide consists of three main components:

  1. A phosphate groups
  2. A deoxyribose (DNA sugar)
  3. A nitrogen-containing base (adenine, guanine, cytosine, and thymine

When put together, these three components form single instructions in the chain. Nucleotides are bound to others via a covalent hydrogen bond that links a deoxyribose to a phosphate group, creating an alternate sequence and half of the helix’s ladder.

Chromosomes

When a DNA strand is matched to its identical pair and combined into a double helix, it forms a surprisingly long chain. Remember that we store billions of nucleotides inside of our DNA sequence, and each copy of our DNA must be stored in our cell nucleus. As a result, each complete DNA chain is further coiled and compacted into a unit known as a chromosome, which stores a full copy of your genome. When we’re conceived, we receive 46 chromosomes from our parents—23 from each. 22 are known as autosomes and handle various functions, while the last determines our biological sex (and is described as either XX for women, or XY for men).