DNA Basics Chapter 2: The Structure of DNA


In November, we introduced the new DNA Basics blog series where we answer some of the most common questions MyHeritage DNA users ask about DNA testing for family history. We’ll delve into MyHeritage DNA features and the science behind the technology, giving you a clearer understanding of the exciting world of DNA and genealogy.

Inheriting DNA

In the last post, we discussed how DNA is inherited by children from their parents. We mentioned that half of each person’s DNA comes from their mother and half from their father. Let’s take a closer look at how this looks under a microscope.

Breaking down DNA: From double helix to nucleotides

Our bodies are made up of more than 30 trillion cells. The headquarters of the cell is the nucleus. Not every single cell actually has a nucleus — red blood cells don’t, for example. But almost all of our cells do have a nucleus and that’s where DNA is stored.

Image Adapted from Owensboro Community and Technical College

DNA takes the shape of a double helix — think of two long (untied!) shoelaces, pressed together along their entire length, and then wrapped around your finger. Each of the two shoelaces is made up of a series of little blocks called nucleotides. There are four nucleotide types, abbreviated as A, T, G and C. The same four nucleotides appear over and over again in different orders to make up the entirety of your individual DNA sequence.

In our analogy of the “instruction manual for you”, think of DNA as the text written in the manual that the body reads in order to make you, you. Nucleotides are the alphabet used to write that text. Nucleotides are the letters, every set of three consecutive letters is called a codon — that’s like the words. Groups of codons make sentences and all together, the whole entire text — all of your DNA together — makes up the whole set of instructions.

When you hold the two shoelaces next to each other, an A on one will always match up with a T on the other; a G on one will always match up with a C on the other. So really, by looking at one shoestring you already know what is written on the other — every place one has an A, the other has a T; every place one has a T, the other has an A. Similarly, every place one has a G, the other has a C; and, every place one has a C the other has a G.

About 99.9% of this very long sequence is identical in every person on earth. The 0.1% that varies from person to person carries the part of the instruction manual that makes us each unique — from the different colors of our skin to our height, and beyond.

Organizing DNA: From histones to chromosomes

The human genome contains approximately 3 billion pairs of nucleotides — that’s two very long shoelaces! Because this is the instruction manual to you, it is extremely important that they are well-organized and don’t get tangled up. There are a number of levels of organization involved, but here we’ll get to know just a few.

Because the strands of DNA are very long, segments are wrapped around proteins called histones, similar to a spool of thread. These spools are then packaged into chromosomes. Think of it as an instruction manual whose text is so long that it is published in many volumes — each volume is a chromosome.

Passing on DNA, one chromosome at a time

Humans have 23 pairs of chromosomes. Or, continuing our instruction manual analogy, we have two editions of the 23-volume instruction manual. For each volume in the series, or each chromosome, we get one of our mother’s two editions and one of our father’s two editions, which they in turn got from their parents. DNA tests like MyHeritage DNA can help you figure out which editions you got from your parents.

By the way, this is also why your Ethnicity Estimate may be different from your siblings’! For example, if your mother is 50% Japanese and 50% Irish, and your father is 50% Italian and 50% Scandinavian, you might end up mostly Japanese-Italian while your sister might be more Irish-Scandinavian. Of course, in reality, the differences are not usually that dramatic. But it is possible, even likely, that siblings will inherit at least slightly different percentages of different ethnicities in their DNA.

More likely, if your mom is 40% Irish and 60% Scandinavian, you might have gotten 30% Irish and 20% Scandinavian (adding up to 50% of your own ethnicities being from your mother), while your brother got 50% Scandinavian and 0% Irish (still giving him 50% of his own ethnicities being from your mother). This is how two biological siblings get different Ethnicity Estimates. Half of your total ethnicities did come from each parent, but you didn’t necessarily get half of each of their ethnicities.

It also works this way with most inheritable traits, not just ethnicity. If one parent is very tall and the other is very short, you and your siblings probably inherited different combinations of your parents’ heights, which is why you will all be different heights from one another, even though your height genes all came from the same original pool. You simply got different combinations from the available genetic options.

Stay tuned

Next month we’ll talk about who reads your instruction manual and how the instructions are carried out. We hope you’ll join us!

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  • Adrian

    January 29, 2018

    I loved it!