Understanding Genome Sequencing

My youngest daughter was born with a rare, mosaic syndrome. In the first year, I didn’t know what to expect from the syndrome but in the second year I noticed that a few of the characteristics that she was exhibiting weren’t part of her syndrome. I expressed this to many doctors but my concerns primarily fell on deaf ears. Finally, I went rogue and started researching on my own. Eventually, I came across a doctor who took interest in her case. This led to genome sequencing on her DNA which eventually resulted in the identification of a gene mutation.

Genome sequencing, in my opinion, is a hope generator. With each gene mutation found, the possibility of a therapy or cure arises.

So, what is genome sequencing and how could it be important to you?

Let’s start by comparing the human body to an onion. When we look at a human body we notice things such as hair, eye color, and smile. What is inside our bodies, however, are intricate layers. When we look at an onion, we see a purple, relatively shiny, pungent vegetable. It’s only when we cut it open do we notice all of the intricate layers.

LAYER 1: Let’s peel the first layer of our bodies off and talk about cells. Inside our bodies are trillions of cells who have specific jobs. More precisely, there are 200 types of cells that include neurons (nerve cells), monocytes (white blood cells), and heart muscle cells. A neuron’s job, for example is to carry messages throughout the nervous system while monocytes’ jobs are to protect the body against infection.

LAYER 2: Inside our cells, are nucleuses. I like to joke and say that nucleuses are like spouses in that they like to give all the orders. But in all seriousness, a nucleus acts like a control center for the cell and gives it directions. For instance, some important things that the nucleus controls are protein production, cell growth, and cell division.

LAYER 3: Inside our nucleuses are chromosomes. Most of us have 23 pairs of chromosomes or 46 individual chromosomes-half of our chromosomes come from our moms whereas the other half comes from our dads. In syndromes like Down’s, however, you will see 47 chromosomes and in Turner’s you will see 45 chromosomes. Although chromosomes are made of protein they are more famously known for containing deoxyribonucleic acid (DNA).

Layer 4: DNA. DNA, as most of you know, carries all of our information. Basically, it determines how we look and function. Perhaps the most fascinating part of DNA, is,”The DNA of any two people on Earth is 99.6 percent identical.” DNA is mostly found in the nucleus but a small amount can be found in a cell’s mitochondria (where energy is created for each cell to function). DNA itself is a long, spiral, ladder-looking molecule that is made up of four nucleotides: adenine, thymine, cytosine, and guanine.

LAYER 5: Nucleotides. Nucleotides are molecules that when paired together, form DNA building blocks (the lines in between the DNA spiral). Let’s stop here for a minute and talk about the English alphabet. The letter “h” by itself doesn’t do much but when paired with the letter “i” it becomes the word “hi”. Nucleotides are a lot like letters and words of the alphabet. They don’t do much alone but when paired together they make up one building block.

Nucleotides are known by their first letters: adenine (A), thymine (T), cytosine (C), and guanine (G) and are often referred to as “bases”. A and T always pair up whereas G and C always pair up. Each DNA strand has between 500,000- 2.5 million nucleotide pairs.

Layer 6: Genes. If nucleotide pairs are like words then a sequence of them become sentences or as we like to call them-genes. Each strand of DNA has over 20,000 genes that write the genetic code for traits or body functions that include eye color, skin color, hair color, height etc. FOR EXAMPLE: I have green eyes but my daughter has blue eyes. The sequence of nucleotides are nearly identical except for the last pair-GENE FOR GREEN EYES: AAACCGGTTTAA GENE FOR BLUE EYES: AAACCGGTTTTT. Do you see the difference?

Individual chromosomes house different genes but to understand this, take a quick look at the following picture as it shows you how different each chromosome is:

FOR EXAMPLE. The chromosome that determines the sex of a child is the 23rd chromosome.

Let’s review our layers quickly:

HUMAN BODY CELLS NUCLEUS CHROMOSOMES DNA NUCLEOTIDE PAIRS

Here comes the fun part-genome sequencing. Genome sequencing or “mapping” as it is often called, goes like this:

Doctors take blood or saliva from a person
A solution is used to break open the cells found in the blood or saliva.
Samples of DNA are carefully taken out of the cells.
DNA is put into a high-tech machine that “reads” every nucleotide pair in the DNA. This is no small task because if each cell has 500,000-2.5 million nucleotide pairs then the entire body has close to 3 billion nucleotide pairs.
Scientists look for new sequencing-AKA “new genes”
Scientists also look for a pattern of mutations-meaning DNA from many people are compared to see if a gene mutation can be found.

Let’s now talk about gene mutations. Our cells are constantly being replenished-every minute, every hour, every day. This is a lot of gene sequencing! Like anything, though, the human body is not perfect and things can go wrong. With gene mutations, something happens in the DNA sequence that makes a change in the gene. This can affect only one nucleotide base OR it can affect many, many nucleotide bases.

FOR EXAMPLE. I have a friend whose child has Cystic Fibrosis. This disease, scientists have found, is associated with a gene mutation off chromosome 7. Let’s use CF as an example of what happens in gene mutations.

Inside chromosome 7 is a gene called CFTR. There are 250,000 nucleotide pairs that make up this gene. This gene’s job is to move choloride ions in and out of cells which helps the body balance salt and water. When a mutation occurs, the nucleotide pairs, depending on the type of mutation, don’t function properly. This results in the body producing abnormally thick mucus that builds up in areas such as the lungs. There are close to 1,000 mutations that are associated with CTFR.

I’ve written a lot in this entry so I think I am going to leave the importance of genome sequencing for another blog entry. In the meantime, I hope this entry was helpful.

Resources and Sources:

http://learn.genetics.utah.edu/content/molecules/builddna/

http://www.sciencekidsathome.com/science_topics/genetics-a.

htmlhttp://genetics.thetech.org/about-genetics/how-do-genes-work

http://www.buzzle.com/articles/cell-nucleus-structure-and-functions.html

http://publications.nigms.nih.gov/insidelifescience/genetics-numbers.html