DNA
Beyond the Gene
DNA Beyond the Gene
Overview
DNA is the chemical name for the molecule that carries genetic instructions that help define how living things look and work. A genome is an organism's complete set of genetic instructions. Only 2% of the human genome encodes for protein. That means 98% the human genome contains non-protein coding DNA. However, non-coding DNA still has a variety of important functions. In this interactive, learn more about the functions of non-coding DNA elements within our genome and how they regulate gene expression, protect our genes, and help shape who we are.
Two Flavors of DNA
Protein-coding vs. Non-coding DNA
Our DNA can be divided into two main categories. Protein-coding DNA (sometimes referred to as a “gene”) is DNA that contains information needed for specific traits, such as height or risk for disease. Non-coding DNA (sometimes referred to as “junk” DNA) is any DNA that does not obviously encode for a protein. Non-coding DNA sequences nevertheless perform a variety of tasks that are critical for maintaining the function of our genome. Scientists are continuing to uncover new roles for non-coding DNA in human health and disease, challenging the concept of ”junk” DNA.
Protein-coding DNA
Blueprint for Proteins
Protein-coding DNA (“genes”) represents the flavor of DNA that provides instructions for the cell to make a protein. The DNA sequence is transcribed to an intermediate RNA message which is then decoded into a protein. The human genome contains approximately 20,000 different protein-coding DNA sequences. However, not all proteins are made at once. The expression of protein-coding DNA is influenced by many different elements of the genome including silencer and promoter sequences. This allows for cell-specific expression and the ability for a cell to respond to changes in the environment.
Silencer DNA
DNA "Off Switch"
One role of non-coding DNA is to help regulate when genes get turned on and off. A silencer is a non-coding DNA sequence that acts like an “off switch” and allows for a repressor protein to bind to that region, signaling to the cell to stop expressing a particular gene. Silencer sequences can help make sure certain genes are turned “off” in specific cells or during certain stages of development. For example, genes required for the development of the fetal heart are normally turned off in adulthood through silencer sequences. Variation in that silencer sequence can allow for improper re-activation of fetal heart genes, potentially leading to heart failure.
Promoter DNA
DNA "ON Switch"
While silencers usually turn genes “off,” promoters can help turn genes “on.” These non-coding DNA sequences are usually located next to the start of a gene and help orchestrate when and where specific enzymes should begin reading the gene (transcription). Transcription is the process of turning a DNA sequence into an RNA sequence. RNA is the single-stranded sister of DNA. Some RNA sequences can then be translated into a protein. Recent studies found that variations in the promoter sequence next to a gene that helps us digest lactose, the sugar found in dairy products, can result in a sustained ability to digest lactose through adulthood.
Repetitive DNA
Non-coding Sequences
Repetitive DNA is a non-coding sequence that occurs in multiple copies throughout the genome. Repetitive DNA comes in many forms, ranging from short stretches with just two repeating nucleotides (such as ATAT) or long stretches with many repeating copies that can span up to 8,000 nucleotides long. Over 55% of the human genome is made of repetitive DNA. Some repetitive sequences can vary from person to person and act like a DNA fingerprint. That’s how DNA is used as biological evidence at the scene of a crime.
Telomeres
Chromosome Protection
Some non-coding DNA sequences provide structure to our genome. Telomeres are the caps at the end of each strand of DNA that protect our chromosomes from damage, like the plastic tip at the end of a shoelace. Centromeres are located at the center of our chromosomes and aid in chromosome duplication, a process that occurs each time our cells divide. Both telomeres and centromeres are made of repetitive non-coding DNA.
Non-coding DNA & Health
Variation
While variants in some protein-coding DNA can lead to disease, we now understand that variants in some non-coding DNA can contribute to disease. Recent studies showed that variation in the copy number of repetitive elements can change the structure of our genome and may impact our health or abilities. For example, additional copies of specific repetitive elements are more frequently observed in individuals with autism spectrum disorder. This highlights the importance of studying variation in non-coding DNA sequences.
ENCODE
Genomic Technology
Scientists across the globe are working together to build an Encyclopedia for DNA Elements (ENCODE) which catalogues the function of DNA sequence elements. Some functions of these DNA sequences were uncovered by studying the consequences of deleting individual elements. Today, powerful computing methods enable scientists to compare whole genome sequences across thousands of samples to more rapidly identify new elements in our genomes. However, defining functions of non-coding regions is an ongoing endeavor and we still have more to learn!