DNA: Life’s Instruction Manual

Through the next several posts I’d like to talk about crop improvement through genetic advancements. This does include transgenics, also known as genetic modification, but that is definitely not the only topic to discuss when addressing the genetics behind crop improvement. The first topic I’ll cover in this series of posts is DNA. Some of you may remember learning about DNA in biology courses in high school and you might be wondering why I’ve chosen devote a whole post to this topic. Before I talk more about the science behind technology like transgenics, I want to be sure to discuss DNA in a context that is relevant to these topics and that may serve, at the very least, as a science refresher.

Unfortunately, many people who do not work directly in biological sciences may not be exposed to much more basic biology than what they learn in a high school science class. This was illustrated by some recent surveys where 80% of people surveyed stated that they want food containing DNA to be labeled as such, forgetting that almost ALL food contains DNA (Survey conducted by McFadden and Lusk). This is a prime example of how most of the discussion surrounding topics like genetic modification, pesticide/antibiotic resistance, and vaccines has come to consist of only opinions, rather than actual information and scientific perspectives. Hopefully, this short post about DNA, and those to follow in this series on crop genetic improvement, can help to clarify some of the science behind current technologies and offer additional perspectives to the overwhelming dialogue of weakly supported opinions.

DNA: Life’s Instruction Manual

Although scientists have studied biology and living organisms for hundreds of years, it wasn’t until the 1950s that DNA, or deoxyribonucleic acid, was identified as the molecule that carries biological information. It was discovered that DNA consists of a linear series of four molecular building blocks, called nucleotides or nitrogenous bases, held together by a sugar-phosphate backbone. The chemical bonds within DNA are arranged in such a way that cause it to be a double helix-shaped molecule. The specific sequence of nucleotides constitutes the genetic code which is the information, or the ‘instruction manual’, needed for living things to develop, grow, and function.

dna structure
(a) The sequence of four nucleotides, or nitrogenous bases, encodes the genetic information necessary for cell development and function.  (b) Of the four nucleotides, Thymine always pairs with Adenine and Guanine pairs with Cytosine, with each nucleotide pair connected to the sugar-phosphate backbone. (c) Double helix structure of DNA  Image:Boundless.com

All living things, from bacteria to mice to sunflowers to humans, have DNA with the same molecular structure, the result of all life on earth having a common evolutionary ancestor. Every single cell in a living organism’s body contains DNA with an identical sequence of nucleotides, the result of mitosis (cell division for growth) and meiosis (cell division for reproduction). All organisms have a unique sequence of nucleotides which is a combination of the DNA sequences of its parents, however, between organisms of the same species, and even between more distantly related species, the percent of the variation in nucleotide sequences can be quite small.  Although sequences may be similar between organisms, this variability is enough to result in obvious and important physical differences.

dna compaction
DNA wraps around histones and is further wound and compacted into chromosomes

In order to fit into cells, DNA molecules are packaged tightly into chromosomes, compacting them down to microns (millionths of a meter) in size. If you were to stretch out all of a human’s DNA from just one cell it would be over 2 meters long, with each chromosome being about 5 cm long (humans have 23 pairs of chromosomes). Within a cell, DNA exists within the nucleus, however, there is also DNA in mitochondria (the cell organelle for respiration and energy production) and chloroplasts (the plant cell organelle where photosynthesis occurs), known as the extranuclear genome. This additional DNA can play important roles in certain cell functions, and is inherited separately from the nuclear DNA.

 

So, all organisms have DNA in each of their cells, which provides the instructions for their growth, development, and function, but how does this actually work within organisms? All organisms consist entirely of proteins or materials made by proteins. This is made possible by the DNA code, which provides the instructions for protein synthesis through a series of steps. Within the DNA nucleotide sequence, there are functional regions called genes. These functional regions are the parts of the ‘instruction manual’ that code for proteins and provide the information important to the organism’s development and function. Within the nucleus, these functional regions of DNA are transcribed into another molecule, RNA (ribonucleic acid). Through this process of transcription, several copies of RNA are made from the single copy of DNA. This RNA is then processed to remove nonessential regions, and transported out of the nucleus into the cytoplasm where ribosomes attach to it to translate the code from the nucleotide sequence into amino acids. Each sequence of three nucleotides (called a codon) codes for one of 20 specific amino acids. These amino acids then form proteins which can function within the cell or within the entire body of the organism.

dna-replication-transcription-and-translation-1-638
The nucleotide sequence of DNA is transcribed into an RNA molecule, which is then translated into a sequence of amino acids, or polypeptide.

An important thing to remember about the first step, transcription, is that the gene and other regulatory elements are both transcribed by RNA. These regulatory elements flank the gene and determine when and in what cells it will be transcribed and translated into proteins. Therefore, different sets of genes are active in different cell types and under different environmental conditions. Amazingly, DNA molecules contain all of the information needed for a living organism to grow through its entire life cycle, from a single-celled embryo to a reproductive adult, as well as maintaining all of an organism’s daily functions and physiological responses, it is just a matter of different genes being active at different times.

DNA and the expression of genes underly all of an individual organism’s traits, be it the color of your eyes, the shape of a leaf, or how quickly a puppy grows into an adult dog. In crop plants, these traits are most significant when they impact the ease of producing food and the quality of the end product available to consumers. As discussed in the past post regarding ancient agriculture, crop plants have changed significantly since the time when our ancestors first began cultivating them. All of these changes trace back to alterations in DNA, gene expression, and the resulting proteins produced. Watch for the next post in this series which will address natural recombination, until then, hold on to these takeaway points regarding DNA.

Takeaway Points:

  • DNA exists in ALL living things: humans, animals, insects, plants, fungi, bacteria, microorganisms, etc.  Therefore it also exists in all food derived from living organisms.
  • Functional DNA, or genes, provide the instructions for the production of proteins, which then act within the cell to serve a function or to trigger another series of cell activities.
  • Genes determine an organism’s development, growth, and function, which results in unique physical characteristics, however, gene expression and the resulting characteristics can also be influenced by external forces like environment and nutrition.
  • Every cell of an organism contains DNA with the same nucleotide sequence, but not all genes are expressed in each cell. Regulatory DNA elements control when certain genes are expressed, and in what cells. These elements are triggered by changes in the environment or signals from other cells.
  • The DNA of crop plants has been changing since they were first cultivated thousands of years ago, long before plant breeding and improvement became scientific fields.

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