It's in our DNA - Part 1

It's in our DNA - Part 1
It's in our DNA - Part 1

As a supplier of quality seed, we know that many factors contribute to the successful growth and production of a crop. The soil conditions and climate at planting and throughout the season, the amount and timing of fertilizer applied, insect pressure and pathogen spread all need to be managed carefully. The quality of the seed and availability of seed treatments can assist seedlings to establish well. Another vital aspect is the choice of the particular variety and its suitability to perform under these varying conditions. As a seed company, one of our biggest challenges is to develop vegetable varieties that are adaptable to a wide range of different conditions. To assist with this, we have research and breeding activities on five different stations across South Africa, and have development trials in other countries. We do this to expose our breeding material to a range of different environmental conditions, day lengths, soil types and pathogen pressures, so that the material that excels will be widely adaptable. This is almost entirely determined by the DNA – genetic make-up and gene combinations – of the plant.

What is DNA? How does it work? Why is it important?

Inside every single plant and animal (and human) cell is a control centre – the “headquarters” – where instructions originate about the metabolic functioning, growth, adaption, flowering, yield, disease resistance – pretty much everything an organism needs to do. This HQ is called the nucleus, and contains the blueprint of instructions: the DNA. DNA – or deoxyribonucleic acid – is a long string of coded “letters” or bases arranged into chromosomes. In a human, there are 23 pairs of chromosomes (one set from each parent), with about three billion bases. For tomatoes, there are 12 pairs of chromosomes with a total of 950-million bases, while butternuts have 20 pairs of chromosomes with about 372-million bases. Chromosomal DNA contains genes and non-coding DNA (like a spacer or filler between the genes). The specific sequence of the DNA bases of a gene determines the structure and function of its corresponding protein. This sequence directs the “construction” machinery within the cytoplasm of each cell to make the protein by assembling amino acids into the sequence coded by the bases. Once the protein is assembled and put together, it is sent to the location it needs to function in. This could be the membrane, cell wall, or even a different part of the organism. Proteins are vital for life – they are the tireless engines that help build every part of an organism, they assist in nearly all metabolic reactions from breaking down waste products, to synthesizing vitamins, or helping release energy from carbohydrates. 

Just as people have different eye and hair colour (determined by their genes), so do plants. The reason why two different zucchini varieties look and grow differently, is a direct result of the combination of genes (that code for proteins) – some “versions” (alleles) of the same gene have a slightly different sequence and are thus more efficient at their task. Some alleles of genes determine the shape, size, colour, nutrition and yield of that zucchini variety. Combining these favourable alleles into a single variety is the goal of plant breeding efforts. Some characteristics are controlled by one or two genes (qualitative traits such as resistance to certain diseases, flower colour, plant height), while other traits require complimentary action from many genes each playing a small role (qualitative traits like yield, fruit size and shape). Qualitative characteristics are easier to select for since the environment doesn’t really play a big role in the effect of the gene, while the effect of genes involved in quantitative traits are more challenging to observe directly. Genes are usually inherited in a predictable way. The first studies on the inheritance of traits were conducted by an Austrian monk named Gregor Mendel in the 1850s. He worked with self-pollinating pea plants, and observed the transmission of seven different traits: plant height (short or tall), pea colour (yellow or green), pea shape (wrinkled or smooth), flower colour (purple or white), pod colour (yellow or green), flower position (axil or terminal), pod shape (inflated or constricted). From his studies he determined that these different traits segregated when passed on to the progeny, and they are independently inherited. His work remained unnoticed until the early 1900s, where similar studies confirmed his findings that the theory of these independent units of inheritance were given the name “genes”. These principles discovered by Mendel are the same ones taught in universities today, and form the basis for selecting traits in all breeding programmes around the world. Another significant discovery in the world of DNA was made by James Watson and Francis

Crick in 1953 when they published the precise structure of DNA – a double helix. Fifty years later, the complete sequence of the three billion bases in the human genome was published. It had taken 13 years to complete, and cost about $3-billion to do. Today, the human genome can be sequenced in about 48 hours and cost $1 000 – an incredible improvement thanks to advanced technology and increased computing power.

Here are some interesting facts about our DNA:

  • If we unraveled the DNA from all cells in a person, it would stretch to Pluto (located 7.5-billion kilometres from Earth) and back.
  • It would take you 50 years to type the entire human genome sequence if you typed at 60 words per minute for eight hours a day, every day.
  • The human genome has about 22 000 genes, which accounts for only 3% of the sequences – the other 97% is thought to be involved in controlling how the genes are expressed.
  • Even though DNA encodes so many genes and results in such diversity, it is made up of only four building blocks: adenine, thymine, cytosine, guanine.
  • The DNA of all humans is 99.9% identical – this means that all the variation we see in people around us comes from differences in a tiny proportion of our DNA.
  • DNA is damaged (broken, mutated) easily by certain enzymes, UV light – thankfully an effective repair process sorts out most of this damage. In most cases, the damage has no effect, but sometimes it can lead to disease like cancer.
  • We share 41% of our DNA with bananas, 40% with fruit flies, 80% with mice, and 98% with chimpanzees.
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