A genome contains all the genetic information needed to create and maintain an individual. Sequencing a genome helps scientists better understand genomes and their role in creating organisms. When scientists sequence a genome, they take an organism’s DNA and determine the order of its base pairs, which are coded by the letters A, C, T and G. These lettered pairs comprise the “rungs” of the ladder-like DNA double helix, which serves as an organism’s unique genetic blueprint.
“The genome is broken down into chromosomes,” explained Luca Comai, a plant biology professor at the UC Davis Genome Center. “Each chromosome is a continuous, linear DNA piece. So, it’s a chapter — a relatively long chapter — in your book.”
Current sequencing methods allow scientists to “extract lines out of each book,” said Comai. From there, the genome is reassembled line by line, as if taping together the individual letters of a book that’s been torn apart by a paper shredder. There are multiple ways to do this and the processes are still error-prone, so scientists evaluate each sequencing read, comparing it to previous ones. They then compile the reads into an order that represents the organism’s genome.
“The problem with assembling a genome is it’s not like when you fly to the moon and you get there and you land on the surface and you’re like, ‘Oh, I’m here. I’ve made it,’” said Ian Korf, professor of molecular and celluar biology at the UC Davis Genome Center.
Korf likened it to collecting coins from a country with no historical record. How would the collector know the collection is complete if there’s nothing to compare it to? Scientists solve this problem by comparing their genome collections to others.
“One of the ways to assess whether you’ve completed a genome is that there are certain types of genes that exist in every organism,” Korf said. “You can’t live without these genes.”
If the assembled genome lacks known essential genes, then scientists know something is missing and they return to sequencing. This process eventually creates a readable draft genome. A draft genome may cover only a percentage of the genome and may have gaps and errors. As scientists continue to create more sequences, they refine the draft to create a finished genome.
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