The genome is the set of genes that an organism has. And as you say in your question, in an organism there are two types of genome, the nuclear and the mitochondrial. The name already refers to the place where it is located and that is the first difference, but not the only one. Nuclear DNA is found within the nucleus of eukaryotic cells, which are those that make up animals, plants or fungi, among others. Our organisms are made up of cells that are the most basic structural unit. And inside these cells there is a nucleus which is where the nuclear DNA is. Outside the nucleus is what we call cytoplasm and in it there are some elements that we call organelles. Mitochondria, which are responsible for the respiration functions of cells, are one of these organelles and within them is the mitochondrial genome.
The mitochondrial genome is a small molecule that has only between 16,000 and 17,000 nucleotides on average in vertebrate animals, although there are important differences in size between different organisms, especially in plants and fungi. Nucleotides are the bases that make up DNA, we could say that they are the building blocks that make it up. These bases are adenine (A), cytosine (C), thymine (T), and guanine (G). And the combination of these four bases is what gives us the DNA sequence. In the case of the nuclear genome, it is a much larger molecule that, for example, in humans has more than 3,000 million nucleotides or base pairs. As you can see, it is much larger and its size also varies significantly from one organism to another, from a few million nucleotides in some fungi to more than one hundred billion in some salamanders, to give just a couple of examples. The variability in size is enormous and there is no direct relationship between the size of the nuclear genome and the complexity of the organism.
There is also a lot of difference in the number of genes that make up each of the genomes. For example, in animals, all mitochondrial DNA generally ends up giving rise to 37 different genes, many of which are involved in the process of cellular respiration. On the contrary, in nuclear DNA we can find thousands of different genes that are responsible for all the functions of the organism, in essence, that there is a living being. In humans there are between 20,000 and 25,000 genes. Furthermore, in the nuclear genome there are many regions that do not give rise to genes. They are regulatory regions and others whose function is still unknown
In terms of structure, the mitochondrial genome is a double strand of DNA, two strands wound on themselves, which make a circular molecule, that is, the ends are closed. In the case of nuclear, it is also a double chain, but linear, with open ends. When the cell does not divide, the nuclear DNA forms chromatin which is like a tangle of DNA strands in the nucleus. At the moment of dividing the cell is when the chromosomes are formed. Chromosomes are the organized structures that make up the nuclear DNA of an organism.
Within each mitochondrion there may be between 100 and 1000 copies of DNA that are generally only inherited from the mother, although there are exceptions
There is also a difference in the number of copies of nuclear and mitochondrial DNA that we find in cells. In general, organisms have two different copies of nuclear DNA, one that is inherited from the mother and one that is inherited from the father. While within each mitochondrion there may be between 100 and 1000 copies of DNA that, generally, are only inherited from the mother, although there are exceptions. And it is that in biology we can often say that the exception is the norm.
One of the things you’ve probably heard about is mutations, which are the errors that appear when DNA is duplicated in new cells. There is a battery of proteins that are responsible for fixing the DNA when one of these errors appears. This repair system makes the rate of mutations in nuclear DNA quite low. In the mitochondrial, this mutation repair machinery is much less efficient. That makes mitochondrial DNA have a much higher error rate.
Ainhoa Agorreta Calvo She has a doctorate in Biology, researcher in the Evolutionary and Conservation Biology group at the Complutense University of Madrid
Question sent via email by Diana Rivera
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