Why is the mitochondrial genome of plants so much larger than that of animals?

The structure of plant mitochondrial genomes varies greatly, but mitochondrial genes are extremely conserved, which is the most conservative and slowest evolution rate among the three sets of plant genomes. Cucumbers have such a large mitochondrial genome, but there are only four more genes than Arabidopsis. It is precisely because the mitochondrial genes in plants are very conserved and the discrimination is insufficient, so they are generally not selected as molecular markers for systematics research.

This is the opposite of animals, where mitochondrial genes evolve at a faster rate, so they are the most commonly used molecular markers in animal systematics research.

So what is it that causes the plant mitochondrial genome to be so large and flood with so many non-coding sequences? What is the reason why in such a crazy mutation of the genome, the mitochondrial gene itself can stand alone, not surprised, and as stable as Mount Tai? At present, we explain it by two different sets of DNA repair mechanisms that occur in the non-coding region and the coding region of the partichondria.

Base shear repair and gene switching-mediated precision repair were performed in the mitochondrial coding region of plants, while non-homologous end joining and break-induced replication-mediated non-precise repair was performed in the non-coding region. <>

Plant plastid genome evolution is just one part of my project, and here is a brief discussion of the characteristics, causes, and consequences of the plant mitochondrial genome.

The main characteristics of the plant mitochondrial genome are: the genome has huge variation in size and structure, but the genes are extremely conserved; The gene distribution is very sparse and contains a large number of non-coding sequences; There is a large amount of RNA.

The circular mitochondrial genome of most animals is about 15-17 kb in size, and the structure is relatively conserved, and the genes are arranged compactly, which are similar to the chloroplast genome of plants, which are between 100-200 kb. However, the mitochondrial genome of plants has very different characteristics from the first two, and its size is generally between 200-750 kb. Some plants, such as cucumbers, have mitochondrial genomes as large as 1556 kb.

And this difference in genome size can be dramatic, even between closely related species. For example, in the genus Silene, S. nocturnalis (S. nocturnalis).Noctiflora) has a mitochondrial genome size, while S. officinalis (S. s

latifolia) has a mitochondrial genome of 253 kb. The two are of the same genus, and the mitochondrial genome size of the latter is more than 30 times that of the former. Even within the same species, the differences in the mitochondrial genome are significant.

As in the white jade grass (svulgaris), only about half of the mitochondrial genome sequence is identical between pairs in any different population.

Although the mitochondrial genome of plants is very large, there are not many coding genes on it, and they are very sparsely arranged. There are about 100 genes in the chloroplast genome of plants, but there are only about 50 genes in the mitochondrial genome of Arabidopsis, which is larger than the chloroplast genome, compared to 37 genes in the mitochondrial genome of humans. Arabidopsis thaliana has less than twice as many mitochondrial genes as a human being, but its genome is 22 times the size.

In other words, most of the mitochondrial genome of plants are non-coding sequences, which account for more than 60% of the entire Arabidopsis mitochondrial genome. These non-coding sequences are made up of repetitive fragments, sequences transferred from the chloroplast genome and genomes, and even sequences from other species obtained from horizontal gene transfer. For example, the mitochondrial genome of the oldest angiosperm plant Amborella trichopoda contains a large number of sequence fragments from mosses, green algae, and other angiosperms.

Let's not discuss which plant or animal is higher. In terms of genomes alone, there is a reason why plant genomes are larger.

In general, the larger the genome, the larger the protein molecules synthesized, the more complex the functions, and the more complex the organism. A chromosome if the genome is larger. Due to the different ways of individual polyploidy, non-coding sequences, repetitive sequences, and gene arrangement, there are great differences in biological genomes.

Genomic polyploidy is more common in plants, but it tends to be diploid and simpler in animals. And the plant genome will have a variety of recombination combinations in the evolutionary process.

The arrangement of genes is also inconsistent, which is manifested in the preference of plants for short clusters of long intergenic sequences, while animals prefer long genes and long introns. The result is a high concentration of animal coding protein DNA. <>

Mitochondria is a two-layer membrane-coated organelle that exists in most eukaryotic cells, which is the structure that produces energy in the cell and is the main site for the cell to carry out aerobic respiration, which is called the "energy factory" of the cell.

In addition to powering cells, mitochondria are involved in processes such as cell differentiation, cell messaging, and apoptosis, and possess the ability to regulate cell growth and cell cycle.

Mitochondria possess their own genetic material and genetic system, but their genome size is limited and is a semi-autonomous organelle.

Mitochondrial DNA is the genetic material in the mitochondria and is double-stranded in a loop. There can be one or several mitochondrial DNA molecules in a mitochondria.

Although mitochondrial DNA can synthesize proteins, its types are very limited, and the number of genomes is generally less than 1% of the nuclear genome.

To date, it is known that the RNA and peptides encoded by mtDNA are: 2 RNA species (12S and 16S species), 22 types of tRNA, and 13 peptides (each containing about 50 amino acid residues) in mitochondrial ribosomes. The vast majority of the proteins that make up the various parts of the mitochondria are encoded by nuclear DNA and synthesized on cytoplasmic ribosomes before being transported to their respective functional sites in the mitochondria.

Because of this, the mitochondrial genetic system is still dependent on the genetic system of the nucleus.

Thus, mitochondria are semi-autonomous organelles.

Mitochondrial structure.

What is known is that the mitochondrial genome of mammals is the smallest, the ones of fruit flies and frogs are slightly larger, the yeast ones are larger, and the mitochondrial genomes of plants are the largest. The complete mitochondrial genome sequences of human, mouse, and bovine have been determined, all of which are about 16 5 kb. There are thousands of copies of mitochondrial genomic DNA in every cell.

There are no exact numbers on how many mitochondria are in the cells of fruit flies and frogs, and how many copies of DNA each mitochondria have. The total amount of mitochondrial DNA is estimated to be only 1% less than that of nuclear DNA. The mitochondrial genome of Saccharomyces cerevisiae is about 84 kb long, with 22 mitochondria in each cell, and each mitochondria has 4 genomes.

The mitochondrial DNA of growing yeast cells can account for up to 18% of the total DNA in the cells.

Because the plant mitochondrial genome contains a large number of non-coding sequences, many of which are short repeats, the plant mitochondrial genome is very easy to recombine. Therefore, unlike the chloroplast genome, which often exists in a complete ring in the cell, the plant mitochondrial genome will form a threaded structure due to recombination, and even many small rings of different sizes, such as cucumber, in addition to the large mitochondrial genome, there are two small mitochondrial genomes of 84kb and 45kb. In the actual sequencing results, many different small ring sequences may be obtained, which also brings a lot of trouble to the sequencing of plant mitochondrial genomes.

Therefore, the current fully sequenced plant mitochondrial genome may not even be 1 10 of the chloroplast genome.

Due to the extreme complexity of the mitochondrial genome of plants, it is far less studied than the chloroplast genome. Moreover, the low density of coding genes also makes the study of them thankless, and few people are willing to spend effort on it. Therefore, the study of plant mitochondrial genome is a very new field, and there are still many problems to be solved.