We all know that mitochondria are the powerhouse of the cell because their main role is to perform aerobic cellular respiration and produce ATP energy for the survival of the cell.
Mitochondria (singular: Mitochondrion) are really very small in size, double-membrane bound organelle, and strictly sausage-shaped with having a diameter of 0.1.0 μm and length of 1.0-4.1 μm only. That’s incredibly small, right?
And yes, mitochondria contain both DNA and ribosomes inside them. The DNA inside the mitochondria is called mitochondrial DNA, while the ribosomes found inside the mitochondria are called mitochondrial ribosomes.
Mitochondrial DNA appears small and circular in nature with only around 16,000 or so base pairs in it as in the case of humans.
While mitochondrial ribosomes contain a different ratio of rRNA & protein due to the presence of several specific proteins and fewer rRNAs as compared to that of the cytoplasmic ribosomes.
Both the DNA and ribosomes found in mitochondria are a lot different from those that are found inside the nucleus and cytoplasm.
And, the functioning of mitochondrial DNA and ribosomes are mostly dedicated to the functioning of the mitochondria only, thus aiding in the production of cellular ATP energy mainly.
Let’s know more about this topic and get the queries cleared. So, let’s get started…
Do mitochondria have DNA? Let’s Know In Detail
Yes, mitochondria have their own DNA that is completely different from the one present in the cell’s nucleus. The mitochondrial DNA is also known as mtDNA.
There are numerous copies of mitochondrial DNA inside the inner membrane of the mitochondria that look small and circular in nature. So, that’s why mitochondrial DNA is also called small circular DNA.
The mitochondrial DNA is not chromosomal DNA. Meaning that mtDNAs don’t form chromosomes and they mostly constitute as a few genes only.
And also that, unlike the nuclear DNA, they are not paired and so we say that mitochondria are haploid. This is also a reason why offspring inherit mtDNA from mother and not from father, although there are various other additional reasons as well to finalize this statement.
In the case of humans, there can be somewhere between 1 to 15 numbers of DNA per mitochrondria.
The two strands of the human mitochondrial DNA are distinguished as one being the Heavy (H) strand, and the other being the Light (L) strand.
Each of the small circular mitochondrial DNA has about 16,000 or so base pairs. It is double-stranded DNA with 37 genes (in the case of human cells) on their respective Heavy and Light strands.
One of the strand is the Heavy (H) strand because it is rich in Guanines, and so this strand have a larger mass due to different proportions of heavier nucleotides.
Another reason for the one strand being heavier than the other because of more Guanines in it, as Adenine and Guanine (Purines) are heavier than Cytosine and Thymine (Pyrimidines) due to their extra ring.
Mitochondrial DNA in offspring’s cellular mitochondria is inherited from the mother’s egg cellular mitochondria, and its main role is to take part in the cellular respiration pathways and ATP energy production by forming the various genes and proteins that will remain confined to the mitochondria.
Do mitochondria have ribosomes? Let’s Know In Detail
Yes, mitochondria have ribosomes called mitoribosomes. During the process of mtRNA translation to mitochondrial proteins, these mitoribosomes actively take part in the formation of various mitochondrial proteins that will lead to cellular ATP production.
We all know that the various steps in cellular respiration occur in the matrix of the mitochondria due to its high concentration of enzymes.
And, it is here in the matrix that these organelles called mitochondrial ribosomes do their translational activities of mitochondrial mRNA causing the production of mitochondrial proteins like the various Chaperone proteins, Outer membrane translocase import receptor proteins, Motor proteins, Tom channel complex proteins, Inner membrane translocase proteins, etc.
It is here to be that noted that in the case of mammals each mitoribosome will have a small 28S and large 39S subunits, together forming a 55S mitoribosome.
And, in the case of plants, each mitoribosome will have a small 33S and large 50S subunits, together forming a 78S mitoribosome.
However, this ratio of rRNA and proteins making up the subunits of mitoribosomes is a lot different from that of the cytoplasmic ribosomes due to different evolutionary inheritance and functioning.
These mitoribosomes stay highly active inside the inner membrane of the mitochondria and so function as riboprotein complex.
Also, note that not all of the proteins that are used and present inside the mitochondria are made by mtDNA and mitoribosomes.
Just for example, in the case of humans, only about 0.80% to 0.87% of proteins, which is around 13 proteins out of the 1300 different proteins present in mitochondria are actually produced by mtDNA and mitoribosomes.
Here, it is to be highly noted from the evolutionary and molecular biology point of view that the mitochondrial ribosomes resemble the same as bacterial ribosomes rather than being a eucaryotic ribosome type with a slight change between the genetics of the two.
Why do mitochondria have DNA?
Mitochondria have DNA called mitochondrial DNA (mtDNA) because:
1. The mtDNA has various genes that encodes for proteins that are essential for the normal functioning of the mitochondria.
2. The haploid (unpaired) DNA present in the mitochondria helps the mitochondria to divide and get inherited from mother to offspring with all the same copies of genes.
3. The mtDNA conducts its own self-replication, transcription, and translation with the help of mitoribosomes as per the cell’s and mitochondria’s genetic requirements.
4. Mitochondria contain their own genome in the mtDNA that is separate and distinct from the nuclear genome of a cell. This not only aids the organelle to be inherited but also to be self-replicated and form various other mitochondrial copies within the same call as per the cell requirements.
5. mtDNA provides the mitochondria its genetic capability to aid in performing key functions for the cell ranging from the formation of ATP and GTP to the synthesis of various other nucleotides, Fe2+/Ca2+ handling, Fe-S clusters, haem, and amino acids, inflammation, and also to perform apoptosis for the cell.
6. In humans, mtDNA encodes only 11 mRNAs, 22 tRNAs, and 2 rRNAs which together aids in forming 13 different types of mitochondrial proteins that will help in the functioning of the mitochondria.
7. It also supports the phenomenon known as heteroplasmy which indicates that the multi-copy and self-replicating nature of mtDNA supports the pretty good existence of mixed populations of mtDNA molecules within the cell, where not all these mitochondrial genomes are identical thus proving that the evolutionary factor of mutations only affect a proportion of the molecules in a cell.
8. The principle phenomenon of mtDNA mutation heteroplasmy plays an important role in the development of mitochondrial dysfunction both in disease and non-disease states.
9. The mtDNA has evolved via. duplications, deletions, and inversion in its various genes over the period of evolution thus causing a great cellular adaptation to make the mitochondria suit to the specific animal cell leading to its proper functioning, as the genome shape and content is different between the various eukaryotes lineages.
10. Unlike nuclear DNA that replicates only during the S-phase of the cell cycle, mtDNA can be replicated independently from nuclear DNA if the need arises. Just like during many of the cases like in embryogenesis, the cells of the inner cell mass of the primordial embryo restrict mtDNA replication until they receive the signals to differentiate to specific cell types.
Why do mitochondria have ribosomes?
Mitochondria have ribosomes called mitochondrial ribosomes because:
1. The main function of mitoribosomes is to aid in performing protein synthesis inside the mitochondria during the translation of mitochondrial mRNA to proteins as the ribosome decodes the mRNA sequence into polypeptides or new proteins.
2. The mitoribosomes are also responsible for synthesizing the various mitochondrial membrane proteins that aid in the formation of the various membrane porins and the various membrane-bound structures of the mitochondria.
3. We know that most of the mitochondrial proteins are synthesized in the cytoplasm by the cytoplasmic ribosomes. But, there exist a few specific mitochondrial proteins that are the key components of the electron transport chain mechanism and so they are one and only translated in the mitochondria by the mitochondrial ribosomes while acting over the mitochondrial mRNA.
4. Different proteins are known to be present in the mitochondrial ribosomes as compared to the cytoplasmic ribosomes. So, a number of these mitochondrial-specific ribosomal proteins are also known to take part in the mitochondria by conducting proper coordination between the mitochondrial translation and other additional well-known cellular-specific processes.
5. There are various mitochondrial ribosome structural proteins that are actually formed or encoded in the various nuclear genes that are specific to this function only. Later on, as the need arises, these mitochondrial ribosome structural proteins enter into the mitochondria and resultantly they assemble coordinately with mitochondrial rRNAs to become full-fledged ribosomes. And so, as a result, they become active and start translating the mRNAs for encoding the essential proteins of the Oxidative Phosphorylation System.
6. Mitochondria is the powerhouse of the cell, so it needs to run independently for processing and providing cellular ATP energy to itself and to the other cell organelles leading to cell survival. So, in order to cope up with this, the cell organelle harbour their own genomes and translational apparatus called mitoribosomes.
7. Mitoribosomes’ presence inside the mitochondria is also an indication that during the evolution of the ribosomes, proteins took over some of the functions of rRNAs, including much of their participation in the inter-subunit communication.
8. Because of the dynamic protein composition of mitoribosomes of the mother’s egg cell, especially after inheritance and later on during embryonic development, results in the rise of heterogeneous populations of mitoribosomes fulfilling specific functions inside the mitochondria.
Are all mitochondrial DNAs (mtDNAs) the same?
Yes, all mitochondrial DNAs present inside all cells’ mitochondria of a particular species are the same. It’s because the mitochondria organelle contains almost identical copies of mtDNA inside it in all cells of the same species type.
However, the mtDNA content is different from species to species with a little to a lot of distinctive similarities.
Just like, if we compare the complete mitochondrial DNA sequences of humans and the common chimpanzee based on non-chimeric sequences then we will see that there is an 8.9% nucleotide difference between the mtDNAs of these two different species.
The mitochondrial DNA is not the same in all species because of the fact that the mtDNA has evolved a lot during the course of evolution in order to fit with the cells’ adapting and ATP production changes with response to the environment.
Giving light to this statement, it is to be noted here that the rate of mtDNA evolution in primates is 5 to 10 times higher than in the rate of nuclear DNA evolution.
So, as the nuclear DNA is different from species to species, the same way and with a lot more changes the mtDNA is also a lot different.
So, the differences between the mitochondrial DNA from species to species is due to the permanent mutational changes that were randomly occurring during the course of evolution.
So, hundreds to thousands of such mutational changes over the mtDNA sequence were added together overtime to bring out various evolutionary changes in the mitochondrial function and type from species to species causing different mtDNA variations as specified in cell biology.
So, maybe that’s why you can also state that the mean amino acid difference between the polypeptides of the 13 peptide-coding mitochondrial genes is 4.4% or so.