Mitochondrial DNA
Mitochondrial DNA (mtDNA) is the genetic material found within mitochondria, the organelles responsible for energy production in eukaryotic cells. Each mitochondrion contains multiple copies of mtDNA, which vary in size and organization across different species. In humans, mtDNA is relatively small, encoding only 37 genes and about 16,500 base pairs, while some higher plants possess larger and more complex mitochondrial genomes. Mitochondria not only house their own DNA but also contain the necessary machinery for DNA replication, transcription, and protein synthesis, functioning independently to produce essential proteins for cellular respiration.
MtDNA is typically inherited maternally, a trait observed in many higher animals and plants, though some organisms, like yeast, demonstrate different inheritance patterns. The intricate structure of mtDNA can include segments originally derived from nuclear DNA and chloroplast DNA, indicating a history of genetic exchange among organelles. Additionally, mitochondria can edit RNA molecules, modifying their sequences to ensure the proper functioning of encoded proteins. Overall, the study of mitochondrial DNA offers insights into cellular energy production, evolutionary biology, and genetic inheritance patterns.
Mitochondrial DNA
Categories: Cellular biology; genetics
Mitochondria play an essential role in the generation of energy in eukaryotic cells. Mitochondria are the organelles that are the main “chemical factories” of the cell, where cellular aerobic respiration—using the Krebs (citric acid) cycle and respiratory electron transport to produce NADH (nicotinamide adenine dinucleotide) and ATP (adenosine triphosphate)—occurs. In a light microscope, mitochondria look like short rods or thin filaments about 0.5 to one micrometers long. A mitochondrion is made up of a smooth outer membrane and an inner membrane that is folded into tubular shapes called cristae. Many aerobic respiration reactions are catalyzed by enzymes that are bound to mitochondrial membranes. Other reactions occur in the space between the inner and outer mitochondrial membranes. Cells may contain anywhere from one to ten thousand mitochondria, depending on the organism and the function of the cell. A typical animal cell contains between 750 and two thousand mitochondria, for example, while the human eye contains ten thousand mitochondria per cell. Cells that are dividing and cells that are metabolically active need larger amounts of ATP and usually have large numbers of mitochondria.
Size and Structure
All eukaryotic cells except some primitive protozoans contain mitochondria. All mitochondria contain their own deoxyribonucleic acid (DNA), which in most species is inherited by the mother. There are hundreds to thousands of copies of mitochondrial DNA (mtDNA) per cell, depending on the organism and the cell's function. The size and organization of mitochondrial DNA vary widely from one species to another. Electron micrographs of mtDNA show linear and circular DNA in a variety of sizes and complex, branched molecules that are larger than the size of the gene. The mtDNA of humans codes for only thirty-seven genes and contains only about 16,500 base pairs. In contrast, the mitochondrial DNA of higher plants is larger, more complex, and present in many different molecules. The mtDNA of some organisms, such as some protozoa, algae, and fungi, is organized in linear molecules with the ends of chromosomes (telomeres) much like nuclear chromosomes.
Cloning mitochondrial DNA and comparing the sequences of the clones show that the entire complexity of a plant's mtDNA can be represented as a “master circle.” Also, it has been learned that sequences are repeated on the master chromosome. The repeated sequences differ for different plant species. A series of recombination events between these identical repeated sequences results in a series of rearrangements of mitochondrial DNA and forms the complex, multiple molecules of varying sizes that are the physical structure of a plant's coding.
Bilaterian animal mtDNA, in contrast, is simpler and smaller due to a limited coding capacity; this mtDNA is often viewed as an economically organized molecule with nearly invariable gene content. Several genetic and genomic features are unique to bilaterian mtDNA, including highly modified structures of encoded transfer RNA, unusual translation initiation codons, a low rate of gene rearrangements and a high rate of sequence evolution, and the presence of a single non-coding "control" region. Nonbilaterian animals show more variation in size and gene content.
Genes Encoded by Mitochondrial DNA
In addition to containing their own DNA, mitochondria contain enzymes for DNA replication and transcription, and ribosomes and transfer RNA for protein synthesis. (Transfer RNA, or tRNA, carries the building blocks of proteins, called amino acids, to the ribosome, where they are assembled according to the instructions found in messenger RNA.) The ribosomes of mitochondria are different from those of chloroplasts and cytoplasm, using a slightly different genetic code (a sequence of three bases that codes for a particular amino acid). Mitochondrial genes code for all of the ribosomal RNA found in mitochondria and for most of the tRNA. Mitochondria make only a small number of proteins that are needed for electron transport and ATP production. The other proteins needed in mitochondria are coded by nuclear DNA, translated in the cytoplasm of the cell, and then transported into the mitochondria. Even though the mitochondrial DNA of higher plants is much larger than that of animals, plant mtDNA codes for only a few more genes.
Exchange of DNA
Mitochondrial DNA contains segments of DNA that originally were in the nucleus and chloroplast. There appear to have been exchanges of DNA between all three of the organelles. For example, some mitochondrial tRNAs appear to be of chloroplast origin. Changes in nuclear genes have been shown to lead to changes in the copy number of the different mitochondrial DNA configurations. There is evidence that the majority of mitochondrial proteins originated in the nucleus before being synthesized and transported to the mitochondria. Therefore, the proteins required for cellular respiration are a mixture of proteins encoded by nuclear genes and proteins encoded by mitochondrial genes. These include the proteins involved in DNA replication and transcription, ribosomal proteins, and the enzymes necessary for the citric acid cycle.
RNA Editing
Mitochondria and chloroplasts contain the biochemical machinery to alter the sequence of the final messenger RNA (mRNA) product in a process called RNA editing. The most common editing is the changing of a cytosine to a uracil, two of the bases found on the “rungs” of DNA molecules and that are responsible for determining the nucleotide sequences that form the genetic code.
Inheritance of Mitochondrial DNA
Given the complexity of mitochondrial DNA, it is difficult to see how the inheritance of a complete genome is ensured. It is not completely understood how this complex network of DNA is passed to daughter cells in a way that assures that all of the genetic information is maintained. However, it has been found that in many eukaryotic organisms, particularly in higher animals and higher plants, organelle genes—including mtDNA—are inherited from the mother (an exception is yeast, which inherits from both parents). Mitochondrial genes seem to also be inherited in a non-Mendalian manner. For example, in yeast, when a chloramphenicol-resistant haploid cell mates with a chloramphenicol-sensitive haploid cell, the resulting daughter cells each have the same mitochondrial genes, which are a mixture of wild and mutant.
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