Extrachromosomal inheritance class 2: organelle DNA

Introduction to chloroplast and mitochondrial DNA

Image credit: theispot.com


Organelle DNA

Cytoplasmic DNA is present in membrane-bounded organelles inside the cell. Chloroplast and mitochondria are most common organelles having their own DNA. This DNA helps them for the synthesis of their own protein molecules to perform different biological functions.

Chloroplast DNA

Photosynthesis occurred within chloroplast provides an energy source to all living organism on earth as the animal cannot produce their own food.

Carl Correns is a pioneer in the discovery of non-mendelian inheritance. He observed that cytoplasmic DNA is responsible for the colour difference in Mirabilis jalapa and the DNA is located in the chloroplast.

DNA in chloroplast in also called as a plastid. Some of the variegations are controlled by not only nuclear DNA but also by plastids. Variegation is different coloured patches on leaves of the plant.

The chloroplast is an organelle dedicatedly working for the photosynthesis. Chlorophyll presents in chloroplast help in photosynthesis and production of energy in plants by the systemic reaction of photosystem I and photosystem II. 

Schematic representation of the cell, chloroplast, and cpDNA

Recent evidence suggests that chloroplast can synthesize protein in the presence of light or ATP, it has fully functional protein synthesis machinery. Chlorophylls, cytochrome a and b and other molecules are synthesized in the chloroplast.

Each chloroplast has multiple copies of DNA molecules. The copy numbers may differ in different species of plant ranging from 40 to 60 copies per chloroplast.

It was believed that chloroplast and mitochondria were free-living bacteria, evidently, some of the drug resistant genes are found on chloroplast DNA.

Ruth Sager’s experiment:

RuthSagerr experiments on chlamydomonas algae, and concluded that 10% of antibiotic resistance genes are located in chloroplast DNA. Chlamydomonas algae cells are cultured on streptomycin rich media.

Almost all cells are killed but one in some million cells are developed resistance and survived. By experimenting repeatedly, he created streptomycin resistant algae cells. 90% of resistant mutant involved nuclear genes but 10% is, however, created by genes located in the cytoplasm.

It is presumed that these 10% genes are located in chloroplast and have non-chromosomal uniparental inheritance. Interestingly, reciprocal crosses between two mating type + and – (with respect to the resistance of streptomycin here denoting as female or male is not possible) indicate that,

When the mating type + is resistance, then all the progenies developed resistance and when this + mating type is susceptible all the colonies become susceptible. The result indicates that the inheritance of non-chromosomal DNA is controlled by maternal side and also by a nuclear gene.

Image represents streptomycin + and streptomycin – mating type in Chlamydomonas

In higher plants the cpDNA ranges in size from 120 to 160 kb. Though all the chloroplast DNAs are circular in nature, some species of algae contains linear cpDNA. All the genes present in cpDNA are almost the same in all organisms. Majorly, cpDNA is grouped into two broad categories:

  1. Genes that encoded for protein synthesis machinery. Chloroplast has its own genome and central dogma machinery. Genes for RNA polymerase subunits, tRNA, the component of translation and ribosomes are present in the chloroplast.
  2. Genes that encoded for a specific component of photosystems. Component of Protein involves in photosystem I and II are encoded by genes present in chloroplast and furthermore, chlorophyll encoding genes are present on cpDNA.

With the help of their own protein synthesis machinery chloroplast synthesize their own protein and by using this molecule in photosynthesis they can create food of their own. Animals do not have chloroplast or genes which can code for chlorophylls, it was the photosynthetic bacteria which were localized in plan cell and symbiotically gave them a power for generating food.

Mitochondrial DNA

Mitochondria is a membrane-bounded organelle present in eukaryotic cells (not present in prokaryotes). It is believed that mitochondria were small moving bacteria in their previous life and symbiotically situated in the cytoplasm of eukaryotic cells.

Mitochondria provide energy to cells in form of ATP, both in plants and animals. The process of oxidative phosphorylation and fatty acid synthesis generates ATP molecules which are the main energy form for all biological reactions.

schematic representation of an animal cell, mitochondria, and mtDNA

Since evolution, it contains unique circular DNA molecules which code for the small number of protein. Mitochondrial DNA has its own protein synthesis unit. It has specific tRNA, aminoacyl tRNA synthetases, ribosome and mRNA.

Mutational changes in mitochondrial DNA leads to heritable alterations in an organism. Some of the protein need for the replication of mtDNAs are not located on mtDNA rather it is located on nuclear DNA.

The genome of mitochondria is smaller and circular, contains only a few genes. It is ranging from 16kb to 100kb in multiple copies as like cpDNA. Interestingly, mtDNA is highly conserved in higher animals from evolution.

mtDNA has different properties as compared to the nuclear genome, as the entire mtDNA has a single promoter site which means it is transcribed as one single unit. Endonuclease helps to cleave this giant transcribed DNA into individual segments of tRNA, rRNA and mRNA. It is somehow equivalent to the operon model of bacteria.

Read the previous article: Extrachromosomal inheritance class 1

Scientific evidence suggest that mitochondria is more similar to bacteria. Mitochondria transcribed mtDNA in one single unit as like bacterial DNA. Mitochondrial rRNA size is same as bacterial rRNA but smaller than eukaryotic rRNA. The protein synthesis process in mtDNA is more similar to bacterial DNA. It has its own membrane as like bacterial cell membrane and can able to synthesize energy-rich molecules (ATP).

Interestingly, not all the protein involved in ATP synthesis are encoded by mtDNA. Some of the proteins are encoded by nuclear DNA. For example, cytochrome oxidase enzyme is encoded by nuclear DNA but plays important role oxidative phosphorylation in mitochondria.

Molecules which are synthesized in mitochondria are governed by both mtDNA and nuclear DNA hence the pattern of inheritance is non-mendelian and complex in understanding. As mitochondria are inherited maternally, the general inheritance pattern is  shown in the figure:

The image represents the inheritance of mitochondrial DNA, in which only female individuals of the population can inherit mitochondrial DNA

In humans, some of the complex genetic disorders are occurred due to mutations in mtDNA.

Muscle weakness and muscle dis-coordination, poor growth, visual and hearing loss, autism spectrum disorders, heart, liver and kidney problems, learning and developmental disability, neurological defect and adrenal and thyroid gland defects are some of the common symptoms of mutations in mtDNA.

Congenital hearing loss, cytochrome C oxidase deficiency, ataxia, Leigh syndrome, Pearson syndrome and creatine deficiency syndrome are the major type of disorders reported in which mtDNA is involved. Look at the comparison of all three types of DNA: 

Comparison of nuclear DNA, mtDNA and cpDNA.

PropertyNuclear DNAmtDNA cpDNA
Arrangement Double helix and noncircularCircular Circular or linear
Copy 2 copies one on each chromosome ~10 to 100 copies per mitochondria ~40 to 60 copies per chloroplast
Inheritance Mendelian Non Mendelian/ maternal Non Mendelian
Nuclear envelope Present Absent bounded in mitochondrial membrane Absent
Location Located on the chromosome and packed into chromatins Circular and do not arranged on chromatin Circular or linear and do not arranged on chromatin
Size More than 3 billion base pairs~16kb to 100kb ~120 to 160 kb

This is all about extrachromosomal inheritance. The scientist has less knowledge about cytoplasmic DNA and how it evolved. We can hope that in the future, information about the exact mechanism of photosynthesis might permits scientist to built an artificial mechanism by which we can produce food for us as like plants.

Attend class: mutation


Article written by: Tushar Chauhan

Article reviewed by: Ravi Parmar

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