Extranuclear inheritance

Cells have tiny parts that help them work.

Some parts turn things into energy. Other parts make food from light.

These parts have their own sets of instructions. These instructions come from the mother. They are inside the egg. The father's parts do not pass them on. This happens because the sperm leaves its parts behind. This helps the child grow well. It is a special way to pass things on.

Most genes live in a part of the cell called the nucleus. But some genes live outside the nucleus. This is called extranuclear inheritance.

These genes are found in tiny parts called organelles. One type is mitochondria. These help turn things into power. Another type is chloroplasts. These make sugar from light in plants.

Mitochondria have their own DNA. They pass this DNA from mothers to children. This happens through the egg. When a sperm meets an egg, the sperm head enters. The middle part of the sperm stays behind. This means the father's mitochondria are not passed on. Because of this, mitochondrial diseases come from the mother.

These parts can also copy themselves. They do this when a cell needs more power. Sometimes, both parents pass on these genes. This is called biparental inheritance. It happens in some yeast. Most of the time, only one parent helps. This is called uniparental inheritance. Other things like viruses can also pass genes this way.

Most genes live inside the cell nucleus. However, some genes live outside that center. This is called extranuclear inheritance.

These genes are found in tiny parts called organelles. One type is the mitochondria. These help turn things into energy. Another type is the chloroplast. These make sugar from light in plants and algae. These small parts are very important for a cell to work well.

These organelles have their own DNA. They can copy themselves on their own. They do not wait for the nucleus to tell them to. Instead, they copy themselves when a cell needs more energy. This happens throughout the life of a cell. Because they copy themselves separately, they rarely mix with other genes. This is different from the genes in the nucleus. Nuclear genes usually mix during a process called recombination.

Scientists have studied how these genes move for a long time. They looked at a fungus called Neurospora crassa. This fungus has a type called Poky. Poky grows very slowly. It has a defect in how its mitochondria work. By studying Poky, researchers learned about extranuclear inheritance. They found that Poky is passed from mothers to their offspring. They also found a tiny missing piece in its mitochondrial DNA.

In humans, we see uniparental inheritance. This means only one parent gives these genes. Mothers pass their mitochondria to children through the egg. When a sperm meets an egg, only the head enters. The middle part of the sperm stays behind. This part contains the father's mitochondria. Because of this, mitochondrial diseases come from the mother. Some plants like tobacco show different ways of passing genes. In tobacco, temperature can change how chloroplasts are inherited.

There are three main ways these genes move. Uniparental inheritance comes from just one parent. Biparental inheritance comes from both parents. This happens in some yeast called Saccharomyces cerevisiae. Vegetative segregation is a third way. This happens when organelles are split randomly during cell division.

Sometimes, even viruses or bacteria can pass genes this way. For example, a mother can pass a virus to a baby. This can happen through the blood or breastmilk. This is called perinatal transmission. It happens near the time of birth.

Most genetic information is stored within the cell nucleus. However, some genes exist outside of this central area. This process is known as extranuclear inheritance or cytoplasmic inheritance.

It involves the transmission of genes located in the cytoplasm. This occurs in most eukaryotes, which are complex organisms. These genes are often found within specialized structures called organelles. They can also be transmitted through cellular parasites like bacteria or viruses.

Organelles like mitochondria and chloroplasts play a vital role in cellular life. Mitochondria are responsible for transforming energy through a process called cellular respiration. Chloroplasts are found in plants and algae to produce sugars via photosynthesis. Both organelles contain their own unique DNA. This extranuclear DNA replicates independently of the DNA in the nucleus. While nuclear DNA replicates once before cell division, extranuclear DNA replicates in response to energy needs. This replication happens throughout the entire lifespan of a cell. Because they replicate separately, these genomes rarely undergo recombination. Recombination is the mixing of genes that is common in nuclear genomes.

There are three general types of extranuclear inheritance. The first is vegetative segregation. This occurs when organelles replicate and partition randomly during mitotic cell division. This results in daughter cells receiving a random sample of the parent's organelles. An example is seen in the mitochondria of asexually replicating yeast cells. The second type is uniparental inheritance. This occurs when only one parent contributes organellar DNA to the offspring.

The third type is biparental inheritance. This happens when both parents contribute organellar DNA. This is less common and usually only occurs in specific species. For instance, the yeast Saccharomyces cerevisiae shows biparental mitochondrial inheritance when two haploid cells fuse.

Uniparental inheritance is a very common way for these genes to move. In humans, mitochondrial inheritance is strictly maternal. The mother's mitochondria are passed to the offspring through the egg cell.

During fertilization, the head of the sperm typically enters the egg. The middle part of the sperm, which contains its mitochondria, is left behind. Consequently, the father's mitochondrial DNA is usually excluded from the zygote. This explains why mitochondrial diseases are inherited from the mother rather than the father. While very rare cases of paternal inheritance have been reported, they require more investigation.

Chloroplast inheritance can also be uniparental, but it is more complex. While often thought to be maternal, researchers have identified paternal inheritance in many species. In the tobacco plant, Nicotiana tabacum, the mode of inheritance can change. It can be maternal, paternal, or even biparental within the same species. The way chloroplasts are inherited in tobacco is affected by temperature. It is also influenced by the enzymatic activity of an exonuclease during male gametogenesis.

Extranuclear transmission can also involve parasites like viruses or bacteria. One specific example is perinatal transmission of viral genomes. This refers to the movement of material from a mother to a fetus. The perinatal period begins before birth and ends about one month after birth. During this time, viral material can be passed through the bloodstream or breastmilk. This is a major concern for mothers carrying viruses such as HIV or hepatitis C. Additionally, symbiotic bacteria in the cytoplasm can be inherited by insects and protists.

Scientists first established the basis for extranuclear inheritance by studying a fungus. The fungus Neurospora crassa has a specific mutant known as Poky. This mutant is characterized by slow growth and defects in mitochondrial ribosome assembly. It also shows deficiencies in several cytochromes. Early studies using genetic crosses proved that Poky is maternally inherited. Researchers eventually discovered the primary defect was a deletion in the mitochondrial DNA. Specifically, the deletion occurred in the sequence encoding the small subunit of mitochondrial ribosomal RNA. This discovery helped prove that certain genotypes are controlled by extranuclear DNA.