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Facts About Genetics & Neuromuscular Diseases
What is a Genetic Disorder?
A genetic disorder is a condition that's caused by a change, called a mutation, in a gene. A disease-causing mutation generally interferes with the body’s production of a particular protein.
What is a gene?

Genes, made of the chemical known as DNA (deoxyribonucleic acid), are strings of chemicals that form a “rough draft” of the recipes (often called codes) for the thousands of proteins our bodies use to build cellular structures and carry out the functions of our cells.
DNA is stored on strands called chromosomes, located mostly in the nucleus of each cell in the body.
How do genes lead to proteins?
The final copies of the protein recipes are actually carried in RNA (ribonucleic acid), a very close chemical cousin of DNA. The cell converts DNA to RNA in its nucleus. Each RNA recipe then leaves the cell’s nucleus and becomes the instruction manual for the manufacturing of a protein outside the nucleus.
How do mutations in genes lead to problems in proteins?

A mutation in the DNA for any protein can become a mutation (error) in the RNA recipe and then an error in the protein made from those RNA instructions. Some mutations are worse than others for the cell. Some mutations lead to production of a slightly abnormal protein, while others lead to a very abnormal protein or to the complete absence of a particular protein.
How do protein problems affect people?
The effects of a mutation in DNA in a person depend on many factors, among them exactly how the mutation affects the final protein (whether the protein is made at all and, if so, how close to normal it is), and how crucial that protein is in the body.
For example, some mutations in the gene for the protein dystrophin lead to relatively mild muscle weakness, while others lead to very severe weakness, depending on how much dystrophin is produced and how close it is to normal dystrophin. The mutations leading to severe weakness ultimately threaten life because dystrophin is needed for cardiac and respiratory muscle functions.
All this may seem like a lot of explanation, but it’s the basis for how you and the professionals you consult can make decisions about any genetic disorder that may be in your family.
What do proteins do?
The functions of proteins include such things as sending or receiving signals to or from other cells, breaking down large molecules into smaller ones, combining smaller molecules to make larger ones, and producing energy for all cellular activities. These activities ultimately result in functions like muscle contraction, digestion and metabolism of food, and regulation of blood pressure and temperature, as well as seeing, hearing, thinking and feeling.
Are There Genes Outside the Cell’s Nucleus?
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Yes. There’s actually another small set of genes that we all possess, inside our cells but outside the cell nucleus. The cell nucleus is where most of our genes reside on the 23 pairs of chromosomes already discussed.
The additional genes, which make up less than 1 percent of a cell’s DNA, are the mitochondrial genes, and they exist as circular strands of DNA inside mitochondria, the “energy factories” of cells. (The singular for mitochondria is mitochondrion.)
What are genes doing inside the mitochondria?

There are 37 genes, mostly involved in energy production, inside the mitochondria. Scientists believe that mitochondria were once independent organisms resembling today’s bacteria, and that when they became part of human and animal cells, they kept their own genes. These genes, arranged on structures that are like the nuclear chromosomes but are ring-like in shape, carry the recipes for 13 proteins needed for mitochondrial functions. They also carry codes for 24 specialized RNA molecules that are needed to assist in the production of other mitochondrial proteins. For reasons that will become clear, it’s important to know that mitochondria also use proteins made by genes in the cell’s nucleus. These proteins are “imported” into the mitochondria.
Can disease-causing mutations occur in mitochondrial genes?
Yes. Disease-causing mutations can occur in the mitochondrial genes. The disorders are often, as one would predict, associated with energy deficits in cells with high energy requirements, such as nerve and muscle cells. The disorders as a whole are called mitochondrial disorders. Mitochondrial disorders affecting muscle are known as mitochondrial myopathies.

How are mitochondrial mutations inherited?
Mitochondrial DNA inheritance comes only through the mother and is therefore completely different from nuclear (from the nucleus) DNA inheritance. The rules for recessive, dominant and X-linked inheritance don’t apply at all.
An embryo receives its mitochondria from the mother’s egg cell, not the father’s sperm cell, at conception. (Research suggests that sperm mitochondria are eliminated by the egg cell.)
Mutations can exist in some of the mitochondria in a person’s cells and never cause much, if any, trouble. (In fact, one theory of aging says that it’s caused by an accumulation of mutations in mitochondrial DNA.) The normal mitochondria are usually enough to produce the needed energy for the body. But once a person has a certain percentage of mutated mitochondria (perhaps 30 percent or so), the energy deficits become crucial and a mitochondrial disorder can result.
Mothers can pass on mitochondrial mutations to their children, but fathers can’t, so mitochondrial DNA inheritance follows a pattern called maternal inheritance. The severity of the child’s disorder depends on how many normal versus abnormal mitochondria the child receives from the mother.
Mitochondrial DNA mutations can also occur during development of an embryo. Not all mitochondrial mutations are inherited. Some occur as an embryo is developing in the womb. Researchers have found that embryonic mitochondrial mutations generally occur after sperm or egg cells have formed in the affected embryo, so, as far as has been observed, these mutations are not passed on to the next generation.
Does DNA from the cell’s nucleus affect the mitochondria?
Yes. DNA from the nucleus also affects mitochondrial function, so some mitochondrial disorders are inherited according to the same rules as are other genetic disorders.
Most mitochondrial proteins aren’t made in the mitochondria but come from genes in the cell’s nucleus. These nuclear proteins are later imported into the mitochondria, where they too help with energy production.
As you may have guessed, mutations also can occur in these nuclear genes that affect mitochondria. So, that’s another way to get a “mitochondrial disorder” — but one that’s not caused by mutated mitochondrial DNA.