Extranuclear Inheritance Flashcards
What is extranuclear inheritance?
Extra DNA found outside the nucleus.
Where is extra nuclear DNA found?
Typically found in organelles
- Mitochondria in eukaryotes
- Chloroplasts in plants
How does extranuclear DNA affect phenotype?
The phenotype of an organism can be influenced by this extranuclear DNA
-Different from Mendelian principles
How is mitochondrial DNA transmitted?
Mitochondrial DNA is transmitted from maternal gamete known as organelle heredity.
-recall secondary oocyte’s and mature ovum get most of the cytoplasm and the polar bodies get very little cytoplasm
Mitochondrial DNA mutations have…
Been linked to human disease affecting mainly energy production
Define extranuclear inheritance
Transmission of genetic information to offspring through the cytoplasm and not the nucleus
-usually from one parent
What is organelle heredity?
DNA from mitochondria or chloroplast determines certain phenotypic characteristics of offspring
What is infectious heredity?
Symbiotic or parasitic association with a microorganism
What is the maternal effect?
- nuclear gene products are stored in egg then transmitted through ooplasm to offspring
- Gene products in ovule distributed to embryo cells can influence phenotype
Describe the mitochondrial genome
Mitochondrial genome is circular with ~16500 bases and 37 genes.
There are hundreds of copies of the mitochondrial genome in each mitochondria and hundreds of mitochondria per cell
What are the abbreviations of mitochondrial and chloroplasmic DNA?
mtDNA and cpDNA
How do chloroplasts and DNA get their needed DNA?
Both chloroplasts and mitochondria also need proteins which were encoded for the on the nuclear DNA
Nuclear proteins are imported into the organelle
Compare and contrast cpDNA with mtDNA in terms of locations and number
Both are mainly circular DNA within non-nuclear organelles
-mtDNA in mitochondrial matrix. Many mtDNA per mitochondria, many mitochondria
-cpDNA in chloroplast stroma. Many chloroplast in the cell, many cpDNA per chloroplast but more in young growing leaves
Compare and contrast cpDNA with mtDNA in terms of their replication
Both are replicated within their particular organelle and use their own DNA to produce RNA and proteins required for their function
-both mitochondria and chloroplasts can divide inside the cell
How did we fund out about organelle inheritance?
1908 a German botanist/scientist , Carl corresponds who followed Mendel’s work was studying four o’ clock plants that had a variety of leaf colors on different branches
Had all white shoots And all green shoots
Main shoot is variegated
Where were the seed(eggs) from in the Crossing experiments with the 4 O’clock plant?
Seeds(eggs) from:
Green branches or white branches
Mixed with pollen from:
Green branches or white branches
Produced plants according to the source of ovule(seeds)
Since leaf coloration is a function of the chloroplast, there must be genetic information in the organelle to influence leaf color
White color resulted from the non-nuclear (cpDNA) genetic information that has a “defect”
What are the 3 types of ovule found in variegated regions ?
Type 1: all chloroplast contains mutated cpDNA
Type 2: all chloroplast contain wild-type cpDNA
Type 3: mixture of chloroplast, some containing wild-type cpDNA and others containing mutated cpDNA
What do seeds from variegated branches produce?
Seeds from variegated branches mixed with pollen from any of the branches STILL produced plants according to the source of ovule
How do organelle DNA contribute to the endosymbiosis theory?
- mtDNA and cpDNA look remarkably similar to bacterial DNA
- mitochondria and chloroplasts replicate their own DNA and contain their own machinery (proteins and RNA) for transcription and translation
- Mitichondria and chloroplasts have a double membrane. (Not seen in other organelles)
How was the endosymbiosis theory proposed?
Dr. Lynn Margulis proposed the theory in 1967:
- Mitochondria and chloroplasts(organelles) arose independently 2 billion years ago from free living bacteria
- Organelles possessed attributes of aerobic respiration and photosynthesis, respectively
- was not believed until genetic evidence substantiated in 1980s
What are the main points of the endosymbiosis theory?
- bacteria were engulfed by larger eukaryotic cells
- beneficial symbiotic relationship developed
- bacteria lost ability to function autonomously
- eukaryotic cells gained oxidative respiration and photosynthesis
How does mtDNA support the endosymbiotic theory?
- Exists in eukaryotes as double-stranded circular DNA
- Smaller than DNA in chloroplasts
How does cpDNA support the endosymbiotic theory?
Genes encode products involved in photosynthesis and translation
How many genes are in human mtDNA?
37
How many RNA in human mtDNA?
Two ribosomal RNAs (rRNA)
22 transfer RNAs (tRNA)
How many polypeptides are essential for oxidative respiration in human mtDNA ?
13 polypeptides are essential for oxidative respiration
-these 13 polypeptides combine with nuclear encoded proteins to make 4 important complexes found in the inner mitochondrial membrane required to make energy for the cell
-ATP is energy for the cell
Which of the DNA encode mitochondrial proteins ?
The majority of proteins for mitochondrial function are encoded by nuclear genes
Include:
DNA polymerase
RNA polymerase
Other tRNAs
Ribosomal proteins
Other respiratory proteins amino acyl tRNA synthases
Initiation and elongation factors of transcription and translation
What are the mitochondrial proteins encoded for by the mtDNA?
Some respiratory proteins
Some tRNAs (comeback to video)
What are respiratory complexes in the mitichondria by which DNA ?
Respiratory protein complexes I, III, IV and V are made of both nuclear and mitochondrial encoded proteins
Give a dangerous by-product of ROS
A by-product of respiration is the generation of Reactive Oxygen Species (ROS)
Why are reactive oxygen species dangerous ?
ROS is very mutagenic to mtDNA, mitochondrial proteins and lipids
-mutation rate for mtDNA is 10X higher than nuclear DNA
Also DNA repair mechanisms limited in the mitochondria
What mutated mtDNA lead to?
Mutated mtDNA can result in defective aerobic cellular respiration
- reduces the production of energy fir the cell
- Can greatly INCREASE the formation of reactive oxygen species
Defective mitochondria have…
Defective mechanism have increase in ROS production
What generates ROS in mitochondria?
The ETC
How is mtDNA inherited?
All zygotes essentially get mitochondria and mtDNA from mother
- A mature human oocyte contains 100,000 to 600,000 mitochondria, each with one mtDNA - Mitochondria all come from the mature oocyte, sperm 50-75
What is heteroplasmy?
As primordial germ cells divide some primary oocytes contain more mutated mtDNA than others
-Mutant load of mtDNA in each mature oocyte can vary
Offspring will have heteroplasmy in somatic cells, some tissues will have more mutant load than other tissues
Each offspring will have variable expressivity of disease
Phenotypes are highly pleiotropic, what does this mean?
Depends on the particular cell and requirements of mitochondrial function
Compare heart cells vs skin cells
Define heteroplasmy
The presence of more than one type mitochondrial mtDNA within a cell. It is an important factor in considering the severity of a mitochondrial disease
Define pleiotropy
One disease causing mutation influences two or more seemingly unrelated phenotypic traits
Define variable expressivity
A phenotype is expressed to a different degree among individuals with the same mutation (example siblings with same mother ea may have different percentage mutated mtDNA )
Explain pleiotropy of mitochondrial disease
Main reason for most mitochondrial pathologies is failure to produce enough ATP
Leads to multisystemic disorders
Severe clinical presentations in high energy demanding tissues
- skeletal muscle - central nervous system (CNS) - Heart muscle
Which organs and patients are affected by mitochondrial dysfunction?
Mitochondrial dysfunction can affect any organ of the body
Some patients do not precisely fit in any disease category, subtle features
Describe the inheritance of mitochondrial diseases
- Non-Mendelian inheritance
- Both males and females can be affected
- Only females can can transmit the disease to their offspring
- 100% of the female’s offspring will manifest
What does the severity of mitochondrial diseases depend on?
The severity of mitochondrial disease will often depend on percentage heteroplasmy
-Mitochondrial genetic disorders can present at any age with almost any affected body system
Give examples of hereditary mitochondrial diseases
Laber’s hereditary Optic Neuropathy-defect in Complex 1
Deafness induced by aminoglycoside antibiotics (rRNA)
Mitochondrial myopathies
States what are mitichondrial myopathies and give examples
Mitochondrial myopathies( disease of the muscle)
Kearns-Sayre syndrome(deletion in mtDNA)
MELAS syndrome- mitichondrial myopathy, encephalopathy, lactic acidosis and stroke like episodes
MERRF syndrome- myoclonic epilepsy; ragged red fibers
What is Laber’s hereditary optic neuropathy ?
Characterized by degeneration of the retinal ganglion cells (RGCs) and atrophy of the optic nerve
Most common cause of optic atrophy
Describe the typical onset and critical threshold of Leber’s hereditary optic neuropathy
- Usually begins between the ages of 25 and 35 (but can occur at any age) and leads to legal blindness
- Critical threshold (90%) in proportion of mutations in mtDNA must be exceeded before disease appears
Describe the resulting deafness of deafness induced by aminoglycoside antibiotics
- Hearing loss occurs within a few days to weeks after administration of any amount of aminoglycoside antibiotic
- mitochondrial hearing loss characterized by moderate to profound hearing loss
Explain: deafness induced by aminoglycoside antibiotics
- Maternally inherited mutation most commonly in mitochondrially encoded rRNA
- Mutation may cause predisposition to aminoglycoside toxicity causing deafness and/or late onset.
- An individual with many mitochondria containing mutations may have childhood onset of hearing loss
Describe onset of Kearns-Sayer
Onset before 20 and includes:
- paralysis of eye muscles and degeneration of retina - Cardiac problems or congestive heart failure - Muscle and skeletal weakness - Ataxia (coordination problems) - Diabetes, dementia and other mental illness
What Kearney-Sayer ?
- Deletion within mtDNA
- most common region is 4997 bp
- Affects systems with higher energy requirements
What is the most common mitochondrial disease?
MELAS
What are the symptoms of MELAS?
Mitochondrial myopathy/(weak muscles), encephalopathy, lactic acidosis, and stroke-like episodes, muscle twitching
When does MELAS presentation occur?
Presentation occurs with the first stroke-like episode 14-15 yrs of age
What usually causes MELAS?
Due to mutation in mitochondrial tRNA
How can lactic acidosis affect the brain?
Lactic acid accumulates in blood and is toxic to the brain
What is MERRF?
Myoclonic epilepsy ; ragged red fibers
-most common mutation in mitochondrial tRNA gene
What are the symptoms of MERRF?
Symptoms of MERRF typically begin in age 6-16 years
Myoclonus is usually the first followed by:
-seizures, ataxia, muscle weakness, worsening eye sight and hearing loss
Explain the “ragged red flags”
Defective mitochondria have mutations in mtDNA or genes on nuclear DNA targeted to mitochondria
- low energy production leads to mitochondria proliferating(multiplying)
- defective mitochondria accumulate in specific regions of muscle
- they appear as “Ragged Red Flags” when muscle is stained with Gomoritrichrome stain as shown to the right
How can mitochondrial proliferation may be observed?
Mitochondrial proliferation in muscle fibers can be seen by staining Complex II, Succinate dehydrogenase
What is the effective oxidative stress?
Causes damage to mtDNA, membranes, proteins and respiratory chain which in turn increases oxidative stress
What can be caused oxidative stress and energy decline?
Mitochondrial defects
Increased mtDNA mutations
What is the energy decline of mitochondria?
Reduced production of ATP with increased production of free radicals
What can result from oxidative stress and energy decline?
Cellular aging and degeneration
Mitochondrial energy production continues to decline and cells stop dividing(senescence)
What can result from mitochondrial defects, energy decline and cellular aging and degeneration?
Oxidativevstress
What can lead to energy decline?
- Oxidative stress
- Mitochondrial defects
- cellular aging and degeneration
What are ROS avengers?
There are proteins and molecules in the mitochondria that clean up the free radicals produced from respiration
As we age, these proteins are not expressed enough keep up with the damage
Mitochondrial disfunction has been implicated:
Has been implicated in:
- aging
- anemia
- blindness
- type II diabetes
- autism
- infertility
- neurodegeberatjve diseases: Parkinson’s and Alzheimer’s
Give the steps mitochondrial replacement therapy
Step 1: nucleus is removed from healthy donor’s egg
Step 2: healthy nucleus is removed from mother’s damaged egg
Step 3: egg carrying the genetic material from two women is fertilized with the father’s sperm and implanted in the mother
