and Organelles prions
Centromeres ensure the proper segregation of genes during mitosis and meiosis. !
Molecules that not linked to chromosomes bearing nuclear centromeres behave differently during mitosis and do not obey Mendel's rules.! !Organelle chromosomes! !DNA and RNA plasmids! !Protein structure!
Organelle Genetics!
Mitochondria and Chloroplasts ! Involved in producing energy for the cell though either oxidative phosphoryation or photosynthesis ! Each contain their own small genome !
Mitochondria!
Use oxygen to produce energy efficiently (aerobic metabolism).! --muscle cells are loaded with them.! Contain own small genome! ~17,000 bp circular DNA in humans! Encodes 2 rRNAs and 22 tRNAs for protein synthesis! 13 proteins for energy metabolism! 99.9% of the mitochondrial proteins encoded by nuclear genes. !
Electron micrograph of a mitochondrion!
Electron micrograph of a mitochondrial DNA!
Nuclear vs Mitochondrial DNA!
Nuclear DNA ! 3 x 106 kbp ! Linear ! >30,000 genes ! Cell contains two copies of each gene ! Encodes all cellular functions, including mitochondrial !
Mitochondrial DNA ! 17 kbp ! Circular ! 37 genes ! Cell contains 1000s of copies ! Encodes only mitochondrial functions !
Map of human mtDNA !
Hypervariable region!
How did mitochondria (and chloroplasts in plants) arise? !
The endosymbiont theory was proposed by Lynn Margulis in the 1980s. ! It proposed that an anaerobic cell engulfed aerobic bacteria, and the two cells developed a symbiotic relationship. With time most bacterial genes ended up in the nucleus, but a few genes remained in what became mitochondria. !
Evidence for Endosymbiont Theory!
Similar lipid compositions in membranes of bacteria and mitochondria. ! Bacterial and mitochondrial genomes circular and lack associated histones. ! Protein synthesis in bacteria and mitochondria similar. ! rRNAs similar in bacteria and mitochondria !
Mitochondria and mutation
In yeast mitochondria are contributed by both parents. Mutations in essential mitochondrial genes, or nuclear genes that afect mitochondrial function, results in a slow growth phenotype (known as petite) on nonfermentable sugars. A second class of mitochondrial mutant has also been isolated., These are mutations that block antibiotic function, allowing cell survival in the presence of the antibiotic. Matings between cells with different genotypes affecting mitochondrial function, followed by sporulation, illustrates several features of mitochondrial genetics. 1. Segregational petites. 1: 1 petite: grande. This indicates the role of a nuclear gene. 2. Suppressive petites. A mixture of petite and grande cells is produced. The mitochondria with defects are segregated preferentially into some cells (which are petite), but not others (which are grande). 3. Neutral petites. All grande. There is either no mitochondria at all in the petite cell or the mitochondria has severe defects in replication.
In many organisms mitochondrial and chloroplast contribution to the zygote is uniparental, usually maternal. 1. Poky in Neurospora
In animals the rules for mitochondrial inheritance are different.! mtDNA is inherited maternally.!
mtDNA A! mtDNA B!
mtDNA C! mtDNA D!
Mitochondria and human disease!
1. Maternal inheritance 2. Variable penetrance and expressivity 3. Variable age of onset
All offspring of diseased mother can express the disease phenotype, while none of the offspring of the diseased fathers do.
!
The proportion of mutant mitochondria determines the severity of the MERF phenotype and the tissues that are affected. Tissues with higher energy requirements (e.g., brain) are least Tolerant of mutant mitochondria. Tissues with low energy Requirements (e.g., skin) are only affected when the proportion Wildtype mitochondria is greatly reduced.
!
Cells in which a population of mitochondria are mutated are typically heteroplasmic, containing Both wildtype and mutant versions of the mitochondria. However, several factors can result in drastically altered ratios of wildtype to mutant mitochondria in daughter cells. 1. Random drift 2. Increased replication rate of some mutated mitochondria = competition 3. Population bottleneck at certain stages of female germ cell development
The frequency of mitochondrial disease is high when one considers that it is roughly 6000 times smaller than the nuclear genome. There may be several reasons for this.
1. Mitochondrial coding DNA is 93% of the mitochondrial genome, but only 3% for the nuclear genome. 2. Mitochondrial DNA has a high mutation rate. Genomes with deletions may replicate at a higher rate then their wildtype counterparts. 3. There are thousands of mitochondria per cell. How does an egg come to have a large number of damaged mitochondria? It seems very unlikely that such mutant molecules would become dominant in a population undergoing random segregation during cell division.
Explanation. There is a population bottleneck during oogenesis during which primary oocytes may ultimately only receive one or a small number of mitochondria, which are subsequently amplified.
*! *! *! *! **!**!! ! *! **! !*! !* * * ! * *! *!! ! !**! * * !* ! *! ! *! ! **! **! * * * *!*! *!!! *! * *! *! !*!! *!*!!*!! **! *! * *! *! **!*! !*! ! ! ***! * ! *! *! ***! ! *! *! *! *! *!*!*!!! * *! *!
Transmission of mitochondrial DNA polymorphisms from heteroplasmic mother to offspring
(Chinnery et al. Trends. Genet.16:500)
Mitochondrial inheritance versus maternal effect!
Maternal effect genes also display a form of maternal inheritance.
Mitochondrial mutations display maternal inheritance, in which the progeny always have the maternal phenotype. In maternal effect the progeny have the phenotype specified by the maternal nuclear genotype. Maternal effect does not involve extranuclear genes. In many organisms the development of the embryo is prominently regulated by molecules that are deposited in the oocyte/egg. Since these molecules derive from the mother their composition is dependent on the maternal genotype. Mutations that disrupt the function of these genes in the female, but not the male, lead to developmental abnormalities. A classic example comes from the study of snail shell coiling. The direction of shell coiling is determined by a pair of nuclear alleles, D (for dextral or rightward coiling), and d (for coiling in the sinistral or leftwards direction). That the phenotype is dependent on the maternal genotype is shown through the results of the reciprocal crosses diagramed below. Cytological analysis of the developing eggs shows that the orientation of the mitotic spindle during the first division determines the direction of shell rotation. This led to the idea that perhaps the D gene product either was, or indirectly influenced the formation of something that determined mitotic spindle orientation. Experiments demonstrating that this is in fact the case involve taking cytoplasm from D/- eggs and introducing it into d/d eggs. The recipient eggs now develop with a dextral orientation. in contrast cytoplasm from d/d eggs is unable to influence the development of D/- eggs. These observations are consistent with the idea that the D gene specifies a product that is deposited in the egg and promotes a particular mitotic spindle orientation. The d product produces no product or a defective product. These embryos coil in the sinistral direction by default. There are examples many of important developmental regulators that are maternal effect genes. we will discuss some of these when we talk about developmental genetics.
Between 1976 and 1983, the military of Argentina kidnapped, jailed, and killed more that 10,000 dissidents. Along with their parents, many infants and toddlers disappeared. In 1977, the grandmothers of these children began to hold vigils in the main square of Buenos Aires to inform others that about the disappearance of their grandchildren. This group became known as the "Grandmothers of the Plaza de Mayo." They contacted many organizations for help, including the American Association for the Advancement of Science (AAAS). ! First, different alleles of the HLA locus were used. Compared the human lymphocyte antigens (HLAs) on white blood cells. Children should share one allele with one of the paternal grandparents and the other with one of the maternal grandparents. Mary Claire King, then a geneticist at Cal, taught Argentine medical workers to analyze HLA markers on white blood cells, and the workers typed HLAs from family members of the missing children. ! The probability that a tested child belonged to the family claiming him or her on the basis of eye-witness accounts of abduction varied from 75-99%. !
Then, mtDNA analysis was used.!
By the mid-1980s, the simpler cases had been solved. Professor King turned to her former mentor and colleague Allan Wilson, a molecular biologist at Cal who was studying mtDNA. PCR amplification and sequencing of the highly variable region of mtDNA made it possible to match a child with his or her ! maternal grandmother. The probability that two unrelated individuals would be identical for this 131 bp region is almost nil. In addition, the high copy number of the mtDNA aided in the identification of the remains of individuals killed by the military regime. !
Human migrations can be studied using Y chromosome and mtDNAs !
Analysis of human mtDNA led to the Mitochondrial Eve Hypothesis !
In the 1980s, Allan Wilson pioneered the use of mtDNA to study human evolution.! In two papers published in 1987 and 1991, he and his colleagues at Cal proposed that we all come from a population of humans that lived in Africa approximately 200,000 years ago.!
Evidence for Mitochondrial Eve hypothesis!
Compared sequences from the highly variable region of mtDNA.! Greater sequence differences among native Africans than any other group, which included Europeans, American Indians, Papua New Guineans, Asians, aboriginal Australians and individuals of Middle Eastern origin.! The diversity of Africans is equivalent to the diversity of all groups combined.! These observations led Wilson to propose that the African population has had the longest time to evolve variation and thus humans originated in Africa.!
But when did Mitochondrial Eve live? !
2.8% of the mitochondrial base pairs differed in the population of subSaharan Africans studied.! Chimpanzees and humans diverged about 5 million years ago, and human and chimpanzee mtDNAs differed at 15% of the base pairs of the mtDNA.! Adjusting the data to account for multiple substitutions at the same base pair, they calculated that mtDNA has been diverging at a rate of 13.8% per million years.! Assuming that this "molecular clock" is ticking at a constant rate, they determined that a "Mitochondrial Eve" from whom we are all derived lived 2.8/13.8 or 0.20 million years ago. Although there has been much argument about the assumptions and statistical methods used, most evolutionary geneticists accept that the women carrying our ancestral mtDNA lived in sub-Saharan Africa approximately 200,000 years ago.!
Many mtDNAs /cell!
2 for nuclear DNAs; 100s-1000s for mtDNAs ! After death DNA slowly degrades ! Can often amplify mtDNA but not nuclear DNA from preserved tissues and skeletons. !
In 1883, the last existing Quagga died in an Amsterdam zoo.!
More than 100 years later, analysis of mtDNA from museum Quagga tissue revealed that Quaggas were closely related to the extant plains zebra. !
Science Times 3/11/03!
Did Neanderthals interbreed with modern humans?!
Two theories for interactions between modern humans and archaic Homo species.! The replacement theory: As early Homo sapiens migrated out of Africa, they entered regions occupied by archaic Homo species and competed with them for resources. H. sapiens won the competition, leading...