Nuclear-encoded rRNAs

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

S, 5S and 4.5S rRNAs) and a small 30S subunit (containing 16S rRNA). Chloro-plast ribosomal proteins are encoded by both nuclear and chloroplast genes. 

All eukaryotic cells in our bodies contain the same 23 chromosomes with the same DNA base sequences. The lone differences are the mitochondria. The mitochondria in typical somatic cells contain less than 0.1% of the cell s DNA but in fertilized and dividing egg cells this number is greater. mtDNA is much smaller, often containing fewer than 20,000 base pairs. The value for humans is 16,569 base pairs. The mtDNA is a circular duplex. mDNA codes for the mitochondrial tRNAs and rRNAs but only a fraction of the mitochondrial proteins. Over 95% of the mitochondrial proteins are encoded by nuclear DNA. The mitochondria divide when the cell divides. 

The RNA product may encode transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), or small nuclear RNAs (snRNAs) that have end point functions in the cell. 

Mitochondrial DNA codes for the mitochondrial tRNAs and rRNAs and for a few mitochondrial proteins. More than 95% of mitochondrial proteins are encoded by nuclear DNA. Mitochondria and chloroplasts divide when the cell divides. Their DNA is replicated before and during division, and the daughter DNA molecules pass into the daughter organelles. 

A mutation in any of the 13 protein subunits, the 22 tRNAs, or the two rRNAs whose genes are carried in mitochondrial DNA may possibly cause disease. The 13 protein subunits are all involved in electron transport or oxidative phosphorylation. The syndromes resulting from mutations in mtDNA frequently affect oxidative phosphorylation (OXPHOS) causing what are often called "OXPHOS diseases."3-6 Mitochondrial oxidative phosphorylation also depends upon 100 proteins encoded in the nucleus. Therefore, OXPHOS diseases may result from defects in either mitochondrial or nuclear genes. The former are distinguished by the fact that they are inherited almost exclusively maternally. Most mitochondrial diseases are rare. However, mtDNA is subject to rapid mutation, and it is possible that accumulating mutants in mtDNA may be an important component of aging.h k... 

The gene encoding the smallest structural RNA component of the nuclear ribosome is the 5S rRNA gene. This gene is not co-located with the remaining nuclear rRNA genes. Neither sequence nor chromosomal location of the 5S gene is known for any neodermatan and it will not be discussed further here. 

Approximately 1000 proteins comprise the mitochondrion the majority are encoded on genes located on nuclear DNA. In fact, as seen in Figure 8-5, the mtDNA encodes only 13 proteins. These mtDNA-encoded proteins are the seven subunits (ND1,2,3,4,4L, 5, and 6) of the NADH-dehydrogenase (RC I) one subunit (cytochrome b) of RC III three subunits (CO I, II, and III) of cytochrome c oxidase (RC IV) and two subunits (A6 and A8) of the ATP synthase (RC V).A11 of these proteins are components of the ETC or the ATP synthase involved in OXPHOS. In addition to these 13 proteincoding genes, the mtDNA encodes 22 mitochondrial transfer ribonucleic acids (tRNAs) and two ribosomal RNA (rRNA) molecules (the large 16S rRNA and the small 12S rRNA). 

The human genome contains approximately 3 billion base pairs (bp) of DNA. The DNA is folded to fit within the nucleus. It is divided among chromosomes and compactly packed into chromatin by histones and other accessory proteins. Each normal somatic cell contains two copies of 22 different somatic chromosomes and two sex chromosomes (XX or XY). Less than 5% of DNA actually encodes protein and other functional products, such as tRNA, rRNA, miRNA, and other small nuclear RNAs. The majority (>95%) of human DNA consists of non-coding sequences, typically repetitive sequences such as minisatellites, microsatellites, SINEs, and LINEs. Microsatellites are short tandem repeats with each repeat unit of 1 to 13 bp long. Mini-satellites are tandemly repeated DNA sequences with the size of repeat unit of 14 to 500 bp. Microsatellite and minisatellite repeats are also known as short tandem repeats (STRs). Highly repetitive sequences containing thousands of repeated units are also found at the... 

I /iCi/oocyte) is used to label the RNAs and can either be coinjected with the gene into the nucleus or injected into the cytoplasm following an initial nuclear injection of the gene. Because RNAs encoded by RNA polymerase III (e.g., tRNAs, 5S rRNAs) are more strongly transcribed than those encoded by RNA polymerase II (mRNA and snRNA) in oocytes, a reasonable amount of DNA to inject is 0.1-lng and 1-5 ng/oocyte for genes containing RNAP III and RNAP... 

There are at least four types of RNA-P. from eukaryotic cells RNA-P.I is found in the nucleolus and preferentially catalyses the synthesis of rRNA. RNA-P.II is present in the nucleoplasm, is mainly responsible for the synthesis of mRNA, and is specifically inhibited by a-amanitin. RNA-P.III transcribes tRNA and other classes of small RNA. The fourth RNA-P. is smailer than the other three and transcribes RNA from mitochondrial DNA however, it is encoded by a nuclear gene. The RNA-P. of chloroplasts and mitochondria show similarities with prokaryotic RNA-P., e.g. inhibition by rifamycins. 

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