线粒体基因组课件

1、线粒体基因组课件 Mitochondrial molecular genetics 线粒体基因组课件 Mitochondrial molecular genetics Mitochondria are the main site of ATP synthesis in eukaryote cells and as such are vital for the health and survival of the cell They are also one of the sites at which apoptosis is mediated These lectures will explo

2、re the molecular genetics of mitochondria, how they are made, the structure of their genome, how they evolved , and how mitochondrial gene expression is controlled. 线粒体基因组课件 Mitochondrial molecular genetics focus on mitochondria: brief overview of their function and structure mtDNA structure and rep

3、lication: - animals - yeast - plants inheritance of mitochondria - petite mutants of yeast biogenesis of mitochondria by fission 线粒体基因组课件 MITOCHONDRIA essential for cell life - ATP synthesis - many metabolic intermediates essential for cell death - unprogrammed death: necrosis ( eg, due to loss of e

4、nergy status) - programmed cell death (apoptosis - controlled cell destruction) 线粒体基因组课件 Two membranes Inner membrane invaginated Numbers of mitochondria per cell vary but usually 100s/cell Matrix contains the TCA cycle (and other) soluble enzymes Inner membrane contains metabolite transporters and

5、the electron transport chain Mitochondrial structure 线粒体基因组课件 The ribosomes can actually be visualized in some mitochondria. In these figures, they are seen in the matrix as small dark bodies. DNA can also be visualized in mitochondria. The DNA is circular and resembles that of a bacterium in its ba

6、sic structure. Mitochondria also have their own ribosomes and tRNA: 22 tRNAs rRNAs (16S and 12S) 线粒体基因组课件 Mitochondria contain DNA molecules with an assortment of genes. Mitochondrial genetic system consist of DNA and the molecular machinery needed to replicate and express the genes contained in thi

7、s DNA. This machinery includes the macromolecules needed for transcription and translation. Mitochondria even possess their own ribosomes. Many of these macromolecules are encoded by mitochondrial genes, but some are encoded by nuclear genes and are therefore imported from the cytosol. 线粒体基因组课件 Mito

8、chondria have their own DNA and Ribosomes Mitochondria have some of their own DNA, ribosomes, and can make many of their own proteins. The DNA is circular and lies in the matrix in structures called nucleoids. Each nucleoid may contain 4-5 copies of the mitochondrial DNA (mtDNA). mitochondrial DNA 线

9、粒体基因组课件 Mitochondrial DNA (mtDNA) mtDNA was discovered in the 1960s; revealed as DNA-like fibers within mitochondria. The complete nucleotide sequences of mtDNA molecules from many different species have now been determined. mtDNA vary enormously in size, from about 6kb in Plasmodium to 2500 kb in s

10、ome of the flowering plants. Each mitochondrion appears to contain several copies of DNA. 线粒体基因组课件 Mitochondrial DNA (mtDNA) In a vertebrate oocyte, for example, it has been estimated that as many as 100 million copies of the mtDNA are present. Somatic cells, however, have fewer copies, perhaps less

11、 than 1 000. Most mtDNA molecules are circular, but in some species, such as alga Chlamydomonas reinhardtii 莱茵衣藻 , they are linear. In the vertebrates 37 distinct genes are packed into a 16 to 17- kb circle leaving little or no space between genes. 线粒体基因组课件 MITOCHONDRIAL DNA (YELLOW) IN THE UNICELLU

12、LAR ORGANISM EUGLENA GRACILIS. THE NUCLEAR DNA (RED) IS ALSO VISIBLE. 线粒体基因组课件 Plant mtDNA In some of the flowering plants an unknown number of genes are dispersed over a very large circular DNA molecule hundreds or thousands of kilobase in size. In these plants the mitochondrial genes may become se

13、parated onto different circular molecules by a process of intramolecular recombination. 线粒体基因组课件 Plant mtDNA This recombination is mediated by repetitive sequences located in the mtDNA. An exchange between two of the repetitive sequences can partition the “master” mtDNA circle into two smaller circl

14、e, a process that superficially resembles the excision of a lambda prophage from E. coli chromosome. In some species, several DNA circles of different sizes are formed by recombination between pairs of repetitive sequences located at different positions around the master DNA circle. These molecules

15、is difficult to study, and more research is needed to elucidate the mechanism that produces them. 线粒体基因组课件 Intramolecular recombination in the mtDNA of the Chinese cabbage, Brassica campestris油菜. Recombination between the repeated elements in the large circular DNA molecule partitions this molecule

16、into two smaller ones. Alternatively, the repeated elements in the two small molecules may recombine with each other to produce a single large molecule. 线粒体基因组课件 The structure of mtDNA The structure of mtDNA molecules has been studied by DNA sequencing. Animal mtDNA is small and compact. In human be

17、ings, for example, the mtDNA is 16,659 base pairs long and contains 37 genes, including two that encode ribosomal RNAs, 22 encode transfer RNAs, and 13 that encode polypeptides involved in oxidative phosphorylation, the process that mitochondria use to recruit energy. In mice, cattle, and frogs, the

18、 mtDNA is similar to that of human beings an indication of a basic conservation of structure within the vertebrate subphylum. 线粒体基因组课件 Map of human mtDNA showing the pattern of transcription. Genes on the inner circle are transcribed from the L strand of the DNA, whereas genes on the outer circle ar

19、e transcribed from the H strand of the DNA. Arrows show the direction of transcription. ND1-6 are genes encoding subunits of the enzyme NADH reductase; the tRNA genes in the mtDNA are indicated by abbreviations for the amino acids. 线粒体基因组课件 The structure of mtDNA Invertebrate mtDNA is about the same

20、 size as vertebrate mtDNA, but it has a somewhat different genetic organization. These differences seem to have been caused by structural rearrangements of the genes within circular mtDNA molecule. 线粒体基因组课件 The structure of mtDNA In fungi, the mtDNA is considerably larger than it is in animals. Yeas

21、t, for example, possesses circular mtDNA molecules 78 kb long. These molecules contain at least 33 genes, including 2 that encode ribosomal RNAs, 23 to 25 that encode transfer RNAs, 1 that encodes a ribosomal protein, and 7 encode different polypeptides involved in oxidative phosphorylation. The yea

22、st mtDNA is larger than animal mtDNA because several of its genes contain introns and there are long noncoding sequences between some of the genes. Animal mtDNA does not contain introns. 线粒体基因组课件 The structure of mtDNA Plant mtDNA is much larger than the mtDNA of others organisms. It is also more va

23、riable in structure. These conclusions come from crude physical and chemical analysis and from DNA sequencing. One of the first plant mtDNAs to be sequenced is from the liverwort地钱 , Marchantia polymorpha. The mtDNA from this primitive, nonvascular (非 维管) plant is a 186-kb circular molecule with 94

24、substantial open reading frames (ORFs) , some corresponding to known genes and others having still unassigned genetic functions. The latter ORFs are therefore called URFs, for unassigned reading frames. 32 distinct introns have been found in the Marchantia mtDNA, accounting for about 20% of the mole

25、cule. In vascular (维管) plants, the mtDNA is larger than it is in Marchantia; for example, it is a 570-kb circular molecule in maize and a 300-kb circle in the watermelon西瓜 . 线粒体基因组课件 The structure of mtDNA Higher plant mtDNA molecules contain many noncoding sequences , including some that are duplic

26、ated. The actual number of genes per mtDNA molecule is unknown. Physical mapping of some of these genes has shown that they are located in different position in the mtDNA circles of different species, even when the species are fairly closely related. This implies that mtDNA of higher plants has unde

27、rgone many genetic rearrangements during its evolution. 线粒体基因组课件 Expression of Mitochondrial genes The simple mtDNA of vertebrates are organized into two large transcription unit, each encoding the information of several genes. When the two strand of human mtDNA are separated by centrifugation, one

28、proves to be denser the H strand (for heavy), than the other referred as the L (for light). The promoters for the H and L transcription units are situated just upstream of the phenylalanine tRNA gene. The transcripts are extended in opposite directions around the mtDNA molecule. 线粒体基因组课件 The transcr

29、ipt from the H strand encodes 2 ribosomal RNAs, 14 tRNAs, and 12 polypeptides. The transcript from the L strand encodes 8 tRNAs and 1 polypeptide. Each transcript is cleaved to separate the tRNAs from the rRNAs and mRNAs, and mRNAs are polyadenylated. Each mRNA is then translated into polypeptides,

30、using the mitochondrial ribosomes and a combination of nuclear and ribosomal tRNAs. Expression of Mitochondrial genes 线粒体基因组课件 Translation in the mitochondria proceeds much as it does on the ribosomes of the cytosol, except that some of the codons have a different meaning. AGA and AGG are terminatio

31、n codons in mammalian mitochondria, whereas in the cytosol they specify the incorporation of arginine; UGA, which is termination codon in the cytosol, is a tryptophan codon in the mitochondria; and AUA, which encodes isoleucine in cytosol, is the methionine initiation codon in the mitochondria. Expr

32、ession of Mitochondrial genes 线粒体基因组课件 In fungi and plants the mtDNA is organized into many separate transcription units, some containing the information for more than one gene. Little is known about the details of transcription, but in yeast, the mitochondrial RNA polymerase is a single polypeptide

33、 encoded by a nuclear gene. RNA processing separates plant mitochondrial transcripts into their constituent parts and also removes the introns, which are present in several plant mitochondrial genes. THE MECHANICS OF THESE EVENTS ARE POORLY UNDERSTOOD. Expression of Mitochondrial genes 线粒体基因组课件 Anot

34、her peculiarity of plant mitochondrial gene expression is that many of the mtRNA transcripts undergo editing; that is, some of the nucleotides are changed after transcript has been synthesized. The most frequent change is C to U (occasionally U to C). Thus, RNA editing alters the composition of codo

35、ns in plant mitochondrial transcript. Editing alters the information that is actually encoded in the mtDNA and allows functional polypeptides to be synthesized. Editing is not found in the nonvascular plants (mosses and algae藻类和苔藓植物). The editing mechanism probably evolved sometime after plants had

36、become established on the land. Expression of Mitochondrial genes 线粒体基因组课件 Yet a third peculiarity of plant mitochondrial gene expression is that some mitochondrial mRNAs are formed by the process of trans-splicing. It occurs when segments of a gene are scattered over the mtDNA molecule. Each gene s

37、egment is transcribed independently, and exons of the different transcripts are spliced together by interactions between the introns that flank them. Expression of Mitochondrial genes 分子内(intramolecular) 剪接(cis splicing) 以及分子间 (intermolecular) 剪接(trans splicing) 线粒体基因组课件 Trans-splicing in wheat mito

38、chondria. Four different RNAs contribute to the final mRNA encoding a polypeptide of the enzyme NDH reductase. 线粒体基因组课件 INTERPLAY BETWEEN MITOCHONDRIAL AND NUCLEAR GENE PRODUCTS Most perhaps all-mitochondrial gene products function solely within mitochondrion. However, they do not function alone. Ma

39、ny nuclear gene products are imported to augment or facilitate their function. Many of the polypeptides needed for aerobic metabolism are also synthesized in cytosol (ATPase that is responsible for binding the energy of aerobic metabolism into ATP). However, because some of the subunits of this prot

40、ein are synthesized in the mitochondria, the complete protein is actually a mixture of nuclear and mitochondrial gene products. 线粒体基因组课件 INTERPLAY BETWEEN MITOCHONDRIAL AND NUCLEAR GENE PRODUCTS This dual (双重的) composition suggests that nuclear and the mitochondrial genetic systems are coordinated i

41、n some way so that equivalent amounts of their products are made; possible molecular mechanisms for this coordination are currently under investigation. 线粒体基因组课件 KEY POINTS mtDNA molecules range from 6-kb to 2500-kb in size, and most of them appear to be circular. mtDNA molecules contain genes for s

42、ome of the ribosomal RNAs, transfer RNAs, and polypeptides used within the mitochondrion. The structure, organization, and expression of mitochondrial genes vary among species. In some organisms, the transcripts of mitochondrial genes are edited after they are synthesized. Both mitochondrial and nuc

43、lear gene products are needed for normal mitochondrial function. 线粒体基因组课件 mt DNA and human disease Recent research has demonstrated that several human diseases are caused by mitochondrial defects, and in some cases, these defects are due to mutations in the mtDNA. One such disease is Lebers heredita

44、ry optic neuropathy (LHON), a condition characterized by the sudden onset of blindness in adults. This disease is associated with the death of the optic nerve (at a physiological level), and with mutation in any of several mitochondrial genes (at a molecular level). Each mutation changes an amino ac

45、id in one of the mitochondrial proteins reducing the efficiency of oxidative phosphorylation. The reduction is great enough to destroy the function of the optic nerve and cause total blindness. It is not known why this effect is limited to the optic nerve. LHON is inherited strictly through the mate

46、rnal line. 线粒体基因组课件 Another disorder caused by a mutation in the mtDNA is a Pearson marrow-pancreas syndrome, characterized by a loss of bone-marrow cells during childhood, is frequently fatal. It is caused by a fairly large deletions in the mtDNA. People with this syndrome almost never have affecte

47、d parents. Thus, the causative deletion probably occurs spontaneously during development in the child or during oogenesis in the mother. Individuals with Pearson syndrome actually have a mixture of deleted and normal mtDNA an example of mitochondrial heteroplasmy. Homoplasmic individuals have never

48、been observed. mt DNA and human disease 线粒体基因组课件 The molecular genetics of Chloroplasts Chloroplasts contain DNA molecules with an assortment of genes. Chloroplast are specialized forms of a general class of plant organelles called plastids质体质体 . Botanists distinguish among several kinds of plastids

49、, including chloroplasts (plastids containing pigments), amyloplasts (plastids containing starch造粉体：一种形成淀粉的植物性白色体), and elaioplasts (油质体 plastids containing oil or lipid). All three types seem to develop from small membrane-bounded organelles called proplastids 前质体, and, within a particular plant sp

50、ecies, all seem to contain the same DNA. This DNA is generally referred to as chloroplast DNA, abbreviated simply as cpDNA. 线粒体基因组课件 CHLOROPLAST DNA In higher plants, cpDNA typically range from 120 to 160 kb in size, and in algae, from 85 to 292 kb. In a few species of green algae the cpDNA is much

51、larger, about 2000 kb. The cpDNA seems to be organized as a closed circular molecule, but in some species (with large cpDNAs) a linear arrangement cannot be ruled out. The number of cpDNA molecules in a cell depends on two factors: the number of chloroplasts and the number of cpDNA molecules within

52、each chloroplast. 线粒体基因组课件 CHLOROPLAST DNA All cpDNA molecules carry basically the same set of genes, but in different species these genes are arranged in different ways. The basic gene set includes genes for ribosomal RNAs, transfer RNAs, some ribosomal proteins, various polypeptide components of p

53、hotosystems that are involved in capturing solar energy, four subunits of a ribulose 1,5-biphosphate carboxylase磷酸核酮糖梭化酶, and four subunit of a chloroplast-specific RNA polymerase. Most cpDNAs have a pair of large inverted repeats that contain the genes for ribosomal RNAs. These repeats range anywhe

54、re from 10 to 76 kb in length and are variously located in different cpDNA molecules. 线粒体基因组课件 . Genetic organization of the chloroplast DNA in the liverwort Marchantia polymorpha. Symbols: rpo, RNA polymerase; rps, ribosomal proteins of small subunit; rpl and secX, ribosomal proteins of large subun

55、it; 4.5S, 5S, 16S, 23S, rRNAs of the indicated size; rbs, ribulose bisphosphate carboxylse; psa, photosystem I; psb, photosystem II; pet, cytochrome b/f complex; atp, ATP synthesis; infA, initiation factor A; frx, iron-sulfur proteins; ndh, putative NADH reductase; mph, chloroplast permease (?); tRN

56、A genes are indicated by abbreviations for the amino acids. 线粒体基因组课件 CHLOROPLAST BIOGENESIS All plastids develop from proplastids. Chloroplasts development is stimulated by light, and involves the transcription of many genes, some located in the nucleus. All the proteins and chlorophyll pigments nee

57、ded for photosynthesis are made and targeted to their appropriate locations within the emerging chloroplast. The formations of functional chloroplasts is a process referred to as biogenesis. Only some of the details are known. Light plays an important role. Some genes are transcribed when light is p

58、rovided. 线粒体基因组课件 CHLOROPLAST BIOGENESIS A special class of pigmented proteins called phytochromes (光敏色素) seems to mediate this and others responses to light. By absorbing light energy, these proteins acquire the ability to trigger other proteins to stimulate the transcription of genes involved in c

59、hloroplast biogenesis. The formation of chloroplasts and the maintenance of their structure and function during the life of a plant depend on the coordinated expression of nuclear and chloroplast genes. 线粒体基因组课件 Chloroplast biogenesis. A mature chloroplast containing stacks (堆) of thylakoid类囊 体 memb

60、ranes (grana基粒) within its protoplasmic stroma (基质) develops from a proplastid after exposure to light. 线粒体基因组课件 KEY POINTS Chloroplast DNA (cpDNA) molecules are typically 120 to 292 kb in size, and they contain at least 100 genes. The organization of cpDNA molecules varies among species of plants a