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S.U.4: Organelle DNA
★ Mitochondria, chloroplasts & a few other cell organelles possess their own DNA, which encodes some of the proteins &
RNA molecules found in the organelles.
★ Mitochondrial DNA (mtDNA) has a no. of advantages for the study of evolutionary relationships.
✓
Why Beja-Pereira ET. Al used it in their study of donkeys.
1. DNA found in mitochondria is shorter than DNA in chrm found in nuclei of euk cells.
2.
☞ Makes it easier to analyse.
mtDNA is abundant, because each cell has numerous mitochondria, each having several copies of
mitochondrial chrm.
3.
☞ mtDNA is easier to isolate & study than nuclear DNA.
mtDNA in animals tends to evolve more rapidly than nuclear DNA.
4.
☞ Useful for looking at relationships among closely related organisms.
mtDNA is typically inherited from only 1 parent (mother).
☞ Its genes aren’t reshuffled every generation by recombination, which obscures genetic relationships.
★ DNA sequences found in mitochondria & other organelles possess unique properties that make these sequences useful.
22.1 Mitochondria & chloroplasts are euk cytoplasmic organelles- Fig 22.2
●
Mitochondria & chloroplasts are membrane-bounded organelles located in the cytoplasm of euk cells.
●
Mitochondria are found in almost all euk cells, chloroplasts are found in plants & some protists.
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Both organelles generate ATP.
Mitochondria
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Tubular 0.5-1.0μm in diameter
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4- 6μm in diameter
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Have 3rd membrane (thylakoid membrane); highly
folded & stacked forming granas, bearing pigments
& enzymes for phosphorylation
Surrounded by 2 membranes (mt= matrix, cp=
stroma) that contain RNA, DNA, enzymes &
ribosomes.
Inner membrane is highly folded; embedded within
enzymes that catalyse ETC & ox phosphorylation
●
Chloroplast
NB!!! Do not skip this!!
Mitochondrion & chloroplast structure
New mitochondria & chloroplasts arise by division
of existing organelles- divisions that take place
throughout the cell cycle & are independent of
mitosis & meiosis.
Possess DNA that encodes polypeptides used by
the organelle, as well as rRNAs & tRNAs needed for
translation of the proteins.
The genetics of organelle encoded traits
●
Mitochondria & chloroplasts are present in the cytoplasm, & are usually inherited from a single parent.
∴
Traits encoded by mtDNA & cpDNA exhibit uniperental inheritance.
▪
In animals, mtDNA is inherited almost exclusively from the ♀ parent, but occasional ♂ transmission of mtDNA
has been seen.
▪
Parental inheritance of organelles is common in gymnosperms & a few angiosperms.
▪
Some plants exhibit biparental inheritance of mtDNA & cpDNA.
GTS 261- Anya OberholzerPage 1
Replicative segregation- Fig 22.3 & Fig 22.4
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Individual cells may contain many organelles, each with numerous copies of the organelle genome.
▪
●
Each cell typically possesses many copies of mitochondrial & chloroplast genomes.
Mutation arising within 1 organelle DNA generates a mixture of mutant & wild-type DNA sequences within the cell.
▪
The occurrence of 2 distinct varieties of DNA within the cytoplasm of a single cell is called heteroplasmy.
*
●
When a heteroplasmic cell divides, the organelles segregate randomly into the 2 progeny cells in a process
called replicative segregation.
❖
Chance determines the proportion of mutant organelles in each cell.
❖
Most progeny cells will inherit a mixture of mutant & normal organelles, by chance; some cells may
receive organelles with only mutant or only wild-type sequences.
•
Homoplasmy: situation in which all organelles are genetically identical.
•
Fusion of mitochondria also takes place frequently.
When replicative segregation takes place in somatic cells, it may create PT variation within a single organism.
▪
Diff cells of the organism may possess diff proportions of mutant & wild-type sequences, resulting in diff
degrees of PT expression in diff tissues.
●
When replicative segregation takes place in germ cells of a heteroplasmic cytoplasmic donor there may be diff PT
among offspring.
●
Myoclonic epilepsy & ragged-red fibre syndrome (MERRF) is caused by a mutation in an mtDNA gene.
●
▪
A 20 year old who carried this mutation in 85% of his mtDNA displayed a normal PT.
▪
A cousin who had the mutation in 96% of his mtDNAs was severely affected.
In diseases caused by mutations in mtDNA, the severity of the disease is related to the proportion of mutant
mtDNA sequences inherited at birth.
Traits encoded by mtDNA- Fig 22.5
●
A no. of traits encoded by organelle DNA have been studied.
▪
1 of the first to be examined in detail was the PT produced by petite mutations in yeast.
*
When grown on solid medium, some colonies of yeast were much smaller than normal.
*
Examination of petite colonies revealed that growth rates of the cells within the colonies were greatly
reduced.
❖
Results of biochemical studies showed that petite mutants were unable to carry aerobic respiration.
•
❖
Some petite mutations are defects in nuclear DNA, but most occur in mtDNA.
•
Mitochondrial petite mutations often have large deletions in mtDNA or are missing mtDNA.
•
mtDNA encodes enzymes that catalyse aerobic respiration.
∴
Petite mutants are unable to carry out aerobic respiration & can’t produce normal amounts of ATP, inhibiting
their growth.
▪
Another known mtDNA mutation occurs in neurospora.
*
●
Obtained all of their energy from anaerobic metabolism, which is much less efficient than aerobic
respiration & results in smaller colony size.
Poky mutants grow slowly, display cytoplasmic inheritance, & have abnormal amounts of cytochromes.
❖
Cytochromes are protein components of the ETC of the mitochondria & play an NB role in ATP
production.
❖
Most organisms have 3 primary types of cytochromes: cytochrome a, b, & c.
•
Poky mutants have cytochrome c but no cytochrome a or b.
•
Poky mutants are defective in ATP synthesis & ∴grow slower than normal wild-type cells.
No. of genetic diseases that result from mutations in mtDNA have been identified in humans.
▪
Leber hereditary optic neuropathy (LHON), which leads to sudden vision loss during middle age.
*
Results from mutations in the mtDNA genes that encode ETC proteins.
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▪
Neurogenic muscle weakness, ataxia, & retinis pigmentosa (NARP) is characterised by seizures, dementia, &
development delay.
▪
Kearns-Sayre syndrome (KSS) & chronic external opthalmoplegia (CEOP) both result in the paralysis of the eye
muscles, droopy eye muscles, droopy eyelids, & in severe cases, vision loss, deafness, & dementia.
GTS 261- Anya OberholzerPage 3
●
A trait in plants that is produced by mutations in mitochondrial genes is cytoplasmic ♂ sterility, a mutant PT found
in more than 140 diff plant species & inherited only from the ♀ parent.
▪
●
Inhibit pollen development but don’t affect ♀ fertility.
No. of cpDNA mutants also have been discovered.
▪
1st to be recognised was leaf variegation in the 4 o’clock plant Mirabilis jalapa.
▪
In the green alga Chilamydomonas, streptomycin-resistant mutations occur in cpDNA.
*
No. of mutants exhibiting altered pigmentation & growth in higher plants have been traced to defects in
cpDNA.
!SEE WORKED PROBLEM P605!
The endosymbiotic theory- Fig 22.6
●
Chloroplasts & mitochondria are similar to bacteria.
▪
●
Compelling evidence that these organelles evolved from eubacteria.
Endosymbiotic theory proposes that mitochondria & chloroplasts were once free-living bacteria that became
internal inhabitants (endosymbionts) of early euk cells.
▪
Btwn 1 billion & 1.5 billion years ago, a large, anaerobic euk cell engulfed an aerobic eubacterium that
possessed the enzymes needed for ox phosphorylation.
*
The eubacterium provided the cell with the capacity for ox phosphorylation & allowed it to produce more
ATP for each organic molecule digested.
*
The endosymbionts became an integral part of the euk host cell, & its descendants evolved into presentday mitochondria.
❖
Similar relation arose btwn photosynthesising eubacteria & euk cells, leading to chloroplasts.
●
Many protists are hosts to endosymbiotic bacteria.
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Mitochondria & chloroplasts are similar in size to present day eubacteria & possess their own DNA.
●
▪
Has many characteristics in common with eubacterial DNA.
▪
Mitochondria & chloroplasts possess ribosomes, some of which are similar in size & structure to eubacteria
ribosomes.
▪
Antibiotics that inhibit protein synthesis in eubacteria but don’t affect protein synthesis in euk cells also inhibit
protein synthesis in these organelles.
Strongest evidence for endosymbiotic theory comes from the study of DNA sequences in organelle DNA.
▪
rRNA & protein-encoding gene sequences in mitochondria & chloroplasts have been found to be more closely
related to sequences in the genes of eubacteria than they are to those found in the euk nucleus.
▪
mtDNA sequences are most similar to sequences found in eubacteria α-proteobacteria.
*
Suggests that the original bacterial endosymbionts came from this group.
▪ cpDNA sequences are most closely related to sequences found in cyanobacteria, a group of photosynthesising
eubacteria.
▪ All this indicates that mitochondria & chloroplasts are more closely related to eubacterial cells than they are to
the euk cells in which they are now found.
22.2 mtDNA varies widely in size & organisation- Table 22.1
●
In most animals & fungi, mitochondrial genome consists of a single, highly coiled, circular DNA- like eubacterial
chrm.
●
Plant mitochondrial genomes exist as a complex collection of multiple circular DNA molecules.
▪
In some, the mitochondrial genome consists of a single, linear DNA molecule.
▪
Each mitochondrion contains many copies of the mitochondrial genome, & a cell may have many
mitochondria.
GTS 261- Anya OberholzerPage 4
●
mtDNA lacks histone proteins normally associated with euk nuclear DNA, but it is complexed with other proteins
that have some histone-like properties.
▪
●
The GC content of mtDNA is often sufficiently diff from that of nuclear DNA that mtDNA can be separated from
nuclear DNA by density gradient centrifugation.
Mitochondrial genomes are small compared with nuclear genomes & vary greatly in size among diff organisms.
▪
Sizes of mitochondrial genomes of most species range from 15,000 bp -65,000 bp, but some are smaller.
▪
No correlation btwn genome size & no. of genes.
*
No. of genes is more constant than genome size.
❖
Most species have only 40-50 genes
❖
Genes encode 5 basic functions.
•
●
Respiration & Ox phosphorylation, Translation, Transcription, RNA processing & Import of
proteins into the cell.
Most of the variation in size of mitochondrial genomes is due to diff in non-coding sequences such as introns &
Intergenic regions.
The gene structure & organisation of mtDNA
●
The genes for most structural proteins & enzymes found in mitochondria are actually encoded for by nuclear DNA,
translated in cytoplasmic ribosomes, & then transported into the mitochondria.
▪
Mitochondrial genome encodes only a few rRNA & tRNA needed for mitochondrial protein synthesis.
▪
The organisation of the mitochondrial genes & how they are expressed is diverse across organisms.
Ancestral & derived mitochondrial genomes
●
●
●
Mitochondrial genomes can be divided into 2 basic types.
▪
Ancestral & derived genomes.
▪
The mtDNA of some organisms don’t fit well into either category.
Ancestral mitochondrial genomes are found in some plants & protists.
▪
Contain more genes than derived genomes.
▪
Have rRNA genes that encode eubacterial-like ribosomes.
▪
Complete or almost complete set of tRNA genes.
▪
Possess few introns & little noncoding DNA btwn genes.
▪
Generally use universal codons.
▪
Have their genes organised into clusters similar to those found in eubacteria.
▪
Ancestral mitochondrial genomes retain many characteristics of their eubacterial ancestors.
Derived mitochondrial genomes are smaller than ancestral genomes & contain fewer genes.
▪
rRNA genes & ribosomes differ from those found in eubacteria.
▪
DNA sequences found in derived mitochondrial genomes differ more from typical eubacterial sequences than
do ancestral genomes.
▪
Contain nonuniversal codons.
▪
Derived mitochondrial genomes have characteristics that differ substantially from those found in typical
eubacteria.
Human mtDNA – Fig 22.7
●
Circular molecule encompassing 16,569 bp that encode 2 rRNAs, 22 tRNAs, & 13 proteins.
▪
2 nucleotide strands of the molecule differ in their base composition
*
Heavy strand (H) strand has more G nucleotides.
❖
*
Is a template for both rRNAs, 14 tRNAs, & 12 proteins.
Light strand (L) has more C nucleotides.
❖
Serves as a template for 8 tRNAs & 1 protein.
GTS 261- Anya OberholzerPage 5
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Origin of replication for the H strand is within the D loop, which contains promoters for both the H & L strands.
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Highly economical in its organisation.
▪
●
*
Almost all mRNA is translated, & there are no introns
*
Each strand has only a single promoter, so transcription produces 2 very large RNA precursors that are
later cleaved into individual RNA molecules.
Many genes that encode polypeptides even lack a complete termination codon, ending in either U or UA.
▪
●
There are a few noncoding nucleotides btwn genes.
Addition of a poly(A) tail to the 3’ end of the mRNA provides a UAA termination codon that halts translation.
Human mtDNA also contains repetitive DNA.
▪
1 region of human mtDNA that does not contain some noncoding nucleotides is the D loop.
Yeast mtDNA- Fig 22.8
●
Organisation of yeast mtDNA is quite diff from that of human mtDNA.
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78,000 bp large.
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Encodes only 6 additional genes to human mtDNA.
▪
2rRNAs, 25 tRNAs, 16 polypeptides.
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Most of the additional DNA in yeast mitochondrial genome consists of introns & noncoding sequences.
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Yeast mitochondrial genes are separated by long Intergenic spacer regions that have no known function.
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The genes encoding polypeptides often include regions that encode 5’ & 3’ UTR of the mRNA.
▪
Also short repetitive sequences & some duplications.
Flowering-plant mtDNA- Fig 22.9
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Flowering plants (angiosperms) have the largest & most complex mitochondrial genomes known.
▪
Mitochondrial genomes range in size from 186,000 bp in white mustard to 2,400,000 bp in muskmelon.
*
●
Even closely related plants may differ greatly in size of mtDNA.
Part of extensive size variation in the mtDNA of the flowering plant can be explained by presence of large direct
repeats, which make up large parts of the mitochondrial genome.
▪
Crossing over btwn these repeats can generate multiple circular chrms of diff sizes.
Nonuniversal codons in mtDNA- Table 22.2
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The same codons specify the same A.A in most bacterial & euk DNA.
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Exceptions to universal code are seen in mtDNA.
▪
Exceptions often differ among organisms.
*
E.g. AGA specifies Arg in the universal code.
❖
AGA encodes Ser in Drosphilia mtDNA & stop codon in mammalian mtDNA.
The replication, transcription, & translation of mtDNA
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mtDNA doesn’t replicate in the same way as nuclear DNA.
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mtDNA is synthesised throughout the cell cycle & isn’t coordinated with synthesis of nuclear DNA.
●
Which mtDNA molecules are replicated at any particular moment seems to be random.
▪
●
Within the same mitochondrion, some molecules are replicated 2 or 3 times, whereas others are not
replicated at all.
Replication.
▪
mtDNA is replicated by DNA poly γ.
▪
2 strands of human mtDNA may not replicate synchronously.
▪
Single-strand-binding proteins, helicases & topoisomerases are needed in mt DNA replication.
*
●
These proteins are encoded by nuclear genes in many organisms.
Transcription & translation of mitochondrial genes exhibit extensive variation among diff organisms.
GTS 261- Anya OberholzerPage 6
▪
●
In human mtDNA, eubacterial-like operons are absent, & there are 2 promoters- one for each nucleotide
strand-within the D-loop.
Transcription.
▪
Transcription of 2 strands takes place in opp directions.
*
Generates 2 giant precursor RNAs that are cleaved to generate individual rRNAs, tRNAs & mRNAs.
❖
As tRNAs are transcribed, they fold up into 3D configurations.
•
❖
tRNA genes generally flank the proteins & rRNA genes.
•
●
●
Configurations are recognised & cut out by enzymes.
Cleavage of tRNAs releases mRNAs & rRNAs.
▪
In mitochondrial genomes of fungi, plants & protists, there are multiple promoters, although genes are
occasionally arranged & transcribed in operons.
▪
Most mRNA molecules produced by transcription of mtDNA are not capped at their 5’ ends.
*
Unlike mRNA transcribed from nuclear genes.
*
Poly(A) tails are added to the 3’ ends of some mRNAs encoded by animal mtDNA, but poly(A) tails are
missing from those encoded by mtDNA in fungi, protists & plants.
*
Poly(A) tails added to animal mitochondrial mRNAs are shorter than those attached to nuclear-encoded
mRNAs& probably added by diff mechanisms.
Some of the genes in yeast & plant mtDNA contain introns.
▪
These introns are mostly self-splicing.
▪
RNA encoded by some mitochondrial genomes undergoes extensive editing.
Translation.
▪
In mitochondria, protein synthesis is initiated at AUG start codons N- formylmethionine.
*
Same as in eubacterial initiation of translation.
▪
Mitochondrial translation uses elongation factors similar to those used in eubacterial translation.
▪
Same antibiotics that inhibit translation in eubacteria also inhibit translation in the mitochondria.
▪
Mitochondrial ribosomes are variable in structure & diff from those in eubacteria & euk cells.
▪
Initiation of translation in mitochondria must be diff from that of both eubacterial & euk cells.
*
Animal mitochondrial mRNA contains no Shine-Dalgarno ribosome-binding site & no 5’ cap.
▪ Diversity in tRNAs encoded by various mitochondrial genomes.
*
Human mtDNA encodes 22 of the 32 tRNAs required for translation by the cytoplasm.
*
In human mitochondrial translation, there is more wobble than in cytoplasmic translation.
❖
Many mitochondrial tRNAs will recognise any of the 4 nucleotides in the 3rd position of the codon.
•
❖
∴
Permits translation to take place with fewer tRNAs.
Increased wobble means that any change in a DNA nucleotide at the 3rd position of the codon will be a
silent mutation & will not alter the A.A sequence of the protein.
More of the changes due to wobble in mtDNA are silent & accumulate over time, contributing to a
higher rate of evolution.
☞ In some organisms, fewer than 22 tRNAs are encoded by mtDNA; in these organisms, nuclearencoded tRNAs are imported from the cytoplasm to help carry out translation in the mitochondria.
☞ In other organisms, the mitochondrial genome encodes a complete set of all 32 tRNAs.
The evolution of mtDNA
●
Comparisons of mtDNA sequences with DNA sequences in bacterial strongly support a common eubacterial origin
for all mtDNA.
▪
●
Patterns of evolution seen in mtDNA vary greatly among diff groups of organisms.
Sequences of vertebrate mtDNA exhibit an accelerated rate of evolution.
▪
E.g. sequences in mammalian mtDNA typically change 5-10 times faster than those in mammalian nuclear DNA.
▪
No. of genes present & the organisation of vertebrate mitochondrial genomes are relatively constant.
GTS 261- Anya OberholzerPage 7
●
Sequences of plat mtDNA evolve slowly, but their gene content & organisation change rapidly.
●
Reasons for basic diff in rates of evolution are not yet known.
▪
●
Possible reason for accelerated rate of evolution seen in vertebrate mtDNA is a high mutation rate.
*
Allow DNA sequences to change quickly.
*
Increased errors associated with replication, the absence of DNA-repair functions, & the frequent
replication of mtDNA may increase the no. of mutations.
*
Large amount of wobble in the mitochondrial translation may allow mutations to accumulate.
At conception, a mammalian zygote inherits approx. 100,000 copies of mtDNA inherited from the egg.
▪
Due to the large no. of mtDNA molecules in each cell & high rate of mutation in mtDNA, most cells would be
expected to contain ma mixture of wild-type & mutant mtDNA (heteroplasmy).
▪
In early development, mtDNA goes through a bottleneck, during which the mtDNAs within the cell are reduced
to just a few copies, which replicate & give rise to subsequent copies of mtDNA.
*
Through this process, genetic variation in mtDNA within a cell is eliminated & most copies of mtDNA are
identical.
Mitochondrial DNA variation & human history
●
Samples of mtDNA have been analysed from thousands of people belonging to hundreds of diff ethnic groups
throughout the world & are helping to unravel many aspects of human evolution & history.
●
Sequences from mtDNA have been used to study the spread of agriculture in Europe.
▪
Agriculture first arose in the Middle East, & spread to many parts of the world.
▪
Agriculture replaced the hunter-gatherer life style.
▪
Two theories have been proposed to explain how agriculture spread into Europe.
1. Demic fusion theory.
↳
Proposes that farmers from the Near East migrated into Europe & replaced the original huntergatherers.
↳
2.
*
Suggests that the modern European gene pool is derived largely from people who migrated from the
Near East.
Cultural diffusion theory.
↳
Proposes that the indigenous hunter-gatherers adopted the practice of farming
↳
Suggests that modern Europeans derive their genes largely from the ancient hunter-gatherers.
Early studies of languages & blood types suggested that agriculture may have spread through demic
fusion.
❖
Analyses of the mtDNA of living Europeans indicate that less than 20% of the modern European gene
pool is descended from immigrating Near Eastern farmers.
•
●
Supports cultural diffusion theory.
Scientists succeeded in extracting DNA from the bones & teeth of 20 early European hunter-gatherers from sites in
Lithuania, Poland, Germany, & Russia.
▪
Used PCR to amplify and sequence mtDNA from the remains.
▪
Compared hunter-gatherer mtDNA sequences with those from 25 early European farmers & 484 modern
Europeans.
▪
Found large genetic differences in all 3 groups, with less than 20% of hunter-gatherer mtDNA types also present in
modern Europeans.
*
Suggest that the first farmers in Europe were not descendants of the early hunter-gatherers.
❖
Supports demic diffusion model.
•
The genetic relationship of the early farmers to modern Europeans is unclear & conclusions are
controversial.
22.3 cpDNA exhibits many properties of eubacterial DNA- Fig22.10 & Table 22.3
●
Many traits associated with chloroplasts exhibit cytoplasmic inheritance.
▪
Indicates that these traits aren’t encoded by nuclear genes.
GTS 261- Anya OberholzerPage 8
*
●
Chloroplasts were shown to have their own DNA.
Among diff plants, the chloroplast genomes ranges in size from 80,000 – 600,000 bp.
▪
Most chloroplast genomes range from 120,000 – 160,000 bp.
●
cpDNA is usually a single, double-stranded DNA molecule that is circular, highly coiled, & lacks associated histone
proteins.
●
Multiple copies if the chloroplast genome are found in each chloroplast, & multiple organelles per cell.
▪
There are several thousand copies of cpDNA in a typical plant cell.
The gene structure & organisation of cpDNA
●
cpDNA is now recognised to be basically eubacterial in its organisation.
▪
●
The order of some groups of genes is the same as that observed in E.coli, & many chloroplast genes are
organised into operon-like clusters.
Among vascular plants, chloroplast chrms are similar in gene content & gene order.
▪
A typical chloroplast genome encodes 4rRNA genes, 30-35 tRNA genes, a no. of ribosomal proteins, many
proteins engaged in photosynthesis, & several proteins having roles in nonphotosynthesis processes.
▪
Key protein encoded by cpDNA is RuBisCO, which participates in carbon fixation in photosynthesis.
*
Makes 50% of the protein found in green plants & is considered the most abundant protein.
*
Complex protein consisting of 8 identical large subunits & 8 identical small subunits.
❖
●
▪
Some chloroplast genes have been identified on the basis of the presence of an operon reading frame.
*
Operon reading frame is a sequence of nucleotides that contains a start codon & stop codon in the same
reading frame.
❖
●
Large subunit is encoded by cpDNA, & small subunit is encoded by nuclear DNA.
Circular chloroplast genome has genes on both of its strands.
Most likely a gene that encodes a protein.
Prominent feature of most chloroplast genomes is the presence of a large inverted repeat.
▪
In rice, this repeat includes genes for 23S rRNA, 4.5S rRNA, 5S rRNA & several genes for tRNAs & proteins.
▪
In some plants, these repeats constitute most of the genome, whereas, in others, the repeats are absent
entirely.
●
Much of cpDNA consists of noncoding sequences, & introns are found in many chloroplast genes.
●
Many of the sequences in cpDNA are similar to those found in eubacterial genes.
The replication, transcription, & translation of cpDNA.
●
●
Little is known of cpDNA replication.
Studies suggest that replication starts within 2 D loops & spreads outwards to form a theta structure.
▪
●
After an initial round of replication, DNA synthesis may switch to a rolling-circle-type mechanism.
Transcription & translation of chloroplast genes are similar to the processes in eubacteria.
▪
Promoters found in cpDNA are almost identical with those found in eubacteria & possess sequences similar to
the -10 & -35 consensus sequences of eubacterial promoters.
▪
Same antibiotics that inhibit protein synthesis in eubacteria inhibit protein synthesis in chloroplasts.
*
▪
●
Indicates that protein synthesis in eubacteria & chloroplasts is similar.
Chloroplast translation is initiated by N-formylmethionine.
Most genes in cpDNA are transcribed in groups
▪
Only a few genes have their own promoters & are transcribed as separate mRNA molecules.
▪
RNA poly that transcribes cpDNA is more similar to eubacterial RNA poly than to any of the RNA poly that
transcribe euk nuclear genes.
▪
Chloroplast mRNAs aren’t capped at the 5’ ends, & poly(A) tails aren’t added to the 3’ ends
▪
Introns are removed from some RNA molecules after transcription.
*
5’ & 3’ ends may undergo additional processing before molecules are translated
GTS 261- Anya OberholzerPage 9
▪
●
Many chloroplast mRNAs have a Shine-Dalgarno sequence in the 5’ UTR, which may serve as a ribosomebinding site.
Chloroplasts contain 70S ribosomes that consist of 2 subunits.
▪
A large 50S subunit & a smaller 30S subunit.
*
Small subunit includes a single RNA molecule that is 16S in size.
*
Large subunit includes 3 rRNA molecules: 23S rRNA, 5S rRNA, & 4.5S rRNA.
❖
4.5S rRNA in the chloroplast ribosomes is homologous to the 3’ end of the 23S rRNA found in
eubacteria.
•
●
Structure of the chloroplast ribosome is very similar to that of ribosomes found in eubacteria.
Initiation factors, elongation factors, & termination factors function in chloroplast translation & eubacterial
translation in similar ways.
▪
Most chloroplast chrms encode 30-35 diff tRNAs.
*
Suggests that the expanded wobble in mitochondria doesn’t exist in chloroplast translation.
❖
Only universal codons have been found in cpDNA.
The evolution of cpDNA
●
DNA sequences of chloroplasts are very similar to those found in cyanobacteria.
▪
Chloroplast genomes clearly have eubacterial ancestry.
●
cpDNA sequences evolve slowly compared with sequences in nuclear DNA & some mtDNA.
●
For most chloroplast genomes, size & gene organisation are similar.
●
cpDNA are inherited only from 1 parent.
●
cpDNA is useful for determining the evolutionary relationships among diff plant species.
Connecting concepts- Genome comparisons- Table 22.4
●
Endosymbiotic theory indicates that mitochondria & chloroplasts evolved from eubacterial ancestors.
∴
mtDNA & cpDNA might be assumed to be similar to DNA found in eubacterial cells.
▪
mtDNA & cpDNA possess a mixture of eubacterial, euk, & unique characteristics.
●
The genomes of mitochondria & chloroplasts aren’t typical of the nuclear genomes of the euk cells in which they
reside.
●
In sequence, organelle DNA is most similar to eubacterial DNA.
▪
Organisation & expression of organelle genomes are unique.
●
Endosymbiotic theory doesn’t propose that mitochondria & chloroplasts are eubacterial in nature, but that they
arose from eubacterial ancestors.
●
Throughout time, the genomes of the endosymbionts have undergone considerable evolutionary change & have
evolved characteristics that distinguish them from contemporary eubacterial & euk genomes.
22.4 Through evolutionary time, genetic info has moved btwn nuclear, mitochondrial, &
chloroplast genomes.
●
Many proteins found in modern mitochondria & chloroplasts are encoded by nuclear genes.
▪
Suggests that much of the original genetic material in the endosymbionts has probably been transferred to the
nucleus.
*
Supported by observation that some DNA sequences found in mtDNA have been detected in the nuclear
DNA of some strains of yeast & maize.
*
Chloroplast sequences have been found in the nuclear DNA of spinach.
GTS 261- Anya OberholzerPage 10
*
●
Evidence that genetic material has moved from chloroplasts to mitochondria.
▪
DNA fragments from the 16S rRNA gene & 2 tRNA genes that are normally encoded by cpDNA have been found
in the mtDNA of maize.
▪
Sequences from the gene that encodes the large subunit of RuBisCO.
*
●
The sequences of nuclear genes that encode organelle proteins are most similar to eubacteria.
Normally encoded by cpDNA, are duplicated in maize mtDNA.
Evidence that some nuclear genes have moved into mitochondrial genomes.
▪
The exchange of genetic material btwn the nuclear, mitochondrial, & chloroplast genomes have given rise to
the term “promiscuous DNA” to describe this phenomenon.
22.5 Damage to mtDNA is associated with aging.
●
Hypothesis to explain the late onset & progressive worsening of mitochondrial disease is related to the decline in
ox phosphorylation with aging.
▪
Ox phosphorylation is the process that generates ATP→ primary carrier of energy in the cell.
*
Takes place in the inner membrane of the mitochondrion & requires a no. of diff proteins, some encoded
by mtDNA & others encoded by nuclear genes.
*
Ox phosphorylation normally declines with age & if it falls below a critical threshold, tissues don’t make
enough ATP to sustain vital functions & disease symptoms appear.
❖
Start life with an excess capacity for ox phosphorylation.
•
Capacity decreases with age, but most people reach old age or die before the critical threshold is
passed.
•
People born with mitochondrial diseases carry mutations in their mtDNA that lower their ox
phosphorylation capacity.
o
At birth, their capacity may be sufficient to support ATP needs but, as their ox
phosphorylation capacity declines, they cross they critical threshold & begin to experience
symptoms.
➢ Symptoms usually appear first in the tissues that are most critically dependent on
mitochondrial energy.
➢ CNS, heart & skeletal muscles, pancreatic islets, kidneys, & the liver.
▪
Because mtDNA is physically close to the enzymes taking part in ox phosphorylation, mtDNA may be more
prone to oxidative damage than is nuclear DNA.
*
When mtDNA has been damaged, the cell’s capacity to produce ATP drops.
*
To produce sufficient ATP to meet the cell’s energy needs, more ox phosphorylation must take place.
❖
May stimulate further production of oxygen radicals→ leads to a vicious cycle.
✓
Evidence that mtDNA damage is associated with aging comes from a study in which geneticists increased the
rate of mutations in the mtDNA of mice.
✓
In mice, DNA poly γ catalyses the replication of mtDNA.
✓
Poly γ synthesises DNA & proofreads.
✓
Geneticists created transgenic mice in which the proofreading activity was unaffected.
✓
Somatic tissues of these transgenic mice accumulated extensive mutations in their mtDNA.
✓
Mice showed symptoms of premature aging, including weight & hair loss, reductions in fertility, curvature of
the spine, & reduced life span.
✓
Results support the hypothesis that mutations in mtDNA can lead to some degree of aging.
✓
▪
The cause of mtDNA damage in the course of normal aging is not clear.
Elevated levels of mtDNA defects have been observed in some patients with late onset degenerative diseases
*
Diabetes mellitus, ischemic heart disease, Parkinson’s, Alzheimer’s, & Huntington.
❖
These diseases appear in middle- old age & have symptoms associated with tissues that critically
depend on ox phosphorylation for ATP production.
GTS 261- Anya OberholzerPage 11
❖
Because Huntington & some cases of Alzheimer are inherited as autosomal dominant conditions,
mtDNA defects can’t be the primary cause of these diseases.
GTS 261- Anya OberholzerPage 12