A prelude to genetics of Mitochondria and Chloroplasts the theory provides an explanation for the presence and source of organellar genome in eukaryotic cell

1. DR. VIBHA KHANNA
ASSO. PROF. (BOTANY)
S.P.C. GOVERNMENT COLLEGE
AJMER (RAJASTHAN)

2. CYTOGENETICS
• BLOCK 1
• PRESENTATION 1: ENDOSYMBIOTIC THEORY:
A theory on the Origins of Eukaryotic Cells:
Mitochondria and Chloroplasts

3. Historical aspect:
• The Endosymbiotic Theory was first proposed by former
Boston University Biologist Lynn Margulis in the 1960's and
officially in her 1981 book "Symbiosis in Cell Evolution".
• Her hypothesis originally proposed that:
– mitochondria are the result of endocytosis of aerobic bacteria
– chloroplasts are the result of endocytosis of photosynthetic
bacteria
– in both cases by large anaerobic bacteria who would not
otherwise be able to exist in an aerobic environment.
– this arrangement became a mutually beneficial relationship for
both cells (symbiotic).
• The result : a cell with a double-membrane bound
organelle. The inner lipid bilayer would have been the
bacterial cell's plasma membrane, and the outer lipid
bilayer came from the cell that engulfed it.

4. Evidence to support her hypothesis:
• 1. . The timeline of life on Earth:
• a. Anaerobic bacteria: Scientists have fossil evidence of bacterial
life on Earth ~3.8 billion years ago. At this time, the atmosphere of
the Earth did not contain oxygen, and all life (bacterial cells)
was anaerobic.
b. Photosynthetic bacteria: About ~3.2 billion years ago, fossil
evidence of photosynthetic bacteria, or cyanobacteria, appears.
These bacteria use the sun's energy to make sugar. Oxygen,
released as a byproduct, began to accumulate in the atmosphere.
As a result, anaerobic cells were now a disadvantage in an oxygen-
containing atmosphere.
c. Aerobic cells appear in the fossil record : ~2.5 Billion years ago.
These cells were were able to use oxygen and convert it into energy
(ATP) and water. Organisms that could thrive in an oxygen-
containing atmosphere were now 'best suited to the environment'.

5. Evidence to support her hypothesis
(Contd.):
2. Organelles have their own DNA, and divide
independently of the cell they live in:
When Margulis initially proposed the Symbiotic
Theory, she predicted that, if the organelles were
really bacterial (prokaryotic) symbionts, they
would have their own DNA.
• In the 1980's this was proven to be the case for
two classes of organelles, the mitochondria and
chloroplasts, that they have DNA that resembled
bacterial DNA, which is different from the cell's
DNA (located in the nucleus membrane)

7. Ancestry of Mitochondria and Plastids:

8. Evidence from genetics for the
endosymbiotic theory
There are multiple, independent lines of evidence to support the
hypothesis that eukaryotes evolved from an endosymbiotic event between
an ancient archaean cell and an ancient aerobic bacterium:
• Mitochondria (and chloroplasts) each have their own DNA,
– Their DNA is organized in a circular chromosome like typical
prokaryotic genomes, and
– Their genomes contain genes that are very similar to genes found in
prokaryotic genomes.
• Mitochondria (and chloroplasts) reproduce by binary fission, the process
that prokaryotes use to reproduce. In contrast, eukaryotic cells reproduce
by mitosis.
• If the mitochondria (or chloroplasts) are removed from a eukaryotic cell,
the cell has no way to produce new ones. In other words, the
“instructions” to make new mitochondria/chloroplasts is not present in
the eukaryotic nuclear genome; they are present in the
mitochondria/chloroplast genomes.

9. Mitochondria:
Notable facts
• One of the major features distinguishing prokaryotes from
eukaryotes is the presence of mitochondria.
• Eukaryotic cells may contain anywhere from one to several
thousand mitochondria, depending on the cell’s level of
energy consumption.
• Each mitochondrion measures 1 to 10 or greater
micrometers in length and exists in the cell as an organelle
that can be ovoid to worm-shaped to intricately branched.
• Mitochondria arise from the division of existing
mitochondria; they may fuse together; and they may be
moved around inside the cell by interactions with the
cytoskeleton.
• However, mitochondria cannot survive outside the cell.

10. Origin of Mitochondria
• Eukaryotes evolved during the Proterozoic eon. Prior to the
origin of eukaryotes, all life on Earth was prokaryotic.
• According to the endosymbiotic theory, eukaryotes arose
as a result of a fusion of Archaean cells with bacteria,
where an ancient Archaean engulfed (but did not eat) an
ancient, aerobic bacterial cell.
• As the atmosphere was oxygenated by photosynthesis, and
as successful aerobic prokaryotes evolved, evidence
suggests that an ancestral cell with some membrane
compartmentalization engulfed a free-living aerobic
prokaryote, specifically an alpha-proteobacterium.

11. Origin of Mitochondria (Contd.)
• The engulfed (endosymbiosed) bacterial cell
developed a mutualistic relationship within the
archaean cell : the engulfed bacterium allowed
the host archean cell to use oxygen to release
energy stored in nutrients, and the host cell
protected the bacterial cell from predators.
• Over many generations, a symbiotic relationship
developed between the two organisms so
completely that neither could survive on its own.
Thus mitochondria came into existence within
eukaryotic cell.

12. Evidence For Endosymbiotic Origin Of
Mitochondria
• Several lines of evidence support that mitochondria are
derived from this endosymbiotic event.
– Most mitochondria are shaped like alpha-proteobacteria and
– are surrounded by two membranes, which would result when
one membrane-bound organism was engulfed into a vacuole by
another membrane-bound organism.
– The mitochondrial inner membrane is extensive and involves
substantial infoldings called cristae that resemble the textured,
outer surface of alpha-proteobacteria.
– The matrix and inner membrane are rich with the enzymes
necessary for aerobic respiration.
– Mitochondria divide independently by a process that resembles
binary fission in prokaryotes.

13. Evidence For Endosymbiotic Origin Of
Mitochondria (Contd.)
– Specifically, mitochondria are not formed from scratch (de
novo) by the eukaryotic cell; they reproduce within it, as if
they are independent organisms and are distributed with the
cytoplasm when a cell divides or two cells fuse.
– Mitochondria have their own (usually) circular DNA
chromosome
– Mitochondrial DNA is stabilized by attachments to the inner
membrane and carries genes similar to genes expressed by
alpha-proteobacteria.
– Mitochondria also have special ribosomes and t-RNAs that
resemble these components in prokaryotes.
These features all support that mitochondria were once
free-living prokaryotes.

14. Ancestry of Mitochondria
Mitochondria that carry out aerobic respiration have
their own genomes, with genes similar to those in
alpha-proteobacteria.
– Many of the genes, of alpha-proteobacterial origin, for
respiratory proteins are located in the nucleus.
– Additionally, in some eukaryotic groups, such genes are
found in the mitochondria, whereas in other groups, they
are found in the nucleus.
This has been interpreted as evidence that genes have
been transferred from the endosymbiont chromosome
to the host genome.
*This loss of genes by the endosymbiont is probably
one explanation why mitochondria cannot live without
a host.

15. Timeline of Endosymbiosis
• Some living eukaryotes are anaerobic and cannot survive in
the presence of too much oxygen. Some appear to lack
organelles that could be recognized as mitochondria. Some of
these eukaryotes were descended from ancestors whose
lineages had diverged from the lineage of mitochondrion-
containing eukaryotes before endosymbiosis occurred.
• Reduced organelles are found in most, if not all, anaerobic
eukaryotes, and that all eukaryotes appear to carry some
genes in their nuclei that are of mitochondrial origin.
• In addition to the aerobic generation of ATP, mitochondria
have several other metabolic functions. One of these
functions is to generate clusters of iron and sulfur that are
important cofactors of many enzymes. Such functions are
often associated with the reduced mitochondrion-derived
organelles of anaerobic eukaryotes. Therefore, most biologists
accept that the last common ancestor of eukaryotes had
mitochondria.

16. Plastids
• The cells of photosynthetic eukaryotes contain,
an organelle called a plastid.
• The plastids of photosynthetic cells are rich in
chlorophyll a and a range of other
pigments(accessory pigments), which are
involved in harvesting energy from light.
Photosynthetic plastids are called chloroplasts
• Like mitochondria, plastids appear to have an
endosymbiotic origin. This hypothesis was also
supported by Lynn Margulis.

17. Ancestry of Chloroplasts
• Plastids are derived from cyanobacteria that lived inside
the cells of an ancestral, aerobic, heterotrophic eukaryote.
This is called primary endosymbiosis, and plastids of
primary origin are surrounded by two membranes.
• The best evidence is that this has happened twice in the
history of eukaryotes.
– In one case, the common ancestor of the major
lineage/supergroup Archaeplastida took on a cyanobacterial
endosymbiont;
– in the other, the ancestor of the small amoeboid rhizarian
taxon, Paulinella, took on a different cyanobacterial
endosymbiont.
• Almost all photosynthetic eukaryotes are descended from
the first event, and only a couple of species are derived
from the other.

18. Ancestry of Chloroplasts (Contd.)
• Cyanobacteria are a group of Gram-negative bacteria
with all the conventional structures of the group.
However, unlike most prokaryotes, they have
extensive, internal membrane-bound sacs called
thylakoids. Chlorophyll is a component of these
membranes, as are many of the proteins of the light
reactions of photosynthesis. Cyanobacteria also have
the peptidoglycan wall and lipopolysaccharide layer
associated with Gram-negative bacteria.
• Chloroplasts of primary origin have thylakoids, a
circular DNA chromosome, and ribosomes similar to
those of cyanobacteria. Each chloroplast is surrounded
by two membranes.

19. Ancestry of Chloroplasts (Contd.)
• In the group of Archaeplastida called the
glaucophytes and in Paulinella, a thin
peptidoglycan layer is present between the
outer and inner plastid membranes. All other
plastids lack this relictual cyanobacterial wall.
The outer membrane surrounding the plastid
is thought to be derived from the vacuole in
the host, and the inner membrane is thought
to be derived from the plasma membrane of
the symbiont.

20. Evidence For Endosymbiotic Origin Of
Plastids
• There is also, as with the case of mitochondria, strong
evidence that many of the genes of the endosymbiont
were transferred to the nucleus.
• Plastids, like mitochondria, cannot live independently
outside the host.
• In addition, like mitochondria, plastids are derived
from the division of other plastids and never built from
scratch.
• Researchers have suggested that the endosymbiotic
event that led to Archaeplastida occurred 1 to 1.5
billion years ago, at least 5 hundred million years after
the fossil record suggests that eukaryotes were
present.

21. Evidence For Endosymbiotic Origin Of
Plastids (Contd.)
• Not all plastids in eukaryotes are derived directly from
primary endosymbiosis. Some of the major groups of
algae became photosynthetic by secondary
endosymbiosis, that is, by taking in either green algae
or red algae (both from Archaeplastida) as
endosymbionts
• Secondary plastids are surrounded by three or more
membranes, and some secondary plastids even have
clear remnants of the nucleus of endosymbiotic alga.
• There are cases where tertiary or higher-order
endosymbiotic events are the best explanations for
plastids in some eukaryotes.

22. Definition of the Basic Concepts:
• Endosymbiotic theory: The theory that states -
‘eukaryotes may have been a product of one cell
engulfing another, one living within another, and
evolving over time until the separate cells were
no longer recognizable as such’.
• Endosymbiosis: engulfment of one cell within
another such that the engulfed cell survives, and
both cells benefit; the process responsible for the
evolution of mitochondria and chloroplasts in
eukaryotes

23. GUESS!!!!
• What evidence is there that mitochondria
were incorporated into the ancestral
eukaryotic cell before chloroplasts?
– All eukaryotic cells have mitochondria, but not all
eukaryotic cells have chloroplasts.

24. A QUESTION?...
• Describe the hypothesized steps in the origin of eukaryotic
cells.
– Eukaryotic cells arose through endosymbiotic events that gave
rise to the energy-producing organelles within the eukaryotic
cells such as mitochondria and chloroplasts.
– The nuclear genome of eukaryotes is related most closely to the
Archaea, so it may have been an early archaean that engulfed a
bacterial cell that evolved into a mitochondrion.
– Mitochondria appear to have originated from an alpha-
proteobacterium, whereas chloroplasts originated as a
cyanobacterium.
– There is also evidence of secondary endosymbiotic events.
Other cell components may also have resulted from
endosymbiotic events.

26. REFERENCES
• Alberts B, Johnson A, Lewis J, et al. Molecular
Biology of the Cell. 4th edition. New York:
Garland Science; 2002. The Genetic Systems
of Mitochondria and Plastids. Available from:
https://www.ncbi.nlm.nih.gov/books/NBK269
24/
• The information was adapted from OpenStax
Biology