Lecture 3: Origins and importance of organelles Flashcards
Origins of chloroplasts & mitochondria:
Endosymbiotic Theory
-idea that ancestral eukaryotic cells engulfed bacterium ‘cells’
steps of endosymbiotic theory:
-Ancestral eukaryotic cell (got internal membrane system)
-englufs an aerobic bacterium –> endosymbiosis occurs = mitochondrion
-englufs photosynthetic bacterium –> endosymbiosis occurs = chloroplast
== Eukaryotic cell with mitochondrion and chloroplast
Evidence for prokaryotic origins of chloroplasts and mitochondria: DNA
Prokaryotes, chloroplasts & mitochondria: 1 single, circular chromosome
Eukaryotes: Multiple linear chromosomes in nucleus
Evidence for prokaryotic origins of chloroplasts and mitochondria: Replication
Prokaryotes, chloroplasts & mitochondria: Binary fission (1 cell splits into 2)
Eukaryotes: Mitosis
Evidence for prokaryotic origins of chloroplasts and mitochondria: Ribosomes
Prokaryotes, chloroplasts & mitochondria: 70S
Eukaryotes: 80S
Evidence for prokaryotic origins of chloroplasts and mitochondria: Size (approx)
Prokaryotes, chloroplasts & mitochondria: 1-10 micrometres
Eukaryotes: 50-500 micrometres
Evidence for prokaryotic origins of chloroplasts and mitochondria: Porins
Prokaryotes, chloroplasts & mitochondria: present
Eukaryotes: not present
Evidence for prokaryotic origins of chloroplasts and mitochondria: Initiating amino acid
Prokaryotes, chloroplasts & mitochondria: N-formylmethionine
Eukaryotes: methionine
Cardiolipin constitutes about 20% of..
the inner mitochondrial membrane lipids. Only found elsewhere in bacteria… evidence for endosymbiotic theory
Genes for cyanobacteria peptidoglycan (cell wall) synthesis is still
present in Arabidopsis… evidence for endosymbiotic theory
Secondary endosymbiosis: alga steps
Cyanobacterium engulfed –> Alga
Alga then engulfed by secondary host –> algae. Plastids with 3 or 4 envelopes are common in algae
Porins
transport across membrane (channels)
Repeated endosymbiosis has led to
plant and algal diversity. Depending upon how many stages of endosymbiosis (up to tertiary) & at each stage what cells fuse
Apicocomplexa are
animal parasite, including Plasmodium (malaria)
Some sea slugs run on
& young..
Solar power by ingesting chloroplasts. If young animals are fed algae for 2 weeks, they can survive 8-10 months without eating.
Endosymbiotic zooxanthellae in..
coral polyps
The transfer of organelle DNA…
has abolished organelle autonomy & increased nuclear complexity
~4,500 (~18% of total) Arabidopsis protein-coding genes..
were acquired from cyanobacteria
Gene transfer from organelles to the nucleus is a..
continuing process
Arabidopsis..
(rock cress) small flowering plant related to cabbage & mustard
Mitochondria DNA:
encodes rRNAs, tRNAs, and 13 out of ~85 components of the oxidative phosphorylation system
Chloroplasts DNA;
encodes some rRNAs, tRNAs, ribosomal proteins, RNA polymerase subunits & some genes for photosynthesis
Maternal inheritance of mitochondrial (and plastid) genome
- simple dilution
- degradation of sperm mtDNA in the fertilised egg
- failure of sperm mtDNA to enter the egg
egg & sperm contains mtDNA molecules
- egg contains 100,000 to 1,000,000 mtDNA
- sperm 100 to 1000 mtDNA
3 person IVF is use to stop
mother passing on faulty mtDNA
3 person IVF steps:
Step 1: parents embryo (unhealthy mitochondria) and healthy donor embryo (healthy mitochondria)
Step 2: Parents’ nucleus removed & donor nucleus removed & destroyed.
Step 3: Parents nucleus now in donor embryo
How do proteins cross chloroplast membranes?
Most proteins are nuclear encoded. A signal peptide is a target sequence of amino acids that is recognized and cleaved by a signal peptidase at the chloroplast envelope (for thylakoid: two signal sequences)
How do proteins cross chloroplast membranes? entering molecule
- > Top;
- chloroplast signal sequence
- thylakoid signal sequence
- thylakoid precursor proteins
Plants without plastids: Rafflesia lagascae
a parasite that has plastid‐like structures but no intact plastid genome.
Plants without plastids: Polytomella
a freshwater alga. Has a plastid but no trace of a plastid genome
Apicomplexa
- parasitic single‐celled eukaryotic organisms deriving from red algae
- 1.2 ‐ 10 million apicomplexan species, only about 0.1% described to date?
- apical complex of microtubules
- 200 million people incapacitated by malaria; 600,000 die annually
- Eimeria spp. responsible for annual $1.5 billion loss to the poultry industry worldwide
Apicomplexa - MAlaria
- Plasmodium still retains its plastid genome
- Apicoplasts of Plasmodium falciparum have four membranes
- Suitable target for drug treatments
Apicomplexa - Cryptosporidium
- Parasite which causes sever infection sin young children and people with weak immune systems
- -infection begins with the ingestion of water or food containing spore-like oocysts
- parasite enter cells of small intestine
- simple single-host life cycle
- oocysts pass into the colon and are released in the faeces
- crypto infects others
Advantages of chloroplasts and mitochondria: The cellular power struggle
- Energetic cost of genes is trivial (2% of cell’s energy budget), but cost of expressing them as protein is 75% of the energy budget. An average bacterium (E. coli) has ~ 13,000 ribosomes, whereas a human liver cell has 13 million on the rough ER alone.
- Mitochondrial genes enabled a 200,000‐fold rise in genome size in eukaryotes compared with bacteria by allowing oxidative phosphorylation across a large area of internal membranes.
- Mitochondria increased the number of proteins that a cell can evolve, inherit and express by 4‐6 orders of magnitude.
