Lecture 3: Origins and importance of organelles

Question 1

Origins of chloroplasts & mitochondria:

Answer 1

Endosymbiotic Theory

-idea that ancestral eukaryotic cells engulfed bacterium ‘cells’

Question 2

steps of endosymbiotic theory:

Answer 2

-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

Question 3

Evidence for prokaryotic origins of chloroplasts and mitochondria: DNA

Answer 3

Prokaryotes, chloroplasts & mitochondria: 1 single, circular chromosome

Eukaryotes: Multiple linear chromosomes in nucleus

Question 4

Evidence for prokaryotic origins of chloroplasts and mitochondria: Replication

Answer 4

Prokaryotes, chloroplasts & mitochondria: Binary fission (1 cell splits into 2)

Eukaryotes: Mitosis

Question 5

Evidence for prokaryotic origins of chloroplasts and mitochondria: Ribosomes

Answer 5

Prokaryotes, chloroplasts & mitochondria: 70S

Eukaryotes: 80S

Question 6

Evidence for prokaryotic origins of chloroplasts and mitochondria: Size (approx)

Answer 6

Prokaryotes, chloroplasts & mitochondria: 1-10 micrometres

Eukaryotes: 50-500 micrometres

Question 7

Evidence for prokaryotic origins of chloroplasts and mitochondria: Porins

Answer 7

Prokaryotes, chloroplasts & mitochondria: present

Eukaryotes: not present

Question 8

Evidence for prokaryotic origins of chloroplasts and mitochondria: Initiating amino acid

Answer 8

Prokaryotes, chloroplasts & mitochondria: N-formylmethionine

Eukaryotes: methionine

Question 9

Cardiolipin constitutes about 20% of..

Answer 9

the inner mitochondrial membrane lipids. Only found elsewhere in bacteria… evidence for endosymbiotic theory

Question 10

Genes for cyanobacteria peptidoglycan (cell wall) synthesis is still

Answer 10

present in Arabidopsis… evidence for endosymbiotic theory

Question 11

Secondary endosymbiosis: alga steps

Answer 11

Cyanobacterium engulfed –> Alga

Alga then engulfed by secondary host –> algae. Plastids with 3 or 4 envelopes are common in algae

Question 12

Porins

Answer 12

transport across membrane (channels)

Question 13

Repeated endosymbiosis has led to

Answer 13

plant and algal diversity. Depending upon how many stages of endosymbiosis (up to tertiary) & at each stage what cells fuse

Question 14

Apicocomplexa are

Answer 14

animal parasite, including Plasmodium (malaria)

Question 15

Some sea slugs run on

& young..

Answer 15

Solar power by ingesting chloroplasts. If young animals are fed algae for 2 weeks, they can survive 8-10 months without eating.

Question 16

Endosymbiotic zooxanthellae in..

Answer 16

coral polyps

Question 17

The transfer of organelle DNA…

Answer 17

has abolished organelle autonomy & increased nuclear complexity

Question 18

~4,500 (~18% of total) Arabidopsis protein-coding genes..

Answer 18

were acquired from cyanobacteria

Question 19

Gene transfer from organelles to the nucleus is a..

Answer 19

continuing process

Question 20

Arabidopsis..

Answer 20

(rock cress) small flowering plant related to cabbage & mustard

Question 21

Mitochondria DNA:

Answer 21

encodes rRNAs, tRNAs, and 13 out of ~85 components of the oxidative phosphorylation system

Question 22

Chloroplasts DNA;

Answer 22

encodes some rRNAs, tRNAs, ribosomal proteins, RNA polymerase subunits & some genes for photosynthesis

Question 23

Maternal inheritance of mitochondrial (and plastid) genome

Answer 23

• simple dilution
• degradation of sperm mtDNA in the fertilised egg
• failure of sperm mtDNA to enter the egg

Question 24

egg & sperm contains mtDNA molecules

Answer 24

• egg contains 100,000 to 1,000,000 mtDNA

- sperm 100 to 1000 mtDNA

Question 25

3 person IVF is use to stop

Answer 25

mother passing on faulty mtDNA

Question 26

3 person IVF steps:

Answer 26

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

Question 27

How do proteins cross chloroplast membranes?

Answer 27

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)

Question 28

How do proteins cross chloroplast membranes? entering molecule

Answer 28

• • • • > Top;
• chloroplast signal sequence
• thylakoid signal sequence
• thylakoid precursor proteins

Question 29

Plants without plastids: Rafflesia lagascae

Answer 29

a parasite that has plastid‐like structures but no intact plastid genome.

Question 30

Plants without plastids: Polytomella

Answer 30

a freshwater alga. Has a plastid but no trace of a plastid genome

Question 31

Apicomplexa

Answer 31

• 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

Question 32

Apicomplexa - MAlaria

Answer 32

• Plasmodium still retains its plastid genome
• Apicoplasts of Plasmodium falciparum have four membranes
• Suitable target for drug treatments

Question 33

Apicomplexa - Cryptosporidium

Answer 33

• 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

Question 34

Advantages of chloroplasts and mitochondria: The cellular power struggle

Answer 34

• 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.