Cycle 1: Chlamydomonas and how it uses light Flashcards
Major structural features of a Chlamydomonas cell (we will fill these in over the next few cycles).
Chlamy is a eukaryotic green algae:
- About 10microns across
- Possess chloroplasts
- 2 flagella that enable motion and can be regrown using the basal body
- Eyespot, partially in chloroplast and partially on membrane, that senses the light environment to indicate to flagella which direction to move in
Features of Chlamy grown in the lab.
- Usually grown as haploid cells, only 1 copy of every chromosome
- Under moderate light and temperature (24-28deg)
- GROWS by binary fission (with cell division occuring every ~10h if optimum condition)
Growth media…what is the difference between a Macronutrient and a Micronutrient.
Chlamy growth media is liquid media: TAP, with various essential salts
Macronutrient: nutrients required in larger amounts by organisms
Micronutrient: nutrients required in smaller amounts by organisms
Look up why Chlamy (and humans) need PO4 (phosphate) and Fe (iron).
Phosphate required for DNA sugar-phosphate backbone, in phospholipid cell bilayer, forms ATP in cellular respiration/photosynthesis.
Iron required to bind to hemoglobin as a cofactor and transport oxygen.*
Chlamy grows with a doubling time of about 10 hours at 25C….what does that actually mean…grows and doubling time?
Chlamy’s DOUBLING/GENERATION TIME refers to the time required for a colony to double in number (for enough binary fission to occur that each cell splits in two) - occurs during exponential growth phase.
Microbial growth curve….what explains its shape….three major phases…(Lag, Exponential, Stationary) what’s going on in each.
Graph - Cells/mL as a function of time
LAG - shallow slope, slow growth rate (cells adjust, synthesize enzymes, prep for growth)
EXPONENTIAL - steep slope, doubling time phase (high growth rate, exponential increase in cell number) (abundant nutrients, favourable conditions for reproduction)
STATIONARY - plateau slope, growth slows again (nutrients depleted, waste accumulated, cell growth equals cell death)
Bacteria have flagella too…but nothing like eukaryotic flagella….analagous structures.
Bacteria (prokaryotes) have flagella too but they are structurally completely different from eukaryotic flagella (they spin instead of waving like in eukaryotes, they have filament and hooks instead of microtubules), because they evolved in parallel - the flagella are ANALOGOUS! (Analogs refer to traits that are similar but evolved independently as a response to a similar challenge. Homologs refer to traits that are similary because they share a common ancestry.)
Chlamydomonas phylogeny….relationship to plants, animals.
Chlamy (and all algae) are photosynthetic eukaryotes but are not plants! They share a common ancestor with plants a billion years ago.
How can we explain that Chlamydomonas and humans have flagella but plants do not. What is the simplest explanation?
Chlamy and humans have flagella because they share a common ancestor, however, plants lost the ability to have flagella during evolution because it wasn’t evolutionarily advantageous to keep them.
Basics of cilia structure (proteins involved, structures)
Flagella and cilia are the same thing!
- Contain microtubules (protein polymer (hollow tube) made from alpha and beta tubulin dimers)
- Flagella have 9 microtubule doublets around and 2 central microtubules
- Dynein arms (motor protein) which move along one side of adjacent doublets (from - to + end) causing flagella to move in a whip-like fashion
Mutations that alter cilia structure/function cause many human disease states (ciliopathies) (recall, we make no distinction between cilia and flagella).
Ciliopathies are distinct diseases linked to mutations in genes involved in cilia structure/function. If dynein is missing (MUTATION IN DYNEIN SYNTHESIS), all cilia become non-motile even if they’re supposed to be motile.
Distinctions between motile and non-motile (sensory) flagella. You do not need to memorize that figure with all the diseases!
Motile cilia (lungs, intestinal tract) move and non-motile cilia (ears, eyes, nose) do not move.
Ciliopathies are often genetically heterogenous…what does that mean?
The same ciliopathies can be caused by mutations in more than one gene, since more than one gene codes for microtubules. It is said to be heterogenous because the same phenotype can be caused by mutations in more than one gene.
Proteins required for flagella can be isolated using SDS-PAGE to compare WT and bbs4 proteins (have no flagella, incapable of phototaxis).
Analysis of 7,476 Chlamy proteins…comparision with proteins in humans, arabidopsis and both those species.
- 26% are only homologs to arabidopsis (1968 Chlamy proteins that are evolutionarily related to 2396 plant proteins), likely related to photosynthesis, etc.
- 10% are only homologs to humans (774 Chlamy proteins ~ 806 human proteins), likely related to flagella, etc.
- 33% are homologs to both arabidopsis and humans, likely related to general eukaryotic function?
What characteristics does Chlamy have that makes it a “model experimental system”? [IMPORTANT]
- Chlamy’s flagella is identical to human cilia and has the same proteins SO it is used to study ciliopathies
- Chlamy is easier/less expensive to study than human cells
