Transcript Lecture 05 - Intro to Eukaryotes - Cal State LA

Prokaryotes
Eukaryotes
small, simple cells
large, complex cells
cell wall of peptidoglycan
cell wall absent (animals) or
made of cellulose (plants)
circular chromosome
nucleus containing many
straight-line chromosomes
no membrane-bound organelles
membrane-bound organelles
(nucleus, plastids, Golgi)
aerobic or anaerobic
mostly aerobic (need O2)
conjugation through a
pilus
sexual reproduction via
meiosis + fertilization
Domain Eukaryota vs. Kingdom “Protista”
“Protist” is now a slang term for a diverse group of Eukaryotes,
including many unrelated lineages that are structurally simple
Metabolically diverse (autotrophic, heterotrophic, both)
Many are single celled organisms, but with more complex cells
than prokaryotes
Others are multi-cellular, but have morphologically simple bodies
that lack true tissues
- some algae (such as giant kelp) resemble plants, but they
lack true leaves, stems or roots
PROTISTS
Plants
Fungi
Animals
The name “Protists” has no phylogenetic validity
It is not a monophyletic group...
8 major lineages of Protists
Freeman 2005
Mitochondrion – organelle responsible for aerobic respiration
- Enclosed within a double membrane

Mitochondrion – organelle responsible for aerobic respiration
- Enclosed within a double membrane
Aerobic Respiration (heterotrophy)
CH2O + O2  CO2 + H2O
sugar
Extracts maximum energy
from organic compounds
by using O2 as the final
acceptor for electrons
Chloroplast – organelle responsible for photosynthesis
Series of stacked membranes called thylakoids, which contain the
pigment chlorophyll
Traps the energy of light, uses it
to energize electrons and rip
them off of H2O
Drives carbon fixation...
Lynn Margulis championed the Endosymbiotic Hypothesis,
which held that eukaryotic mitochondria and chloroplasts were
originally prokaryotes
- first presented this idea in 1981; no one believed her
Suggested:
(a) mitochondria were
descended from an
anaerobic bacterium
(b) chloroplasts from a
phytosynthetic
cyanobacterium
Primary Endosymbiosis
Ancient, anaerobic eukaryote engulfed
an aerobic bacterium
Aerobic respiration yields more ATP
(= cellular energy) than less efficient
anaerobic respiration
Instead of eating this engulfed cell, they
struck up a partnership, or symbiosis
- bacteria got safe place to live, steady
supply of carbon compounds from its
host cell
- eukaryote got a more efficient form of
metabolism
Evidence for Endosymbiosis
- mitochondria and plastids have their own circular chromosomes
encoding genes needed to replicate their DNA
- reproduce by binary fission, like bacteria (splitting in 2, cloning)
- same size as bacteria
- have their own ribosomes for protein synthesis, which are very
similar to bacterial ribosomes
- each is surrounded by a double membrane, consistent with
proposed engulfing mechanism (end up with membrane of
original bacterium, plus 1 layer of the host’s membrane)
Molecular Evidence for Endosymbiosis
Freeman 2005
Confirmation came a decade later, when DNA sequence analysis
showed that DNA in mitochondria is closely related to
a-proteobacteria, while chloroplast DNA is closely related to
cyanobacteria
Plant Cell: a combination of 3 distinct genomes
nucleus (eukaryotic ancestor)
light,
H2O
O2,
sugar
ATP
mitochondrion
(proteobacteria ancestor)
chloroplast
(cyanobacteria ancestor)
sugar,
O2
Cyanobacterium
Heterotroph 1
Primary Endosymbiosis
- one eukaryotic cell engulfs
a prokaryote, which
becomes its chloroplast Alga 1
(red)
Primary Endosymbiosis
Alga 1
(red)
Secondary Endosymbiosis
- one eukaryotic cell engulfs
another eukaryote
- the mitochondria & nucleus of
the engulfed cell are eventually lost;
all that remains is its original
chloroplast
Alga 2
(brown)
Heterotroph 2
Alga 1
(red)
Study Question:
- how many membranes would you
expect there to be around the plastid
of a brown alga?
Why?
Alga 2
(brown)
Heterotroph 2
Secondary Endosymbiosis - multiple origins
1
2
3
Primary endosymbiosis
occurred here
Secondary endosymbiosis
occurred 3 different times
A red alga was engulfed by the ancestor of:
1) Euglenoids
2) Dinoflagellates
3) Diatoms + Brown algae (kelp)
Nuclear division in Eukaryotes
Mitosis: copies nucleus & preserves chromosome number
(2N or N, whichever it started as)
- cloning: produces 2 daughter cells identical to the mother cell
diploid
haploid
Meiosis: halves chromosome number (2NN), producing
haploid (N) daughter cells
- how animals produce sperm and eggs
- putting 2 haploid cells together to form a diploid zygote is
called syngamy
Chromosomes:
DNA in eukaryotic nuclei,
wound around proteins
46 human chromosomes,
arranged in 23 pairs of
homologous chromosomes
– diploid (2N) state
In animals, only gametes
(sperm + eggs) are
haploid
– all other cells are diploid
Fig. 13.3, Campbell & Reece 2005
Mitosis
preserves chromosome #
Diploid (2N)
Diploid (2N)
Fig. 13.9, Campbell & Reece 2005
Meiosis
halves chromosome #
Diploid (2N)
Haploid (N)
Sex in Eukaryotes
Chromosome number is cut in half by meiosis, at some point
2N  N
adult cell
Diploid  Haploid
gametogenesis
sperm
egg
Babies, made diploid
Chromosome number later doubles during syngamy (fertilization)
N  2N
Haploid  Diploid
syngamy
+
sperm
egg
Sex, made boring
Chromosome number is cut in half by meiosis, at some point
2N  N
adult cell
Diploid  Haploid
Most important thing to know: during meiosis, through a process
called crossing over, sections of homologous chromosomes
get swapped
- you end up with new combinations of alleles, together on one
chromosome for the first time
generates diversity
Life Cycles
Life cycles depict the various stages in an organism’s life
- different stages can look exactly the same, or totally different
(like caterpillars and butterflies)
Sexual reproduction occurs via meiosis & syngamy; haploid (N)
and diploid (2N) states alternate
N  2N  N  2N  N  2N …
Asexual reproduction occurs via mitosis, makes exact duplicates