The nuclear envelope consists of a lipid bilayer.
Long and fibrous lamin proteins form a layer of structural support for the nuclear envelope. Lamin is phosphorylated in prometaphase, causing a conformational change and the loss of laminal structural properties. Without laminal support, the nuclear membrane breaks apart and absorbs into the smooth endoplasmic reticulum. The endoplasmic reticulum breaks apart, but is bound to the lamin via the inner nuclear membrane Lamin B Receptor; and the lamin binds to chromatin.
As anaphase ends, dephosphorylation of existing lamin begins. Once the genetic material has fully segregated at the completion of anaphase, production of new lamin is well underway. The new lamin drags tubes of smooth endoplasmic reticulum across the surface of the chromatin; these tubes flatten and merge, forming a solid nuclear membrane. The endoplasmic reticulum and lamin detach themselves from the chromatin. In the mature daughter cell, the lamina is a continous layer that is bound to the inner membrane of the nuclear envelope by emerin proteins.
Eukaryotic organelles are not present in prokaryotes.
Summary of Eukaryotic Organelles (list format)
Summary of Eukaryotic Organelles (table format)
| Organelle | Role | Membranes |
| Nucleus | Contain & protect DNA. Site of transcription and partial assembly of ribosomes | 2 |
| Mitochondria | Produce ATP via Krebs Cycle and oxidative phosphorylation | 2 |
| Ribosomes | Synthesize proteins | 0 |
| Rough Endoplasmic Reticulum (RER) | Synthesis & modification of secretory, membrane-bound & organelle proteins | 1 |
| Smooth Endoplasmic Reticulum (SER) | Detoxification & glycogen breakdown in liver; steroid synthesis in gonads | 1 |
| Golgi Apparatus | Modification & sorting of protein, some protein synthesis | 1 |
| Lysosomes | Contain acid hydrolases which digest various substances | 1 |
| Peroxisomes | Metabolize lipids & toxins using H2O2 (peroxide) | 1 |
Eukaryotic cells are defined as having a nucleus, thus distinguishing eukaryotes from prokaryotes. The nucleus is an organelle containing the genome and complex machinery to control gene expression, and is separated from the cytoplasm by a double-membrane. From the nucleus, genetic information flows to the cytoplasm for utilization. In contrast, while eukaryotes encase their genome in a double-membrane, prokaryotes allow their genome to float in the cytoplasm (sometimes localized to one part of the cell).
| Event | Eukaryotes | Prokaryotes |
| Replication | Nucleus | Cytoplasm |
|---|---|---|
| Transcription | Nucleus | Cytoplasm |
| Splicing | Nucleus | Cytoplasm |
| Translation | Cytoplasm | Cytoplasm |
The nucleus is surrounded by a double-membrane (aka nuclear membrane, nuclear envelope and nucleolemma) and structural scaffolding (the nuclear lamina). This double-membrane keeps DNA and proteins that function in the nucleus segregated from the cytoplasm. Some argue that the nuclear envelope originated as an endosymbiont; others argue the nuclear envelope originated as an invagination of the cellular membrane. The nuclear membrane is contiguous with the endoplasmic reticulum membrane and is reinforced by a cytoskeletal scaffold called the nuclear lamina. Also, the nuclear membrane is laden with huge protein complexes that create nuclear pores that control the passage of molecules in and out. The nucleus also contains specialized structures such as: the nucleolus, a large dense structure where rRNA is transcribed and processed and where ribosomes are assembled; the smaller cajal body, where snRNPs are assembled.
Prokaryotic mRNAs are polycistronic, meaning they encode multiple proteins and do not require a special structure to specify the correct reading frame. However, eukaryotic mRNAs are monocistronic, meaning they only encode a single protein and require a special 5’ cap structure to initiate translation. In eukaryotes, the DNA is transcribed into precursor RNA; pre-mRNA is processed to form mRNA; and the finished mRNA proceeds to the cytoplasm for translation. Translation of unprocessed RNA is detrimental. The nucleus is essential for eukaryotes, separating this unprocessed RNA from the translation machinery in the cytoplasm.
Hydrogenosomes metabolize carbohydrates into ATP and H2. Metabolism in the hydrogenosome is similar to anaerobic bacteria (via enzymes including PFO and ferredoxin) and mitochondria (via Krebs cycle enzymes that convert acetyl CoA into acetate and succinyl CoA which is then converted to succinate and ATP).
Mitochondria and hydrogenosomes likely share an ancestral organelle: similar machinery used for protein translocation; similar signals used for protein translocation; phylogenetically related proteins (Hsp60); and a phylogenetically related ADP/ATP carrier protein. Techniques to explore this theory are shown below.
There are three different theories as to how the hydrogenosome could have arisen: as converted mitochondria; from a common ancestor with mitochondria; or arose independently from mitochondria through different endosymbionts. Hydrogenosome origins are difficult to pinpoint because unlike mitochondria it has no genome. Over time the endosymbiont that evolved into the hydrogenosome transferred all of its genes to the nucleus.
Hydrogenosome function can be characterized as follows: isolate hydrogenosomes; fractionate samples; run on a 1D gel, using Zn chelating chromatography or Na2CO3 extraction; perform a gel trypsin digest; recover the tryptic peptides; use mass spectrometry to get the protein sequence; and run the sequence through databases. From this, a pie chart can be made of hydrogenosomal proteins’ function, revealing most to be small GTP-ases.
| Technique | Overview |
|---|---|
| Phylogenetics | It turns out that the Hsp60 protein in the hydrogenosome branches with a monophyletic group (99%) composed exclusively of mitochondrial homologues and this branches right next to the proteobacteria group (73%) from which the mitochondria are thought to have arisen. |
| Characterization | Organelles with a common origin are predicted to have common protein translocation signals and machinery. Incidentally, hydrogenosomal matrix proteins have mitochondrial-like targeting presequences. To determine if this signal is necessary for organelle targeting in vitro and in vivo, membrane translocation components may be characterized. These components must have evolved as the endosymbiont was converted to an organelle, revealing the evolutionary history. |
The apicoplast (aka plastid) is an organelle found exclusively in the apicomplexan phylum. The apicoplast has a 35kb genome and is surrounded by 4 membranes. Although its genome can be isolated and sequenced, the apicoplast itself cannot be isolated. The apicoplast is also known as a plastid, as drugs against chloroplasts and prokaryotes also kill apicomplexan parasites.
Phylogenetics and GFP tagging reveal that nuclear-encoded proteins of apicoplast origin localize to the apicoplast. Thus, localization of apicoplast proteins is regardless of where they are encoded. Apicoplast proteins have three domains: hydrophobic signal sequence; transit peptide; and mature protein domain. Localization requires the signal sequence and transit peptide.
Identifying protein function via proteomics requires isolated organelles. Unfortunately, the apicoplast cannot be isolated. Thus a common sense approach has been used: all the proteins required for fatty acid synthesis were found in the apicoplast; thus, the apicoplast can probably synthesize fatty acids.
The apicoplast likely became encased in four membranes via a double endosymbiotic event. The chloroplast arose by engulfment of a cyanobacteria by a plant/algae ancestor. An algae was then engulfed by the ancestor of all apicomplexans. Thus an apicoplast organelle arose with four membranes.
The apicoplast genome was sequenced and found to be plastid-like. It encodes rRNA, tRNAs, ribosomal proteins and 5-6 genes related to chloroplast genes. Furthermore, drugs targeting prokaryotic and chloroplast enzymes also kill apicomplexans. Thus, it seems that the apicoplast arose from a chloroplast. Also, nuclear-encoded plastid-like proteins — ie, acyl carrier protein (ACP) or p59 — localize to the apicoplast. The same goes for proteins of plastid origin. But how?
Deletion and restoration identified an evolutionarily conserved apicoplast targeting signal at the N-terminus. Known as the ACP, this bipartite signal contains: a signal sequence to engage secretion; and a plastid-targeting domain to target the plastid. Fusing GFP to ACP targeted it the plastid. Analyzing numerous ACPs led to a consensus amino acid sequence. All translated proteins were screened for this motif at their N-terminus. This identifies putative apicoplast proteins.
Nuclear genes, morphology, biochemistry and pharmacology places apicomplexans with ciliates and dinoflagellates. However, presence of plastids suggest placement with plants and algae. This paradox is resolved by two stage lateral genetic transfer: an ancestral plant/algae cell engulfed a cyanobacteria; this evolved into a free-living algae; this algae was engulfed by an ancestral apicomplexan. Plastids and mitochondria arose via engulfment of eubacteria by eukaryotes.
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