Gene expression in eukaryotic cells is regulated by repressors as well as by transcriptional activators. Like their prokaryotic counterparts, eukaryotic repressors bind to specific DNA sequences and inhibit transcription. Other repressors compete with activators for binding to specific regulatory sequences.
Overview of Prokaryotic Transcription
Prokaryotic transcription and translation can occur simultaneously. This is impossible in eukaryotes, where transcription occurs in a membrane-bound nucleus while translation occurs outside the nucleus in the cytoplasm.Genetic material in eukaryotes is contained within a nucleus within the cell and DNA is organized into chromosomes. Eukaryotic organisms may be multicellular or single- celled organisms. Eukaryotic cell membrane contain sterols, whereas no prokaryotes except the wall of Mycoplasma, has sterol in its membrane.
Operons occur in prokaryotes, but not eukaryotes. In eukaryotes, each gene is made on individual mRNAs and each gene has its own promoter. Operons are prokaryotic arrangements of multiple genes (with common functions) under the control of a single promoter.
Prokaryotes can't have introns, because they have transcription coupled to translation. They don't have time/space for that, since intron splicing will stop the coupling. Eukaryotes evolved the nucleus, where splicing can be done.
When an operon is transcribed, all of the genes on the operon are on the same mRNA. Operons occur in prokaryotes, but not eukaryotes. In eukaryotes, each gene is made on individual mRNAs and each gene has its own promoter. Cells can't afford to waste energy making genes if they don't need them.
Bacteria are examples of the prokaryotic cell type. In general, prokaryotic cells are those that do not have a membrane-bound nucleus. In fact "pro-karyotic" is Greek for "before nucleus". Besides bacteria, the cyanobacteria (blue-green algae) are a major group of prokaryotes.
There is no such structure seen in prokaryotes. Another main difference between the two is that transcription and translation occurs simultaneously in prokaryotes and in eukaryotes the RNA is first transcribed in the nucleus and then translated in the cytoplasm. These events do not occur in prokaryotes.
In eukaryotes, protein synthesis occurs in the cytoplasm. In prokaryotes, protein synthesis begins even before the transcription of mRNA molecule is completed. This is called coupled transcription - translation. In eukaryotes, most of the gene have introns or non coding sequences along with exons or coding sequences.
The poly(A) tail is important for the nuclear export, translation, and stability of mRNA. mRNA molecules in both prokaryotes and eukaryotes have polyadenylated 3′-ends, with the prokaryotic poly(A) tails generally shorter and less mRNA molecules polyadenylated.
There are two classes of polysomes or polyribosomes in eukaryotic cells. A polysome contains a single mRNA and several attached ribosomes, one ribosome for every 100 or so nucleotides. It takes about 30 s for a ribosome in an eukaryotic cell to synthesize a protein containing 400 amino acids.
Protein synthesis begins at a start codon AUG near the 5' end of the mRNA. mRNA binds to the P site of the ribosome first. The ribosome is able to identify the start codon by use of the Shine-Dalgarno sequence of the mRNA in prokaryotes and Kozak box in eukaryotes.
Polycistronic mRNA is a mRNA that encodes several proteins and is characteristic of many bacterial and chloroplast mRNAs. Polycistronic mRNAs consist of a leader sequence which precedes the first gene. Monocistronic mRNA is a mRNA that encodes only one protein and all eukaryotic mRNAs are monocistronic.
mRNA is found in the nucleus and the cytoplasm in cells. Transcription happens in the nucleus in eukaryotic cells, and so the original transcript of
Following are the substantial difference between Prokaryotic Cells and Eukaryotic Cell: Organelles like mitochondria, ribosomes, Golgi body, endoplasmic reticulum, cell wall, chloroplast, etc. are absent in prokaryotic cells, while these organelles are found in eukaryotic organisms.
Eukaryotic cells contain membrane-bound organelles, such as the nucleus, while prokaryotic cells do not. Differences in cellular structure of prokaryotes and eukaryotes include the presence of mitochondria and chloroplasts, the cell wall, and the structure of chromosomal DNA.
Thus, two types of
cells are found in the organisms:
eukaryotic and
prokaryotic depending on whether
cells contain membrane-bound organelles or not.
Shikha Goyal.
| Prokaryotic Cell | Eukaryotic cell |
|---|
| Nucleus is absent | Nucleus is present |
| Membrane bound nucleus absent. | Membrane bound Nucleus is present. |
Both prokaryotic and eukaryotic cells have structures in common. All cells have a plasma membrane, ribosomes, cytoplasm, and DNA. The plasma membrane, or cell membrane, is the phospholipid layer that surrounds the cell and protects it from the outside environment.
The cells of all prokaryotes and eukaryotes possess two basic features: a plasma membrane, also called a cell membrane, and cytoplasm. However, the cells of prokaryotes are simpler than those of eukaryotes. For example, prokaryotic cells lack a nucleus, while eukaryotic cells have a nucleus.
Eukaryotic cells have membrane-bound organelles, while prokaryotic cells do not. Eukaryotic cells have a nucleus that contains genetic information called DNA, while prokaryotic cells do not. In prokaryotic cells, the DNA just floats around in the cell.
Cell wall. Many kinds of prokaryotes and eukaryotes contain a structure outside the cell membrane called the cell wall. With only a few exceptions, all prokaryotes have thick, rigid cell walls that give them their shape. Among the eukaryotes, some protists, and all fungi and plants, have cell walls.
Eukaryotes include such microorganisms as fungi, protozoa, and simple algae. Viruses are considered neither prokaryotes nor eukaryotes because they lack the characteristics of living things, except the ability to replicate (which they accomplish only in living cells).
