Cell Theory holds that all living things consist of cells, that cells are the basic functional units of life, and that all cells arise from pre-existing cells. This foundational idea emerged in the mid-nineteenth century, heavily influenced by advancements in microscopy and the work of scientists such as Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.
Early observations date back to Robert Hooke, who in 1665 described small “cells” in a slice of cork. Schleiden and Schwann’s collaborative insights published in the 1830s formally proposed plants and animals alike are built from cells. Soon after, Virchow added the concept that new cells must come from existing ones, effectively dismissing earlier theories of spontaneous generation.
Over time, cell theory expanded, integrating discoveries about DNA as hereditary material. This modern perspective underscores that cells not only provide structure to organisms but also govern the passing of genetic information through cell division. Such insights reshaped biology, paving the way for specialized fields such as cell biology, molecular genetics, and biotechnology.
Within the prokaryotic domain, everyday species belong to eubacteria, whereas archaea inhabit extreme environments (e.g., high salt or temperature).
Bacterial shapes are commonly categorized as bacilli (rod-shaped), spirilli (spiral), or cocci (spherical).
Bacteria lack a membrane-enclosed nucleus, so their genetic material resides in a region called the nucleoid. Because they do not form a mitotic apparatus, they rely on their cytoskeleton to separate replicated DNA. Unlike eukaryotes, they also lack typical organelles such as the Golgi and mitochondria.
The bacterial cell wall is composed of peptidoglycan, which differs from the cellulose in plants or the chitin in fungi. Many bacteria employ flagella made of flagellin, rotating like a rotor to propel the cell, driven by a proton or sodium gradient—an approach distinct from ATP-powered eukaryotic flagella.
Bacteria reproduce through binary fission, an asexual reproduction method where the DNA is duplicated and the replicated copies segregate as the cell elongates, eliminating the need for spindle fibers.
This straightforward division leads to rapid, exponential growth until resources become scarce, at which point multiplication slows and eventually plateaus.
Genetic adaptability
Transposons (also present in eukaryotic cells), known as “jumping genes,” are mobile DNA elements that can relocate within the genome, increasing genetic diversity and altering gene expression. They move via cut-and-paste or copy-and-paste mechanisms and significantly impact evolution and genome structure.
Genetic diversity also arises from mutations as well as horizontal gene transfer methods:
Variations of aerobic and anaerobic metabolism
Interbacterial relationships and transportation
Prokaryotic cells often carry plasmids—small, double-stranded DNA molecules that replicate independently of the bacterial chromosome or sometimes integrate into it. These plasmids can be transferred between bacteria via conjugation, where a bacterium with a specialized plasmid (F⁺) forms a pilus to pass genetic material to an F⁻ recipient, potentially including portions of chromosomal DNA. Another mechanism of horizontal gene transfer is transformation, in which free DNA fragments released by lysed bacteria are taken up by other cells and integrated into their genome, sometimes conferring traits like antibiotic resistance.
Within prokaryotes, gene regulation primarily involves controlling transcription through activators, inhibitors, and operons (e.g., inducers that increase gene expression or repressors that decrease it). Translation and transcription are coupled—ribosomes can begin translating an mRNA before its transcription is complete—allowing specialized regulatory processes like attenuation.
For instance, under high levels of tryptophan, rapid ribosome movement on the newly made mRNA can trigger early termination of the trp operon. Conversely, scarce tryptophan slows the ribosome, allowing full mRNA synthesis.
Prokaryotes do not undergo the extensive RNA processing seen in eukaryotes; there is no intron removal or complex post-transcriptional modifications. Instead, regulation at the transcription level provides a straightforward but highly effective way of coordinating cellular functions and rapidly adapting to environmental shifts.
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