Based on cell structure, all cells are widely grouped into two categories: prokaryotic cells and eukaryotic cells. In this article, the prokaryotic cell is discussed in detail.
What is a Prokaryotic Cell
Where are Prokaryotes Found
Characteristics of Prokaryotic Cell
Components
Prokaryotic Cell Structure
Examples of Prokaryotic Cells
How do Prokaryotic Cells Divide
They lack a true nucleus, i.e., they do not have a nuclear membrane.The entire genetic material or DNA is located in a central region of the cell called the nucleoid. It typically consists of a single, circular double-stranded DNA.Membrane-bound organelles such as mitochondria, chloroplast, Golgi bodies, and lysosomes are absent.The cell wall consists of carbohydrates and amino acids.The plasma membrane also acts as the mitochondrial membrane.Reproduce asexually through binary fission and sexually by conjugation, transformation, and transduction.
How Big are Prokaryotic Cells
Typically, prokaryotic cell sizes range from 0.1 to 5.0 μm in diameter and thus are significantly smaller than eukaryotic cells. They have a surface area to volume ratio higher than eukaryotes because of their small size.
Shapes
The three most common prokaryotic cell shapes are spiral (coiled-shaped), bacillus (rod-shaped), and coccus (spherical or oval-shaped). Other than the three basic shapes, cells can be sheathed, stalked, filamentous, star-shaped, spindle-shaped, lobed, and pleomorphic, to name a few.
Parts and Functions
1) Bacterial Cells
They are single-celled organisms found almost everywhere on earth, having diverse sizes, shapes, and structures. Their cell wall is made of peptidoglycan, a molecule made of sugar and amino acids, making the cell wall tough and thick. They contain some unique structures such as the capsule, flagella, and pili. Some bacteria form rigid and dormant structures called endospores that help them survive adverse environmental conditions such as high temperature and food scarcity. Their genetic material consists of one circular coiled chromosome with about 160,000 base pairs (bp) to 12,200,000 bp. Bacterial cells also contain plasmids, which are smaller circular pieces of extrachromosomal DNA. Escherichia coli (E.coli), Bacillus spp, Streptomyces spp, Clostridium spp, and Pseudomonas spp are typical bacteria examples.
2) Archaeal Cells
They are unicellular organisms that are similar in size and shape to bacteria. Archaea are commonly found in extreme environments such as springs, oceans, marshlands, and even inside other organisms, including humans. Their cell membrane has a monolayer of phospholipid that protects the cells against harsh environments. Also, archaeal cell walls are made of pseudopeptidoglycan, made of proteins. Like the bacterial cell, archaea also contain external appendages such as flagella and pili. Archaea have one circular chromosome, as bacteria do. The archaeal chromosome ranges between 491,000 bp to 5,700,000 bp. They can also contain plasmids. Halobacterium spp, Thermoplasma spp, Sulfolobus spp, Desulfurococcus spp, and Acidilobus spp are typical archaea examples.
Asexual Reproduction
It occurs by binary fission. Binary Fission: It starts when the parent cell DNA replicates. The cell then starts to increase in size, and the cell wall extends outwards. The DNA then segregates, and a new cell wall starts to develop in the middle of the cell. Finally, the cell splits to form two individual daughter cells, completing the process of reproduction. Binary fission does not produce genetic variation.
Sexual Reproduction
It occurs through recombination, which brings genetic variation to the population. Here, genes from one prokaryote are incorporated into the other to have a genetic mix between them. Recombination is achieved through three processes: Conjugation: Transfer of genes between cells occurs through a protein tube-like structure, called pilus, connecting the two cells. Transformation: Bacteria take up DNA from the surrounding medium, transport it across the cell membrane, and incorporate it into the cell’s DNA. Transduction: It involves the exchange of bacterial DNA through viruses. Bacteriophages, viruses that infect bacteria, transfer bacterial DNA from previously infected bacteria to the next bacteria they infect.
