How Do Microbes Grow and Replicate?
How prokaryotes replicateBacteria and archaea reproduce asexually by splitting one cell into two equal halves in a process called binary fission (Figure 1). Before a cell divides, it must first replicate the genome so that each daughter cell gets a copy of the DNA instruction manual. Prokaryotes do not undergo sexual reproduction, but as we will see later in the course, they are able to exchange genetic information through other processes.
Figure 1: The stages of binary fission in prokaryotic cell replication © Ecoddington14 CC BY-SA 3.0The diagram above shows the process of binary fission.
- The parent cell contains a large circular chromosome and a smaller plasmid.
- The chromosome and plasmid are replicated
- A copy of the chromosome and plasmid move to each end (pole) of the cell.
- The cell wall begins to grow inwards at the middle point (septation).
- The growing cell walls meet in the middle to form a septum.
- The cells separate into two identical daughter cells (cytokinesis).
How eukaryotic microbes replicateMany unicellular fungi, like the Brewer’s yeast Saccharomyces pombe, also replicate asexually by a process similar to binary fission. In eukaryotes the DNA genome is packaged in chromosomes within the nucleus and so the process of asexual replication in yeast looks a bit more complicated than binary fission in prokaryotes. The first step is to replicate the chromosomes to form two copies of each chromosome (two sets of sister chromatids), which are then separated to the two poles of the cell via the process of mitosis (Figure 2).
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Small and Mighty: Introduction to Microbiology
Figure 2: The stages of mitosis in eukaryotic cell replication © Lady of Hats [Public Domain]The diagram above shows the process of mitosis in eukaryotic cells.
- The chromosomes condense and the mitotic spindle begins to form (prophase).
- The nuclear envelope disintegrates, and the chromosomes bind to microtubules in the mitotic spindle (prometaphase).
- The chromosomes align in the middle of the cell (metaphase).
- The two sister chromatids separate and move to the opposite poles of the cell (anaphase).
- Two new nuclear envelopes form (telophase).
- The cell divides into two identical daughter cells (cytokinesis).
Figure 3: Scanning electron micrograph (SEM) image of Baker’s yeast (Saccharomyces cerevisiae) © Mogana Das Murtey and Patchamuthu Ramasamy CC BY-SA 3.0Under conditions of stress, some yeasts can switch from asexual to sexual reproduction (Figure 4). Saccharomyces cerevisiae cells are either mating type a or mating type (alpha) (alpha) which produce different signalling molecules (pheromones). This species of yeast has 16 chromosomes, and a single cell either has one copy of each chromosome (haploid) or two copies of each chromosome (diploid) in the nucleus. When a haploid a-type and haploid (alpha)-type cell meet they recognise the pheromones produced by the other mating type. This triggers them to join together (fuse) to form a diploid cell with 32 chromosomes (2 copies of each chromosome, one from a and one from (alpha)). This diploid cell can either replicate asexually by budding, and remain as a diploid or undergo meiosis to form four haploid spores. When conditions become more favourable, the spores germinate into haploid cells that then start to reproduce.
Figure 4: Asexual and sexual reproduction in the budding yeast Saccharomyces cerevisiae © pl.wiki: Masurcommons: Masurirc: [Public domain]The diagram above shows the asexual and sexual reproduction cycles in a budding yeast:
- Asexual reproduction via budding
- Fusion of two haploid cells to form a diploid cell.
- Formation of haploid spores via meiosis.
Figure 5: Digitally colourised scanning election microscope (SEM) images of Giardia lamblia. Left: single cell. Right: undergoing binary fission (via mitosis) © CDC/ Dr. Stan Erlandsen.
Figure 6: Scanning electron microscope (SEM) images of amoebal cells undergoing binary fission (via mitosis) © CDC/Janice Haney CarrThe amoeba in these SEM images are replicating by binary fission. You can see they have plenty of pseudopodia, like the amoeba in our 3D model.
How viruses replicateIn order to replicate, viruses need to infect a host cell and hijack the host machinery to make many copies of the viral genome, and lots of viral proteins which are then assembled into new virus particles. There are six main stages in a typical virus replication cycle
- attachment of the virus to a host cell,
- penetration of the host cell;
- uncoating of the viral genome;
- replication of viral genome and production of viral proteins;
- assembly of new virus particles;
- release of new virus particles from the cell.
Figure 7: The Influenza virus replication cycle.Viruses obtain the building blocks and energy they need to complete their replication cycle from the host cell. In the next few Steps we’ll explore the types of nutrients cellular microbes need to fuel their own growth and replication and the many different places they get these from.
Small and Mighty: Introduction to Microbiology
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