How do microbes grow and replicate?
In the previous Steps, you looked at how the different groups of cellular microbes evolved and how all living organisms, including you, share a common ancestor called LUCA. The staggering diversity of microbes on Earth today is, in part, due to their ability to replicate rapidly. In this Step we consider how the five different groups of microbes do this.
Cellular microbes replicate asexually and/or sexually. In asexual reproduction, a single microbe produces two identical offspring (clones) without the help of a partner. In sexual reproduction, two microbes mix their genetic information and so their offspring are genetically different.
How prokaryotes replicate
Bacteria 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 3.0
The 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).
Under optimal growth conditions, the bacterium Escherichia coli divides once every 20 minutes. If it takes 20 minutes for a single cell to divide and produce two cells: after 40 minutes there will be 4 cells; after 6 hours there will be 262,144 cells; after 12 hours the population will soar to 68,719,476,736 cells. You may like to watch this video on Wikimedia.
How eukaryotic microbes replicate
Many 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).
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).
Other unicellular fungi, like the Baker’s yeast Saccharomyces cerevisiae, replicate in a different asexual process called budding. A daughter cell grows out as a small bud from the mother cell, which starts to replicate the chromosomes via mitosis. The daughter cell receives one of the two nuclei generated via mitosis and a few membrane-bound organelles and then separates, leaving a scar on the surface of the mother cell (Figure 3).
Figure 3: Scanning electron micrograph (SEM) image of Baker’s yeast (Saccharomyces cerevisiae) © Mogana Das Murtey and Patchamuthu Ramasamy CC BY-SA 3.0
Under 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) 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 -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 ). 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.
Protists are a varied group, made up of all the eukaryotes that aren’t fungi, plants or animals. They use a range of mechanisms to reproduce. The majority replicate via asexual binary fission, including Giardia (Figure 5) and amoeba (Figure 6). Other protists undergo asexual reproduction via multiple fission, budding or spore formation. Some protists undergo sexual reproduction when they encounter stressful environmental conditions or as part of complicated life cycles in different hosts. One such example is Plasmodium, which causes malaria.
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 Carr
The 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 replicate
In 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.
Many viruses are released through a process called lysis, which bursts the host cell apart. They do this by producing enzymes late in the infection cycle that form holes in the host cell membrane causing water to rush into the cell via osmosis. Some viruses, including Influenza virus, are released by budding through the cell membrane.
You can see an overview of the influenza replication cycle in Figure 7. Influenza virus attaches to receptors on the surface of a human cell via a viral spike protein called Hemagglutinin. The virus particle is taken into the cell via endocytosis and the viral nucleocapsid is transported into the nucleus. The viral RNA genome is transcribed into viral mRNA and replicated in the nucleus. Viral proteins are produced from viral mRNA using host ribosomes in the cytoplasm and the viral spike proteins are incorporated into the host cell membrane. When newly formed viruses are released from the cell by budding, they acquire a lipid envelope from the host cell membrane embedded with viral spike proteins.
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.
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