Fungal Distribution and Importance ,Fungal Structure,Fungal Reproduction

   

  Unlike protists, fungi are primarily terrestrial organisms. They have a global distribution from polar to tropical regions. Fungi are saprophytes, securing nutrients from dead organic material by re­ leasing degradative enzymes into the environment.  

     This enables their absorption of the soluble products-a process sometimes called osmotrophy. Fungi are important decomposers. They degrade complex organic materials in the environment to simple organic compounds and inorganic molecules. In this way, carbon, nitrogen, phosphorus, and other critical constituents of dead organisms are released and made available for living organisms. Many fungi are pathogenic, with over 5,000 species known to attack economically valuable crops and many other plants.

 About20 new human fungal pathogens are documented each year. Conversely, fungi also form beneficial relationships with other or­ ganisms. For example, the vast majority of vascular plant roots form  important  associations  with  fungi  called mycorrhizae.

Mycorrhizae 

Fungi, especially yeasts (single-celled fungi), are essential to many industrial processes involving fermentation. Examples in­ clude the making of bread, wine, beer, cheeses, and soy sauce. They are also important in the commercial production of many organic acids (citric, gallic) and certain drugs (ergometrine, cor­ tisone), and in the manufacture of many antibiotics (penicillin, griseofulvin) and the immunosuppressive drug cyclosporine. 

 In addition, fungi are important research tools in the study of fun­damental biological processes. Cytologists, geneticists, bio­ chemists, biophysicists, and microbiologists regularly use fungi in their research. The yeast Saccharomyces cerevisiae  is  the best understood eukaryotic cell. It has been a valuable model organism in the study of cell biology, genetics, and cancer.

Fungal Structure

The body or vegetative structure of a fungus is called a thallus (pl., thalli). It varies in complexity and size. Single-cell microscopic fungi are referred to as yeasts, while multicellular masses are called molds.

 Fungi also include macroscopic puffballs and mushrooms. Like most bacteria, fungi possess cell walls; however,fungal cell walls are usually made of chitin. Chitin is a strong but flexible nitrogen containing polysaccharide consisting of N-acetylglucosamine residues. Instead of chitin, some fungal cell walls are composed of other polysaccharides such as man­ nans, galactosans, or cellulose.

A yeast is a unicellular fungus with a single nucleus that reproduces either asexually by budding and transverse division or sexually through spore formation. Each bud that separates can grow into a new cell, and some group together to form colo­ nies. 

 Generally yeast cells are larger than bacteria and are com­monly spherical to egg-shaped. They lack flagella and cilia but have most other eukaryotic organelles. The thallus of a mold consists of long, branched, threadlike filaments of cells called hyphae (s., hypha; Greek hyphe, web) that form a tangled mass called a mycelium (pl., mycelia). In some fungi, protoplasm streams through hyphae,uninterrupted by cross walls.

 These hyphae are called coenocytic or aseptate hy­phae. The hyphae of other fungi  have cross walls called septa {s., septum) with either a single pore  or multiple pores  that enable cyto­plasmic streaming. These hyphae are termed septate hyphae.

Hyphae are composed of an outer cell wall and an inner lumen, which contains the cytosol and organelles. A plasma membrane sur­ rounds the cytoplasm and lies next to the cell wall. The filamentous nature of hyphae results in a large surface area relative to the volume of cytoplasm. This makes adequate nutrient absorption possible.

Fungal Reproduction

Reproduction in fungi can be either asexual or sexual.

 Asexual reproduction is accomplished in several ways: 

               1) a parent cell undergoes mitosis and divides into two daughter cells by a cen­ tral constriction and formation of a new cell  wall    

               2) mitosis in vegetative cells may be concurrent with budding to produce a daughter cell. 

                This is very common in yeasts. The formation of asexual spores often accompanies asexual reproduction and is usually used as a means of dispersal. There are many types of asexual spores, each with its own name. Arthroconidia (arthrospores) are formed when hyphae frag­ ment through splitting of the cell wall or septum. Sporangiospores develop within a sac (sporangium; pl., sporangia) at a hyphal tip. CONIDIOSPORES are spores that are not enclosed in a sac but produced at the tips or sides of the hy­pha. BLASTOSPORES are  produced  from a  vegeta­tive mother cell budding.

Sexual reproduction in fungi involves the fusion of compat­ible nuclei. Homothallic fungal species are self-fertilizing and produce sexually compatible gametes on the same mycelium. Heterothallic species require outcrossing between different but sexually compatible mycelia. Depending on the species, sexualfusion may occur between haploid gametes, gamete-producing bodies called gametangia, or hyphae. Sometimes both the cyto­plasm and haploid nuclei fuse immediately to produce the dip­loid zygote,  as seen in higher  eukaryotes.  Usually,   however,there is a delay between cytoplasmic and  nuclear  fusion.  

  Thisproduces a dikaryotic STAGE in which cells contain two separate haploid  nuclei  (N  +  N), one  from  each  parent.After a period of dikaryotic existence, the two nuclei fuse and undergo meiosis to yield haploid spores. This is seen in both ascomycetes and basidiomycetes, so these are sometimes re­ ferred to as dikaryotic fungi.

Fungal spores, both asexual and sexual, are important for fusionHaploid stage (N)several reasons. They enable fungi to survive environmental stresses such as desiccation, nutrient limitation, and extreme temperatures, although they are not as stress resistant as bacterial endospores. 

They aid in fungal dissemination, which helps ex­ plain their wide distribution. Because spores are often small and light, they can remain suspended in air for long periods  and  are often spread by adhering to the bodies of insects and other animals.  

     The bright colors and fluffy textures of many molds often are due to their aerial hyphae and spores. Finally, the size, shape, color, and number of spores are useful in the identification of fungal species.

Eumycetozoa,slime molds,

First described in the 1880s, members of Eumycetozoa or "slime molds" have been classified as plants, animals, and fungi. 

 As we examine their morphology and behavior, the source of this con­ fusion should become apparent.

 Recent analysis of certain pro­ teins (e.g., elongation factor EF-1, a-tubulin, and actin) as well as physiological, behavioral, biochemical, and developmental data point to a monophyletic group.

 Eumycetozoa includes Myxogas­ tria and Dictyostelia. 

The acellular slime  mold  (Myxogastria) life cycle includes a distinctive stage when the organisms exist as streaming masses of colorful protoplasm. 

The protoplasm creeps along in amoeboid fashion over moist, rotting logs, leaves, and other organic matter, which it degrades .

 Acellular slime molds derive their name from the presence of a large, mul­ tinucleate mass called a plasmodium; there can be as many as 10,000 synchronously dividing nuclei within a single plasmo­ dium.

 Feeding is by endocytosis.

 When starved or dried, the plasmodium develops ornate fruiting bodies.

 As these mature, they form stalks with cellulose walls that are resistant to environmental stressors .

 When conditions im­ prove, spores germinate and release haploid amoeboflagellates. These fuse, and as the resulting zygotes feed, nuclear divisions give rise to the multinucleate plasmodium.

Cellular slime molds (Dictyostelia) are strictly amoeboid and use endocytosis to feed on bacteria and yeasts. 

Their com­ plex life cycle involves true multicellularity, despite their primi­ tive evolutionary status .

 The species Dictyostelium discoideum is an attractive model organism.

 During its life cy­ cle, a pseudoplasmodium is formed. 

This consists of many, many individual vegetative cells moving together as a mass. Thus it differs from the acellular slime mold's true plasmodium. 

The pseudoplasmodium forms when starved cells release cyclic AMP and a specific glycoprotein, which serve as molecular sig­ nals.

 Other cells sense these compounds and respond by forming an aggregate around the signal-producing cells. 

In this way, large, motile, multicellular slugs develop and serve asprecursors to fruiting body formation.

 Fruiting body morpho­ genesis commences when the slug stops and cells pile on top of each other.

 Cells at the bottom of this vertically oriented struc­ ture form a stalk by secreting cellulose, while cells at the tip differ­ entiate into spores .

 Germinated spores become vegetative amoebae to start this asexual cycle anew.

Dictyostelium spp. display complex behaviors. 

In addition to farming, described in the chapter opening story, they also differ­ entiate to resemble primitive immune cells.

 During slug forma­ tion, some cells become "sentinel cells" and vanquish harmful bacteria.

 Sentinel cells accomplish this by producing proteins that are similar to those involved in immune responses in higher or­ ganisms.

 These cells roam within the slug as if patrolling for pathogenic bacteria such as Legionella pneumophila, which are known to infect Dictyostelium spp.

 Sentinel cells have been found in several species related to Dictyostelium discoideum; immunolo­ gists are not too surprised, noting that all multicellular organisms need protection against bacterial pathogens.

Sexual reproduction in D. discoideum involves the formation of special spores call macrocysts.

 These arise by a form of conju­ gation that has some unusual features. 

First, a group of amoebae become enclosed within a wall of cellulose.Conjugation occurs between members of different mating types, of which there are three, as well as those capable of self-fertilization. 

Mating type (denoted as Type I, II, or III) is controlled by the nucleotide se­ quence of a single gene. Following conjugation, a large amoeba forms and cannibalizes the remaining amoebae. 

The now-giant amoeba matures into a macrocyst. Macrocysts can remain dor­ mant within their cellulose walls for extended periods. Vegetative growth resumes after the diploid nucleus undergoes meiosis to generate haploid amoebae.

 

Overview of Protists,Protist Morpholog,Encystment and Excystment,Protist Reproductive Cells and Structures,Protist Taxonomy

Most eukaryotes are microbes. 

 It is therefore not surprising that the vast diversity of protists is a function of their capacity to thrive in a wide variety of habitats.

 Their one common require­ ment is moisture because all are susceptible to desiccation.

 Most protists are free living and inhabit freshwater or marine environ­ ments.

 Many terrestrial chemoorganotrophic forms can be found in decaying organic matter and soil. 

Whether terrestrial or aquatic, protists play an important role in nutrient cycling.

Protozoa, or chemoorganoheterotrophic protists, may be saprophytes, securing nutrients from dead organic material by releasing degradative enzymes into the environment. 

 They then absorb the soluble products-a process sometimes called osmotrophy.

 Other protozoa employ holozoic nutrition, in which solid nutrients are acquired by phagocytosis.

 Photo­ autotrophic protists are strict aerobes and, like cyanobacteria, use photosystems I and II to perform oxygenic photosynthesis.

 It is difficult to classify the nutritional strategies of some protists be­ cause they simultaneously use both reduced organic molecules and C02 as carbon sources. 

This strategy is called mixotrophy.

Protist Morphology

Despite their diversity, protists share many common features. 

 In many respects, their morphology and physiology are the same as the cells of multicellular plants and animals.

 However, because many protists are unicellular, all of life's various functions must be performed within a single cell.

 Those that are multicellular lack highly differentiated tissues. Therefore the structural complexity observed in protists arises at the level of specialized organelles.

The protist cell membrane is called the plasmalemma and is identical to that of multicellular organisms.

 In some protists, the cytoplasm immediately under the plasmalemma is divided into an outer gelatinous region called the ectoplasm and an  inner  fluid  region,  the  endoplasm.  

The  ectoplasm  imparts rigidity to the cell body.

 Many protists also have a supportive mechanism called the pellicle.

 The pellicle consists of the plasma­ lemma and a relatively rigid layer just beneath it. The pellicle may

be simple in structure. 

For example, Euglena spp. are protists with a series of overlapping strips with a ridge at the edge of each strip fitting into a groove on the adjacent one, much like how the "tongue-and-groove" boards of a hardwood floor fit together.

 In contrast, the pellicles of ciliate protists are exceptionally complex with two membranes and a variety of associated structures. 

Al­ though pellicles are not as strong and rigid as cell walls, for those that possess them, pellicles impart the characteristic shape asso­ ciated with that particular species.

One or more vacuoles are usually present in the cytoplasm of protozoa.

 These are differentiated into contractile, secretory, and food or phagocytic vacuoles.

 Contractile vacuoles function as osmoregulatory organelles in those protists that live in hypotonic environments, such as freshwater lakes. 

Osmotic balance is maintained by continuous water expulsion.

 Phagocytic vacuoles are conspicuous in protists that ingest food by phagocytosis ( ho­ lozoic protists) and in parasitic species. Phagocytic vacuoles are the sites of food digestion.

 In some organisms, they may occur anywhere on the cell surface, while others have a specialized structure for phagocytosis called the cytostome (cell mouth). 

When digestion commences, the phagocytic vacuole is acidic, and as digestion proceeds, the vacuole membrane forms small blebs.

 These pinch off and carry nutrients throughout the cyto­plasm. 

The undigested contents of the original phagocytic vacu­ ole are expelled from the cell either at a random site on the cell membrane or at a designated position called the cytoproct.

Several energy-conserving organelles are observed in protists.

 Most aerobic chemoorganotrophic protists have mitochondria, while photosynthetic forms have chloroplasts.

 A dense protein­ aceous area, the pyrenoid, which is associated with the synthesis and storage of starch, may be present in chloroplasts. 

The majority of anaerobic chemoorganotrophic protists lack mitochondria; some of these organisms have hydrogenosomes.

Many protists feature cilia or flagella at some point in their life cycle. Their formation is associated with a basal bodylike or­ ganelle called the kinetosome. In addition to aiding in motility, these organelles may be used to generate water currents for feed­ ing and respiration.

Encystment and Excystment

Many protists are capable of encystment. During encystment, the organism becomes simpler in morphology and develops into a resting stage called a cyst. The cyst is a dormant form marked by the presence of a cell wall and very low metabolic activity. Cyst formation is particularly common among aquatic, free-living protists and parasitic forms. Cysts serve three major functions:

(1) they protect against adverse changes in the environment, such as nutrient deficiency, desiccation, adverse pH, and low levels of 02

 (2) they are sites for nuclear reorganization and cell division (reproductive cysts) and

 (3) they serve as a means of transfer between hosts in parasitic species (i.e., they are the infective stage).

 Protists escape from cysts by a process called excystment . Although the exact stimulus for excystment is un­ known for most protists, it is generally triggered by a return to favorable environmental conditions. For example, cysts of para­ sitic species excyst after ingestion by the host.

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Sexual reproduction involves the formation of gametes. Protist cells that produce gametes are termed gamonts. The fusion of haploid gametes is called syngamy. Among protists, syngamy can involve the fusion of two morphologically simi­ lar gametes (isogamy) or two morphologically different types . Meiosis may occur before the formation and union of gametes, as in most animals, or just after fertilization, as is the case with lower plants. Furthermore, the exchange of nuclear material may occur in the familiar fashion-between two different individuals (conjugation)-or by the develop­ ment of a genetically distinct nucleus within a single individ­ ual (autogamy).

With this level of reproductive complexity, perhaps it is not surprising that the nuclei among protists show considerable di­ versity. Most commonly, a vesicular nucleus is present. This is 1 to  10  1-1m in diameter,  spherical,  and has a distinct nucleolus and uncondensed chromosomes . Ovular nuclei are up to 10 times this size and possess many peripheral nucleoli. Still others have chromosomal nuclei, in which the chromo­ somes remain condensed throughout the cell cycle. Finally, many ciliated forms have two types of nuclei: a large macronu­ cleus with distinct nucleoli and condensed chromatin, and a smaller, diploid micronucleus with dispersed chromatin but lacking nucleoli .

Protist Reproductive Cells and Structures

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Protist Taxonomy

Ever since Antony van Leeuwenhoek described the first proto­ zoan "animalcule" in 1674, the taxonomic classification of the protists has remained in flux. 

 During the twentieth century, classification schemes were based on morphology rather than evolutionary relationships.

 Protists were often classified into four major groups based on their means of locomotion: flagel­ lates (Mastigophora), ciliates (Infusoria or Ciliophora), amoebae (Sarcodina), and stationary forms (Sporozoa).

 Although these terms may still be encountered, they are without evolutionary context and should be avoided.

 While it is now agreed that the old classification system is best abandoned, little agreement exists on what should take its place. Here we use the higher-level classification system for the eukaryotes based on morphologi­ cal, biochemical, and phylogenetic analyses proposed by the International Society of Protistologists in 2005. This scheme does not use formal hierarchical rank designations such as class and order, reflecting the fact that protist taxonomy remains an area of active research. 

The Classification of the Protists  as proposed by the International Society of Protistologists is presented in a table on the text website.

 

 

Comparison of Bacterial, Archaeal, and Eukaryotic Cells

 A comparison of the bacterial and eukaryotic cells in  demonstrates that there are many fundamental differences be­tween these cells. 

These differences are also observed between archaeal and eukaryotic cells, because archaea are similar to bacteria at the gross structural level.

 Eukaryotic cells have  a membrane-enclosed nucleus.

 In contrast, bacterial and archaeal cells lack a true, membrane-delimited nucleus.

 Most bacteria and archaea are smaller than eukaryotic cells, often about the size of eukaryotic mitochondria and chloroplasts.

Many other major distinctions between these groups exist.

 that bacterial and archaeal cells are much simpler structurally.

 In particular, an extensive and diverse col­ lection of membrane-delimited organelles is missing.

 Further­ more, bacterial and archaeal cells are functionally simpler in several ways.

 They lack mitosis and meiosis, and have a simpler genetic organization.

 Many complex eukaryotic processes are ab­ sent in bacteria and archaea: endocytosis, intracellular digestion, directed cytoplasmic streaming, and ameboid movement are just a few.

Despite the many significant differences, all cells are re­ markably similar on the biochemical level, as we discuss in succeeding chapters. 

With a few exceptions, the genetic code is the same in all, as is the way in which the genetic informa­ tion in DNA is expressed. 

The principles underlying meta­ bolic processes and many important metabolic pathways are identical.

 Thus beneath the profound structural and func­ tional differences between bacterial, archaeal, and eukaryotic cells, there is an even more fundamental unity: a molecular unity that is basic to all known life processes.

Retrieve, Infer, Apply

1. Outline the major differences between bacterial, archaeal, and eukaryotic cells. How are they similar?

2. What characteristics make members of Archaea more like eukaryotes? What features make them more like bacteria?

 

Chloroplasts

 Chloroplasts

Plastids are cytoplasmic organelles of photosynthetic protists and plants.

 They often possess pigments such as chlorophylls and carotenoids, and are the sites of synthesis and storage of food reserves. 

The most important type of plastid is the chloro­ plast.

 Chloroplasts contain chlorophyll and use light energy to convert C02 and water to carbohydrates and 0 that is, they are the site of photosynthesis.

 Two major types of chloroplasts have been identified: those that evolved from a primary endosymbi­otic event and those that evolved from a secondary or tertiary event .

 The chloroplasts of plants and some photosynthetic protists are primary plastids and are the focus of this discussion.

Chloroplasts are quite variable in size and shape, but they share many structural features.

 Most are oval with dimen­ sions of 2 to 4 flm by 5 to 10 flm, but some photosynthetic protists possess one huge chloroplast that fills much of  the cell.

Like mitochondria, chloroplasts are encompassed by two membranes . 

A matrix called the stroma is en­ closed by the inner membrane.

 The stroma contains DNA, ribosomes, lipid droplets, starch granules, and a complex in­ternal membrane system whose most prominent components are flattened, membrane  delimited  sacs  called  thylakoids.

Mitochondria,the powerhouse of the cell

Found in most eukaryotic cells, mitochondria frequently are called the "powerhouses" of the cell .

 Metabolic processes such as the tricarboxylic acid cycle and the generation of ATP, the major energy cur­ rency of all life forms, take place here.

 When viewed with a transmission electron microscope, many mitochondria are cy­ lindrical structures and measure approximately 0.3 to 1.0 fliD by 5 to 10 11m.

 (In other words, they are about the same size as bacterial cells.)

 Some cells possess 1,000 or more mitochon­  have a single, giant, tubular mitochondrion twisted into a continuous network permeating the cytoplasm.

The mitochondrion is bounded by two membranes: an outer mitochondrial membrane separated from an inner mitochon­ drial membrane by a 6 to 8 nm intermembrane space . 

The outer mitochondrial membrane contains porins and thus is similar to the outer membrane of Gram-negative bacteria.

 The inner membrane has infoldings called cristae (s., crista), which greatly increase its surface area.

 The shape of cristae differs in mitochondria from various species. 

P latelike (laminar) cristae, cristae shaped like disks, tubular cristae, and cristae in the shape of vesicles have all been observed.

 The inner membrane encloses the mitochondrial matrix, a dense material containing ribo­ somes, DNA, and often large calcium phosphate granules.

 In many organisms, mitochondrial DNA is a closed circle, like most bacterial DNA. However, in some protists, mitochondrial DNA is linear.

Each mitochondrial compartment has a characteristic chemical and enzymatic composition. For example, the outer and inner mitochondrial membranes possess different lipids.

 Enzymes and electron carriers involved in electron transport and oxidative phosphorylation are located only in the inner membrane.

 Enzymes of the tricarboxylic acid cycle and those involved with the catabo­ lism (breaking down) of fatty acids are located in the matrix.

The mitochondrion uses its DNA and ribosomes to synthe­ size some of its own proteins.

 In fact, mutations in mitochondrial DNA often lead to serious diseases in humans.

 However, most mitochondrial proteins are manufactured under the direction of the nucleus and must the nucleus and must be transported into the mitochondrion.

 Mitochondria reproduce by binary fission, a reproductive pro- cess  used  by  many   bacteria.

 

Nucleus , nucleus function , nucleus definition , cell nucleus , nucleus diagram

The nucleus is by far the most visually prominent organelle in eukaryotic cells. 

It was discovered early in the study of cell struc­ ture and was shown by Robert Brown in 1831 to be a constant feature of eukaryotic cells.

 The nucleus is the repository for the cell's genetic information.

Nuclei are membrane-delimited spherical bodies about 5 to 7 11m in diameter . 

They contain more than one chromosome; the exact number depends on the organism, cell type, and stage in the life cycle.

 Each eukaryotic chromosome is composed of chromatin. 

Chromatin is a complex of DNA and proteins, including histones.

 Histones are small basic proteins rich in the amino acids lysine, arginine, or both.

 There are five types of histones in most eukaryotic cells: HI, H2A, H2B, H3, and H4.

 Eight histone molecules form an ellipsoid about 11 nm long and 6.5 to 7 nm in diameter around which the DNA wraps to form a "beads-on-a-string" formation . 

Each bead is called a nucleosome.

Chromosomes are very dynamic and vary in terms of their degree of compaction.

 When the cell is not dividing, there is less compaction.

 The highest degree of compaction occurs dur­ ing cell division.

 Compaction is brought about in part byhistones and other proteins, including condensins.

 Recall that condensins are also used by bacterial and archaeal cells to compact their chromosomes during cell division.

 Tethering of one region of the chromosome to another and tethering of the chromosome to nuclear structures also contribute to chromo­ some folding and compaction.

a complex structure consisting of inner and outer membranes separated by a perinuclear space.

 The nuclear envelope is contin­ uous with the ER at several points, and its outer membrane is covered with ribosomes.

 A network of intermediate filaments, called the nuclear lamina, is observed in animal cells.

 It lies against the inner surface of the nuclear envelope and supports it.

 Many nuclear pores penetrate the envelope, and each pore is formed by about 30 proteins; each pore plus the associated pro­ teins is called a nuclear pore complex .

 Pores are about 70 nm in diameter and collectively occupy about 10 to 25% of the nuclear surface. 

The nuclear pore complexes serve as trans­ port routes between the nucleus and surrounding cytoplasm.

 Small molecules move through the nuclear pore complex un­ aided.

 However, large molecules are transported through the nuclear pore complex. Some nuclear pore complex proteins are involved in these transport processes.

Often the most noticeable structure within the nucleus is the nucleolus .

 A nucleus may contain from one to many nucleoli.

 Although the nucleolus is not membrane-enclosed, it is a complex organelle with separate granular and fibrillar regions.

 It is present in nondividing cells but frequently disappears during mi­ tosis. After mitosis, the nucleolus reforms around the nucleolar organizer, a particular part of a specific chromosome.

The nucleolus plays a major role in ribosome synthesis.

 The DNA of the nucleolar organizer directs the production of ribo­ somal RNA (rRNA).

 This RNA is synthesized in a single long piece that is cut to form the final rRNA molecules.

 The processed rRNAs combine with ribosomal proteins (which have been syn­ thesized in the cytoplasm) to form partially completed ribosomal subunits.

 The granules seen in the nucleolus are probably these subunits.

 Immature ribosomal subunits then leave the nucleus, presumably by way of the nuclear pore complexes, and mature in the cytoplasm.