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a) A micrograph of a spherical cell approximately 4 µm in diameter. B) A micrograph of wavy ribbon shaped cells approximately 10 µm in length. C) a micrograph of a bell shaped cell approximately 50µm in diameter with a tail approximately 200 µm in length. D) An oval shaped cell approximately 100 µm in length. A ring shaped cell approximately 4 µm in diameter; the ring shaped cell is inside a red blood cell.
Eukaryotic cells come in a variety of cell shapes. (a) Spheroid Chromulina alga. (b) Fusiform shaped Trypanosoma . (c) Bell-shaped Vorticella . (d) Ovoid Paramecium . (e) Ring-shaped Plasmodium ovale . (credit a: modification of work by NOAA; credit b, e: modification of work by Centers for Disease Control and Prevention)
  • Identify two differences between eukaryotic and prokaryotic cells.

Nucleus

Unlike prokaryotic cells, in which DNA is loosely contained in the nucleoid region, eukaryotic cells possess a nucleus , which is surrounded by a complex nuclear membrane that houses the DNA genome ( [link] ). By containing the cell’s DNA, the nucleus ultimately controls all activities of the cell and also serves an essential role in reproduction and heredity. Eukaryotic cells typically have their DNA organized into multiple linear chromosomes. The DNA within the nucleus is highly organized and condensed to fit inside the nucleus, which is accomplished by wrapping the DNA around proteins called histones.

A micrograph of a portion of an oval cell. In the center is a darker spherical structure.
Eukaryotic cells have a well-defined nucleus. The nucleus of this mammalian lung cell is the large, dark, oval-shaped structure in the lower half of the image.

Although most eukaryotic cells have only one nucleus, exceptions exist. For example, protozoans of the genus Paramecium typically have two complete nuclei: a small nucleus that is used for reproduction (micronucleus) and a large nucleus that directs cellular metabolism (macronucleus). Additionally, some fungi transiently form cells with two nuclei, called heterokaryotic cells, during sexual reproduction. Cells whose nuclei divide, but whose cytoplasm does not, are called coenocytes .

The nucleus is bound by a complex nuclear membrane , often called the nuclear envelope , that consists of two distinct lipid bilayers that are contiguous with each other ( [link] ). Despite these connections between the inner and outer membranes, each membrane contains unique lipids and proteins on its inner and outer surfaces. The nuclear envelope contains nuclear pores, which are large, rosette-shaped protein complexes that control the movement of materials into and out of the nucleus. The overall shape of the nucleus is determined by the nuclear lamina , a meshwork of intermediate filaments found just inside the nuclear envelope membranes. Outside the nucleus, additional intermediate filaments form a looser mesh and serve to anchor the nucleus in position within the cell.

A micrograph showing an oval cell with a large oval nucleus. The nucleus is red with a bright green outline labeled nuclear lamina. Green lines criss-cross the rest of the cell outside the nucleus.
In this fluorescent microscope image, all the intermediate filaments have been stained with a bright green fluorescent stain. The nuclear lamina is the intense bright green ring around the faint red nuclei.

Nucleolus

The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) biosynthesis occurs. In addition, the nucleolus is also the site where assembly of ribosomes begins. Preribosomal complexes are assembled from rRNA and proteins in the nucleolus; they are then transported out to the cytoplasm, where ribosome assembly is completed ( [link] ).

a) A diagram showing the nucleus. A sphere in the center of the nucleus is labeled nucleolus. Lines within the nucleus are labeled chromatin. The fluid of the nucleus is labeled nucleoplasm. The outer region just inside the nuclear envelope is labeled nuclear lamina. The outside of the nucleus is labeled nuclear envelop and pores in the envelope are labeled nuclear pores.  The nuclear envelope is continuous with and becomes the endoplasmic reticulum; a webbing of membranes outside the nucleus. B) A micrograph showing these same structures. The nucleolus is a dark region inside the nucleus which is composed of many lighter lines. The nuclear envelop forms the outside of the nucleus and a pore is seen as a light region in the envelope. Outside the envelope are many lines labeled rough endoplasmic reticulum. A smaller set of lines is labeled mitochondrion overlaying part of the RER.
(a) The nucleolus is the dark, dense area within the nucleus. It is the site of rRNA synthesis and preribosomal assembly. (b) Electron micrograph showing the nucleolus.

Questions & Answers

characteristic of Gram negative bacteria
jane Reply
Characteristics of Gram Negative Bacteria As with Gram positive bacteria, Gram negative bacteria also contain the peptidoglycan polymer in their cell wall. While this polymer is thin (2 to 4 nanometers in thickness with just about 3 layers of peptidoglycan) in Gram negative bacteria, it's also com
Kaviya
it's also composed of long glycan strands that are cross-linked by peptide molecules. This composition serves a number of functions including protecting the bacterial cell from lysis
Kaviya
Good shot
Enoch
Thanks 😊
Kaviya
what was Hans Christian Gram's supported in the modern Microbiology?
Wilson Reply
what is microbial growth
Chisa Reply
The organism responsible for vulva ulcers
nyiter Reply
Why are vascular pathogen poorly communicable from person to person?
Aj Reply
Most vascular pathogens are poorly communicable from person to person because they need a medium to be communicated i,e a vector that would carry them from one person to other
Kaviya
what's the habit of protista
Afieahngwi Reply
They show both autotrophic and heterotrophic mechanisms...
Swetha
thanks
Afieahngwi
welcome...
Swetha
let me mention some water. Air .Food and so on
Gattiek Reply
causes of infectious diseases
Afieahngwi Reply
water.Air
Gattiek
infectious disease are caused by pathogenic micro organisms like bacteria ,fungi..
Swetha
What is pasteurization?
Wilson
are fungi prokaryote or eukaryotes?
Afieahngwi Reply
fungi are eukaryotes.
Swetha
All fungi are eukaryotes. Even micro fungi.
Lad
have..complex cellular organization and membrane bound nucleus ...and..also... having loops of DNA( like plasmids) as.bacteria
Swetha
what enzyme replaces rna nucleotides with dna nucleotides during replication?
Remi Reply
an enzyme called DNA ligase.
Jael
describe the acid fast staining procedure used in the diagnosis of tuberculosis
Salma Reply
bacterial morphology
lf_ Reply
what is the difference between biogenesis & abiogenesis
Mayuri Reply
biogenesis is when living comes out from other living things as a result of reproduction while a biogenesis is the process where living things comes out from non living things
Usman
living things come form other form living things is biogenesis. ....right?
Mayuri
what is mean by pasturation method?
Mayuri Reply
Pasteurization is a process that kills harmful bacteria and creates an extended shelf life for your milk. ... It's pretty simple—we take the milk from the cows, we rapidly heat it to a high enough temperature to kill the bacteria, and then we cool it back down before packaging and shipping it to you
Kaviya
tell me about abiogenessis &biogenesis
Mayuri
discribe aristol spontaneous generation theory in brif
Mayuri Reply
The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation, the notion that life can arise from nonliving matter. Aristotle proposed that life arose from nonliving material if the material contained pneuma (“vital heat”).
Kaviya
thank you 😊
Mayuri
No mention dear 😊
Kaviya

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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