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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

what is infectious
Fred Reply
Many poeple are die due to covid-19 virus
AMINU Reply
they become sick due to millions bacteria, fungi and some insects that make plants as their host.
charles Reply
what are microbes and what are their effects to humans
charles Reply
classify microorganisms.
Satarupa Reply
protozoa' bacteria' virus' algai' archea'
Cabdifitaax
protozoa
Ayesha
Thank you
Satarupa
mutant having a requirement for a certain growth factor is called a(n)
Purushoth Reply
what is atome
Marie Reply
is the smallest part of any material that cannot be broken up by chemical means
Feisal
How does forest tree become sick
Emmanuel
Aseptate hyphae are seen in
lab Reply
yes
Ahmed
w
Ahmed
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Apdikrim
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Feisal
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Salman
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Feisal
CHEMISTRY
Apdikrim
biology
Ahmed
I.m student
Ahmed
I.m question what is difference between arthropod and insect?
Ahmed
Arthropod is a large division of jointed-foot Invertebrates such as Insects, centipedes and others so Insect is a part of the division or the phylum
hilina
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Ahmed
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hilina
describe binary fission
Queen Reply
in which parent cell devides into two daughter cells
Iqra
single parent cell
Tean
who is the father of microbiology
Mary Reply
Antonie van Leeuwenhoek , he is father of microbiology and Louis Pasteur is father of modern microbioloy.
Arshad
Anton van leeuwenhoek ( He descovered bacteria in 1679)
Satarupa
what is the role of protein in virus genome
Rana Reply
what is microbiological
Apdikrim Reply
Science is a good Subject
AMINU Reply
yes it is
Madrine
what is amoeba
Joy Reply
a single celled organism with finger like extensions
janet
they could either be free living or parasitic
janet
is an organisms with no shape
Joy
What is microbiology
Ashaq Reply
is the branch that deals with microorganisms
Nakaweesi
is the science which deals with the study of microorganisms whether they may be plants or animals .
Sona
it is also deals instruments?
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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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