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7.23: The Prokaryotic Cell - Biology

7.23: The Prokaryotic Cell - Biology


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All cells share four common components: (1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment; (2) cytoplasm, consisting of a jelly-like region within the cell in which other cellular components are found; (3) DNA, the genetic material of the cell; and (4) ribosomes, particles that synthesize proteins. Prokaryotic cells differ from eukaryotic cells in several key ways.

A prokaryotic cell is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. Prokaryotic DNA is found in the central part of the cell: a darkened region called the nucleoid (Figure 1).

Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion, while most pili are used to exchange genetic material during a type of reproduction called conjugation. Many prokaryotes also have a cell wall and capsule. The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment.

Reproduction

Reproduction in prokaryotes is asexual and usually takes place by binary fission. Recall that the DNA of a prokaryote exists as a single, circular chromosome. Prokaryotes do not undergo mitosis. Rather the chromosome is replicated and the two resulting copies separate from one another due to the growth of the cell. The prokaryote, now enlarged, is pinched inward at its equator and the two resulting cells, which are clones, separate. Binary fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share genes by three other mechanisms.

In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it too may become pathogenic. In transduction, bacteriophages, the viruses that infect bacteria, sometimes also move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism. Archaea are not affected by bacteriophages but instead have their own viruses that translocate genetic material from one individual to another. In conjugation, DNA is transferred from one prokaryote to another by means of a pilus, which brings the organisms into contact with one another. The DNA transferred can be in the form of a plasmid, a small circular piece of extrachromosomal DNA, or as a hybrid, containing both plasmid and chromosomal DNA. These three processes of DNA exchange are shown in Figure 2.

Reproduction can be very rapid: a few minutes for some species. This short generation time coupled with mechanisms of genetic recombination and high rates of mutation result in the rapid evolution of prokaryotes, allowing them to respond to environmental changes (such as the introduction of an antibiotic) very quickly.

Try It

How do scientists answer questions about the evolution of prokaryotes? Unlike with animals, artifacts in the fossil record of prokaryotes offer very little information. Fossils of ancient prokaryotes look like tiny bubbles in rock. Some scientists turn to genetics and to the principle of the molecular clock, which holds that the more recently two species have diverged, the more similar their genes (and thus proteins) will be. Conversely, species that diverged long ago will have more genes that are dissimilar.

Scientists at the NASA Astrobiology Institute and at the European Molecular Biology Laboratory collaborated to analyze the molecular evolution of 32 specific proteins common to 72 species of prokaryotes.[1] The model they derived from their data indicates that three important groups of bacteria—Actinobacteria, Deinococcus, and Cyanobacteria (which the authors call Terrabacteria)—were the first to colonize land. (Recall that Deinococcus is a genus of prokaryote—a bacterium—that is highly resistant to ionizing radiation.) Cyanobacteria are photosynthesizers, while Actinobacteria are a group of very common bacteria that include species important in decomposition of organic wastes.

The timelines of divergence suggest that bacteria (members of the domain Bacteria) diverged from common ancestral species between 2.5 and 3.2 billion years ago, whereas archaea diverged earlier: between 3.1 and 4.1 billion years ago. Eukarya later diverged off the Archaean line. The work further suggests that stromatolites that formed prior to the advent of cyanobacteria (about 2.6 billion years ago) photosynthesized in an anoxic environment and that because of the modifications of the Terrabacteria for land (resistance to drying and the possession of compounds that protect the organism from excess light), photosynthesis using oxygen may be closely linked to adaptations to survive on land.

Learning Objectives

Prokaryotes (domains Archaea and Bacteria) are single-celled organisms lacking a nucleus. They have a single piece of circular DNA in the nucleoid area of the cell. Most prokaryotes have a cell wall that lies outside the boundary of the plasma membrane. Some prokaryotes may have additional structures such as a capsule, flagella, and pili.



Cell (biology)

The cell (from Latin cella, meaning "small room" [1] ) is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to as the "building blocks of life". The study of cells is called cell biology, cellular biology, or cytology.

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. [2] Most plant and animal cells are only visible under a light microscope, with dimensions between 1 and 100 micrometres. [3] Electron microscopy gives a much higher resolution showing greatly detailed cell structure. Organisms can be classified as unicellular (consisting of a single cell such as bacteria) or multicellular (including plants and animals). [4] Most unicellular organisms are classed as microorganisms.

The number of cells in plants and animals varies from species to species it has been estimated that humans contain somewhere around 40 trillion (4×10 13 ) cells. [a] [5] The human brain accounts for around 80 billion of these cells. [6]

Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. [7] [8] Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells. [9] Cells emerged on Earth at least 3.5 billion years ago. [10] [11] [12]


7.23: The Prokaryotic Cell - Biology

BIOL 201: Cell Biology
Fall 2002

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Instructor: Lauren Yaich, Ph.D.
Telephone: 362-0260
e-mail: [email protected]
Office: 203F Fisher Hall

Lecture Times: M W F 9-9:50 am
Lab Times: Th 8-12 am or Th 2:30-6:30 pm
Office Hours: 10 -11 am 2-3 pm Monday, Wednesday, and Friday
Also by appointment and on a "drop-in" basis.

Textbook: Becker, Kleinsmith, and Hardin. (2003) The World of the Cell (Fifth Edition).
Benjamin Cummings: San Francisco.

Lab Manual: Lab exercises will be provided in the form of handouts.
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Class Description and Philosophy

Cell biology is the study of the structure and function of prokaryotic and eukaryotic cells. In this course we will examine many different areas of cellular biology including: the synthesis and function of macromolecules such as DNA, RNA, and proteins control of gene expression membrane and organelle structure and function bioenergetics and cellular communication. Examples of relevant human disorders will also be used to help the student understand what happens when cells don't work as they should! Laboratories will focus both on exercises that help illustrate cellular phenomena, as well as on the introduction of techniques and procedures commonly utilized in modern cell and molecular biology research. The development of critical thinking processes and proficiency in scientific reading and writing will be emphasized throughout the course.
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Lecture Schedule:

August
26 Introduction (Chapter 1)
28 Cell Chemistry (Chapter 2)
30 Macromolecules: Part I (Chapter 3)

September
2 Labor Day: No Classes !
4 Macromolecules: Part II (Chapter 3)
6 Cells and Organelles (Chapter 4)
9 Microscopy Techniques (Appendix)
11 Bioenergetics (Chapter 5)
13 Enzymes (Chapter 6)
16 Discussion Session
18 Lecture Exam #1
20 Cell Membranes (Chapter 7)
23 Membrane Transport (Chapter 8)
25 Electrical Signaling (Chapter 9)
27 Signal Transduction: Part I (Chapter 10)
30 Signal Transduction: Part II (Chapter 10)

October
2 Extracellular Structures (Chapter 11)
4 Discussion Session
7 Lecture Exam #2
9 Intracellular Compartments and Trafficking: I (Chapter 12)
11 Intracellular Compartments and Trafficking: II (Chapter 12)
14 Chemotrophic Energy Metabolism (Chapter 13)
16 Aerobic Respiration (Chapter 14)
18 Photosynthesis: Part I (Chapter 15)
21 Photosynthesis: Part II (Chapter 15)
23 Discussion Session
25 Lecture Exam #3
28 Chromosomes and the Nucleus: Part I (Chapter 16)
30 Chromosomes and the Nucleus: Part II (Chapter 16)

November
1 Cell Cycle (Chapter 17)
4 Control of Cell Cycle and Cancer (Chapter 17)
6 Meiosis and Gamete Formation (Chapter 18)
8 Recombination and Genetic Variability (Chapter 18)
11 Discussion Session
13 Lecture Exam #4
15 Transcription and the Genetic Code (Chapter 19)
18 RNA Processing (Chapter 19)
20 Translation and Protein Sorting (Chapter 20)
22 Regulation of Gene Expression: Part I (Chapter 21)
25 Regulation of Gene Expression: Part II (Chapter 21)
27 No Class: Thanksgiving Break !
29 No Class: Thanksgiving Break !

December
2 Cytoskeleton (Chapter 22)
4 Cell Motility (Chapter 23)
6 Last Day Class - Discussion Session

Final Exam:
Wednesday, December 11 9-11 am

The final exam is partly cumulative. A list of terms and concepts for the final will be distributed in early December.
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Laboratories

Laboratory attendance is required. An unexcused absence from lab will result in a 10 point deduction from your lab grade. Occasionally you may be asked to set up or take down part of an experiment at other times than the standard lab period. The tentative lab schedule is listed below. We will try to follow this as closely as possible, but late-arriving orders or uncooperative "critters" may necessitate a last minute change at times. The lab handouts will be provided to you in advance. You will be expected to have read through the lab handouts carefully before coming to lab.

Lab Safety Policy:
Students are expected to demonstrate suitable laboratory conduct and to practice standard laboratory safety procedures. Approved safety glasses must be worn at all times when working with chemicals or doing procedures which may involve potential eye hazard. It is the student's responsibility to provide their own safety glasses (and/or lab coat if desired). Safety glasses can be purchased in the bookstore at a modest cost. Lab coats are available at most uniform supply stores. While a lab coat is not required, it is recommended that you do not wear "good" clothes to lab. If you must do so, cover them up with a lab coat or an old shirt. No eating, drinking, or smoking will be tolerated in the lab. Treat all chemicals, biological materials, and lab equipment with respect. Failure to do so can adversely affect one's experimental results or personal well being. Remember to report all accidents to the instructor, no matter how minor they seem to be. Additional safety rules specific to each individual lab will be specified in the relevant laboratory handout.

Lab Schedule (Tentative)

August
29 Introduction to Cell Biology Lab / Computers in Biology

September
5 Microscopy
12 Spectrophotometry: Enzyme Kinetics
19 Cell Membranes
26 Proteins I: Ion-Exchange Chromatography

October
3 Proteins II: SDS-PAGE
10 ELISA Immunoassay
17 Organelle Isolation
24 Drosophila Chromosome Squash
31 Spectrophotometry: Cell Growth

November
7 DNA I: Competent Cell Transformation
14 DNA II: DNA Extraction / Gel Electrophoresis
21 Cell Motility
28 Thanksgiving Break - No Lab !

December
5 Recitation: Final Exam Review
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Watch the video: Prokaryotic cell model (October 2022).