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Learning objectives

  • Define gene and genotype and differentiate genotype from phenotype
  • Describe chromosome structure and packaging
  • Compare prokaryotic and eukaryotic chromosomes
  • Explain why extrachromosomal DNA is important in a cell

Thus far, we have discussed the structure and function of individual pieces of DNA and RNA . In this section, we will discuss how all of an organism’s genetic material—collectively referred to as its genome —is organized inside of the cell. Since an organism’s genetics to a large extent dictate its characteristics, it should not be surprising that organisms differ in the arrangement of their DNA and RNA.

Genotype versus phenotype

All cellular activities are encoded within a cell’s DNA. The sequence of bases within a DNA molecule represents the genetic information of the cell. Segments of DNA molecules are called gene s, and individual genes contain the instructional code necessary for synthesizing various proteins, enzymes, or stable RNA molecules.

The full collection of genes that a cell contains within its genome is called its genotype . However, a cell does not express all of its genes simultaneously. Instead, it turns on (expresses) or turns off certain genes when necessary. The set of genes being expressed at any given point in time determines the cell’s activities and its observable characteristics, referred to as its phenotype . Genes that are always expressed are known as constitutive genes ; some constitutive genes are known as housekeeping genes because they are necessary for the basic functions of the cell.

While the genotype of a cell remains constant, the phenotype may change in response to environmental signals (e.g., changes in temperature or nutrient availability) that affect which nonconstitutive genes are expressed. For example, the oral bacterium Streptococcus mutans produces a sticky slime layer that allows it to adhere to teeth, forming dental plaque ; however, the genes that control the production of the slime layer are only expressed in the presence of sucrose (table sugar). Thus, while the genotype of S. mutans is constant, its phenotype changes depending on the presence and absence of sugar in its environment. Temperature can also regulate gene expression . For example, the gram-negative bacterium Serratia marcescens , a pathogen frequently associated with hospital-acquired infections, produces a red pigment at 28 °C but not at 37 °C, the normal internal temperature of the human body ( [link] ).

A photo of an agar plate with pink cells on the left and one with beige cells on the right. Both plates are labeled S. marcescens. The pink culture was grown at 28 degrees; the beige culture at 37 degrees.
Both plates contain strains of Serratia marcescens that have the gene for red pigment. However, this gene is expressed at 28 °C (left) but not at 37 °C (right). (credit: modification of work by Ann Auman)

Organization of genetic material

The vast majority of an organism’s genome is organized into the cell’s chromosomes , which are discrete DNA structures within cells that control cellular activity. Recall that while eukaryotic chromosomes are housed in the membrane-bound nucleus, most prokaryotes contain a single, circular chromosome that is found in an area of the cytoplasm called the nucleoid (see Unique Characteristics of Prokaryotic Cells ). A chromosome may contain several thousand genes.

Questions & Answers

full life cycle of plasmodium parasite
Emmah Reply
what are ways of handling sharps
namugenyi Reply
never recap or bend a sharp objects
benita
describe the process of platelet formation
Joy Reply
what is parasitic helminths
Sadiya Reply
list three categories of symbiotic relationships.
mary Reply
what's the difference between microbial intoxication and infectious diseases
dranimva Reply
microbial intoxication results when a person ingests a toxin or a poisonous substance that has been produced by a microbe while infectious disease results when a pathogen colonize the body and subsequently cause disease.
mary
weighing balance is needed in lab of microbiology for weighing?
amna Reply
It is mainly used for media preparation and product testing purpose.
Nakaweesi
thank you for your joining
nuur
what is microbiology
Shamsuddeen Reply
study of living organisms that are too small to be visible with naked eye
Marah
yes
Ahmed
microbiology is the study of microscopic organisms, either unicellular(single cell), multicellular( cell colony), or acellular (lacking cell).
munachimso
is the word Atypical or a typical bacteria. .am confused pliz help
MUWANGUZI Reply
typical bacteria
Zulpha
okay thank you what does that mean
MUWANGUZI
atypical means that it has some characters from bacteria not all characters ...but tybical means that it has all the characters that bacteria have
Reham
thank you Zulpha
MUWANGUZI
some examples please
MUWANGUZI
thank you Reham
MUWANGUZI
1) typical bacteria contain a cell wall whereas atypical bacteria usually do not contain a cell . 2) typical bacteria can be either Gram-positive or Gram-negative while atypical bacteria remain colorless with Gram staining. 3) cells of typical bacteria are large ,while cells of the atypical small
Marah
Example of atypical : Mycoplasma pneumoniae , chlamydophila pneumoniae , legionella
Marah
what is micro biology
Jauharah Reply
is the study of organisms which can't be viewed by our necked eyes
Egumat
Because it preexisting causing secondary infection after collateral damage of normal microbota
Rafaa Reply
combinations of drugs that can't be taken together and why
Grace Reply
Antagonism: the combined action is less than that of the more effective agent when used alone). All these effects may be observed in vitro (particularly in terms of bactericidal rate) and in vivo والله اعلم
Lenovo
....fermentros have 1-15litre capacity
AMAR Reply
which of the following microorganisms are classified as eukaryotic?
Semugab Reply
how do I see the list?
Melissa
what are the choices?
Melissa
dear tell us the choices
MUWANGUZI
mr semugab give us the list please
Nambi
Fungi
munachimso
what are the new discoveries of microorganisms
Ezibon Reply
bacteriology,viriology,micrology
Egumat

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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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