<< Chapter < Page Chapter >> Page >

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

what is the size of virus
Beatrice Reply
What is the difference between TVC and Bioburden test
Mohamed
?
Mohamed
structure of bacteria and 10 types
Jennifer Reply
what is accidental host?
Domingo Reply
what is endomembrane system
Ikpi Reply
what is human anatomy
Ikpi
okay. Go ahead and ask
Blessing Reply
Industrial microbiology mcq
mohamed
Okay. What's your question?
Blessing
life arises from living matter or live organism.
Swami Reply
yes
sildra
I think live matter arises from non living matter
sildra
I dont think so...can u explain with an example
Manya
living maters made by non living matters
sildra
non living matters like stones? rocks?
Manya
eno
sildra
no
sildra
then?
Manya
cells are made by C N O minerals etc
sildra
I mentioned these as non living maters
sildra
that's all
sildra
cells are made up of those things but they originate from living things..
Manya
ok
sildra
Ok..good chat:-)
Manya
where are you from
sildra
thanks
sildra
India...u?
Manya
Tamil nadu
sildra
I am from Maharashtra
Manya
what about your studies
sildra
completed bsc.. preparing for msc entrance...wbu?
Manya
same
sildra
are you microbiologist
sildra
yes i am
Manya
good
sildra
what s the scope for micro in ur state?
Manya
did you find your college to higher studies
sildra
have to give an entrance exam for every college here...so lets c
Manya
food industries, medical lab, vaccine industries ,etc
sildra
hoping for pune University...wbu?
Manya
great!
Manya
is that centeral University right
sildra
what is your namr
sildra
hello
Udhaya
Hi
Maruf
Family kindly help me with this question? 1) Shortlist the configurative measurements of the following human anatomical ranges of÷ - Blood ( haemeglobin) in both male and female - Haematocytes in both male and female - Hepatocytes in both male and female - Lymphocyte / T. Lymphocytes in both male
Gifted
My names are Gift Mwale and am a Zambian. Kindly help me with this research which goes like this... 1) Shortlist the configurative measurements of the following human anatomical ranges of ÷ - Blood ( haemeglobin) in both male and female - Hepatocytes in both male and female - Haematocytes in both
Gifted
please what is the full meaning for TCDS
UDEME
from a single cell
Freedom
tcds means transcranial direct current stimulation...in this small electric currents are given to brain( specific parts) to help increase brain performance or to help with depression.. current should be in range 0.5-2.0mA
Manya
what's underlying disease relating unsanitary diet microorganism with the highest rate of epidemology solution and efficacy leading molecules elucidated structural solutions
feven
please can anybody talk about brain tumour and its cure.
BELLO Reply
enlargement of the thyroid gland resulting in over production of hormone.
Kamal Reply
What can u say on Thyroid Cancer?
Abdulkareem
Please, talk about the thyroid cancer.
BELLO
explain the Grave's disease
John Reply
what is cell
Avi Reply
is unit of life
Kamaluddeen
Ok
mohamed
who is an industrial microbiologist
Cynthia Reply
I want to know the biochemical composition of bacteria
Josh Reply
It contains peptidoglcon, DNA nd RNA
Asiya
what are Carrier protein
Ikpi
bacteriophage disadvantage
Momina Reply
disease due to __________ abnormalities are termed primary immunodeficiencies
Tayee Reply
Some primary immunodeficiencies are due to a defect of a single cellular or humoral component of the immune system.
Prince
Examples of primary immunodeficiencies include: chronic granulomatous disease, X-linked agammaglobulinemia, selective IgA deficiency etc
Prince
thank you
Nana
explain microbial mutation
Emerald
what is mutation
Cynthia Reply
alteration in genetic makeup
UDEME

Get the best Microbiology course in your pocket!





Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Microbiology' conversation and receive update notifications?

Ask