Lecture 2 - Molecular and Cell Biology

• relatively simple structure, ease and speed of growth [divides every 20-60 min], clonal populations can be easily isolated on soft agar, excellent models for studyting fundamental aspects of cellular biochemistry and molecular biology.
• Genome 4 million base pairs - encodes about 4000 different proteins
• In contrast - the human genome is about 1,000 times more complex and encodes ~ 100,000 different proteins HUMAN GENOME PROJECT

Yeasts

• E. coli not useful for studying aspects of cell structure/function or molecular processes which are unique to eukaryotes.
• Yeasts - the simplest eukaryotes
• easy to grow, fairly rapid reproduction [once every 2 hours] etc.
• Most studied species - Saccharomyces cerevisiae
• Genome contains ~14 million base pairs [roughly 3 x that of E. coli] 16 linear chromosomes
• Exhibits the typical features of a eukaryotic cell although much more simple than most eukaryotes.
• Easily grown as a colony on agar - different nutrient requirements
• General principles seem to apply tø all eukaryotic cells.

Dictyostelium discoideum

• cellular slime mold
• 10X the genome of E. coli
• easily grown, amenable to genetic manipulation
• If plentiful food - it grows as a single-celled amoeba, feeding on bacteria and yeasts
• Highly mobile - good model for animal cell movement
• Aggregation if adequate food supply not available - forms wormlike structures called slugs composed of ~ 10,000 individual cells
• Model for cell signaling and cell/cell interactions

Caenorhabditis elegans [aka C. elegans]

• nematode (round worm)
• one of the most widely used models for studies of animal development and cell differentiation
• genome 100million base pairs
• adult worms are composed of 959 somatic cells + 1000 to 2000 germ cells
• Easily grown
• embryonic origin and lineage of ALL the cells has been traced

Drosophila melanogaster

• fruit fly
• crucial model organism in developmental biology and genetics
• Genome similar in size to C. elegans
• 2 week reproductive cycle
• many many genes analyzed in detail
• has led to striking advances in understanding the molecular mechanisms that govern animal development.
• ESPEC. formation of body plan of compelx multicellular organisms.

Arabidopsis thaliana

• Plant - contributed greatly to our understanding of plant development and plant molecular biology
• genome 70 million base pairs
• ~ 5X that of yeast and similar to Drosophila and C. elegans
• easy to grow
• easy to derive mutants, etc.

African clawed frog - Xenopus laevis

• large eggs, large #s of eggs - develop outside the mother
• complete cycle in lab

Laboratory mouse

• inbred strains which are genetically identical

• specific genes have been introduced into the mouse germ line, so that their effects on development or cell function can be studied

• specific genes have been deleted in the mouse germ line

• All cells are composed of molecules
• MOLECULE a collection of atoms held together by chemical bonds
• The water molecule is a collection of two atoms of hydrogen and one atom of oxygen.

The molecules which cells are made of fall into two general classes: small molecules and large molecules

1. The smaller molecules fall into four general classes: simple sugars, fatty acids, amino acids, and nucleotides.

2. The larger molecules are:
   • synthesized from the small molecules
   • small molecules which form the subunits of the structure
   • 4 types of macromolecules:
     1. polysaccharides (assembled from simple sugars)
     2. lipids (assembled from fatty acids)
     3. proteins (assembled from amino acids) *most emphasis
     4. nucleic acids (assembled from nucleotides) *most emphasis
   • These larger molecules are POLYMERS
     As a general rule, a biological polymer is assembled by a process in which individual subunits are added one by one to a chain. Addition of each subunit to the polymeric chain involves the formation of a COVALENT BOND (instrinsically stable under physiological conditions. Each polymerization reaction is accompanied by the loss of a water molecule for every subunit added, giving rise to the name CONDENSATION REACTION (otherwise known as DEHYDRATION SYNTHESIS). To reverse the polylmerization reaction - add water molecule HYDROLYSIS.
   • Proteins and nucleic acids are very large molecules MACROMOLECULES
   • Nucleic acids carry genetic information
   • Proteins provide the means of executing the genetic information
   • For both, the sequence in which the individual building blocks are joined together is the critical factor that determines the property of the resulting macromolecule.
   • Polysaccharides also may be large enough to qualify as macromolecules, although they do not have the complexity of proteins and nucleic acids.
   • Lipids may also be large but again lack the complexity of proteins and nucleic acids.

We have talked about large, and small molecules, how does one measure molecular size?
Basically three ways:

1. -length
   • - based on the metric sytem Used more commonly in microscopy and cell biology
   • 1 nanometer (nm) = 10-9 meters at the molecular level
   • 1 micrometer (um) = 10-6 meters
   • 1 micrometer (um) = 1,000 nanometers at the cellular level
   • Another dimension 1 nanometer = 10 Angstroms ( 1 Angstrom = 0.1 nanometer)
   • To give you an idea of relative sizes:
     • prokaryotic cells are approximately 1 um (or 1000 nm) in diameter
     • eukaryotic cells are approximately 10 - 50 um (or 10,000 to 50,000 nm) in diameter
     • protein (polypeptide) .002 um to .010 um or 2-10 nm in length as far as molecules go, quite large

2. -molecular weight
   • molecular mass
   • very useful measurement
   • unit dalton
   • dalton = mass of one hydrogen atom
     • so, the molecular mass of hydrogen is = 1 dalton
     • the molecular mass of carbon is = 12 daltons
     • the molecular mass of IgG= 150,000 daltons
   • kilodalton = 1000 daltons
     • the molecular mass of IgG = 150 kDa

3. -number of monomers of a polymer
   • number of monomers
   • unit of a protein is an amino acid (# of amino acids)
   • unit of a nucleic acid is nucleotide (# of nucleotides) - used commonly as number of bases

Comments or questions should be directed to Cheryl.Davis@wku.edu
Last Modified: August 16, 1997
All contents copyright (C) 1997.
Western Kentucky University.