Download Molecular Cell Biology Prof. D. Karunagaran Department of

Molecular Cell Biology
Prof. D. Karunagaran
Department of Biotechnology
Indian Institute of Technology Madras
Module 1
Prokaryotic and Eukaryotic Cells
Lecture 2
Eukaryotes
Morphological diversity of Eukaryotes
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Eukaryotes are characterized by the presence of a well-defined nucleus
surrounded by a nuclear envelope.
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Cell membrane and cytoplasm are present in eukaryotic cells.
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Obviously, eukaryotic cells have more complex organization in terms of structure
and function.
Structure of a typical plant cell
Animal cell structure
Cell membrane
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Some cells can keep changing their shape while others keep a constant shape.
Ameba and leukocytes change their shapes often.
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Nerve cells and plant cells do not change their shapes often.
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Functional adaptations, adjacent cells, surface tension, viscosity of protoplasm,
rigidity of cell membrane etc are the factors affecting the shape of a cell.
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Cell membrane is about 6-10 nm in thickness.
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Consists of a lipid bilayer with proteins extrinsic and intrinsic to the membrane.
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Entry and exit of materials are controlled by the membrane.
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Water and large molecules enter the cell by the process of endocytosis.
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The cell products exit out by the process of exocytosis.
Structure of a plasma membrane
Cell wall
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Cell wall provides rigidity to the plant cells. Cellulose is the major component.
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Communication between plant cells takes place via some tunnels called
plasmodesmata that interconnect two cells.
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In animal cells there is no cell wall but there is a cell coat made up of
glycoproteins, glycolipids and polysaccharides that control the thickness of the
cell membrane.
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In addition the cell coat is involved in molecular recognition between cells and
also contains enzymes and antigens.
Nucleus
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Nucleus is an important component of eukaryotic cells and its shape is generally
spheroid but can be ellipsoid in certain cells.
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Size of the nucleus depends upon the no. of chromosomes and thus on the DNA
and protein content.
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Mononucleate, Binucleate and poynucleate cells are present in tissues.
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Nucleus stores the genetic information. DNA duplication and transcription take
place in the nucleus.
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The nuclear envelope has two membranes interrupted with nuclear pores.
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Nucleoplasm contains the uncondensed chromatin that is essentially made up of
DNA and proteins.
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In the nondividing cell the dispersed form of chromatin is present called
euchromatin.
Nucleolus
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Certain regions of chromatin close to the nuclear envelope and attached to the
nucleolus are actually condensed and called heterochromatin.
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Nucleolus is spheroidal in shape and is the location in which RNA is synthesized.
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During cell division the chromatin becomes condensed with a concurrent
disappearance of nucleolus and nuclear envelope forming the chromosomes
(Chroma – color; soma – body).
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DNA is associated with basic proteins called histones forming the nuclear bodies
or nucleosomes of about 8.5 nm.
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A thick chromatin fiber is formed with strings of nucleosomes measuring about 30
nm.
Chromosomes
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In eukaryotes every cell contains the same no of chromosomes and so the same
amount of DNA and the same no of genes.
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This is true for all the somatic cells but the gametes have only half the no of
chromosomes.
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Somatic cells divide by the process of mitosis preserving the diploid no of
chromosomes in the daughter cells.
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Gamete formation reduces this to half the number (haploid) by the process of cell
division called meiosis.
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When the sperm and ovum unite to form the zygote diploid number is restored
and thus we inherit one haploid no of chromosomes from each parent.
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This homologous pair of chromosomes pairs up longitudinally each having two
chromatids and thus forming a tetrad.
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Thus there is a pairing of one chromatid with the other in the homologous
chromosome.
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Portions of these chromatids get exchanged during meiosis (crossing over).
Microtubules
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The cytoplasm contains several organelles.
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The microtubules, microfilaments and microtrabeculae constitute the dynamic
and spongy cytoskeleton that is important in maintaining the shape of the cell.
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It is also involved in cell mobility and the colloidal changes.
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Microtubules are thin but rigid tubular structures of about 25 nm in diameter.
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The walls of microtubules are made up of 13 filaments.
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Microtubules contain tubulin as the major protein and the tubulin subunits can
polymerize and depolymerize.
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Microtubules can constitute asters and spindle which are important during cell
division.
Centrioles
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Centrioles are cylinderical in shape and open at both ends and their walls contain
nine groups of microtubular triplets arranged in the form of a circle.
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Centrioles are doublets and each one is perpendicular to the other.
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In animal cells during mitosis centrioles are pulled to the opposite poles and are
involved in the formation of spindles.
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Plant cells do not have centrioles but still the spindle formation occurs
Cilia and flagella
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Basal bodies or kinetosomes are very similar to centrioles usually present at the
base of cilia and flagella.
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Cilia are short processes that extend into the surrounding medium and flagella
are similar but longer processes.
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Cilia are useful in locomotion for protozoan cells or helpful in moving particles in
medium such as the air passages or oviduct.
Flagella are also useful in locomotion of cells.
Filaments
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Microfilaments are thin structures consisting of actin associated with myosin and
other proteins - important in muscle contraction.
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Cell motility in nonmuscle cells also involve the interactions of actin and myosin.
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Intermediate filaments that play a mechanical role contain various fibrous
proteins and are intermediate in size between microtubules and microfilaments.
Nuclear envelope
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Nuclear envelope consists of flat sacs called cisternae
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They have two membranes and contain nuclear pores that allow transfer of
materials in and out of nucleus.
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The outer membrane is covered with ribosomes and the inner membrane
remains in touch with chromatin fibres.
Endoplasmic reticulum (ER)
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Endoplasmic reticulum is made up of tubules and flat sacs
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There are two types, rough/granular and smooth/agranular ER
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Rough ER is covered with ribosomes in which proteins are synthesized.
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Both are continuous with each other and are involved in the transport of products
and secretion.
Mitochondrion
Plastids
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Plant cells contain Plastids that are not present in animal cells
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The colorless plastids are called leukoplasts that participate in the storage and
metabolism of starch.
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Plastids containing pigments that give them a color are called chromoplasts and
the most well known in this group are the chloroplasts that contain chlorophyll,
the green pigment in plants.
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Chloroplast also has a double membrane structure and is the site of
photosynthesis.
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Its stroma is filled with a no of soluble enzymes.
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Both mitochondria and chloroplast contain their own DNA. They also have
ribosomes and RNA molecules and can synthesize some of their proteins.
Chloroplast structure
Lysosomes and peroxisomes
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Lysosomes are organelles with a single membrane containing a variety of
enzymes.
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These enzymes digest the foreign substances entering the cells through
endocytosis and also for the digestion of parts of the cytoplasm. They originate
from ER and Golgi complex.
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Peroxisomes are also organelles with a single membrane containing enzymes
concerned with production and breakdown of peroxides and have a protective
role.
Chemical components
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Chemical components of the cell are
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Water 75-85%
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Protein 10-20%
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Inorganic salts 2-3%
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Monomeric units
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Polymers/macromolecules
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Carbohydrates
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Proteins
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Lipids
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Nucleic acids
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Enzymes
Water
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Water is a good natural medium for dissolving many substances present in a cell
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Water is also in a bound form with proteins participating in hydrogen bonding
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Water has high specific heat coefficient and can absorb heat
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Salts dissociated into anions and cations are important to maintain the acid-base
balance and osmotic pressure in a cell.
DNA and the Central Dogma
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As mentioned before all living organisms contain DNA and RNA
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DNA is the genetic material that stores genetic information to be passed on to the
offspring.
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DNA is transcribed into RNA
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DNA is made up of nucleotides and three consecutive nucleotide sequences
contain the codons coding for a specific amino acid.
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Translation of RNA results in the synthesis of proteins
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DNA makes RNA makes proteins is often called the central dogma
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DNA is mainly present in the nucleus within the chromosomes but a small
amount of DNA is present in mitochodria and chloroplasts.
DNA and RNA
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Nucleic acids are made up of bases, sugars and phosphates
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Deoxy ribose in DNA and ribose in RNA
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The bases are Adenine, Thymine, Cytosine and Guanine in DNA
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The bases in RNA are Adenine, Uracil, Cytosine and Guanine
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Adenine and Guanine are purines
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Cytosine, Uracil and Thymine are pyrimidines
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Nucleotides are linked together by phosphodiester linkages
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Nucleotides such as ATP are important to store and transfer chemical energy in
a cell.
Double helix structure:
RNA
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The structure of RNA varies from DNA in that it has ribose as the sugar instead
of the deoxy ribose in DNA. In addition it is single stranded.
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mRNA carries the information on the sequence of amino acids
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tRNA identifies and transfers the correct amino acid to the ribosomes
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rRNA is present in ribosomes
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RNA forms hair pin loops with the help of base pairing
Carbohydrates
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Carbohydrates are the main sources of energy in living cells
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They are also components of cell wall and intercellular materials
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Monosaccharides are simple sugars triose, pentose, hexose etc
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Glucose is hexose and is important source of energy
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Galactose, fructose and mannose are other hexoses
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Disaccharides areMaltose, Lactose and sucrose
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Polysaccharides are Starch, cellulose and glycogen.
Mono, Di and Polysaccharide
Glycoproteins
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Glycoproteins are proteins having covalent linkage with monosaccharides such
as galactose, mannose, fucose, N-acetyl glucosamine and sialic acids.
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On the protein side the linkages are through the amino acids asparagine, serine
or threonine.
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Cellular glycoproteins are present in the cell membranes that participate in the
cell recognition and membrane interactions.
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Secreted glycoproteins include serum glycoproteins, thyroglobulin, ovalbumin,
ribonuclease and deoxyribonuclease.
Lipids
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Lipids are relatively less soluble in water and soluble in organic solvents. Simple
lipids are esters of fatty acids.
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Natural fats are triglycerides that are triesters of fatty acids and glycerol.
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Triglycerides are used as storage forms of energy in adipose tissues
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The energy is derived from the oxidation of fatty acids.
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Compound lipids are more complex and yield many components upon hydrolysis
other than fatty acids and alcohol. They are important components of cell
membrane.
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Phospholipids are important components of the cell membrane. They have two
fatty acids esterified with glycerol and the hydroxyl group of glycerol is esterified
with phosphoric acid bound with choline or ethanolamine or serine or inositol.
Complex lipids
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Glycolipids and sphingolipids are combinations of glycerol with sphingosine.
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Sphingomyelin is an important component of nerves (myelin sheath).
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Cerebrosides contain galactose or glucose.
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Sulfatides contain sulfuric acid esterified to galactose.
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Gangliosides are complex forms of lipids containing sphingosine, fatty acids,
carbohydrates (lactose and galactosamine) and neuraminic acid.
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They are present in cell membranes and may act as receptors of viruses.
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Steroids are also a form of lipids containing a complex structure.
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Best example is cholesterol, estrogen, progesterone and corticosterone.
Proteins
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Amino acids are the building blocks of proteins.
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There are 20 different types of amino acids present in proteins.
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Primary structures of proteins are characterized by the sequence of aminoacids.
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Secondary structure is by the formation of alpha helix or beta pleated sheet
stabilized by the hydrogen bonds.
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Other segments of a protein can adopt to a random coil structure.
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Tertiary structure refers to the 3 dimensional arrangements as globulins or
fibrous proteins.
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Quaternary structure is the arrangement of protein subunits.
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Enzymes are also proteins and act as biocatalysts.
Amino acids present in proteins
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This is only an outline of the various features of a typical eukaryotic cell.
Study Questions
1. Bring out the differences between the structural features of plant and animal cells.
2. How is RNA different from DNA?
3. Secondary structure of proteins is characterized by the presence of
a) globular shape b) alpha helix or beta pleated sheets c) disulphide bonds
d) amino acid sequence
4. Match the following
Ganglioside
Sucrose
Disaccharide
Cellulose
Tyrosine
Sphingosine
Cell wall
Amino acid
5. Rough endoplasmic reticulum contains ---------- whereas smooth endoplasmic
reticulum is ----------------.