2013 Lecture 3 - Cell Organization - II

7/27/2019 2013 Lecture 3 - Cell Organization - II

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Lecture 3

EukaryoticCell Structure


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Eukaryotic cells

` The main feature of eukaryotic cells is theirpossession of membrane-bound organelles` a structure or part that is enclosed within its own

membrane inside a cell and has a particularfunction(s)` more elaborate compartmentalisation of the cell

`

These organelles have specific roles defined bythe chemical reactions that take place withineach

` Characteristic of the` Protists` Fungi` Plants

` Animals


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Prokaryotes cf. Eukaryotes

PROKARYOTES EUKARYOTES

Members of the domainsArchaea and Bacteria. Members of the domainEukarya.

Appeared 3.5 billion years

ago.

Appeared 1.7 billion

years ago.Greater diversification

over longer existence;

more metabolic variationand diversity.

Lesser diversification

over shorter existence;

less metabolic variationand diversity.

Majority of all living

things.

Tiny minority of all

living things.


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Unicellular. Unicellular and multicellular

forms.Typically 0.2-2.0 m. Typically 10-100 m.

Surface area to volume ratio

high; so metabolic rates high,and generation times short.

Surface area to volume ratio

low; so metabolic rates low,and generation times are

long.

No nucleus/nuclearmembrane; genetic material

in cytoplasm (nucleoid).

True nucleus, with nuclearmembrane.


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Membrane-enclosed

organelles absent.'Internal' membranes, if

present, are invaginations

of plasma membrane.

Membrane-enclosed

organelles present; eachwith particular role in

cell.

Temporal, not physical

compartmentalisation:

lower efficiency of cellularfunctions.

Physical

compartmentalisation:

gives division of labourand greater efficiency.

Cells are less structurally

and functionally complex.

Cells are more complex.


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Low DNA complexity; only

a single, loose, circular DNAmolecule and no histones.

High DNA complexity; DNA

divided into multiple tightly-bound, linear bundles

(chromosomes), and

associated with histones

(proteins involved in DNA

replication and gene

expression).

Satellite DNA structures,called plasmids, are

common; store many

important genes.

Plasmids are rare ineukaryotes.


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Circular DNA molecule if prokaryote

(left) and human chromosomes (right).


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Eukaryotic DNA is coiled around groups (cores) of proteinaceous histones.


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Cells contain only one copy

of every gene (haploid, n).

Cells contain two (diploid,

2n) or more (polyploid; e.g.,4n) copies of every gene.

Reproduction usually

asexual (withoutinvolvement of gametes).

Both sexual and asexual

reproduction found.

Sexual reproduction rare;

does not involve meiosis.(Haploid cell cannot divide

to produce gametes with

half its gene complement.)

Sexual reproduction

common and involvesmeiosis. (E.g., one 2n cell

goes to one 4n cell, then to

four n cells, or gametes.)


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Cell division involves binary

fission. (One n cell to one

2n cell, and then to two n

cells.)

Cell division is more complex

and by mitosis. (E.g., one 2n

cell to one 4n cells, and

then to two 2n cells.)


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Binary fission in bacteria. Mitosis in eukaryotes.


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Ribosomes are small (70S). Ribosomes are larger (80S).


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Cytoskeleton is either

absent or poorly developed

in cytoplasm; no or poor

cytoplasmic streaming.

Well developedcytoskeleton in cytoplasm

(with microtubules,

microfilaments, andintermediate filaments);

plays role in cell

organization and shape, andin cytoplasmic streaming.


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Fluorescentmicrograph

showing

cytoskeletalcomponents

in eukaryotic

epithelialcells.


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Cytoskeletal components form scaffolding and conveyor

belts within cell, holding components in place and moving

substances around.


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Cell wall usually present and

is chemically complex; in

most bacteria (but not

archaea), it contains

peptidoglycan (a single large

polymer of protein and

carbohydrate).

Cell wall, when present, ischemically simple and has no

peptidoglycan (e.g., cellulose

cell wall of plant cells).


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` Eukaryotes are composed of four kingdoms:

1. Kingdom Protista.

2. Kingdom Plantae.3. Kingdom Fungi.

4. Kingdom Animalia.

Protista Plantae Fungi Animalia


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Organelles

` Main feature of eukaryotic cells is their

possession oforganelles, i.e., membrane boundcompartments:

` inside the cell; and

` in which specific chemical reactions occur.


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Organelles

` Each organelle type has particular role within

the cell:

` Defined by the types of chemical reactionsorganelle can carry out.

` Organelles may be bound by:` A single membrane (e.g., Golgi, vesicles).

` A double membrane (e.g., nucleus,

mitochondria, chloroplasts).


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Organelles

Golgi apparatus (each compartment

surrounded by a single membrane).

Mitochondrion (surrounded by

double membrane).


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Organelles

` Several membrane-less structures sometimes

called organelles.

` They include:

1. Ribosomes (sites of protein synthesis).

2. The cytoskeleton` protein filaments

` structural and functional roles.

3. The extracellular matrix

` also has structural and functional roles.


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Organelles

` Advantages of compartmentalisation:

1. Division of labour between compartments:

2. Cell not a 'Jack of All Trades.'

3. Organelle's molecules kept away from other

molecules in cell:4. Prevents inappropriate reactions.

5. Allows many (sometimes incompatible) reactions

to coexist in same cell.

6. Allows greater diversity in resources cell can use.


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Organelles

` Advantages of compartmentalisation (contd):

7. Reactions can proceed more efficiently:

8. Chemicals could be concentrated withinparticular regions of the cell.

9. Environment within organelle can be madeoptimal for functions it performs (e.g., different

pH from rest of cell).

10. Membrane can regulate organelles uptake of rawmaterials and expulsion of wastes:

` Greater control of individual processes.


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Organelles

` Advantages of compartmentalisation (contd):

11. Different parts of a multistep process can

occur in different cellular compartments:` e.g., gene transcription takes place in nucleus

while translation occurs at ribosomesimultaneously.

` Greater control of overall process.


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Organelles

` Efficiency of compartmentalisation led to:

1. Greater variety and efficiency in

eukaryotic cell functions.

2. Specialisation of different types of cells

into tissues in multicellular eukaryotes.


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Organelles

Tissues of plant leaf, each with specialised cell types.


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Origin of organelles

` Theorised that organelles developed via the

process ofendosymbiosis:

1. Infolding of the plasma membrane, or

2. One cell ingested another (host), giving rise to

a permanent mutualism.` Eventually the ingested cell:

1. Lost some of its functions.2. Transferred many of its genes to host's DNA.

3. Became an organelle within the host cell.


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Origin of organelles

Endosymbiotic origin of mitochondria and chloroplasts.


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Nucleus

` Largest organelle in the cell` Double membrane:

` Associated with RER` Disintegrates for cell division` Nuclear pores

` ~ 9nm` Selective communication with cytoplasmprovide aqueous channels through the nuclear

envelopecomposed of multiple proteins, collectively referred

to as nucleoporins

` Transport of ribosomes, nucleotides & proteins


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Nucleus

` Site of DNA storage` Genetic control of cell metabolism

` Nucleolus produces ribosomes & RNA` The nuclear lamina is a dense`

~ 30 - 100 nm thick` fibrillar network composed of intermediate

filaments made oflamin that lines the

inner surface of the nuclear envelope


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Nucleus

Diagrammatic representation of structure of the eukaryotic nucleus.


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Nucleus

` Nucleoplasm:` fluid contained within the nucleus of a eukaryote in

which the chromosomes and nucleoli are found` Chromatin is the ground material

` fibrous complex of eukaryotic DNA and histone proteins` The extensive DNA of eukaryotic cells is tightly bound to

small basic proteins (histones) that package the DNA inan orderly way in the cell nucleus` assembled into individual repeating units called

nucleosomes

The basic structural unit of chromatin consisting of DNAwrapped around a histone core

` Histones are involved in a range of activities, including DNAreplication and gene expression


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Nucleosome

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cooper&part=A618&rendertype=figure&id=A622


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Nucleus

`Chromatin (nuclear ground

material) consists of1. Euchromatin2.

Heterochromatin3. Chromosomes


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Nucleus

1. Euchromatin`

found in both prokaryotes &eukaryotes` loosely organised chromatin

` appears as lightly staining bands` appreciable gene activity`

undergoes transcription


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Nucleus

2. Heterochromatin` only in eukaryotes` highly condensed DNA-protein

complex

` little or no transcription or geneactivity` protect chromosome integrity


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Nucleus

`Chromosomes`highly condensed chromatin`visible as strands`The carriers of genes, consisting of

long DNA molecules and associatedproteins


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Nucleus


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Nucleus


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Nucleus

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2013 Lecture 3 - Cell Organization - II