BIOL 1003 Lecture Notes - Lecture 2: Hydrogen Hypothesis, Peptide Nucleic Acid, Protein Biosynthesis

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22 Nov 2017
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PROKARYOTIC CELLS
Prokaryotic cells were the first form of life on Earth, characterised by having vital biological
processes including cell signaling and being self-sustaining. They are simpler and smaller than eukaryotic
cells, and lack membrane-bound organelles such as the nucleus. Prokaryotes include two of the domains of
life, bacteria and archaea. The DNA of a prokaryotic cell consists of a single chromosome that is in direct
contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes
are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter.[13]
A prokaryotic cell has three architectural regions:
Enclosing the cell is the cell envelope generally consisting of a plasma membrane covered by a
cell wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most
prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma
(bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives
rigidity to the cell and separates the interior of the cell from its environment, serving as a protective
filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior
forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a
hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various
sorts of inclusions.[3] The genetic material is freely found in the cytoplasm. Prokaryotes can carry
extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids
have been identified in several species of spirochete bacteria, including members of the genus Borrelia
notably Borrelia burgdorferi, which causes Lyme disease.[14] Though not forming a nucleus, the DNA is
condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
On the outside, flagella and pili project from the cell's surface. These are structures (not present in
all prokaryotes) made of proteins that facilitate movement and communication between cells.
EUCARYOTIC CELLS
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Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about
fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume.
The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the
presence of membrane-bound organelles (compartments) in which specific metabolic activities take place.
Most important among these is a cell nucleus,[3] an organelle that houses the cell's DNA. This nucleus
gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:
The plasma membrane resembles that of prokaryotes in function, with minor differences in the
setup. Cell walls may or may not be present.
The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are
associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the
cytoplasm by a membrane.[3] Some eukaryotic organelles such as mitochondria also contain some DNA.
Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in
chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as a sensory cellular
antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling
to ciliary motility or alternatively to cell division and differentiation."[15]
Motile cells of eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers
and flowering plants.[16] Eukaryotic flagella are less complex than those of prokaryotes.
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CELL COMPONENTS
Membrane
Main article: Cell membrane
The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm
of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and
prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from
its surrounding environment and is made mostly from a double layer of phospholipids, which are
amphiphilic (partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or
sometimes a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules
that act as channels and pumps that move different molecules into and out of the cell.[3] The membrane is
said to be 'semi-permeable', in that it can either let a substance (molecule or ion) pass through freely,
pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor
proteins that allow cells to detect external signaling molecules such as hormones.
Cytoskeleton
Main article: Cytoskeleton
A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red,
and microfilaments are stained green.
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps
during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter
cells after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic
cytoskeleton is composed of microfilaments, intermediate filaments and microtubules. There are a great
number of proteins associated with them, each controlling a cell's structure by directing, bundling, and
aligning filaments.[3] The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance
of cell shape, polarity and cytokinesis.[18] The subunit protein of microfilaments is a small, monomeric
protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate
filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of
the subunit protein of intermediate filaments include vimentin, desmin, lamin (lamins A, B and C), keratin
(multiple acidic and basic keratins), neurofilament proteins (NFL, NFM).
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