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Definition and Classification of cell
Cell junction is the connection between the neighbouring
cells or the contact between the cell and extracellular
It is also called membrane junction.
Cell junction are classified into three types
a-Occluding junction or Tight junction
b-Communicating junction or Gap junction
Tight Junction- occluding junctions / zonulae
occludens - zonula occludens), are the closely associated
areas of two cells whose membranes join together forming
a virtually impermeable barrier to fluid.
A type of junctional complex present only in
Gap junctions are clusters of intercellular channels
that allow direct diffusion of ions and small molecules
between adjacent cells.
At gap junctions, the intercellular space narrows from
25 nm to 3 nm.
General Principles of Cell Signaling
signaling cell => signaling molecule binds to receptor molecule on target cell
Signaling Distances: => Four Types of Signaling F.16-3
The process of reproduction or formation of
new cell from pre-existing cell is referred to as cell
The period in one cycle of cell division is
completed is called cell cycle
Interphase can be subdivided into G1, S and G2
In yeast “Start” is at the end of G1; at this point the cell
is committed to DNA synthesis.
In mammals, this is called the “restriction point”. This
point late in G1 is a “checkpoint”; a cell will exit the cell
cycle if certain requirements to proceed to synthesis are
A second restriction point occurs in G2 before entry into
Interphase M Interphase M Interphase
G1 S G2 M G1 S G2 M G1 S G2
Events during G1
Preparation of chromosomes for replication
Duplication of cellular components
G1 checkpoint (or restriction point); cell commits to
division or exits from cell cycle
INTERPHASE: S PHASE
Duplication of the centrosome
The centrosome is located near the nucleus of the cell
and contains the microtubule organizing center MTOC
in animal cells. It contains two centrioles that migrate to
the poles before cell division and serve to organize the
Checkpoint (restriction point) for entry into M phase
Cell division (mitosis or meiosis for germ cells)
Can be subdivided into four subphases:
Factors that influence M phase entry
Time (During early embryogenesis, divisions may
proceed rapidly, essentially alternating M and S phases,
with little growth between them.)
Completion of DNA Replication