Cell Junctions and Cell Cycle

Intercellular space in closely packed tissue is

about 20nm. The cells are bound together by the specific adhesive glycoprotein.

Epithelial cells adhere to each other through glycoproteins called Cadherins

Modified cell membranes contributing in cohesion and communication are called Cell junctions

Cell Junctions

There are three types of Cell Junctions

1. Occluding Junctions 2. Adhering Junctions3. Communicating Junctions

Types of Cell Junctions

Found in epithelial tissues Also known as “Tight Junctions” Do not allow passage of small molecules form

impermiable membrane. Types:

Zonula Occludens Fascia Occludens

Occluding Junctions

Encircles the entire cell perimeter Occludes the intercellular space Series of focal fusions The adjacent cell membranes approach each

other, outer leaflets fuse, diverge again then fuse again

At fusions sites specific trans membranous proteins named (Occludins, and Claudins) perform the binding function

Less in PCT and more in the intestinal mucosa

Zonula Occludens

A strip like tight junction of limited extent Found between the endothelial cells of the

blood vessels

Fascia Occludens

Anchoring junctions Provide cell-cell or cell to basal lamina

adherence Types:

Zonula adherens Fascia adherencs Macula adherens (Desmosomes) Hemidesmosomes

Adhering Junctions

A belt like junction No fusion of cell membranes Trans membranous glycoprotein “E-cadherin”

occupies intercellular gap E-cadherin links to adherent proteins in

cytoplasm which are: Catenin Vinculin

Zonula Adherens

Structurally it is similar to Zonula adherence But its cell junction is strip-like and (not ring-

like or belt-like) i.e. Cardiac muscle cells.

Fascia Adherens

Macula adherins are commonly known as

desmosomes “Spot-weld” like junctions Randomly distributed along lateral plasma

membranes of the cells in simple epithelium In stratified epithelium it is distributed throughout

the plasma membrane It is also found in cardiac muscle cells

Desmosomes

Cell membrane in the region of junctions are

seen further apart (30mm) than the usual gap Electron dense attachment plaques are located

opposite to each other on the cytoplasmic aspects

Intermediate filaments of the cytoskeleton are anchored to the attachment plaques

Two types of transmembranes glycoproteins named Desmocolins and Desmogleins provide adherence

Desmosomes

These junctions serve to anchor the epithelial

cells to the basal lamina A hemidesmosome is a spot like adhering

junction which gives appearance of a half desmosome

In hemidesmosome transmembrane linker proteins are integrins

The cytoplasmic intermediate filaments of keratin are inserted in to the attachment plaque

Hemidesmosomes

Characterized by presence of minute tubular

passageways Provide direct cell to cell communication Tubular passages allow movement of ions and

other small molecules between adjacent cells

Communicating Junctions

Gap junction also called the “Nexus” which are

communication junctions, occur frequently between the epithelial cells

Also found in cardiac muscle cells, smooth muscles, neurons, astrocytes, and osteocytes

Plasma membrane of the adjoining cells are closely opposed with a gap of only 2nm

The gap junction contains closely packed numerous tubular intercommunicating channels

Gap Junction

The lumens of the channels of gap junction

have an average diameter of 1.5nm These channels permit free passage of ions,

sugar and amino acids In cardiac and smooth muscles the gap

junction provides electrical coupling of the adjacent cells

Gap junctions are frequently found in embryonic cells

Gap Junctions

Static cell population The cells incapable of mitotic division i.e.

Neurons Cardiac Cells

Stable Cell population Division occurs on stimulation as a result of an injury or a disease Renewing Cell population Undergoes regular mitotic division i.e. epidermis of skin and epithelial lining of GIT

Cell Renewal

Cells belonging to renewing population undergoes a sequence of

events which are repeated over and over again

The cycle is divided in to two parts

M PHASE: in which mitosis occurs (30 to 60 minutes) INTERPHASE: it is intervening period between two cell divisions

consist of three sub phases

1- The G1 Phase (GAP-1) - During this phase synthesis of RNA and proteins occur s - Cell size is restored to normal - The duration of G1 is about 8 hours

Cell Cycle

2- The S-Phase: - During this synthesis of DNA takes place - It results in preparation of exact replica of genetic material and duplication of centrioles - Duration is 8 hours

3- G2 (GAP-2) Phase: - It is period between the end of S phase and beginning of mitosis - During this process production and accumulation of energy for mitosis takes place - Duration is 2 to 4 hours

Cell Cycle

Cell Division

Mitosis

• The process of cell division which results in the production of two daughter cells from a single parent cell.

• The daughter cells are identical to one another and to the original parent cell.

Mitosis can be divided into stages

1. Interphase2. Prophase3. Metaphase4. Anaphase5. Telophase

Interphase

The cell prepares for division

DNA replicated Organelles replicated Cell increases in size

Chromosomes become visible under LM Threads become shorter and thicker consist of

two chromatids joined by centromere Nucleoli disappears Centrioles separates and migrate to each pole and starts giving out mitotic spindle

ProphaseThe cell prepares for nuclear

division

Chromosomes line up at the center of the cell Spindle fibers attach from daughter cells to

chromosomes at the centromere Equatorial plate is formed Microtubules of mitotic spindle are attached at centromere Microtubules exert pull on chromosomes

MetaphaseThe cell prepares chromosomes for

division

Spindle fibers pull chromosomes apart ½ of each chromosome (called chromatid)

moves to each daughter cell Chromatids separate and move to respective

poles as an independent chromosome In human cell two identical sets of 46 chromosomes move to the opposite poles

AnaphaseThe chromosomes divide

A constriction called cleavage furrow

appears in the middle of elongated cell Nuclear envelop is formed enclosing

chromosomes 2 nuclei form Cell wall pinches in to form the 2 new daughter cells

TelophaseThe cytoplasm divides

Mitosis -- ReviewInterphase

                                              

              

Prophase

                                             

               

Metaphase

                                              

              

Anaphase

                                             

               

Telophase

                                              

              

Interphase

                                             

               

MEIOSIS

• Meiosis is the type of cell division by which

germ cells (eggs and sperm) are produced. • One parent cell produces four daughter cells. • Daughter cells have half the number of

chromosomes found in the original parent cell

Meiosis

During meiosis, DNA replicates once, but the

nucleus divides twice. Four stages can be described for each division

of the nucleus.

Meiosis

Meiosis I

First division of meiosis

Prophase is much longer consisting of five stages

1. Leptotene: Chromosomes becomes visible in the nucleus 2. Zygotene: Homologus chromosomes come together

along their entire length and synapses are formed 3. Pachytene: Chromosomes become thicker and shorter

Each chromosome pair is called bivalent4. Diplotene: Chromosomes began to separate along their

length. Each bivalent consists of four chromatids 5. Diakinesis: Separation of chromosomes continue.

Nucleolus and the nuclear envelop disappears

Prophase

A spindle of microtubules is produced by

centrioles Equatorial plate is formed The bivalent chromosome pairs align in the

centre of the spindle

Metaphase

Chromosomes of homologous pairs completely

separates and move to the opposite poles No division of centromere occurs and the

whole chromosomes move to opposite poles

Anaphase

Nuclei are reconstructed The parent cell is divided in to two daughter

cells Each daughter cell contains haploid (23)

chromosomes Each chromosome is double structured

consisting of two sister chromatids

Telophase

Meiosis II

Second Division of Meiosis

Meiosis II

Differences in Mitosis &

Meiosis Mitosis

Asexual Cell divides once Two daughter cells Genetic information

is identical

Meiosis Sexual Cell divides twice Four haploid

daughter cells Genetic information

is different

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