Cell Signaling

 Cellular level of organization:  Cell Signaling.

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Introduction:

• Cells need to interact with their environment and other cells around them. This is called Cell Signaling. 

• Single cellular organisms need to detect nutrients in their environment, and cells in multicellular organisms are involved in a complex system of communication with each other.

• Cells detect signals with Cell Receptors on their plasma membrane, which are usually Glycoproteins or Glycolipids. 

• The signaling molecule binds to the Receptor because its shape is complementary to it (Lock and Key).

• This then initiates a chain of reaction within the cell, leading to a response.

Cell Signalling Pathways: 

• Cell Signalling Pathways can be classified on the basis of distance over which the signalling occurs.

• Endocrine Signalling.

• Paracrine Signalling.

• Autocrine Signalling.

1. Endocrine Signalling :

• It involves signalling over large distances.

• Usually where the signalling molecule is transported in the circulatory system.

• Hormones are often used as cell signalling molecules in multicellular organisms. 

• Hormones are produced in special glands called “Endocrine Glands”. 

• The released hormones bind to receptor sites on a Target Cell, which starts a response.

• e.g. Use of Insulin to lower blood glucose levels. In response to high glucose levels, Beta-Cells in the pancreas release the hormone Insulin into the blood which causes lowering of elevated blood glucose levels.

2) Paracrine Signalling: 

• It occurs between cells which are close together, sometimes directly, sometimes via extracellular fluid.

• Often, cells that are near one another communicate through the release of chemical messengers (ligands that can diffuse through the space between the cells).

3) Autocrine Signaling:

• Autos meaning self.

• It is where the cell stimulates a response within itself by releasing signals for its own Receptors.

• In many cases, a signal may have both autocrine and paracrine effects.

Commonly Asked Questions.

1. What are different forms of cell signaling?

2. What is Endocrine signaling? How it works.

Cell Junctions

 Cellular level of organization:  Cell Junctions.

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Introduction:

• In many animal tissues (e.g., connective tissue), each cell is separated from the next by an extracellular coating or matrix.

• However, in some tissues (e.g., epithelium), the plasma membranes of adjacent cells are pressed together.

• Cell junctions primarily act as communicators between cells while also helping in anchoring the cell to the basement membrane.

• There are five different types of cell junctions: 

1. Gap Junctions, 

2. Adherens Junctions, 

3. Hemidesmosomes, 

4. Desmosomes, and 

5. Tight Junctions.

Tight Junction.

• Tight junctions are areas where the membranes of two adjacent cells join together to form a barrier. 

• The cell membranes are connected by strands of transmembrane proteins. 

• The cytoskeletons of the individual cells are linked through the tight junctions.

• The junctions are so tight that nothing can pass through them hence substances have to cross the cell itself.

• They are often present in the epithelial cells, and on the apical surface of the cell.

• Tight junctions bind cells together, prevent molecules from passing in between the cells, and also help to maintain the polarity of cells.

Adherens Junctions

• Also called “zonula adherens, intermediate junction, or belt desmosome".

•  These are protein complexes that occur at cell–cell junctions in epithelial and endothelial tissues.

• They are more basal to the tight junctions and completely encircle the cell.

• The primary function of the Adherens Junction is to stick to an adjacent cell or surface and provide strong mechanical attachments between adjacent cells.

Desmosomes.

• Also known as a macula adherens.

• Desmosomes are localized patches that hold two cells tightly together. They are common in epithelia (e.g., the skin and Heart). 

• Desmosomes are attached to intermediate filaments of keratin in the cytoplasm.

• They are hyper adhesive in nature.

Hemidesmosomes.

• Hemidesmosomes are found in epithelial cells connecting the basal epithelial cells to the basal lamina.

• Hemidesmosomes are also involved in signaling pathways.

• They are similar to the desmosomes but are at the base only allowing cells to anchor to the basement membrane.

Gap Junctions

• They are specialized connections between cells which connect cytoplasms of two cells.

• They act as a tunnel between two cells which helps transport important cell signals and micronutrients.

• They are present in almost all tissues except in some mobile cells like RBCs while stationary mature skeletal muscle cells.

Commonly Asked Questions.

1. Write in short about different types of the cell junctions.

Protein Synthesis

Cellular Level of Organization: 

Protein Synthesis 

Introduction:

• Proteins are responsible for the characteristics of the cell, some form its structure while some form its functional parts.

• All the information for protein synthesis is stored in the DNA of the organism.

• DNA has the instructions for making all proteins required  in an organism but…   DNA CANNOT make proteins!  DNA doesn't leave the nucleus, but proteins are made outside the nucleus in the ribosomes which are present in the cytoplasm. 

• DNA is used to build RNA.  RNA leaves the nucleus and builds proteins in the ribosome.  Proteins create an organism's physical traits and functions.

• 3 Types of RNA are made from DNA template strand:

• Ribosomal RNA (rRNA):  Used to build ribosomes.

• Transfer RNA (tRNA):  Used to carry amino acids from cytosol to ribosomes.

• Messenger RNA (mRNA):  Carries the instructions for making ONE protein from nucleus to ribosome.

RNA STRUCTURE:

• RNA has one chain (strand) of RNA nucleotides

• RNA nucleotides have a ribose sugar (not deoxyribose)

• RNA is made from 4 different RNA nucleotides:

• Adenine (A)          Uracil (U)

•  Guanine (G)         Cytosine (C).

• Uracil bonds ionically to the base Adenine.

• RNA nitrogen bases are attracted to DNA nitrogen bases (example:  RNA Uracil base is attracted to DNA Adenine base). 

PROTEIN SYNTHESIS IS A TWO STAGE PROCESS:

1. Transcription (making RNA from DNA in the nucleus).

2. Translation (using RNA to build proteins in the ribosome).

TRANSCRIPTION

• Takes place in the nucleus.

• During Transcription (in the nucleus) the instructions for building one protein is copied from a DNA template strand gene into messenger RNA (mRNA).

• TRANSCRIPTION PROCESS INVOLVES FOLLOWING STEPS:

1.  Enzyme Helicase opens the Helical structure of DNA.

2. RNA polymerase enzymes sit on DNA strands along a gene code.

3. The point where the code starts is called the Promoter end while where it ends is called  Terminator end.

4. Free RNA nucleotides floating in the nucleus bond ionically to exposed bases on the DNA template strand.

5. The RNA nucleotides that bond to the DNA template are covalently linked together to form a pre-messenger RNA molecule.

6. Pre Messenger RNA contains two regions…

1. Exons: The actual protein code.

2. Introns: The code which is not useful for a particular protein.

7. The special enzymes called  “Spliceosomes” delete the introns and join all exons together to form a mRNA. The process is called “Splicing.”

8. The messenger RNA leaves the nucleus and the two DNA strands of DNA come back together.

9.  When mRNA is made from DNA all of the 3 letter DNA words in a gene are converted to 3 letter RNA words (codons) in the mRNA molecule. 

10. A typical mRNA molecule will contain more than 100 codons, with each codon specifying the placement of one particular amino acid in a protein chain. 

TRANSLATION

• Takes place in cytoplasm.

• Messenger RNA (mRNA) brings instructions for making ONE protein from the nucleus to the ribosome.

• The mRNA binds with a small ribosomal unit.

• This binding initiates binding of a large ribosomal unit with the small ribosome unit-mRNA complex to form an assembly line for protein production.

• The large Ribosomal unit has three binding sites

• A: Attachment site.

• P: Pairing Site.

• E: Exit site.

• The completion of small ribosomal unit-mRNA- large ribosome unit complex activates movement of tRNA.

• Transfer RNA (tRNA) molecules bring amino acids from the cytosol into the ribosome to build the protein.

• Different types of tRNA bring different types of amino acid to the ribosome.

• Different types of tRNA also have different 3 base anticodon sequences that bond to mRNA “codons”. 

• The codon-anticodon binding takes place at Attachment site.

• The start codon is the first codon of a messenger RNA. It is always AUG which stands for amino acid Methionine.

• tRNA’s will only bond to mRNA where the mRNA codon complements the tRNA anticodon.

• Molecules in the ribosome covalently bond adjacent amino acids together to form the protein chain at the P site.

• The empty tRNA molecule leaves the complex at E site to bind with another amino acid.

• As the process continues, the length of amino acids grows.

• When the mRNA stop codon appears in the ribosome, the complete protein chain is released and the small ribosomal unit-mRNA- large ribosome unit complex gets separated.

Commonly Asked Questions.

1. Write a short note on “Protein Synthesis”.

2. Write a note on “translation”.

3. Write a note on “Transcription”.

Cell Division: Meiosis

 Cellular level of organization:  Cell Division: Meiosis.

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Introduction:

• Cell is defined as a basic structural and functional unit of the body.

• Cytology is a branch of science that deals with the study of cells.

• The animal cell is divided into two major parts as,

Plasma Membrane.

Subcellular organelles.

1. Nucleus.

2. Nucleolus

3. Mitochondria.

4. Golgi complex.

5. Ribosomes.

6. Endoplasmic reticulum.

7. Lysosomes.

8. Centrioles.

• The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. 

• These events include the duplication of its DNA (DNA replication) and some of its organelles, and subsequently the partitioning of its cytoplasm and other components into two daughter cells in a process called cell division.

• The events in the cell that include the duplication of its DNA (DNA replication) and some of its organelles, and subsequently the partitioning of its cytoplasm and other components into two daughter cells in a process called cell division.

• Cell Division: The ability of the cell to produce daughter cells.

• Normally, cells divide to replace dead or missing cells,

• intestinal cells divide every 3 days, are broken down by digestion

• blood cells last 3 months, are replaced by new cell division

• nerve cells usually don't divide, last for life

• Embryo: almost constant cell division. Every 30 minutes.

• Sex cells are unique: in humans, males produce 500,000,000 /day. Females 1/month.

Meiosis.

• Definition: It's a special type of cell division where the cell genome is reduced to half to produce “Four Haploid Cells”.

• The cells produced in meiosis are called gametes.

• As genetic material is reduced to half, it is also called as “Reductional Cell Division”.

1. Parent Cell (2n)-------> Daughter Cell (1n).

• Meiosis is divided into following stages,

Meiosis I	Prophase I	Meiosis II
Prophase I	Leptotene	Prophase II
Metaphase I	2) Zygotene	2) Metaphase II
Anaphase I	3) Pachytene	3) Anaphase II
Telophase I	4) Diplotene	4) Telophase II
Cytokinesis I	5) Diakinesis	5) Cytokinesis II

Meiosis I:

• It is the phase where actual reduction takes place.

• It is further divided into,

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

• Cytokinesis I

1. Prophase I:

• It is the longest phase and is divided into following phases for convenience of study.

1.  Leptotene

2. Zygotene

3. Pachytene

4. Diplotene

5. Diakinesis.

1. Leptotene:

• Also called “Leptonema” (Lepto= Thin, Nema= Thread).

• In this stage condensation of Chromosomes takes place.

• Chromosomes get converted into thin thread like structures.

• Each maternal and paternal chromosome starts searching for their “homologous chromosomes”.

2. Zygotene:

• Also called “Zygonema” (Zygo= Pair, Nema= Thread).

• Maternal and Paternal Chromosomes come together to form a pair called “Synapse / Bivalent / Homologous pair”.

3. Pachytene:

• Also called “Pachynema” (Pachy= Thick, Nema= Thread). 

• Most important event takes place i.e. “Crossing Over”.

• Two nonsister chromatids overlap each other, the point of attachment called  “Chiasmata”.

• Due to this transfer of genetic material took place between maternal and paternal chromosomes.

• Pachytene hence is an important step as the basis of evolution takes place here.

4. Diplotene:

• Also called “Diplonema” (Diplo= Two, Nema= Thread). 

• Nuclear membranes and nucleolus start disappearing.

• Centrioles get connected by spindle fibers.

• Centrioles start moving to the opposite ends.

• The overlapped chromosomes start repulsing each other.

5. Diakinesis:

• Also called “Terminalization”.

• Stage is characterized by the ending of cross over.

• The repulsion between crossed over chromosomes continues, still there is attachment at tip.

• Nuclear membranes and nucleolus disappear.

• Centrioles got connected by spindle fibers.

• Centrioles move to opposite ends.

• This stage marks the end of Prophase I.

2) Metaphase I:

• Chromosomes become more prominent.

• Centrioles are moved towards the opposite ends of the cell and are connected with spindle fibers.

• The crossed over chromosomes get arranged at the equatorial plane.

• Crossed chromosomes are still attached at the tip with each other.

• Metaphase is the best stage to study the chromosomes.

3) Anaphase I:

• Spindle fibers start contracting.

• Results in dys-junctioning of chromosomes.

• The separated chromosomes start moving to the opposite ends of the cell.

• The chromosome number gets reduced to half.

4) Telophase I:

• It is just opposite to Prophase I.

• Spindle fibers disappear.

• Nuclear membrane and nucleolus reappears.

• Formation of Two Haploid Nuclei takes place at opposite ends of cells.

• The chromosomes undergo recoiling to form Chromatin threads.

5) Cytokinesis I:

• The furrow formation takes place in the cell membrane.

• The furrow starts growing and contracting outside to inside and results in formation of two daughter cells.

• The formed daughter cells after brief rest undergoes Meiosis II.

Between Meiosis I and Meiosis II there is a short resting period of the cells called “Interkinesis”.

Meiosis II:

• It is just like Mitosis.

• Each haploid cell formed during meiosis I undergoes Meiosis II.

1. Prophase II:

• Nuclear membranes and nucleolus disappear.

• Centrioles start moving to opposite ends of the cell and are connected by Spindle fibers.

• Chromatin material undergoes condensation to form chromosomes.

2. Metaphase II:

• Chromosomes become more prominent.

• Centrioles are moved towards the opposite ends of the cell and are connected with spindle fibers.

• The chromosomes get arranged at the equatorial plane.

• Metaphase is the best stage to study the chromosomes.

3. Anaphase II:

• Spindle fibers start contracting.

• Results in dys-junctioning of chromosomes.

• The separated chromosomes start moving to the opposite ends of the cell.

4. Telophase II:

• It is just opposite to Prophase II.

• Spindle fibers disappear.

• Nuclear membrane and nucleolus reappears.

• Formation of Two Nuclei takes place at opposite ends of cells.

• The chromosomes undergo recoiling to form Chromatin threads.

5. Cytokinesis II:

• The furrow formation takes place in the cell membrane.

• The furrow starts growing and contracting outside to inside and results in formation of two gametes.

Commonly Asked Questions.

1. Write a short note on “Meiosis”.

2. Why is meiosis called “Reductional Cell Division”?