Cell Biology, Chromosome, DNA, Gene Concept

Cell Biology

Cell is structural and functional unit of life. It is the smallest part of a living organism. Every organ of our body our brain, our tongue, our eyes even the bones are made up of so many small cells. Plant leaves, stems and roots are also made up of cells. All organisms start their life from a single cell.

It is called structural unit because every part of an organism is made up of cells.

It is called functional unit because any kind of movement in the body of an organism is because of cells. Like when we walk our muscle cells contract and expand because of which we are able to do so. Likewise movement in a plant is because of cell activity.

There are two types of cells Eukaryotes which contain nucleus and Prokaryotes which do not.

Prokaryotic Cells :

The life on earth has started from this kind of cell. Prokaryotic cells are usually single celled organisms like bacteria, archaea and blue green algae. These cells have no definite nucleus. Ribosome is smaller in size. Chromosomal protein like histones is absent here. Many other cell organelles like mitochondria, endoplasmic reticulum, chloroplasts are absent here. Prokaryotic cells divide by binary fission. Mitochondria and chloroplast is absent here.

Eukaryotic Cells :

These are highly advanced cells. Eukaryotic cells can be either single-celled or part of multi-cellular organisms. These cells have well developed nucleus with nuclear membrane. Chromosomal protein like histones is present here. These cells posses double membrane structure. These cells divide by mitosis and meiosis. Plants, animals, Fungi, Amoeba, Euglena etc. have prokaryotic cells.

Cellular Organelle

Plasma Membrane:

It is the outermost layer in animal cells and in plants it lies next to cell wall. It is made up of lipoprotein. This membrane is very thin flexible and have fine pores which are semi permeable.

It controls the entry of certain solutes and ions. It protects the cell form its surroundings. In animals it also maintains the shape of the cell.

[Note : Here semi permeable means it allows some material to pass through it where as some material are not allowed to pass through to pass.]

Cell Wall (Non-Living) :

It is present in prokaryotic cells and some eukaryotic cells. It is a rigid layer and covers the plasma Membrane. It is composed of cellulose. This layer is fully permeable means it allows every material to pass through it.

It also helps to maintain the shape of the cell and provide the cell protection from outside.

Cytoplasm :

Everything taken together inside the plasma membrane except nucleus is called cytoplasm. Different types of organelles are there inside cytoplasm apart from organic and inorganic compounds.

Any chemical reactions and metabolic activity happens here. Anaerobic respiration takes place in cytoplasm. It is the place for production of piruvic acid.

Endoplasmic Reticulum (ER) :

The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations. The ER has two forms: the rough ER (RER), which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER (SER), which lacks ribosomes.

The smooth ER plays a role in calcium sequestration and release. A special kind of ER is present in muscles called Sarcoplasmic Reticulum. It plays a role of storing Ca++. But this is absent in mature mammalian RBC and embryonic cell.

Mitochondria :

Mitochondria are power generators.  Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria multiply by binary fission like prokaryotes.

Mitochondria play a critical role in generating energy in the eukaryotic cell. Respiration occurs in the mitochondria, which generate the cell's energy and release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP.

Golgi Apparatus (Golgi Body) :

It is present in animal cell. In plant cell it is present with the name of dictyosomes. It is made up of tubules, vesicles and vacuoles.

Chief function includes synthesis and secretion of enzymes and hormones.

Ribosome :

These are small granules, single walled spherical bodies either scattered in cytoplasm or attached to endoplasmic reticulum. It is mainly composed of RNA.

It is the site for protein synthesis.

Lysosome :

Present only in animal cells. These are membranous globular hydrolytic enzyme loaded with lipoprotein. It also contains 40 different types of enzymes.

Intracellular digestion occurs here. It forms bone by digesting cartilages. During starvation they digest the stored food inside the cell. Lysosomes are also called suicidal bag because when a cell becomes old or gets damaged they blast (autolysis) and cell dies.

Centrosome (Only Animal Cells) :

It is located near the nucleus surrounding the centrioles. Centrosome is an organelle that serves as the main microtubule organizing center of the cell.

It regulates the cell division. Forms spindle fibres while cell division.

Plastids (Only Plant Cells) :

These are double membranous structure having different kind of pigments. On this basis plastids are of three different types.

1. Chloroplast

Chloroplast contains the green pigment of plants which is called chlorophyll.  It is made of Mg++ (ion) containing pyrrole head and lipid or phytol tail. Unit of chloroplast is thylakoids and this thylakoid contains the chlorophyll.

Chlorophyll is vital for Photosynthesis, which allows plants to absorb energy from light.

2. Leucoplast (leuco: white)

These are having colorless or white color granules. These granules provide a silver kind of color and helps plant in reflecting sun rays. Potatoes have lots of leucoplast.

3. Chromoplast (chromo : color)

They provide different granules responsible for different colors (yellow, orange, red). The color of corolla of plant flower is such because of this. Fruits color is such because of this.  Yellow color is because of a pigment called xanthophylls while orange and red are because of carotene pigment.

Above different kind of colors have been discussed like green, silvery, yellow, orange and red and now you people would be wondering what about the other colors like blue, violet etc. So for your knowledge plastids are not responsible for all these colors. There are other pigments called anthocyanins which remain dissolved in the cell juice and provide these colors to the plant.

Nucleus :

This is the largest organelle of the cell mostly in spherical shape. It acts as control room or programming center of the cells and hence most important part of a cell. It plays an important role in cell division ………….It is made up of different parts discussed below.

1. Nuclear Membrane

It is two layer membranes surrounding nuclear fluid called nucleoplasm. Its surface is having nuclear pore complexes which serve in transport of molecules between cytoplasm and nucleoplasm.

2. Nucleoplasm

It is clear transparent fluid substance inside nucleus. This provides as the location for nuclear metabolism and functioning.

3. Nucleolus

It is round shaped or spherical DNA codes for rRNA and it provide site for synthesis of ribosome. It also dictates ribosome to synthesize protein by forming and storing RNA.

4. Chromatin Network (or Chromatin Fibre)

These are beaded thread like structure made up of DNA and chromosomal protein. It has genes for different character of a cell and subsequently the organism.

Granules (Non-Living)

These are small particles, crystals or droplets. These are starch (in plant cell), glycogen (in animal cell) and fat-containing granules responsible for providing food to the cell.

Vacuoles (Non-Living)

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some animal and bacterial cells. Vacuoles are enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution.

It isolates materials that might be harmful or a threat to the cell. It contains waste products. It also contains water in plant cells. It allows plants to support structures such as leaves and flowers due to the pressure of the central vacuole.

PROTOPLASM :

Many a time you might face this term ‘protoplasm’. So do not confuse yourself thinking that protoplasm is some other organelle inside a cell. It is nothing but all the living substances inside a cell taken together.

Protoplasm = Cytoplasm + Nucleus

Difference between Plant & Animal Cell

Instead of same basic structure both the plant cell and animal cell shows some difference whish are discussed below –

Inventer/ Discoverer	Year
1. Centrosome in present	1. Centrosome is absent
2. Cell wall absent	2. Cell wall present here
3. Plastids are absent here	3. Plastid present
4. Cytoplasm more denser and granular	4. Cytoplasm not dense
5. Vacuoles are large in size	5. Vacuoles are smaller in size

Chromosome, DNA, Gene Concept

Structure of Chromosome

Let’s have a look at the journey from DNA – to-Chromosome.

DNAs are long tape like structure.

DNA surrounded by 8 histones becomes a structural unit called nucleosome and a long chain of nucleosome is formed.

This chain gets twisted and forms coiled (spiraled) structure. Hence chain converts into coiled chain. This coiled chain further twists and produce a super-coiled chain structure. This super coiled chain is known as chromatin fibre.

Chromatin fibre bunched together is called chromatids.

A pair of chromatids is called chromosome which at the time of cell division divides in to two pairs and looks like ‘X’ shape.

Hope your confusion no more exists now.

In normal state inside a nucleus chromatin fibres remain scattered which during cell division condense and takes the shape of chromosome which is discussed above.

GENE :

It is widely accepted by the scientists as a name given to some stretches of deoxyribonucleic acids (DNA) and ribonucleic acids (RNA).