Mitosis

Mitosis

Mitosis is a type of cell division involves a series of complex changes in both, the nucleus and cytoplasm. During mitosis the diploid (2n) number of chromosomes kept constant in daughter cells.

Interphase

It is an active phase of cell division which occurs before the onset of mitosis. The basic components of cell duplicated during this phase particularly the DNA and histones. Moreover various proteins are also synthesized during interphase, which are essential for proper completion of mitosis.

Main Phases of Mitosis

The process of mitosis has been studied in both plants and animals. Walther studies it in animals wile Strassburger studied it in plants. According to these studies, mitosis takes place in somatic cells and comprises two phases.

1- Cytokinesis (division of cytoplasm)
2- Karyokinesis (division of nucleus)

Cytokinesis

Cytokinesis refers to the division of cytoplasm. In animals, it is carried out by the formation of cell membrane and contraction of cytoplasm. Cell membrane moves inward deeply and divides the cell into two halves. In plants, some special structures are formed by the transformation of spindle fibers at equatorial; these structures are called phragmoplast which leads to the division of one cell into two.

Karyokinesis

Karyokinesis refers to the division of nucleus. It comprises four stages:

1- Prophase
2- Metaphase
3- Anaphase
4- Telophase

1- Prophase

It involves the appearance of chromosomes as thin threads by the condensation of chromatin fiber in the nucleus. Each chromosome comprises two chromatids with centromere present in between. As mitosis proceeds, nucleolus and nuclear membrane also disappears gradually

2- Metaphase

During this phase spindle fibers starts to appear. Chromosomes get attached with spindle fibers and migrates from equator to opposites poles to take the final shape.

3- Anaphase

During anaphase, Spindle fibers attached to centromere pull them and two chromatids get separated from each other and move towards opposite poles.

4- Telophase

This phase is an opposite or reversal phase of prophase. All changes took place during prophase reverse to their original states. On reaching the poles, chromosome start uncoiling and nucleoli and nuclear membrane begins to appear, which results in the formation of two daughter cells.

Significance of Mitosis

Mitosis is a significant cell division which ensures the provision of correct and complete genetic information to the daughter cells. As chromosome have heredity material and number of chromosomes must be kept constant in daughter cells in order to transfer complete genetic information and mitosis keeps this number of chromosome constant. In case of abnormal number of chromosome, several physiological and psychological disorders can cause in an organism. So we can say that the product of mitosis is not only quantitative but qualitative as well. Healing of wounds and formation of new cells takes place due to the process of mitosis.

Nucleic Acid

Nucleic Acid

A nucleic acid is a macromolecule composed of polynucleotide chains. These polynucleotide chains are made up of several monomers which are called nucleotides. These nucleotides carry genetic information or form structures within cells. Ncleic acids are found in every living organism including viruses as well. Nucleic acids were first discovered in 1871 by Friedrich Miescher.

Composition of Nucleic Acid

Each nucleotide comprises three components; Pentose Sugar, Phosphase Group and nitrogenous bases. Nitrogenous bases are of two types; purine and pyrimidine. Purines are based on Adenine (A) and Guanine (G) while Pyrimidine are based on Cytosine (C), Thymine (T) and Uracil (U). Nitrogenous bases found in the two nucleic acid types are different: adenini, cytosine and guanine found in both RNA and DNA, while thymine only occurs in DNA and uracil only occurs in RNA. Some other types of nucleic acid also occur, for example inosine found in strands of mature tRNA.

Pentose Sugar in ribonucleotide is ribose and in deoxyribonucloetides is deoxyribose. Phosphate linkage with pentose sugar is called Phosphodiester Linkage. Phosphoric acid has ability to develop ester linkage with hydro oxyle(OH) groups of pentose sugar.

Types of Nucleic Acid

There are two types of nucleic acids.

1- RNA
2- DNA

1- RNA

RNA stands for Ribonucleic acid, is a single polynucleotide chain comprising nucleotide monomers. These monomers plays significant role during transcription. It transcribed genetic information from deoxyribonucleic acid (DNA) into proteins during protein synthesis. In this way RNA acts as a messenger between DNA and ribosomes which are protein synthesis complexes present in cell. RNA forms vital portions of ribosomes and serves as an essential carrier of amino acids during protein synthesis. RNA exists in three types which include:

1- tRNA (transfer RNA)
2- mRNA (messenger RNA)
3- rRNA (ribosomal RNA)

2- DNA

DNA stands for Deoxyribonucleic acid, comprises two polynucleotide chains linked together by weak hydrogen bonds. There are two hydrogen bonds between A and T and three hydrogen bonds between C and G. DNA is heredity material contain genetic information passed from generation to generation. Evidence from the study of bacterium (Pneumococcus) that DNA is heredity material. This genetic information plays essential role in the development and functioning of all known living organisms.

The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes or are involved in regulating the use of this genetic information. The amount of DNA depends upon the number of chromosomes present.

DNA comprises four types of nitrogenous bases i-e cytosine, thymine, guanine and adenine. These bases are linked together to form a polynucleotide chain. Two polynucleotide chains coil around each other to form the double helical sucture of DNA molecule.

What are Chromosomes?

Chromosomes

Chromosomes are important part of a cell discovered in 1876 by Waldeyer. In 1908 W.S. Sutton observed that genes are located on chromosomes and genetic phenomenon can be explained in terms of chromosomal behavior during cell division particularly during metaphase and anaphase.

Structure of Chromosome

Chromosome comprises two chromatids attached to centromere. Centromere is the point where spindle fibers get attached with them. The position is very important to distinguish the type of chromosome. You can say that on the basis of its position the type and shape of a chromosome can be defined.

Types of Chromosomes

On the basis of centromere’s position, chromosomes can be divided into four types.

1- Telocentric (Centromere present at one end of chromosome)
2- Acrocentric (Centromere present close to one end, so acrocentric chromosomes have very small arms)
3- Metacentric (Centromere present in centre with equal arms)
4- Submetacentric (Centromere present near the centre but not exactly in the centre therefore have an equal arms)

Karytype

It is the term used to describe the total chromosome complement of cell. Study of karytype helps in determining the total number of chromosome present in human beings and in other species.

Chemical Composition of Chromosomes

Chromosomes are chemically composed of DNA and basic protein. These basic proteins are called Histones. DNA and Histones combine together to form a structure which is called Nucleosome. Nucleosome comprises eight molecules of various types of histones with two turns of DNA molecules. Under electron microscope nucleosome appears as a beaded structure of 10nm diameter.

What is Cell Nucleus?

Cell nucleus is spherical or semi spherical body present usually in the center of eukaryotic cells. In prokaryotic cell, nuclear material is dispersed throughout the cytoplasm without a definite membrane. Nucleus is often called 'Brain Of The Cell' as it contains cell's hereditary material DNA and coordinates its all metabolic activities including growth, metabolism, protein synthesis and reproduction (cell division) etc. As far as its size is concerned it occupies about 10 percent of total eukaryotic cell's volume.

Nuclear membrane is nuclear envelope that separates the nuclear material from the cellular cytoplasm. It is double layered membrane that encloses the nuclear material during most of the cell's life cycle. These two unit membranes run parallel to each other with a space between them called perinuclear space which is an electron transport area. At some places outer membrane gives out tubular structures called endoplasmic reticulum. Annuli are openings in nuclear envelope through which nucleoplasm communicates with the cytoplasm. These nuclear pores regulate the passage of molecules between the nucleus and cytoplasm allowing them to pass through the nuclear membrane. It is believed that these pores can open and close but its definite mechanism is not clear. During cell division (mitosis), the nuclear membrane disintegrates into vesicles which disappear in the cytoplasm. But soon at the reformation stage when two cells complete their formation and the chromatin begins to appear these vesicles again combine to reform nuclear membrane.

The semi-fluid matrix bounded by nuclear membrane is called nucleoplasm. Within nucleoplasm, a mass of thin fibrils called chromatin is present. This chromatin network is the actual site of genetic activities as it contains DNA and become highly coiled at the time of cell division to form chromosomes. Chromatin is so organized that it gives reticulate appearance. About 6 feet of DNA is present in every human cell, divided into 46 individual molecules (chromosomes). Chromosomes are chemically composed of DNA, RNA, Histone (basic proteins) and Hertones (non histone proteins). Calcium is also present in it. Genes, the fundamental units of heredity, are present on chromosomes on specific position called locus.

The nucleus also contains one or more dark, spherical, colloidal bodies called nucleoli (singular; nucleolus). Nucleolus is associated with a special region called 'Nucleolar Organizing Region' present in the chromosome. It is rich in RNA which passes to cytoplasm to take part in protein synthesis. Nucleoli are involved in manufacturing ribosomes. A nucleolus consists of RNA, acidic and basic dyes, phospholipids and alkaline phosphates. Under microscope nucleolus looks like a large dark spot within the nucleus. A nucleus may contain several nucleoli which sometimes merge to form a single and large nucleolus.

What is Mitochondria?

A Brief Look at Mitochondria

Mitochondria are sausage-shaped, double membraned cytoplasmic bodies usually called "Power House Of Cell". These are responsible for ATP synthesis through cellular respiration. Mitochondria are present in almost all eukaryotic cell with the exception of protist Chaos (Pelomyxa) carolinensis.They show constant motion within the cell and tend to aggregate at places where energy requiring process is going on or at the parts of cell associated with active processes e.g in flagellated protozoas mitochondria are present around the base of flagella and in cardiac muscle, they surround the contractile elements. One of the richest sources of mitochondria is Hummingbird flight muscle.

Mitochondria with 0.5 to 1.0 micrometer diamete are bounded by outer and inner membrane, composed of phospholipid bilayers and proteins. Both membranes differ in their structure and function. Outer membrane is smooth and elastic helps in its extension & contraction. Porins (integral protein) present in outer membrane form channels through which small ions, nutrient molecules, ATP etc can pass easily. Any disruption in outer membrane allows proteins in the intermembrane space to leak into the cytosol (cytoplasmic matrix) which can lead to cell death.

Explaining the Structure of Mitochondria

Let’s have a look at the structure of mitochondria.

Inner membrane shows folded structure, forms invaginations into the mitochondrial cavity to increase the surface area for cellular respiration. These invaginations are called cristae which can be branched or unbranched. Cristae are the sites for ATP synthesis. Inner membrane is permeable only to oxygen, carbon dioxide, and water etc. About 1/5 of the total protein in a mitochondrion is present here.

Inter Cellular Space is the space between outer and inner membrane play important role in oxidative phosphorylation. Due to the permeablility of outer membrane it is rich in protein especially cytochrome C and other small molecules.

Matrix is the fluid or substance fill the mitochondrial cavity inside the inner membrane. It is rich in ribosomes, DNA, proteins and manganese. Ribosomes (of 70S type) present here are different from cytoplasmic ribosomes. Its DNA is of circular shape called mitochondrial DNA. About 2/3 of the total protein of mitochondrion is present in matrix. The matrix contains highly-concentrated mixture of enzymes. Enzymes for Kreb s cycle are also present here. The matrix play important role in ATP synthesis with the help of the ATP synthase present in inner membrane.

Oxysomes are knobs like structures present on the outer surface of outer membrane and on the inner surface on inner membrane. Oxysomes present on outer side are stalkless while those attached to inner surface have small stalk for attachment, this stalk is called F0 particles and rounded part is called F1 particle. Both have enzyme ATP-ase and are related to ATP synthesis. These are proteinaceous in nature. Oxysomes act as electron carrier and oxygen released results in the formation of water and ATP molecule.

How do a mitochondria works?

Functions of Mitochondria

1- Main function of mitochondria is energy production through ATP synthesis. It makes impossible for multicellular organisms to exist without mitochondria. They provide energy through the break down of respiratory substances for ATP synthesis used in cell metabolism.

2- Through oxidative phosphoryation mitochondria make efficient use of nutrient molecules.

3- Mitochondria are also involved in cell signalling (communication system to control basic cellular activities), cellular differentiation, apoptosis (PCD-programmed cell death), cell cycle and cell growth as well. Certain amount of protein synthesis also takes place here.

4- Mitochondrial perform specific function in specific types of cells e.g mitochondria present in liver cells contain enzymes that allow them to detoxify ammonia (waste product of protein metabolism).

5- Additionally mitochondria also regulates membrane potential, cellular proliferation and certain heme and steroid synthesis reactions. Mitochondria can also store calcium. There is a significant interplay between the mitochondrion and ER (significant storage site of calcium) regarding calcium storage.

Diseases/Disorders Related to Mitochondria

Aftermaths of Mutations or Damage to Mitochondria

Any damage to mitochondria can cause a wide range of disorders which mostly present as neurological disorders. Its improper functioning lead to energy break down which causes poor coordinations, sensory poblems and reduced mental activity etc. Errors in cellular information processing are responsible for diseases such as cancer, autoimmunity and diabetes.

Diseases caused by mutation in the mtDNA include Kearns-Sayre syndrome and MELAS syndrome. Mostly these diseases are transmitted by mother as the zygote derives its mitochondria and its mtDNA from the ovum. Large-scale mtDNA mutations can cause Kearns-Sayre syndrome, Pearson's syndrome and progressive external ophthalmoplegia while MELAS syndrome, Leber's hereditary optic neuropathy, myoclonic epilepsy with ragged red fibers are the outcome of small-scale or point mutations in mtDNA.

Environmental factors may also influence mitochondrial disease. Influence of pesticide exposure on Parkinson's disease is an example of environmental influences.

Aging process also lead to a number of changes in mitochondria. Tissues of elderly patients show a decreased enzymatic activity. Even large deletions in the mitochondrial genome can lead to high levels of oxidative stress and neuronal death in Parkinson's disease. Hypothesized links between aging and oxidative stress show their symptoms after the age of 50 years.

What is Protein Synthesis?

Definition of Protein Synthesis

Protein synthesis, a way of gene expression in which gene produces a product, usually a protein.

Stages of Protein Synthesis

It involves two stages:

1- Transcription
2- Translation.

1- Transcription

Transcription is a process of RNA synthesis in which RNA molecules are synthesized on DNA template. As in eukaryotes DNA is restricted to nucleus does not participate in protein synthesis directly. An intermediate RNA molecule is involved in it which is synthesized in nucleus where genetic information stored in DNA is transferred to this RNA molecule. This RNA is called mRNA (messanger RNA) or transcript as it contains a message to direct the ribosomes to assemble amino acids for given kind of protein. During this process only one of the DNA strand is transcripted which is called sense strand and other strand is called antisense strand.

Transcription comprises three stages; Template binding and initiation, Elongation and Termination. Template binding involves a DNA molecule and RNA polymerase. RNA polymerase is a holoenzyme composed of a core enzyme and a sigma factor. Sigma factor recognises promoters along the sense strand. Promoters (initiation sites) are attachment points where initial binding of RNA polymerase and a DNA molecule occurs. In the absense of sigma factor, core enzyme is responsible for this binding but not specifically to the promoters. During elongation RNA polymerase passes along DNA molecule to unwind the double helix and a structure called open promoter complex is formed at the point of breakage. The synthesized portion of RNA molecule gradually dissociates from the strand allowing it to return to its original helical structure. Finally termination occurs at suitable positions after the ends of genes.

2- Translation

Translation, the final stage of gene expression in which RNA molecule is translated into polypeptides. This complex process involves ribosomes and three types of RNA molecule (mRNA,rRNA,tRNA produced during transcription). The main thing during translation process is that genetic code determines the sequence of nucleotides in mRNA molecule to specify the sequence of amino acids in a polypeptide. There must be a code for each of 20 amino acids found in polypeptide. Translation comprises four stages; tRNA charging (aminoacylation), Initiation, Elongation and finally Termination of polypeptide chain. tRNA charging involves linkage of tRNA molecules to its respective amino acids. This charging is controlled by a group of enzymes called aminoacyl synthetases. Each aminoacyl synthetases is specific for each amino acid. First amino acid become activated by reacting with ATP and then linked with itsrespective enzyme to form a complex. This reacted complex get linked with specific tRNA molecule through a covalent linkage.This process is called amino acid activation.

After this charging stage,initiation, elongation and termination of polypeptide chain is carried out respectively. During initiation process small ribosomal sub-unit binds to a mRNA molecule and then a charged tRNA (charged by Methionine, an amino acid coded by AUG) pairs with this small sub-unit to form a resultant structure called Initiation complex. After the formation of initiation complex elongation process starts. When large sub-unit of ribosome binds with this complex. The large sub-unit have two binding sites for charged tRNA molecule; P-site (peptdyl) and A-site (aminoacyl site). The tRNA binds to P-site and specific charged tRNA become attached to A-site.The attachment of tRNA to A-site is dictated by sequence of second triplet (anticodon) in mRNA. An enzyme peptidyl transferase, present in large ribosomal sub-unit, catalyses the formation of peptide bond to link two amino acids together. At the same time the covalent bond between amino acid and tRNA (on P-site) molecule is broken and a dipeptide is formed. This increase in the length of polypeptide chain by one amino acid is actually called elongation. Before elongation repeats itself another process called translocation occurs. During translocation tRNA (now uncharged) attached to P-site is released from the large sub-unit and A-site becomes vacant again for coming charged tRNA to repeat the process. The final stage termination is signalled when a termination codon UAG,UAA or UGA enters the A-site. Release factors enter the A-site and cleaves the polypeptide chain from tRNA. Ribosomes releases the polypeptide and mRNA and dissociates into its sub-units. Released polypeptide chain folds up into tertiary structure to begin its functional life.

What are Enzymes?

Enzymes are the catalysts or chemical substances synthesized in living cell and responsible for metabolic activities within the organism. Subatances which react with the help of enzymes are called "Substrates" and materials produced as the result of a chemical reaction are called "Products". Active sites are present on the surface of enzyme to combine with a substrate through hydrogen bonds or ionic forces. Substrates and enzyme combine to form enzyme-substrate complex. It involves a "lock and key" machanism in which some part of substrate establishes a close fit into some part of enzymes.

Enzymes are essentially proteins. But some of them consists of protein part called "apoenzyme" ans a non-protein part called "prosthetic group". Prosthetic group is divided into two catagories i-e activators and coenzymes/cofactors. Acticator is the inorganic metal portion attached to apoenzyme to help in binding a substrate to the enzyme. Loss of activator causes the inactivity of an enzyme. Some known acticators are copper, calcium, cobalt, iron etc. Coenzyme is the organic prosthetic group of enzyme. Coenzymes have loose association with enzyme acts as donors or acceptors of atoms in forming enzyme-substrate complex. Some coenzymes are NAD (nicotinamide adenine dinucleotide), NADP (nicotinamide adenine dinucleotide phosphate), FAD (flavin adenine dinucleotide) and COA(coenzyme A). Certain vitamins also act as coenzymes.

Some specific features of enzymes differentiate them from inorganic catalysts as enzymes are specific in their action associated with particular chemical reaction, these are not as passive during reaction as catalysts as sometimes molecules of enzymes get destroyed during reaction and velocity of a reaction is not always proportional to the concentration of the enzyme. But usually enzymes retain themselves during the reaction and can be reused for some other reaction. The chemical reactions involving enzymes are reversible if product get accumulated for a long time.

Enzymes catalyses a reaction by reducing the its energy of activation to make molecules more reactive. It brings the substrates closer either binding them in closer association or putting them under stress or strain. Activation energy is the energy required to start a chemical reaction. Enzymes can bring about those reactions at low temperature which only occur at high temperature. So it also act in the place of "thermal agitation".

Factors Affecting the Activity of Enzymes

Certain factors affect the activity of enzymes including temperature, pH and poisons. Rise in temperature causes rise in enzyme activaty. But at very high temperature denaturation of apoprotein results in the inactivation of enzyme due to the breaking of its hydrogen bonds present in enzyme. Change in pH causes changes in ionic state of a substrate results in the formation of charged particles which may not correspond with the ionic groups present in the active sites of enzymes. As a result no enzyme-substrate complex formed. Certain poison or inhibitors mask the active sites of enzymes forming the enzyme-inhibitor complex. In this situation substrate have to compete for its active site so it is called "competitive inhibition".It can be removed by increasing the concentration of substrate. The conformational change in the enzyme due to the formation of enzyme-inhibitor complex is called "allosteric effect".

Enzyme nomenclature follows some rules. Some enzymes are named after the name of their substrate by adding a suffix 'ase'. For example lipase reacts with lipids (fats), Carbohydrase reacts with carbohydrates and proteinase reacts with proteins. Sometime the name of enzyme refers to the reaction rather than the substrate e.g decarboxylase, dehydrogenase, mutase and oxidase etc. In this case the name of substrate is added to the name of enzyme such as pyruvic acid decarboxylase, succinic acid dehydrogenase, glucomutase and cytochrome oxidase. Old names of enzymes formulated before the nomenclature have been retained.

Classification of Enzymes

Enzymes can be classified into following groups.

1- Hydrolyzing and Oxidizing Enzymes

Hydrolyzing enzymes break down the substrate by the addition of water. e.g Carbohydrases(acts on carbohydrates), proteinases (acts on proteins) and eaterases (acts on easters). Carbohydrases include amylase (converts starch into maltose), maltase ( converts maltose into d-glucose) ,sucrase ( converts sucrose into glucose ) etc. Easterases include lipases (converts fats into fatty acid and glycerols) and phosphatases (converts phosphoric acid esters into phosphoric acid) etc.

Oxidizing enzymes help in oxidation-reduction process by transfer of electrons or hydrogen. These are classified as oxidases and dehydrogenases. Oxidases use molecular oxygen as hydrogen acceptor to form water e.g cytochrome oxidase. While hydrogenases remove hydrogen from the substrate and transfer it to the oxygen or to reducible substance. Flavoproteins (yellow enzymes) also accept hydrogen from substrates.

2- Intracellular & Extracellular Enzymes

Intracellular enzymes are produced and used in the same cell while extracellular enzymes are produced in one cell and used in other cells in other part of body.

3- Desmolases & Phosphorylases

Desmolases catalyse the reaction by breaking carbon to carbon bonds e.g aldolase and carboxylase. while phosphorylases acts by adding phosphate group to the substrate. This process is called phosphorylation Its reversible process is called dephosphorylation. Some known phosphorylases are amylo-phosphorylases, trans-phosphorylases, phospho-isomarase and phospho-mutase etc.

Uses of Enzymes

Enzymes are utilized in variety of ways. They play a major role in all metabolic activities going on in a living organism. In digestive process they help in breaking up of food particles to release the energy stored in them. Salivary amylase present in saliva converts starch into maltose. In stomach, pepsin present in gastric juice acts on proteins to converts it into polypeptides and peptones. Proteases,amylase and lipase present in pancreatic juice acts on protein,starch and lipids respectively to convert them into digestible food particles. In infants rennin is present in place of pepsin to digest the milk.

In food industry a variety of enzymes are used for food processing e.g in the production of various types of syrups, in dairy industry, in fruit juice and brewing industries. Enzymes are also present in our food as a raw material. Foods that are high in enzymes include raw meat, fresh fruits, unpasteurized milk, vegetables, fermented foods and nuts etc. Breast milk is also enzyme rich food in its uncooked state, as in this state it contains natural enzymes to help in its digestion.

Commercial uses of enzymes includes detergents, leather, antibiotics and fructose production. Infact detergents are the first large scale application for microbial enzymes. Bacterial proteinases is considered an important detergent enzymes. Lipases decompose fats into more water-soluble compounds by hydrolysing the ester bonds between the glycerol and fatty acid. Amylases are also used in detergents to remove starch based stains.

Enzymes are also responsible for metabolic functions in plants cells. Oxidases and de-hydrogenases are found in the matrix of the mitochondria. Many phosphorylases have been discovered to be attached on the inner membrane of mitochondria. Enzymes necessary for carbon dioxide fixation are present in the stroma. Ribosomes also have enzymes which mediate the production of peptide chains of proteins. Deoxyribonuclease present in nucleus helps in hydrolysis of DNA.