Spherical Shaped Organelle Found In All Eukaryotic Cells Biology Essay


The nucleus in cell biology every now and then referred as the control center. And it is a membrane-enclosed organelle present in eukaryotic cells. It is called as control center for the reason that the function of the nucleus is to maintain the integrity of the genes and to control the activities of the cell by regulating the gene expression. The nucleus is made up of nuclear envelope which is a double membrane that encloses the whole organelle and separates its stuffing from the cellular cytoplasm.

Nucleus is a spherical shaped organelle found in all eukaryotic cells. It is visible when cell is not dividing. The nucleolus plays an indirect role in protein synthesis by producing ribosome which is nothing but the cell organelles made up of rna and proteins. It contains the RNA for protein manufacture. As compared to the other cell organelles, the nucleus is the most important one which accounts to about 10 percent of the cells volume. The structure of a cell nucleus consists of nuclear membrane, nucleoplasm (karyoplasm), nucleolus and chromosomes. Nucleoplasm is the matrix found inside the nucleus.

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It is also known as the nuclear envelope. The nuclear membrane surrounds the nucleus and it composed of two layers(double layered structure). The outer layer is connected to the endoplasmic reticulum. In between the two layers a fluid-filled space or perinuclear space is present. The nuclear communicates with the remaining of the cell through openings called nuclear pores. It also contains a numerous openings for nuclear traffic.

Chromosomes are usually in the form of chromatin and contain the genetic information. It is composed of DNA. Chromatin is also classified into two types; they are heterochromatin and euchromatin based on their function. The cellular division in the chromosomes is thicken and has a set number per species for example 23 pairs for human.

The cell nucleus defines the hereditary distinctiveness of an organism. One of the main functions of the cell nucleus is to organize and control gene expression and the replication of DNA all through the cell cycle. The cell nucleus is responsible for the protein synthesis, cell division or cell compartmentalization, growth and differentiation. The genes in the form of long and thin DNA strands, which is known to as chromatins. It supplies the proteins and RNA in the nucleolus and exchange of hereditary molecules between the nucleus and the rest of the cell. Nucleus is a site for transcription in which messenger RNA are created from the protein synthesis. During the cell compartmentalization the chromatins are arranged into chromosomes. Selective transporation of regulatory factors and energy molecules through nuclear pores. Ribosomes are protein factories necessary organelle. Above mentioned functions of regulating and integrating the genes and expression, the nucleus is defined as control center of the cell.

461 words

Question 3

The citric acid cycle is called as the tricarboxylic acid cycle i.e. TCA cycle, the Krebs cycle, or Szent Györgyi Krebs cycle, is a sequence of enzyme-catalyzed chemical reactions. It is defined as a part of the aerobic derivative process within eukaryotes called as cellular respiration, and it is a process that generates adenosine triphosphate (ATP) by oxidizing energy-rich fuel molecules. The Citric Acid Cycle is one of three stages of cellular respiration. Krebs was awarded the Nobel Prize in 1953 for the discovery of the citric acid cycle. Glycolysis and electron transport/oxidative phosphorylation are some of the important reactions involved. The components and reactions of the citric acid cycle were well-known by seminal work from Albert Szent Györgyi and Hans Krebs.

The citric acid cycle can be described in steps. The process starts with the shift of two carbon acetyl group from acetyl CoA to the 4 carbon acceptor compound to make a six carbon compound like citrate. The citrate then goes in the process of chemical transformation and losses its two carboxyl group and turn into CO2. It evolves from the oxaloacetate and indirectly from the acetyle CoA. The acetyle CoA donated carbon turn out to be a part of the oxaloacetate carbon after the beginning of the citric acid cycle but for the reason that the responsibility of the citric acid cycle in the anabolism, it is not lost.

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There are many TCA cycle intermediates which are also used as precursors for the biosynthesis of other molecules. The energy formed in the oxidative step is transferred as energy-rich electrons to NAD+ and it forms NADH. For each acetyl group that enters into the cycle of the citric acid, three molecules of NADH are formed. QH2 is produced because of the electrons transferred to the electron acceptor Q. The four carbon oxaloacetate has been regenerated and the cycle continues.

The net reaction for the defined cycle:

Acetyl CoA + 3NAD + FAD + GDP + Pi + 2H20  Co ASH + 3 NADH + FADH2 + GTP + 2CO2 + 3H

As it is mentioned in the kerbs cycle about the whole series of mitochondrial chemical reactions which are necessary for cellular respiration in all living things. The Krebs cycle cannot function without the three macronutrient components which are essential to every diet: fats, carbohydrates and proteins.

Electron Transport chain constitutes the re-oxidation of NADH to NAD which starts a sequence of oxidation/reduction reactions which produces two products and defined as bond energy synthesized by the reactions by which ATP is produced. Three ATP are produced for each NADH entering the reaction, whereas each FADH consequences in the synthesis of two ATP molecules and Water. In the electron transport chain the final electron acceptor is the oxygen and this reaction reduces the oxygen to water.

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Question 10

A germ layer referred to as a germinal epithelium and defined as a group of cells produced during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates and animals which are more complex than sponges (eumetazoans and agnotozoans) produce two or more primary tissue layers sometimes called primary germ layers. Animals with radial symmetry such as cnidarians form two germ layers (the ectoderm and endoderm) producing it diploblastic. Animals with bilateral symmetry form a layer between these two layers appropriately called as the mesoderm forming them triploblastic. Germ layers ultimately give rise to all of an animal's tissues and organs through the stages of organogenesis.


Fertilization leads to the formation of zygote. During the next stage, cleavage takes place (i.e.) mitotic cell divisions transforms the zygote into a tiny ball shaped of cells known as blastula. This premature embryonic form undergoes gastrulation, forming a gastrula with either two or more layers i.e. the germ layers. In all vertebrates, these are the forerunners of all adult tissues and organs. The look of the archenteron marks the onset of gastrulation.

After three days, in human the zygote forms a solid mass of cells by mitotic division,which is known as morula. Later the morula converts to a blastocyst, which consisting of an outer layer called atrophoblast, and an inner cell mass called the embryo blast. Filled with uterine fluid, the blastocyst breaks out of the zona pellucida and undergoes implantation. The inner cell mass primarily has two layers: the hypoblast and epiblast. The end of the second week, a primitive streak appears. The epiblast in this area moves towards the primitive streak, dives down and forms a new layer known as endoderm. By approaching the hypoblast out of the way goes on to form theamnion. The epiblast keeps moving and produces the second layer, the mesoderm. The top layer is known as the ectoderm.


The endoderm synthesizes tissue within the lungs, thyroid, and pancreas. The endoderm is the germ layers produced through animal embryogenesis. Cells migrating all along the archenteron form the inside layer of the gastrula, which develops into the endoderm. The endoderm consists of compressed cells at the first layer, which subsequently become columnar, and it forms the epithelial lining in the digestive tube excepting part of the mouth and pharynx and the terminal part of the rectum which are lined by involutions of the ectoderm.

Endoderm forms the lining cells of all the glands which release into the digestive tube including those which open in liver and pancreas. Endoderm helps to organize the epithelium of the auditory tube, tympanic cavity the trachea, bronchi, air cells of the lungs, the urinary bladder, part of the urethra, the follicle lining of the thyroid gland and thymus. The endoderm forms stomach, the colon,

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the liver, the pancreas, the urinary bladder, the lining of the urethra, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestine.

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Question 5

Eukaryotic cell division is complex process due to presence of multiple chromosomes within nucleus of cell generally bounded by membrane. This leads to process of DNA replication and partitioning being a complex. Eukaryotic cell mostly do not divide on a continuously and rather use a control mechanisms to prevent it from being cancerous or developed into tumours. The cells instead of dividing continuously get differentiated and perform various types of specific functions in an organism.

The control mechanism uses variety of check before and after each phase maintaining strict entry and exit points. This is carried out by Tumour suppressors enzymes or proteins. The most important of them are retinoblastoma susceptibility gene (pRB) and the p53 are both tumor suppressors.

The Eukaryotic cell division process consists of two main phases called Interphase and Mitosis. Interphase contributes to major part of the time taken for cell division whereas Mitosis is last part of division where cell gets divided into 2 daughter cells having exact replica of parent cell's DNA.


Interphase consists of three sub-processes as follows:

Gap1 or G1: In G1, the cell starts with preparation for division and replication of DNA which consists of protein synthesis for replication and analysis of external and internal environments to check that whether the all the conditions are favourable for cell division. The eukaryotic generally contains centrioles except plants which began to replicate at this stage and copies starts moving in opposite directions. Each of the centrioles pairs are surrounded by a centrosome.

Synthesis or S:

Synthesis phase performs the DNA replication and produced exact two copies of the chromosomes. This phase is the longest and may take up to 10-12 hours of typical 24hours division of eukaryotic cell.

Gap2 or G2:

Second gap phase or G2 is responsible for synthesis of proteins involve in assembly of processes responsible for the duplication and division of chromosomes called Mitosis. And further to division of parent cell into two identical daughter cells called Cytokinesis. G2 similar to G1 also verifies for optimal conditions of cell replication and try to take any precaution, if required. G2 has very important task of controlling the entry to final Mitosis phase.

The preparations also involve replication of centriole-chromosome complexes hat replicated during G1 now begin to support the formation of spindles to control the chromosomes while getting divided into two daughter cells.


Mitosis or M phase of the eukaryotic cell cycle performs the action of partition and dividing chromosomes, organelles and cytoplasm equally into daughter cell. Mitosis is divided into four stages viz. Prophase, Metaphase, Anaphase and Telophase. The total time taken by the Mitosis phase requires nearly 1-2 hours of the 24hours process cell division.

Prophase: The prophase prepares a package containing both DNA and protein. The prophase also divided sister chromatids formed during Synthesis as mentioned above and joined at Centomere. At the end the prophase, each of the daughter cell have exact replica of entire gene sequence of original cell and the nuclear envelope is partitioned into vesicles. This phase is also responsible for the formation of Kinetochores, a special structure that gets attached to the opposite ends of the centrosomes.

Metaphase: Metaphase commences with the lining of the cell at equatorial plate for cell division so that all the material consisting of chromosomes and other cell material gets divided equally among daughter cells. The chromosomes also have atleast 2 microtubules attached to it.

Anaphase: Anaphase starts with two sister chromatids moving to opposite directions for equatorial plate. This is perfomed to ensure that cell gets divided into exact copy without any concerns. The Chromatids starts moving opposite in directions leading to shortening of microtubules and nuclear envelopes also forms a around each set of chromatids.

621 words

Question 7

The Prokaryotes are a group of micro-organisms that lack a cell nucleus or other membrane-bound organelles. One of the prokaryotic organisms is the EColi. The DNA replication mechanism is very observed in EColi due to its simplicity. The Prokaryotic Cell Division and Replication is given by steps such as Initiation,,Elongation,,Termination and Regulation. Unlike Eukaryotics, the Prokaryotic Cell Division is not carried by dividing cell on an equatorial plate but it is a process of budding out daughter cells out of parent cell.


The initiation process is stated by the protein called DNA-A box that helps in binding the origin in a supercoiled pattern. Then process of melting of DNA-B boxes starts to initiated the process of breaking hydrogen bond between the A & T nucleotides. This is done with the help of Adenosine Tri Phosphate or ATP. Then the formation of replication fork begins by the help attaching DNA-B proteins to DNA-A proteins. DNA-C proteins then get attached to subunit of helicase in 6:1 ratio. Then 5 pairs of DNA-A dimmers and DNA-C released to termination of pre-priming process. The SSB or Single Strand Binding protein then prevents the formation of secondary structures by DNA-A proteins. The unwinding of DNA initiates and each DNA template is formed so that DNA synthesis can be activated.


The priming process as explained in previous phase makes way for DNA to replicate by loading DNA Polymerase III Holoenzyme. This enzyme acts as an catalyst uses mechanisms to differentitate between DNA and RNA. The nucleophilic attack on the alpha-phosphate of DNA and stabilize the negatively charged Tri-Phosphate on the DNA nucleotides. The attack results in formation PyroPhophates that hydrolizes to 2 phosphates which DNA synthesis towards completion.

The DNA Polymerase III also differentiates correct and incorrect pairs by Watson-Crick pairs that generally complement to correct pairing. To prevent any kind of ambiguity the replication occurs in opposite direction. The leading strand that is being replicated performs it in small fragment known as Okazai fragments which requires primers. The Okazai fragments are then used to fill gap between the DNA nucleotides and seal them with enzymes called Ligase.


Termination marks the end of DNA replication by the help of Tus protein and termination sequence which prevents two replication forks to proceed in one direction.DNA start with production of one parental and one synthesised strand. The catenation have two interlinks rings that can be separated by help of proteins.


The DNA replication regulation is a very important factor which can be achieved by several ways. The methods are ratio of ATP to ADP, ration between DNA-A to DNA-A Boxes and sequestering of ORI-C. The ATP to ADP is responsible for cell size and indicating ready to divide. The binding of DNA-A-ATP complex mainly carries out the DNA replication procedure. So, amount of the DNA-A-ATP is responsible for replication and its control. After one replication, the amount of DNA gets reduced and cell replication stops till amount DNA-A again is enough for replication.

499 words

Question 4

Chemiosmosis is the diffusion of ions through a selectively permeable membrane and it relate the generation of ATP by the association of hydrogen ions across a membrane throughout cellular respiration. ATP synthesizes the enzymes that make ATP by chemiosmosis. The protons bypass through the membrane with the kinetic energy to phosphorylate ADP and make ATP. The synthesis of ATP by chemiosmosis takes place in chloroplasts and mitochondria and in several bacteria.

The energy produced by chemosmosis is substantial. Thirty six molecules of ATP can be formed for each glucose molecule through cellular respiration of Krebs cycle, the electron transport system, and the chemiosmosis. Two ATP molecules are produced from glycolysis, the total of thirty eight molecules of ATP. Each ATP molecule is capable of releasing 7.3 kilocalories of energy per mole. These ATP molecules may be used in the cell. The ATP molecules can't be stored for a longer time as usual. Cellular respiration must be generated from ATP molecules as they are used.

The chemosmosis starts after the electron transport chain, letting H+ ions with their gradient to synthesize ATP. The H+ passively diffuses, which is called as passive transport. ATP is also called to be the energy currency, and where as the mechanism by which ATP was generated in the mitochondria was well explainable to be the substrate level phosphorylation. The hypothesis was the source for understanding the actual process to oxidative phosphorylation. The biochemical mechanism of ATP produced by oxidative phosphorylation was unidentified. ATP synthesizes the enzyme and produced ATP by chemiosmosis. The production of ATP by chemiosmosis starts in chloroplasts and mitochondria and in some bacteria.ATP permits protons to bypass through the membrane using the kinetic energy to phosphorylate ADP and generating ATP.

The H+ will diffuse from high proton application to lower proton concentration. Peter Mitchell predicted that an electrochemical concentration gradient of protons across a membrane could be formed to make ATP. The diffusion of water across a membrane is known as chemiosmosis. The transfer of charged ions across a membrane is affected by the electrical forces and thermodynamic forces, the affinity of substance to diffuse from regions of higher concentration.

A number of evidence support the chemiosmotic theory of ATP synthesis:-

(a) The internal of thylakoids can be used to generate low acid pH by suspending the chloroplasts into an acidic medium pH 4.0 for an instance of time. When these chloroplasts are circulated to a slightly alkaline medium pH 8.5, one with a lower concentration of protons and a supply of ADP and inorganic phosphate, spontaneously produces ATP. No light is required. 

(b) When isolated chloroplasts are illuminated, the medium within which they are suspended becomes alkaline - it would lead to a conclusion if protons were being detached from the medium and transferred into the thylakoids.

464 words

Question 9

Cytoskeleton Components









Controlling cell tension, maintain cellular shape, protuberance from cells and transduction

6-7 nm

G-Actin Protein

Helical structure

Maintains cell structure and tension to prevent it from rupture or shape change.

Intermediate Filaments

Controlling cell tension, maintainin cell structure, components for lamina and sacromeres in nucleus.

10-12 nm

Keratin and other keratin like proteins like vimetins, lamins, etc…

Twisted together in cord shape

Stronger bounding than Microfilaments & generally found in animals with few plants as exception.


Form cilia and flagella for cell movement, performs inter-cellular transportation, spindle during cell division, cell wall in plants.

23-25 nm

Globular proteins like α and β tubulin with Guanosine Triphosphate(GTP)  

Star shaped triplets for Centriole and growth of cylindrical tubes for cilia and flagella

Forms the part of cellular extension like Cilia, Flagella, Centriole and even Cell Wall in plants which in contrast to the other cytoskeleton components.