CELL BIOLOGY

Structure and function of cell organelles

Cell is the fundamental structural and functional unit of life playing an integral part in giving structure, shape and performs essential biological or metabolic functions with the help of small structures called cell organelles or subcellular organelles. Subcellular organelles are found inside the cell and unique in their structure and respective functions (Byrne and Selvakumaraswamy, 2021). Their functions are coordinated in such a manner that makes the cell perform functions normally to sustain life. The primary structural difference between different cell organelles lies in the absence, presence or number of membranes surrounding them.

Without membrane- Ribosomes and cytoskeleton that are scattered in the cytoplasm.

Single membrane- Vacuoles, Endoplasmic Reticulum, Golgi apparatus and lysosome.

Double membrane- Mitochondria, chloroplast and nucleus.

 Types of Endocytosis

There are two types of endocytosis

1) Phagocytosis

Phagocytosis or cell eating is the biological process which encompasses engulfing cells, pathogens or particles more than 0.5 micrometre diameter. Leucocytes employ this mechanism to digest foreign materials mainly pathogens to provide immunity against diseases. Most common and primary example of living cells showing phagocytosis are (Chen and Huang, 2018) macrophages and neutrophils that enhances the immunity. This is initiated by binding of the particles to be ingested to the receptor at the cell's surface resulting in the formation of pseudopodia which gets fused with that particle to form a vesicle. This vesicle ultimately detaches from the cell membrane and gets transferred into the cell. Now, the vesicle gets destroyed by the content of the vesicle.  

2) Pinocytosis

This is commonly known as cell drinking in both animal and plant cells in which cell absorbs selective substances from external environment including nutrients and water. One of the advanced or specialised type of pinocytosis is receptor mediated in which macromolecules get attached to the receptors at the surface of cell membrane and gets transported inside the cell. Ex- cholesterol transport. This is initiated by the molecules that need to be ingested bind to the receptor at the cell's surface resulting in inward folding up of cell Assignment Help membrane making a vesicle containing both the molecules and the fluid. Ultimately, the pinocytic vesicle gets detached from the cell surface and gets transferred inside the cell  

Exocytosis

In this, the materials are moved from inside to outside of the cell and responsible for eliminating toxic substances or waste materials produced by the cellular metabolism for maintaining homeostasis, establishing communication between cells through molecules like neurotransmitters and hormones and growth, survival and repair.  In this, vesicles are formed (Cooper and Adams, 2022) inside the Golgi apparatus and ER and reaches the plasma membrane. Now, the vesicles fuse with the cell membrane and its contents are secreted into the space outside the cell leading to detachment of these vesicles from the cell's surface.

There are two types of exocytosis- regulated and constitutive.

Regulated- For molecules that are synthesized and utilised at different sites, this kind of exocytosis helps in packaging by surrounding with a single layer of cell membrane. In this, the release can be regulated and hence called regulated exocytosis.

Constitutive- It doesn't need any kind of signalling molecules like neurotransmitter.

Differences between endocytosis and exocytosis include:

Similarities between endocytosis and exocytosis include both involving cell membrane and mechanism of active transport for moving particles inside and outside of the cell via the formation of vesicle pores.

Structure of DNA

DNA (Deoxyribonucleic Acid) is the molecule that performs the function of carrying and passing on the hereditary or genetic information from parents to their offspring. The structure of DNA resembles to spiral ladder or more scientifically double helical structure composed of nucleotides. Nucleotide is the structural unit of DNA and made up of three various substances phosphate groups, nitrogenous base pairs and sugar backbone (deoxyribose). Phosphate groups and sugar connects the nucleotide to make strand of DNA. Adenine (A), Thymine (T), Guanine (G) and Cytosine (C) are the kinds of nitrogenous bases in which A links with T and G links with C. The sequence in which these nitrogenous bases links with each other decides the genetic code. Two strands combine together to form a DNA molecule. These two strands run parallel and opposite to each other in directions 5'-3' and 3'-5' respectively and are said to be complementary to each other. Two join these two strands together, hydrogen bonds play an important role. These two strands (Cramer, 2019) are twisted helically where one turn is composed of ten nucleotides. Each helix has a pitch of 3.4nm that means the distance between two adjacent nitrogenous base pairs is 0.34nm.  

RNA and its types

RNA (Ribonucleic acid) - It is single stranded, made up of single unit of ribose sugar and derived from DNA. In this the Adenine base pair is linked with Uracil base pair. It performs many essential functions including protein synthesis, producing new cells and an intermediate in many metabolic reactions. There are primarily three types of RNA, mRNA, tRNA and rRNa.

tRNA (transfer RNA) plays a significant role in selecting the protein needed by the body. It is situated at the terminal points of individual amino acid. It is also known as soluble RNA and it acts as a connecting link between the mRNA and that specific amino acid.

rRNA (Ribosomal RNA)

It is the unit of the ribosome and are present inside the cytoplasm, where ribosomes are located. In the living organisms, rRNA plays a significant part in the formation and converting mRNA into proteins called translation. rRNA is primarily made up of cellular RNA and is the most prominent RNA (80%) in the cells of all living organisms. (Del Pino, 2019)

mRNA (Messenger RNA)

This kind of RNA performs its functions by passing on the genetic or hereditary material in the ribosomes and conveys the messages regarding the protein's types, needed by the living cells of body. On the basis of these functions, these types of RNA is known as the messenger RNA. Hence, the mRNA has an integral part to play during the process of transcription or while protein synthesis.

After DNA is synthesized in the nucleus, it combines with another DNA strands to get packed into a structures called chromosomes. These chromosomes are only visible during cell division when they get condensed. RNA is synthesized in the nucleus and then transported to the site where they perform their function. For example- mRNA facilitates synthesis of protein and found in cytoplasm. tRNA is generally found scattered in the cytoplasm but reaches (Ehara, et. al., 2019) the ribosomes where gets attached to the mRNA. rRNA is present on the ribosomes scattered in the cytoplasm. Both mRNA and tRNA can move in the cytoplasm but rRNA is fixed on the ribosomes.

Transcription

Transcription is the biological conversion of DNA into mRNA in which only one strand is duplicated called template. This conversion is facilitated by the main enzyme DNA dependent RNA polymerase and gets completed in three consecutive stages including initiation, chain elongation and termination.

Initiation

The enzyme RNA polymerase gets attached to the DNA molecule followed by the movement along the line of the DNA strand by the time it identifies the promoter. These are called the initiation sites. This activates the unwinding of double helical DNA structure leading to exposure of the nitrogenous bases of both the strands (Kummer and Ban, 2021).

Chain Elongation

Ribonucleotides are attached to the template making the mRNA to grow or increase in length.

Chain Termination

On arriving at the terminator sequence, the polymerase activity is stopped disrupting the transcription process resulting in detachment of RNA polymerase from the template strand of DNA.

Checkpoints in cell cycle

Cell cycle is the sequence of events during a cell division. There are two types of cell division in eukaryotic cell mitosis and meiosis with different stages.( Neidle and Sanderson, 2021) Checkpoints in the cell cycle are the regulatory points or mechanisms that ensures controlled cell division unlike cancerous cells. The decision to proceed further or stop at the current stage is taken at these check points. There are primarily three stages G1, G2 and spindle checkpoints. While proceeding from G1 to S, G1 checkpoint decides whether the size of the cell is suitable to divide has adequate nourishment essential for cell division and without damages in DNA. While moving from G2 to M phase, G2 check point plays an essential role in ensuring that the DNA molecule has been entirely copied. While proceeding from metaphase to anaphase in both meiosis and mitosis, the spindle checkpoint comes into play to make sure that (Pollard, et. al., 2022) the chromosomes are correctly bind to and in right alignment along the spindle.

Differences between mitosis and meiosis are

Processes behind genetic diversity in humans

There are primarily two processes that ensures that DNA of the embryo is different from the parental DNA, crossing over and random assortment of chromosomes while cell division by meiosis. In crossing over, chromosomes having similar structure called homologous chromosomes cross over and exchange fragments of DNA leading to recombination resulting in alleles in new combination creating diversity in genetic makeup. (Tao, et. al., 2020) During random assortment, random segregation of homologous chromosomes occur on the metaphase plate resulting in division of paternal as well as maternal chromosomes arbitrarily during gamete formation with enhancing diversity in genetic makeup.

Influence of yolk on cleavage pattern

Yolk contains lipids and proteins necessary for developing and sustaining the embryo and its amount in eggs has significant implications on determining the pattern of cleavage during embryonic development. For example- in humans and frogs, eggs contain small amount of yolk resulting in symmetrical division of egg into little and approximately same sized fragments or cells on every division called holoblastic whereas in eggs of reptiles, fish or birds having (Voiculescu, et. al., 2020) high amount of yolk leading to its getting concentrated at only one pole leading to unsymmetrical at the site of low concentration (animal pole) while the other region does not divide.

Distribution of yolk in the egg influence the orientation as well as symmetry during division. In the eggs of birds and fish, concentration of yolk at one pole leading to its remaining undivided.

Gastrulation in humans, chick, sea star and frog

Gastrulation is a phase during embryonic development in which the major germ layers ectoderm, mesoderm and endoderm are created.

Sea star

Blastula- initial stage where embryo appears like a hollow sphere with cells. Then, a segment of cell depresses inwards resulting into blastopore.

Endoderm- Now, cell begins moving into the cavity called blastocoel in the blastopore creating the endoderm.

Meso and ecto- More number of cells begins entering into the blastocoel resulting in mesoderm first followed by ectoderm (Wamaitha and Niakan, 2018) 

Chick

Blastula-Initial stage and appears like disc shaped blastula. Formation of endoderm starts with the primordial band or line on the blastula's surface.

Endoderm- cells leave the blastula's surface and shifts the hypoblast resulting in endoderm.

Ecto and meso- More number of cells leave the blastula's surface and enters into the blastula to create mesoderm followed by ectoderm.

Humans- almost same as chick

Blastula-initial stage where formation of endoderm starts with the primordial band or line on the blastula's surface.

Endoderm- cells leave the blastula's surface and shifts the hypoblast resulting in endoderm

Ecto and meso- More number of cells start moving beside the endoderm resulting in the creation of mesoderm while the outermost cells develop into ectoderm.

Frogs- nearly same as sea star

Blastula- initial stage where embryo appears like a hollow sphere with cells. Then, a segment of cell depresses inwards resulting into blastopore.

Endoderm- Now, cell begins moving into the cavity called blastocoel in the blastopore creating the endoderm.

Ecto and meso- Certain cells start moving beside the endoderm resulting in the creation of mesoderm while the outermost cells develop into ectoderm.

Similarities between gastrulation in human and chick

• Gastrulation begins with the primordial band or line in both of them on which cells reach to transform and develop into these three layers.
• Both involves shifting the hypoblast resulting in development of endoderm.
• In both of them endoderm leads to formation of the outer linings of digestive tracts and other organs, mesoderm resulting in muscles, skeletal tissue and circulatory system and ectoderm giving rise to epidermis, nervous system and others.

Evaluation of gastrulation in humans

The formation of different layers in the gastrulation process is evaluated on the basis of which organs they develop into (Winklbauer, 2020). To begin with, endoderm is formed from shifting of hypoblast cells and develops into digestive system (stomach, liver and others), thymus, thyroid gland, lungs, trachea and other respiratory organs. Mesoderm is formed from movement of cells beside endoderm and develop into cartilages, bones, blood vessels and cells, kidneys and organs of reproductive system. Ectoderm is formed from the cells remaining at the outermost position and develop into nervous system, hair, nails, eyes, ears, glands like adrenal medulla and neurons.

Endomembrane System

Membranes perform various functions including serving as the barrier that regulates the entry and exit of molecules into and out of the cell, establishing communication between cells with their immediate surroundings, providing a site for reaction involving energy production like photosynthesis and respiration, formation of proteins, carbohydrates and lipids. Endomembrane system is mainly composed of membranes of ER, lysosomes, vacuoles, cell membrane and Golgi apparatus interconnected with each other. ER and Golgi apparatus are in close vicinity with their membranes almost connected facilitating the transfer of substances (Xiang, et. al., 2020) like lipids and proteins between them giving rising to interdependence between these organelles. The functions of endomembrane system is performed by coordinating and combining various cell organelles and includes: 

Synthesis of protein-

Rough ER having ribosomes are used for secretory process and delivered at other cell organelles. Proteins after synthesis enter into the lumen to go through folding and other modifications like glycosylation and then transferred to the Golgi apparatus in the form of vesicles. 

Modifying cellular products- Takes place in the Golgi apparatus by adding carbohydrates derivatives to the protein and modification of lipid molecules giving its unique structure and function.

Secretion of proteins

Now, the vesicles get detached from the Golgi apparatus and gets fused with the cell membrane releasing its contents like proteins outside the cell by exocytosis.

CONCLUSION

This report has addressed the extensive concepts in cell biology including cell division along with its check points and all the steps, DNA and RNA structure, transcription, how these are stored in the cell, embryonic development in chick, humans, frogs and sea star, cell structure and functions. This report concluded with gaining the understanding of coordination in the activities of cell organelles to carry out essential biological functions for sustaining life. 

REFERENCES

• Books and journals
• Byrne, M. and Selvakumaraswamy, P., 2021. Evolutionary modification of gastrulation in Parvulastra exigua, an asterinid seastar with holobenthic lecithotrophic development. Evolution & Development, 23(2), pp.63-71.
• Chen, B. and Huang, S., 2018. Circular RNA: an emerging non-coding RNA as a regulator and biomarker in cancer. Cancer letters, 418, pp.41-50.
• Cooper, G. and Adams, K., 2022. The cell: a molecular approach. Oxford University Press.
• Cramer, P., 2019. Organization and regulation of gene transcription. Nature, 573(7772), pp.45-54.
• Del Pino, E.M., 2019. Embryogenesis of marsupial frogs (Hemiphractidae), and the changes that accompany terrestrial development in frogs. Evo-Devo: Non-model Species in Cell and Developmental Biology, pp.379-418.
• Ehara, H., Kujirai, T., Fujino, Y., Shirouzu, M., Kurumizaka, H. and Sekine, S.I., 2019. Structural insight into nucleosome transcription by RNA polymerase II with elongation factors. Science, 363(6428), pp.744-747.
• Kummer, E. and Ban, N., 2021. Mechanisms and regulation of protein synthesis in mitochondria. Nature Reviews Molecular Cell Biology, 22(5), pp.307-325.
• Neidle, S. and Sanderson, M., 2021. Principles of nucleic acid structure. Academic Press.
• Pollard, T.D., Earnshaw, W.C., Lippincott-Schwartz, J. and Johnson, G., 2022. Cell Biology E-Book: Cell Biology E-Book. Elsevier Health Sciences.
• Tao, Y., Yang, Y., Zhou, R. and Gong, T., 2020. Golgi apparatus: an emerging platform for innate immunity. Trends in cell biology, 30(6), pp.467-477.
• Voiculescu, O., 2020. Movements of chick gastrulation. Current Topics in Developmental Biology, 136, pp.409-428.
• Wamaitha, S.E. and Niakan, K.K., 2018. Human pre-gastrulation development. Current topics in developmental biology, 128, pp.295-338.
• Winklbauer, R., 2020. Mesoderm and endoderm internalization in the Xenopus gastrula. Current topics in developmental biology, 136, pp.243-270.
• Xiang, L., Yin, Y., Zheng, Y., Ma, Y., Li, Y., Zhao, Z., Guo, J., Ai, Z., Niu, Y., Duan, K. and He, J., 2020. A developmental landscape of 3D-cultured human pre-gastrulation embryos. Nature, 577(7791), pp.537-542.
  Read More- Entrepreneurship and The World Economy

UPTO51%

Avail The Benefit Today!

To View this & another 50000+ free

Enter Email

Submit