Flow of Molecular Genetic Information Within the Cell

The flow of genetic information in cell is illustrated by ‘central dogma’. According to this model, information encoded in DNA is transferred from DNA to RNA and then to proteins via translation.

DNA Replication: The basic property of any living organism is to reproduce. The only way to generate new cells is to divide existing ones. So, cell division is essential for the survival of all creatures. DNA contains the genetic infor­mation that each cell requires. When a cell splits, all of its DNA must be completely replicated in order to copy the biological information to be passed on to the daughter cell. DNA replication is the name for this process. DNA repli­cation occurs in nucleus of eukaryotic cell.

Transcription: Transcription is the initial stage in gene expression, when information from a gene is used to build a functional product like a protein. The objective of transcription is to create an RNA copy of a DNA sequence of gene. The information needed to make a polypeptide is carried by the RNA copy, or transcript, of a protein-coding gene. Before being translated into proteins, eukaryotic transcripts must go through some processing steps. Transcription occurs in nucleus of eukaryotic cells. Transcription is regulated at various level by specific interaction of transcription factors, promoters, and other regulatory sequences of DNA which ultimately leads to gene expression control.

Reverse Transcription: Reverse transcription is the process by which double-stranded DNA is created from an RNA template with the help of reverse transcriptase enzyme. Reverse-transcribing RNA viruses (such as retroviruses) reverse-transcribe their RNA genomes into DNA with reverse transcriptase, becomes integrated into the host genome and replicates along with it.

RNA Processing: After transcription, newly synthesized RNA, also known as the primary transcript, is further processed before becoming functional. RNA processing differs according to the type of RNA viz. messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Translation: Translation is the process of deciphering mes­senger RNA (mRNA) and utilizing its information to produce a polypeptide, or chain of amino acids. This process is carried out in the ribosomes present in cyto­plasm of the cell.

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Genetic Code

In an mRNA, groups of three nucleotides include instructions for making a polypeptide. These are called codon. Sixty-one different codons are there to code different amino acids constituting protein. Three codons (UAA, UAG, and UGA) are ‘stop’ codons that indicate the end of polypeptide synthesis. One codon, AUG, is considered as ‘start’ codon that kicks off translation process. The relationships between mRNA codons and amino acids are known as the genetic code.

Learning Outcomes

• Cell theory: Cell theory is one of the fundamental ideas of biology. The tenets to the cell theory are (1) the cell is the fundamental unit of structure and function in living things, (2) all organisms are made up of one or more cells, (3) ‘Omnis cellula e cellula’, i.e. ‘All cells arise from pre-existing cells’. Theodor Schwann, Matthias Schleiden, and Rudolph Virchow are credited with the formulation of the theory.

• Fluid-mosaic model: This is the most accepted model of biomembrane, depicted it as dynamic

(continued) structure with the ability to fuse or separate, expand, or contract during diverse cellular processes. The model was proposed by Singer and Nicolson in 1972.

• Dictyosome: In plant cells, the Golgi complex is made up of dictyosomes, which are a collection of disconnected components.

• ATP Synthase: Inner membrane and cristae of mitochondria possess small tennis-racket like ATP synthase dimer (earlier known as elementary particles or oxisomes).

• Photorespiration: In photosynthetic plants, photo­respiration or oxidative C2 cycle or the oxidative photosynthetic carbon cycle occurs in peroxisomes. Peroxisomes remain associated with mitochondria and chloroplast. Glycolate from chloroplasts is transported to peroxisomes. Glycolate is oxidized to glyoxylate by catalase in peroxisome. Subse­quently, glyoxylate is converted to glycine.

• Euchromatin: Euchromatin refers to the light- packed, transcriptionally active areas of chromatin.

• Heterochromatin: Heterochromatin refers to the condensed form of chromatin.

• Nucleolar Organizer Region (NOR): NOR is the crucial place of chromosome for the formation of chromosome. NOR connects the nucleolus to the chromatin.

• 9+0 arrangement: A centriole possesses a cart­wheel structure in transverse section. At the periph­ery of the wheel, there are nine peripheral fibrils in 9+0 arrangement.

• 9+2 arrangement: Shaft of flagella or cilia has axoneme structure. Axoneme has 9 doublet fibrils at periphery and 2 singlet fibrils at the centre (9+2 arrangement).

Exercises

Objective Questions

Q1. Which organelle is involved in cell wall synthesis?

Q2. Which type of membrane protein spans the entire width of the membrane?

Q3. Desmosomes are concerned with

Q4. Which is the smallest cell organelle?

Q5. Which organelle is involved in glycosylation of proteins?

Q6. Amphisome is the hybrid organelle produced by fusion of and.

Q7. What is the marker enzyme of lysosome?

Q8. Who coined the term mitochondria?

Q9. Who built the first microscope?

Q10. Which cell organelle helps in the formation of root hair?

Q11.

Q12. Who stated ‘Omnis cellula e cellula'?

Q13. Where is Photosystem I in plants found?

Q14. Which divalent ion is required for attachment of small and larger subunits of ribosome?

Q15. How many types of intermediate filaments are found in cell?

Q16. What is diplosome?

Q17. What is kinetosome?

Q18. In which plant cell centriole is found?

Q19. What are ciliopathies?

Q20. Who proposed endosymbiont theory?

Subjective Questions

Q1. What is cellular autonomy in unicellular organism?

Q2. Describe the structure of plasmodesmata with the help of a diagram.

Q3. Describe the subunit structure of ATP synthase.

Q4. Describe different types of vacuoles.

Q5. Differentiate between microtubules, intermediate filaments, and microfilaments.

Q6. Describe fluid-mosaic model of cell membrane.

Q7. Describe the structure of centriole. What are their functions?

Q8. Describe endosymbiont theory.

Q9. Name the different types of plastids. Describe the structure of chloroplastid.

Q10. Describe the ultrastructure of cilia and flagella.

Q11. What are the functions of Golgi apparatus?

Q12. What are the chemical compositions of cell membrane?

Q13. What is the difference between plasma membrane and cell wall?

Q14. Describe different components of prokaryotic cell.

Q15. What are the different functional sites of ribosomes? Write their functions.

Answer to Objective Questions

A1. Golgi Apparatus

A2. Transmembrane protein

A3. Cell adherence

A4. Ribosome

A5. Golgi complex

A6. Endosomes, autophagosomes

A7. Acid phosphatase

A8. Carl Benda

A9. Zacharias Janssen

A10. Golgi apparatus

A11. Nucleolus

A12. Rudolph Virchow

A13. Appressedpartofgrana

A14. Mg²+

A15. Four types

A16. The pair of centrioles are known as diplosome.

A17. Basal body of cilia and flagella is also known as kinetosome.

A18. Bryophytes, pteridophytes, and cycads

A19.

A20. Lynn Margulis

Keywords for the Answer to Subjective Questions

A1. A cell of a unicellular (single celled) organism can exist independently, i.e. it does not depend upon any other cell for any function, material, or information.

A2. Plasmodesmata are cytoplasmic bridges in the shape of minute pores that connect adjoining plant cells to form a continuous protoplasm. Within plasmodesmata, an endomembrane-derived structure connecting Endo­plasmic Reticula (ER) of two adjacent cells (desmotubule) is present.

A3. F_o, F₁ particles. F₁—α and β, one each of subunits γ, δ, and ε; F_o—a, b, d, F6, and the oligomycin sensitivity­conferring protein (OSCP).

A4. Sap vacuoles, contractile vacuoles, food vacuoles, air vacuoles.

A5. Diameters are different.

A6. Mosaicism, fluidity, asymmetry.

A7. Cart-wheel structure with 9+0 arrangement. (a) Centrioles facilitate in cell division with the forma­tion of MTOC. (b) The distal centriole develops in axial filament or tail in spermatozoa. (c) Centrioles can develop into basal bodies from which cilia and flagella are formed.

A8. According to the endosymbiotic theory, some of the organelles in today’s eukaryotic cells were previously prokaryotic bacteria.

A9. Leucoplast, chromoplast, chloroplast. Chloroplastid— thylakoid, stroma, grana.

A10. Basal body, rootlets, basal plate, shaft.

A11. (a) Secretion of biomolecules (b) Synthesis of glycoproteins and glycolipids (c) Formation of specialized structures (d) Synthesis of cell wall components.

A12. Water-20%, Carbohydrates-1-5%, Lipids-20-79%, Proteins-20-70%.

A13. Size, occurrence, function.

A14. Nuclear material, Vacuoles, Ribosomes, Photosyn­thetic thylakoids, Cell wall, Capsule, Flagella, and fimbriae.

A15. (a) mRNA binding site, (b) A-site or aminoacyl site where mRNA codon and aminoacyl tRNA are directed during translation process, (c) P-site or Peptidyl site where peptide bond formation, elongation, and transfer of peptide chain to site A occurs, and (d) E-site or Exit site.

Further Reading

Textbooks

Alberts B, Bray D, Hopkin K, Johnson AD, Lewis J, RaffM, Roberts K, Walter P (2013) Essential cell biology, 4th edn. Garland Publishing Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H,

Martin KC, Yaffe M, Amon A (2021) Molecular cell biology, 9th edn. Macmillan Learning

Research or Review Articles

Brenner S, Jacob F, Meselson M (1961) An unstable intermediate carrying information from genes to ribosomes for protein synthesis. Nature 190:576-581

Danielli JF, Davson H (1935) A contribution to the theory of permeabil­ity of thin films. J Cell Comp Physiol 5(4):495-508

Koonin EV (2000) How many genes can make a cell: the minimal-gene­set concept. Annu Rev Genomics Hum Genet 1:99-116

Novikoff AB, Beaufay H, de Duve C (1956) Electron microscopy of lysosome-rich fractions from rat liver. J Biophys Biochem Cytol 2(4):179

Singer SJ, Nicolson GL (1972) The fluid mosaic model of the structure of cell membranes. Science 175(4023):720-731