Cells and Genome

• Ancestors didn’t know about DNA or anything about cells still they called something “life”.

• Now as things changed, we describe “life” made up of tiny cells as:

  • Small

  • Membrane-bound entities

  • Covered with a plasma membrane

Key terms

• Cell Biology: It is the study of the structure, function, and behavior of cells.

• Heredity: It is the central definition of life and distinguishes life from other processes.

• Unicellular organisms: They are the simplest organisms with a naked nucleus and are devoid of membrane-bound organelles like mitochondria, chloroplasts, etc. Eg: Amoeba, Bacteria, etc.

• Multicellular organisms: They are organisms with a well-defined nucleus and membrane-bound organelles like mitochondria, chloroplast, etc. Eg: Trees, fungi, humans, etc.

• Genome: Totality of its genetic information as embodied in its complete DNA sequence.

DNA

• Building Block of Life: (Deoxy Ribonucleic Acid (DNA))

  • The basic unit of DNA: Nucleotide

  • Length of DNA is defined as no. of nucleotides, or a pair of nucleotides is referred to as base pairs present in it.

  • Nitrogenous Base + Pentose Sugar + Phosphate Group = Nucleotide

    • Nitrogen Bases:

      • Purines (Adenine and Guanine)

      • Pyrimidines (Cytosine and Thymine)

  • Guanine binds with Cytosine --> Triple Hydrogen bonds

  • Adenine binds with Thymine --> Double Hydrogen bonds

    • Pentose Sugar:

      • Deoxyribose (lacking one oxygen atom, that oxygen is removed from a 2nd-carbon atom of sugar)

    • Phosphate Group:

      • Phosphoric Acid (H3PO4)

Features of DNA

• The two chains have anti-parallel polarity. It means, one chain has polarity.

• It is made up of two polynucleotide chains.

• The distance between purine and pyrimidine makes the structure uniform.

Central Dogma of Life:

• Replication: The DNA replicates itself with the help of mainly the DNA-dependent DNA polymerase enzyme.

• Transcription: The process of copying the genetic material from one strand of the DNA into RNA is termed transcription.

  • There are three major types of RNAs: mRNA (messenger RNA); tRNA (transfer RNA); and rRNA (ribosomal RNA).

  • The mRNA provides the template.

  • The tRNA brings amino acids and reads the genetic code.

  • The rRNA plays a structural and catalytic role during translation.

• Translation: It refers to the polymerization of amino acids to form a polypeptide.

• Proteins: Catalysts

  • Monomers: Amino Acids

  • Amino acids are joined by a peptide bond.

  • The cellular factory for synthesizing proteins is the ribosome.

  • 20 types of amino acids.

  • Functions: Maintains structures, generates movements, senses signals, etc.

• Genetic Code:

  • They are triplets in nature.

  • 64 possible codons.

Plasma Membrane

• Phospholipid bilayer: It is formed of a double layer of lipid molecules sand witched between two layers of protein.

• Hydrophilic (water-loving) head towards the outside and Hydrophobic (water-hating) tail towards inside.

• Selectively Permeable (allows specific substances to pass through the cell).

• Smallest organism: Bacterium Mycoplasma genitalium.

Living organisms obtain energy in different ways:

• Organisms that feed on other living things or organic chemicals are called organotrophic. Eg: Fungi, bacteria in the human gut, etc.

• Organisms that make their food synthesize light are called phototrophic. Eg: Algae, plants, etc.

• Organisms that feed on rocks are called lithotrophic.

• Organisms that can fix nitrogen. Eg: plants of the pea family, cyanobacteria, etc.

Organisms are divided based on cell structure:

• Prokaryotes:

  • Primitive; Small; Simple.

  • No distinct nuclear compartment.

  • No membrane-bound organelles.

  • Live in variable ecological habitats.

  • The prokaryotes are divided into two groups:

    • Bacteria/ Eubacteria

    • Archaebacteria

• Eukaryotes:

  • Advanced; Complex

  • Distinct nuclear compartment.

  • Membrane-bound organelles are present.

• Bacteria (the primitive organisms):

  • Bacteria are the sole members of Kingdom Monera.

    insert bacteria picture

  • Eubacteria are true bacteria characterized by the presence of a cell wall. Eg: Blue-green algae (Cyanobacteria)

  • Bacteria are classified based on shape into four types:

    • Cocci (spherical)

    • Bacilli (rod-shaped)

    • Spirillum (spiral like)

    • Vibrio (comma shaped)

• Archaebacteria:

  • They are special and survive in some of the harshest habitats.

  • Extreme salty (halophiles), marshy areas (methanogens), and hot springs (thermoacidophiles)

Mutations

• An error or random accident which occurs during cell divisions.

Ways to incorporate genetic information into cells:

• Intragenic mutation: an existing gene can be randomly modified by changes in its DNA sequence, through various types of errors that occur mainly in the process of DNA replication.

• Gene duplication: an existing gene can be accidentally duplicated to create a pair of initially identical genes within a single cell; these two genes may then diverge in the course of evolution.

• DNA segment shuffling: two or more existing genes can break and rejoin to make a hybrid gene consisting of DNA segments that originally belonged to separate genes.

• Horizontal (intercellular) transfer: A piece of DNA can be transferred from the genome of one cell to that of another—even to that of another species. This process is in contrast with the usual vertical transfer of genetic information from parent to progeny.
  

Viruses:

• Acellular organism (neither dead nor living).

• Relies on the host organism for duplication.

• Evolved as parasites.

• Bacteria infecting viruses: Bacteriophage

Eukaryotes:

• Evolved from symbiosis.

• Have hybrid genomes.

• Genomes are big and rich in regulatory DNA. Eg: Pufferfish

• Genomes define the program of multicellular development.

• Live as Solitary Cells.

  

• Arabidopsis thaliana was chosen as a model plant to study the close evolutionary relationship between all flowering plants.

• Animal Cells studied are the nematode worm Caenorhabditis elegans, the fly Drosophila melanogaster, the zebrafish Danio rerio, the mouse Mus musculus, and the human, Homo sapiens.

• Fruit Fly Drosophila Melanogaster played a key role in the field of genetics.

• Powerful new technologies such as genome sequencing are producing striking advances in our knowledge of human beings and helping to advance our understanding of human health and disease.