Cell Structure and Components

Study Notes

Prokaryote cells: no nucleus, membrane-bound organelles, genome is a circular plasmid DNA, plasmid is important for gene modification
Eukaryote cell structure: organelles are compartments with specific functions, nucleus contains DNA, nucleoproteins, and RNA
Nucleus: stores DNA, coordinates cell activities, has two forms of DNA: heterochromatin (condensed, dark, inactive) and euchromatin (uncondensed, light, active)

Nucleolus: site of ribosomal RNA synthesis, found within the nucleus
Nuclear envelope: double-layered membrane with nuclear pores, controls what enters and exits the nucleus
Cytoplasm: site of metabolism, protein synthesis, and fatty acid synthesis
Cytosol: involved in metabolism, protein synthesis, and cell signaling
Endoplasmic reticulum (ER): rough ER has ribosomes attached, involved in protein synthesis, smooth ER is involved in lipid synthesis
Golgi apparatus: modifies and packages proteins and lipids for delivery
Mitochondria: double-membraned organelle, involved in ATP production, has its own DNA and ribosomes
Ribosome: site of protein synthesis, composed of two ribosomal RNA subunits
Lysosomes: involved in intracellular digestion, has acid hydrolases
Peroxisomes: involved in chemical detoxification and lipid metabolism
Proteosome: degrades intracellular proteins
Endosome: involved in sorting and delivery of lipid vesicles and their contents

Central dogma: genetic information flows from DNA to RNA to protein
DNA organization: double-stranded DNA is wrapped around histones to form nucleosomes, which form 'beads on a string DNA'
Nucleosomes can tightly pack into solenoid structures, forming 30nm fibres, which are compacted into chromosomes
DNA replication: semi-conservative, each strand of the original DNA serves as a template for the synthesis of a new complementary strand
DNA polymerase: adds nucleotides to the growing DNA chain, has proofreading and editing functions
Okazaki fragments: short, discontinuous DNA segments synthesized in the lagging strand during DNA replication

Types of stem cells: totipotent, pluripotent, multipotent, oligopotent, unipotent
Stem cell differentiation: environmental cues cause changes in gene expression, leading to cell remodelling and differentiation

Semiconservative model: each strand of the original DNA serves as a template for the synthesis of a new complementary strand
Conservative model: the original DNA molecule remains intact, and an entirely new DNA molecule is synthesized
Dispersive model: the original DNA molecule breaks into fragments, which serve as templates for the synthesis of new DNA

Demonstrated the semi-conservative model of DNA replication
Used isotope labelling, density gradient centrifugation, and sampling at intervals to observe the density of DNA molecules

Demonstrated the existence of DNA polymerase
DNA polymerase I has three enzymatic activities: 5' to 3' DNA polymerizing, 3' to 5' exonuclease, and 5' to 3' exonuclease### Cell Division and Telomere Maintenance
Telomere maintenance allows cells to undergo multiple rounds of cell division without losing essential genetic material, making it important in stem cells and rapidly dividing cells.

Telomerase is active in germ cells, stem cells, and certain immune cells, but most somatic cells have limited telomerase activity.
With each round of cell division, telomeres gradually shorten in somatic cells.
This process is thought to contribute to cellular aging, leading to cell senescence or apoptosis (cell death).

Telomeres are maintained by the action of telomerase, which adds repetitive DNA sequences to the ends of chromosomes on the 3’ of the lagging strand during DNA replication.