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Summary Robbins CHAPTER 1 The Cell as a Unit of Health and Disease
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Comprehensive summary of the chapter 1 of Robbins and Cotran Pathologic Basis of Disease 10th edition
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ROBBINS AND COTRAN PATHOLOGIC BASIS OF DISEASE 10TH EDITIONCHAPTER 1 The Cell as a Unit of Health and Disease
Prepared by: Omid J. Siahmard
THE GENOME
Noncoding DNA
Within the human genome, only 20,000 genes encode proteins constituting just 1.5% of
the genome.
Five Major Classes of Functional Non-Protein-Coding Sequences in the Human Genome
o Promoter and enhancer regions
Provide binding sites for transcription factors
o Binding sites for factors that organize and maintain higher order chromatin
structures
o Noncoding regulatory RNA
Not translated BUT regulate gene expression
Examples: micro RNAs (miRNAs) and long noncoding RNAs (lncRNAs)
o Mobile Genetic Elements
Example: transposons; “jumping genes” move around the genome during
evolution resulting in variabilities
o Telomeres and Centromeres (special structural regions of DNA)
Satellite DNA
Major component of centromeres
Repeating sequences
Associated with spindle apparatus attachment
Maintains the dense, tightly packed organization of heterochromatin
The two most common forms of DNA variation in the human genome are single nucleotide
polymorphisms (SNPs) and copy number variations (CNVs)
o SNPs
Variants at single nucleotide positions and are almost always biallelic
Can occur across the genome (introns, exons, intergenic regions)
Roughly 1% of SNPs occur in coding regions
o CNVs
Form of genetic variation consisting of different numbers of large contiguous
stretches of DNA
Approximately 50% of CNVs involve gene-coding sequences
Epigenetics – Heritable changes in gene expression that are not caused by variations in
DNA sequence.
Histone Organization (Epigenetic factors)
Nucleosomes consist of DNA segments 147 bp long that are wrapped around a central core
structure of highly conserved molecular weight proteins called histones.
At the light microscopic level, nuclear chromatin is recognizable as:
o Heterochromatin: cytochemically dense and transcriptionally inactive
o Euchromatin: Disperse and transcriptionally active
Histone methylation
o Lysines and arginines can be methylated by specific writer enzymes
o Methylation of histone lysine can lead to either transcriptional activation or
repression.
Histone acetylation
, o Lysine residues are acetylated by HATs (histone acetyltransferases)
Resulting in opening of the chromatin and transcription
o Histone deacetylases (HDACs)
Reverses action of HATs; leads to chromatin condensation and inactivation.
Histone phosphorylation
o Serine residues can be modified by phosphorylation
o May result in either activation of inactivation of chromatins
DNA Methylation
o Typically results in transcriptional silencing
Micro-RNA and Long Noncoding RNA
Transcribed but not translated
Important role in gene regulation
Micro-RNA
o miRNAs do not encode proteins
o they modulate translation of target mRNAs
o Does posttranscriptional silencing
o miRNA transcription -> primary miRNA -> pre-miRNA -> exported out of the
nucleus -> trimmed by cytoplasmic Dicer enzyme -> mature double-stranded
miRNAs -> unwinds and incorporate with RNA-induces silencing complexes (RISC)
-> posttranscriptional silencing
Long Noncoding RNA
o lncRNAs can bind to chromatin and restrict RNA polymerase from accessing coding
genes within that region
o Example: XIST
Transcribed from X chromosome
Essential role in X chromosome inactivation
Excaped X inactivation but forms a repressive cloak on the X chromosome
resulting in gene silencing
o Can facilitate: Gene activation, gene suppression, chromatin modification, assembly
of protein complexes
CELLULAR HOUSEKEEPING
Maintains normal functioning and intracellular homeostasis
Rough endoplasmic reticulum (RER) and Golgi apparatus
o Assemble new protein destines for the plasma membrane or secretion
Free ribosomes
o Synthesize proteins intended for the cytosol
Breakdown of molecules, proteins, and organelles takes place at three different sites:
o Proteasomes
Disposal complexes that degrade denatured or otherwise “tagged” cytosolic
proteins
o Lysosomes
Site of senescent intracellular organelle breakdown (autophagy) and where
phagocytosed microbes are killed and catabolized
o Peroxisomes
Contain catalase, peroxidase, and other oxidative enzymes
Contribute in the breaking down of very long-chain fatty acides, generating
hydrogen peroxide in the process
Endosomal vesicles – shuttle internalized material to the appropriate intracellular sites
Cytoskeleton
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