APS138 Molecular and Cell Biology Flashcards
How is DNA packed in the nucleus?
- DNA is wrapped around histones to create chromatin
- Density of this varies and acts as a regulator
Tightly packed chromatin
HETEROCHROMATIN
- Prevents RNA Pol binding
- Therefore is ‘silent’
- Characterised by +ve histone tails
- Allows tighter binding of histones to DNA
- Associated with Methylated DNA which is inaccessible
Lightly packed chromatin
EUCHROMATIN
- Decondensed by active regulatory mechanisms that allow transcriptional machinery to access DNA
- Associated with unmethylated DNA and is accessible
What enzymes condense chromatin?
Histone Deacetylase
Histone Demythylase
(Modify the histone tails)
What enzymes decondense chromatin?
Histone acetyl transferases
Histone methyl transferases
(Neutralise +ve charges)
DEFINITION: Epigenetic Trait
A trait that can be passed on by meiosis
Inactive RNA Pol 2
- RNA Pol has a tail that is not phosphorylated
- It is not bound to a promotor
- Gene & Promotor are typically cytosine-methylated and associated with tightly packed chromatin
Paused/ Stalled State of RNA Pol 2
- Gene is almost ready to be transcribed
- Transcription factor is bound to the promotor (could also be sat on gene body)
- RNA Pol 2 is phosphorylated at Serine 5 but remains inactive
Elongation/ Active State of RNA Pol 2
- More transcription factors bind to regulatory elements in the promotor
- RNA Pol 2 gets phosphorylated at Serine 2 and 5 which triggers the transcription progress
RNA Processing- Splicing and Alteration of mRNA ends
- 5’ & 3’ ends are capped
3’ end is also subjected to polyadenylation - Introns are spliced out by the spliceosome
Alternative splicing provides more regulation
DEFINITION: Ubiquitous Regulators
Small RNAs that exist everywhere and throughout cells regulating gene expression
Double stranded and roughly 19-30 nucleotides long
3 Examples of Ubiquitous Regulators
- microRNA (miRNA)
- small interfering RNA (siRNA)
- piwi interacting RNA (piRNA)
Post Transcriptional Gene Silencing- Small RNAs
- Repression of gene translation - miRNA
2. Promote mRNA degradation - miRNA & siRNA
Transcriptional Gene Silencing- Small RNAs
- Repression of gene transcription via chromatin remodelling and DNA methylation (recompensed) - siRNA from double stranded RNA or piRNA from single stranded RNA
miRNA Basics
- Come from non coding genes
- The genes have complimentary sequences to a coding gene elsewhere in the genome
- This produces a tertiary structure that resembles hair pins
- These are recognised by RNA induced gene silencing complex
How miRNA works
- Once recognised by a gene silencing complex the larger pre miRNA is chopped up by enzymes
- They’re downloaded one strand at a time onto Argonaute proteins which protect them
- Helps them bind onto the gene that needs silencing
DEFINITION: Ubiquitin
- A small protein that regulates the turnover of proteins
- Addition of ubiquitin targets a protein for degradation
- Form of gene regulation after synthesis
DEFINITION: Glycosome
Peroxisome involved in glycogen storage and metabolism
DEFINITION: Coenocytic Structure
A multinucleate structure
Reasons for compartmentalisation
- Maintain different environments
- Metabolic Regulation
- Sequestrion of toxic substances
- Cells secrete and internalise large numbers of proteins
Uses for Protein Filaments
- form 3D mesh which creates a rigid shape and structure
- for movement, forms trackways
- almost exclusive for animal cells and associated with location and translocation of the nucleus
DEFINITION: Actin
- Fibres 3-6nm diameter
- Used for gliding, contraction and cleavage
- With myosin responsible for muscle contraction
DEFINITION: Microtubules
- Determine cell shape
- Provide a trackway for movement of cell organelles and vesicles
- Form spindle fibres in mitosis
- Inside the flagella and cilia
DEFINITION: Intermediate Filaments
- 8-12nm diameter
- Give cell flexibility
- Anchor and position the nucleus
Two types of motor protein involved in the movement of molecules and vesicles
- Kinesin
2. Dynein
DEFINITION: Kinesin
- Motor protein
- Travels towards the plus end
- Away from the nucleus
- Requires ATP
DEFINITION: Dynein
- Motor protein
- Travels towards the minus end
- Towards the nucleus
- Requires ATP
DEFINITION: Melanocytes
- Specialised cells containing melanosomes which allow fish such as cephalopods to change colour
- Motor proteins transport these along microtubule/ actin tracts
Function of Cortisol microtubules
- Form a template for the deposition of cellulose in bands
- Leads to turgor driver cell growth which is constrained along the X axis
- Move in parallel with cellulose synthase complexes
Rough Endoplasmic Reticulum
- Processes folding and assembly of proteins
- Protein translocates via translocation pores
- N-Terminal Signal Peptide is removed whilst Nascent Polypeptide emerges into lumen
- Lumen is specialised for folding, assembly, modification, quality control and recycling
- Proteins undergo glycosylation and disulphide bond formation
What makes Endoplasmic Reticulum Dynamic in Plant and Animal Cells
Plant Cells: Movement depends on actin/ myosin
Animal Cells: Movement relies on microtubules
The Golgi Apparatus
- Distribution, shipping and manufacturing department of cells chemical products
Role of Endoplasmic Reticulum & Golgi in milk secretion
- Transport of ions & water across membrane into the lumen
- Smooth ER forms cytoplasmic lipid vesicles & lipid secretion
- Rough ER synthesises milk protein (casein) modified in the lumen, then delivered to Golgi where lactose is synthesised
- Vesicular movement of immunoglobulins protect from diseases
Ricin in Castor Beans
- The beans contain ricin which is a cytotoxin that inhibits protein synthesis and is for anti herbivory
- When seeds germinate its rapidly degraded
- Initially synthesised as proricin and is not catalytically active until proteolytically cleaved within protein bodies so it doesn’t poison itself
DEFINITION: Chaperones
Proteins that assist correct intracellular folding and polypeptide assembly
Quality Control in the ER
- Mutant proteins which have been incompletely or incorrectly folded are retained in the ER and degraded by lysosomes and proteosomes
ER Quality Control and Cystic Fibrosis
- Cystic Fibrosis transmembrane conductance regulator (CFTR) is a chloride ABC Transporter
- Mutated version of the channel protein still works but is identified and degraded
- Not enough channels in the epithelial lining so fail to take up enough Cl- from the lumen
- Causes yucas accumulation
Roles of Vacuoles/ Lysosomes
- Storage of carbohydrates, organic acids, anthocyanin, seed storage proteins
- Isolate toxic substances
- Anti herbivory, produces tannins, ricin
- Maintain internal hydrostatic pressure
Cyanogenesis in plants
- Method for anti herbivory
- Plants containing high levels of cyanide
- If consumed too much can cause chronic cyanide poisoning
- Cyanide ions interfere with ion containing respiratory enzymes
Cyanogenesis in animals
- Larvae of the Burnet Wasp can sequester cyanogenic glucosides
- Stored in viscous droplets in cuticle cavities in the integument
- Adults then release it as a pheromone to attract males
Endosymbiotic Theory
Concept that ancestral eukaryotes encorperated with aerobic bacteria to become more efficient, became the mitochondria
Also occurred for the chloroplast
Secondary Endosymbiosis
- Plastids contains multiple membrane envelopes which suggests multiple endosymbiosis events occurred
Repeated Endosymbiosis
- Termed as the driver of plant/algal diversity
- Believed through multiple endosymbiosis all higher plants evolved from algae
- Multiple membranes support the theory
- Different types of chlorophyll also support this; red and green chlorophyll from green lineage and others form red lineage