Cell structure Flashcards
What is epigenetics?
Control of gene expression by the modification of DNA
Stable, heritable phenotype caused by changes in a chromosome without alterations in DNA sequence
Genes can be switched on/off by internal/external signals
What are epigenetics affected by?
Development (during childhood) Environment Drugs/Pharmaceuticals Ageing Diet
Whats a nucleosome?
DNA wound and packed around histone proteins= chromatin
Whats a chromosome?
Carries genetic info in the form of genes
Threadlike structure containing nucleic acids and proteins
Found in the nucleus
Chromosome structure
Nucleosomes wound together at different levels of superstructure
Charge of DNA
-ve
Charge of histones
+ve
What does DNA’s fractal globule structure mean?
Folded in a way keeping neighbours along NDA helix while remaining completely unknotted
How do gene regulatory proteins recognise DNA sequence info?
Outside of NDA double helix is studded with DNA sequence info that gene regulatory proteins can recognise without having to open the double helix
What 4 ways can genes be regulated?
- Transcriptional
- Post transcriptional
- Translational
- Post-translational
Transcriptional control
Genes can be turned on/off Heterochromatin Euchromatin - heterochromatin is such part of the chromosomes, which is a firmly packed form and are genetically inactive, while euchromatin is an uncoiled (loosely) packed form of chromatin and are genetically active Histone modification
Heterochromatin
DNA tightly wound around histones= chromatin
Causes chromosomes to be visible during cell division
Transcription of genes not possible as RNA polymerase can’t access the genes
So no transcription during cell division
Euchromatin
DNA loosely wound around histones
Present during interphase
So protein synthesis occurs during interphase
Histone modification- Acetylation
Acetylation/phosphorylation reduces positive charge on histones
so DNA coils less tightly
allowing certain genes to be transcribed
Histone modification- Methylation
Makes histones more hydrophobic
Bind more tightly to DNA
DNA coils more tightly
Preventing transcription of genes
Post transcriptional control
RNA processing
RNA editing
Post transcriptional- RNA processing
pre-mRNA modified to mRNA
modified nucleotide added to 5’ end
tail of adenine nucleotides added to 3’ end
Helps stabilise mRNA and prevents degradation in cytoplasm
Splicing- introns (non-coding DNA) are removed and exons are joined together
All happens in nucleus
Post- transcriptional- RNA editing
Nucleotides of some mRNA sequences can be added/deleted/substituted
Causes proteins to be made with different functions
Translational-control
Degradation of mRNA
Binding of inhibitory molecules to mRNA
Activation of initiation factors
Translational-control- mRNA degradation
more resistant mRNA= longer it will stay in cytoplasm
so more protein is synthesised
Translational control- inhibitory proteins
Inhibitory proteins bind to mRNA
prevents mRNA binding to ribosomes
no protein synthesis
Translational control- initiation factors
Initiation factors are activated
help mRNA bind to ribosomes
so more protein synthesis
Post translational control
Modifications of proteins:
- Adding carbohydrate/lipids/phosphates
- Adding bonds such as disulphide bridges
- Folding/shortening of protein
Role of nuclear envelope in an animal cell@?
Separates chromatin from cytoplasm
Peroxisome in animal cell?
Membrane bound organelle
oxidises and breaks down fatty acids
Cytoskeleton
Microfilaments- actin
Microtubules- tubulin
Intermediate fibres
Function of cytoskeleton
Network of fibres that gives cell stability and structure
Organelle movement
Cell division
Microtubules make spindle fibres for cell division
What are centrosomes?
Organelles that produce microtubules
Role of cell membrane
Cell compartmentalisation
Endocytosis/exocytosis
How do enzymes as biological catalysts help?
1) Greater reaction specificity
2) Function in milder (physiological) reaction conditions
3) Can be regulated
4) They make endpoint more favourable
Molecular hierarchy of structure
- Cells and organelles
- Chromatin, plasma membrane, cell wall= supramolecular complexes
- DNA, protein, carbohydrates= macromolecules
- Amino acids, nucleotides, sugars= monomeric units
What is heme?
Organometallic compound
transports O2 in blood
What is stereoisomerism?
Molecules have same molecular formula but different spatial arrangement
2 types of stereoisomerism?
- Enantiomerism (optical isomerism)
2. Diastereoisomerism (cis-trans isomerism)
Enantiomers
- Form of optical isomerism
- Non superimposable images
- Chiral carbon
- Identical physical properties
Diastereoisomers- cis-trans
- E-Z isomerism, E=trans (opposite) , Z= cis (together)
- Have different physical and chemical properties
Levels of protein structure
Primary structure- a.a sequence
Secondary structure- folds- alpha helix or beta pleated sheet
Tertiary structure- 3D: polypeptide chains
Quaternary structure- multiple polypeptide chains
