Eukaryotic Gene Expression Flashcards
Histones
- proteins that wrap around DNA
- about 100 AAs per histone
- positive AAs bind tightly around histones to the negatively charged DNA
- highly conserved protein across species
What are the 4 common types of histones?
- H2A
- H2B
- H3
- H4
Nucleosomes
- 10nm fiber
- “beads on a string”
- DNA that is wound 2x around an octamer of histones (2 histones of each type)
Histone tails protrude outwards from a ____
nucleosome
Histone Tails
- consist of amino acids that are available for chemical modification
- the N terminus of an AA
Histones only briefly leave DNA during ___
replication
30nm Fiber
- consists of interactions between histone tails and linker DNA (DNA “strings” between nucleosomes)
- H1 involved here
- takes the 10nm fiber and folds it into a chromatin fiber
- seen in interphase of cell cycle when the chromosomes are decondensed
300nm Fiber
- looped domains
- formed when the 30nm fibers form loops that attach to a chromosome scaffold made of proteins
Chromosome scaffold is rich in ___ and ___
topoisomerase and H1
Metaphase/Mitotic Chromosome
- most tightly folded
- occurs during metaphase when the looped domains fold to form sister chromatids
Centromere
- middle of the mitotic chromosome that binds to the mitotic spindle
- present during metaphase
Chromatin
- complex of DNA and protein
- changes depending on the stage of the cell cycle
In interphase, there are regions of DNA that are more ___ than others
condensed
Heterochromatin
- tightly packed
- contains less frequently transcribed genes
Euchromatin
- loosely packed
- contains “housekeeping genes” – genes that need to be transcribed all the time
- genes are more accessible
Supercoiling
helps keep DNA wound tightly
Single Copy DNA
one gene with no duplications
Repetitive DNA
- gene families
- lots of copies of the same gene – these duplicates have evolved throughout time
- lots of these are found on the X chromosome
- ex. smell receptors
With age, our ___ get shorter
telomeres
Cancer has a way of replacing ___ so that they can have uncontrolled cell growth
telomeres
Acetylation of histone tails leads to ___ of DNA folding
loosening
tail goes from positively charged to neutral
Lots of acetylation means ___ transcription
increased
Deacetylation of histone tails leads to ___ of DNA folding
tightening
In general (depends on the AAs present), methylation of histone tails leads to ___ of DNA folding
loosening
Phosphorylation of histone tails (addition of negative charges) leads to ___ of DNA folding
loosening
negative charge of DNA and PO4 repel each other
How can you regulate the step between DNA and pre-mRNA
-regulating transcription initiation
How can you regulate at the level of RNA processing (pre mRNA to mature mRNA)?
-alternative splicing (produce different isoforms by varying the exons that we put together during RNA processing)
Where does translation of mRNA occur?
cytosol
What happens to mRNA if it is not translated?
it is degraded in the cytosol
What processing occurs between polypeptide and active protein formation in the cytosol?
protein processing (can be either by chemical modification or cleavage)
Histone tails contain a lot of positively charged, ___ and ___ amino acids
lysine and arginine
Where does methylation of DNA occur?
on the bases A, G, T, and C (mostly C)
Long stretches of inactive DNA are usually ___
methylated
Barr Bodies
- inactive X chromosomes
- contain heavily methylated DNA
Females have one ____ X chromosome
inactive
Transcription Factors
- can bind to DNA
- torque DNA to open it up which allows RNA Polymerase to bind and start transcription
- regulated by cellular signals
Where do transcription factors bind? (2)
-proximal control elements (directly upstream of promoter)
-distal control elements or enhancer
(located thousands of nucleotides upstream of promoter)
What happens when transcription factors bind to the enhancer?
the rate of transcription is greatly increased b/c the enhancer can fold over and interact by the promoter which helps RNA Polymerase bind
What regulation occurs at the RNA level?
- alternate splicing
2. regulate degradation of mRNA – if mRNA has a short half life then less translation occurs
What regulation occurs at the protein level?
- addition of sugar to protein
- phosphorylation
- regulation of the half life of a protein by tagging it with ubiquitin which targets it for destruction by a proteasome
