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14 Cards in this Set
- Front
- Back
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Nucleus structure
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the nucleus has a double concentric membrane supported by a fibrous protein mesh (cytoskeleton) filled with aqueous material containing DNA, RNA and proteins. The outer membrane is continuous with the ER and the inner with the nuclear lamina
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Benefits of having a nucleus
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- separates delicate chromosomes from cytoskeleton filaments (so not damaged when cell changes shape)
- separates RNA transcription from translation machinery, this allows RNA splicing (joining up of different introns), the regulation of RNA export and the export/import of other molecules for gene regulation and control |
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DNA structure
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- Deoxyribonucleic acid
- sugar phosphate back bone joined with phosphodiester bonds giving a 3`(OH of sugar) and 5` (phophate) end, with organic bases protruding inwards (A, T , C , G) - Bases hydrogen bonded together to form double helix of the 2 anti parallel strands - T bonds with A with 2 bonds, C to G with 3 bonds - A and G are purine bases, C and T and pyrimidine. |
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DNA replication
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- two complementary strands separate by the breaking of hydrogen bonds
- each stand acts as a template for a complementary strand to form in semi-conservative replication - replication starts at replication origins/bubble and replication forks move out of this point exposing bases for free nucleotides to bind and form a new strand of DNA - the two strands are glued together to form the centromere |
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Gene expression
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- each chromosome has one or more promoter regions where transcription starts, and stop codon to end the process
- the DNA strands separate at replication origin and transcription starts to form RNA by polymerase II - RNA is processed by capping (top), polyadenylation (tail) and removing the introns (splicing) |
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Chromosomes
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- telomeres: protective ends of chromosome, made of repeat sequence (1000x GGGTTA), can shorten with age and increase chance of damage (cancer)
- centromeres: hold metaphase chromosomes together in highest packaging, region that attaches to mitotic spindle - P (short) arm and q (long) arm consist of banded genes with specifc loci named by letter (chromosome) letter (arm) number (major band) number (minor band) |
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DNA packaging - Nucleosome
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- packaging factor 3x
- DNA winds around histone proteins, each has 2 H2A, H2B, H3 and H4 histone proteins to make a octamer. - hydrogen bonds form between DNA and histone proteins - forms beads on a string formation with up to 80nt of linker DNA between nucleosomes - arrangement in decondensed chromatin (dna bound to proteins) |
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DNA packaging - 30nm fibre
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- packing factor 10x
- H1 histone binds to the outside of the nucleosome and allows interactions with other histones so the nucleosomes can coil into a shorter, thicker strand - interactions between tails of the 4 core histones help attach nucleosomes to one another - linker DNA between nucleosomes binds other proteins, can result in the loss of a nucleosome but can result in higher level of packaging |
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DNA packaging - looped domains
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- chromatin fastens to a nuclear scaffold to form looped domains, which allows controlled assess
- genes can be turned on by less packaging for transcription (euchromatin) - tightly wound genes can be hidden to turn them off and become inactive (heterochromatin) |
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DNA packaging - Mitotic chromosome
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- packing factor 10000x
- condensin proteins use ATP to further wind and pack chromatin to form a 1400nm mitotic chromosome |
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Nuclear envelope - pore complex
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- nuclear envelope is perforated by pores to allow transport in and out of the nucleus
- made of cytoplasmic protrusions (fibres) on top of a central ring of transport proteins, and a nuclear basket structure to catch molecules - small molecules can move without regulation by diffusion, larger molecules use pore and move more slowly |
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Nuclear localisation signals (NLSs)
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- amino acid sequence rich in basic (+ve) amino acids that act as nuclear address
- position in protein my vary but must be accessible to import machinery and not removed so the can be recycled for reimport after mitosis |
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Nuclear import control
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- regulated by chemical binding, phosphorylation, release of chaperones, unmasking of the NLS
- Importin binds to cargo, sometimes with an adaptor protein, these bind to the FG repeats of the nuclear pore complex which pass the cargo from one repeat to the next until it reaches the nucleus, it is then dissociated and the importins recycled |
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The nuclear lamina
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- next to inner membrane
- 2D lattice (mesh) of fibres called the intermediate filaments - provides support and organisation - made of LAMIN proteins, which have a globular head and helix rod like central domain, these bind in pairs to form dimers which twist together into long filaments, attaches to membrane by anchoring a COOH terminus to the membrane |
