Nucleus and DNA Flashcards
Describe the structure of the nuclear envelope.
Double membrane
Inner membrane lined with lamina protein to provide structural support
Outer membrane attached to rough ER
Nuclear pores cross both membranes
What molecules can pass through nuclear pores?
Macromolecules and polar molecules.
What are nuclear pores made of?
They are large hetero multimers of 30 different proteins.
How are proteins imported into the nucleus? What is different about exporting proteins?
This process is cyclic but we will start at an importin that has just left the nucleus:
RAN-GTP bound importin -> RAN-GTP is hydrolysed -> RAN-GDP bound importin
RAN-GDP dissociates from the importin which allows a cargo protein with the correct nuclear localization signal to bind to the importins receptor sequence.
Importin/cargo complex travels through the pore.
Once within, a RAN-GTP binds to the a receptor on the importin causing the protein to dissociate and be deposited in the nucleus.
The importin RAN-GTP complex then leaves via a nuclear pore and the process repeats.
When proteins are exported RAN-GTP and the protein both bind to the exportin and the whole complex travels through the pore. RAN-GTP promotes export.
How does RAN-GTP cause the cargo protein to be deposited in the nucleus? How is RANGTP concentration maintained in the nucleus?
RAN GTP will physically block part of the importins NLS binding site. RANGDP is cycled back into the nucleus where it is phosphorylated by RAN guanine exchange factor (a protein attached to chromatin). RAN GAP is what facillitates the hydrolysis of an importin bound RAN GTP.
What is the role of nuclear fibrils?
The fibrils protrude both in to the cytoplasm and the nucleus. They play a role in the selective transport process through the pore blocking certain molecules leaving/entering based on size and other factors.
How did the nuclear membrane evolve?
Invagination of the plasma membrane.
What are the large and small ribosomal subunits made of?
large: 49 ribosomal proteins, 3 RNA molecules
small: 33 ribosomal proteins, 1 RNA molecule
How are ribosomes made.
Large precursor RNA is made
This pre-RNA is cleaved to make the 4 different RNAs needed in ribosomes. Ribosomal protein genes are transcribed in the nucleus and translated in the cytoplasm. The proteins are imported into the nucleus and assemble on the pre-RNAs in the nucleolus. As the rRNA’s mature, more proteins are added to form pre-ribosomal particles. These are exported via nuclear pores to yield active ribosomal subunits.
What is an intron and exon?
Intron: non coding
Exon: coding
What is a codon?
Three bases that code for an amino acid or a start/stop code.
How are chromosomes spread during 1) interphase and 2) mitosis
1) Distributed throughout nucleoplasm
2) Condensed
What is the Rabl configuration?
When chromosomes are attached to the membrane, typically found in plants.
What proteins are found where in chromatin/nucleosomes.
H1= the linker protein that binds to DNA between nucleosomes.
H2A, H2B, H3, H4 comprise the histone core nucleosomes wrap around.
Define chromatin and what H1 does to it.
Chromatin is a single thread of nucleosomes wrapped around histones. H1 proteins link the DNA pulling the thread together compacting it to a 30 nm fibre. This fibre this further compacts to package DNA.
