BMS1060 - Cell Biology - Wk 3-4 Flashcards
Describe the structure of the nucleus (in relation to membranes).
- Nuclear envelope - Inner and outer membrane (gap between the two is called the perinuclear space).
- Membrane supported by nuclear Lamins (filaments)
- Inner membrane contains proteins that act as anchoring sites for chromatin and nuclear lamina.
- Outer membrane continuous with RER.
- Nuclear envelope contains pores which control the movement of substances in and out of nucleus (- RNA out, proteins in)
Why is the nucleus important?
- Separates fragile chromosomes from cell contents (protection of DNA).
- Important in DNA replication, transcription and RNA processing.
- Separates RNA transcription from translation machinery in the cytoplasm.
- Nuclear envelope allows gene expression to be regulated.
What happens in the nucleolus?
Site at which ribosomes are assembled and RNA is transcribed,
Describe nuclear pores.
NPCs transport molecules in both directions simultaneously very quickly.
Contain numerous repeats of phenylalanine-glycine motifs/sequences. Weak affinity for each -> gel-like mesh inside pore complex.
Mesh acts as a sieve - restricting diffusion of large molecules and allowing smaller molecules to pass through very quickly. Cell proteins and cytosolic ribosomes too large to diffuse passively through - confining protein synthesis to cytosol.
What are Nuclear Localisation Signals?
Signals responsible for allowing substances to move through nuclear pores INTO nucleus.
Occurs through large, continuously open nuclear pores.
Most common signal - 1 or 2 short sequences rich in lysine and arginine. Can be located almost anywhere in the amino acid sequence.
As long as one of the protein subunits displays a NLS, entire complex will be imported into nucleus.
How are Nuclear Localisation Signals generated?
Nuclear import receptors on nuclear pore complexes.
Cargo protein binds to receptor. NL signal produced.
Some receptors use adaptor proteins that form an import receptor bridge.
Variety of import receptors and adaptors to recognise a range of nuclear localisation signals.
Describe Nuclear Import Receptors.
Cytosolic proteins on nuclear pore contain multiple low-affinity binding sites for FG (phenylalanine-glycine) repeats. These form a mesh inside the Nuclear pore.
These repeats attract import receptors and their bound cargo (in the cytosol) proteins to Nuclear Pore Complexes.
Import receptors bind to FG repeats, disrupting interactions between repeats - dissolving the gel-like mesh so receptor-cargo complexes can diffuse into the NPC pore.
Describe Nuclear Lamina
Meshwork of interconnected protein subunits (nuclear Lamins).
Anchored to proteins on inner nuclear membrane.
Gives shape and stability to nuclear envelope.
Interacts directly with chromatin.
Describe the structure of nucleotides.
Composed of:
- a five-carbon sugar (ribose or deoxyribose)
- a phosphate group
- a nitrogen-containing base are bound.
Nucleotides are joined by phosphodiester bonds.
(in red circle -> nucleoside)
Name the four nitrogenous bases found in DNA and their pairs. How many hydrogen bonds do they form?
What about the pairs in RNA?
Describe the 2 different types of structures of bases.
Adenine and Thymine - 2 hydrogen bonds
Cytosine and Guanine - 3 hydrogen bonds
RNA has Uracil instead of Thyamine.
Purines (bigger/two-ring) - Pure As Gold - Adenine and Thymine
Pyrimidines (smaller/single-ring) - the other 3 bases.
What are the strucural differences between ribose and deoxyribose?
Ribose has an extra oxygen molecule here.
How are phosphodiester bonds formed?
Eukaryotic DNA contains ____ pairs of ________ chromosomes and 1 pair of ____________ chromosomes
22 pairs
homologous
non-homologous
What is a karyotype?
An individual’s complete set of chromosomes.
Also referred tot lab-produced images of a person’s chromosomes isolated from the a cell and arranged in numerical order.
What is a centromere?
The attachment site for the 2 sister chromatids in a chromosome.
Not always in centre of chromosome.
Keep chromosomes properly aligned during cell division.
What are telomeres?
How do they affect the lifespan of cells?
WHAT?:
Repetitive stretches of DNA (TTAGGG) located at the ends of linear chromosomes.
They protect the ends of the chromosomes from unravelling.
LIFE SPAN:
In many cells, telomeres lose a bit of their DNA every time a cell divides. When all the telomere is gone, the cell cannot replicate and dies.
Some cells have an enzyme (Telomerase) that prevent their chromosomes from losing their telomeres - cells live longer.
What role do telomeres play in cancer?
ROLE IN CANCER:
Chromosomes of malignant cells usuAly do not lose their telomeres - fueling uncontrolled cell growth.
What happens in interphase?
G1 phase - cell undergoes growth
S phase - cell makes a copy of its DNA
G2 phase - cell continues to grow and prepares for cell division
What is Heterochromatin?
Chromatin regions that are condensed during interphase and transcriptionally inactive.
What does a nucleosome core particle consist of?
- 8 histone proteins
- Double-stranded DNA that is 147 nucleotides long.
DNA wound around histone proteins and consensed, forming a protein core.
Each nucleosome separated from next by linker DNA (approx. 200 nucelotide pairs).
How are nucleosomes condensed?
In a zigzag manner.
Consensed further into loops.
Give a DNA packaging overview.
Describe Okazaki fragment production
Because DNA polymerase only runs 5’–>3’, on lagging strand, polymerase has to work in opposite direction, so…
DNA unzipped, polymerase builds Okazaki fragment, then more DNA unzipped and another fragments built.
DNA ligase joins fragments together.
What are the secondary and tertiary structures of RNA that I need to know?
Secondary structure: Double-stranded loops and hairpins
Tertiary structure: Pseudoknot
All of these are stable.
