Cell nucleus and division Flashcards
The nucleus:
The distinguishing feature of eukaryotic cells.
—– organelle of cell - typically 5 to 8 micrometer diameter but depends on cell type.
Contains 2 metres of DNA, complexed to protein (the DNA/protein complex is called —-).
Some chromatin is highly condensed (Called heterochromatin) especially at the —- and ends of chromosomes and tends to be at the periphery)
The remaining chromatin is more —- (euchromatin). This is a lighter material.
Largest
Chromatin
Centromere
Open
Is euchromatin or heterochromatin more relaxed?
What does this mean?
Euchromatin
Means it is more assessable for transcription factors so this DNA will contain the active genes of the cell.
Nucleolus:
The DNA coding for ribosomal RNA (found at the ends of 5 different chromosome pairs) is localised to a region called the nucleolus - site of —- and —- of ribosomal subunits.
Have genes that code for ribosomal RNA and protein. The whole area is dense so this is dark.
synthesis
assembly
Nuclear Envelope:
The nucleus is bounded by a double nuclear membrane (envelope)
Outer nuclear membrane is —- with the rough endoplasmic reticulum
Inner nuclear membrane is supported by the nuclear —- - a meshwork of filaments composed of proteins called lamins (intermediate filament protein). We think the lamina are involved with maintaining the shape of the nucleus.
continuous
lamina
What two environments does the nuclear envelope?
How does communication work across it?
What does the nuclear pore complex form?
Nuclear and cytoplasmic
Via nuclear pores
A aqueous environment
How do small molecules (<9nm) get across the membrane?
Large ones?
Small = diffusion
Large = active transport
What is a nuclear localisation sequence?
What does this do?
A short sequence of basic amino acids (lysine and arginine).
These amino acids instruct a protein to be imported from the cytoplasm to the nucleus.
Explain the process of moving a protein from the cytoplasm to the nucleus
Cytoplasmic nuclear import receptors bind to the nuclear localisation sequence. The protein and the receptor move across the membrane together and once inside they separate.
This delivers the protein to the pore complex.
The energy for this comes from GTP hydrolysis.
Proteins to be exporter have a nuclear export signal. When not associated with the protein, this sequence is recognised by the channel and the protein is removed.
Explain the summary of chromatin folding and then go into detail on it
The DNA is wound twice around each protein and then these are packaged together. This structure is then folded and makes springs. The chromosomes are the highest level of packing.
- long DNA molecules have to be packaged into chromatin by association with histones.
- The DNA is first wrapped twice around a core particle consisting of 8 histone proteins. (2 of each of H2S, H2B, H3 and H4).
- Nucleosomes are spaced at 200bp intervals
- The nucleosomes are then coiled into a 30nm fibre with the help of histone H1 (linker protein)
- The 30nm fibre in turn forms loops attached to a ‘chromosomal scaffold’
- The loops are further condensed to form the mitotic chromosome
Highly condensed chromatin (e.g. mitotic chromosomes, heterochromatin) are transcriptionally —-
Active genes are found in chromatin with a more —- configuration
Regulation of local chromatin —- is an important aspect of controlling gene expression
silent
open
compaction
What are 3 features of eukaryotic chromosomes?
- At least one origin of replication (where copying begins)
- A centomere
- A telomere
What are centromeres? Give details on this
The centromeres are special DNA sequences that function during cell division: all found on chromosomes.
Help hold the replicated daughter chromosomes (chromatids) together until they are ready to be separated and enable assembly of a protein complex (the kinetochore), which binds to microtubules, and attached to the mitotic spindle.
What is a telomere?
What are they necessary for?
A tandem repeat of short G-rich sequences (GGGTTA . in humans) at the ends of the chromosomes.
Necessary to prevent chromosome fusion, to solve the problem of replicating linear DNA molecules. (DNA polymerase is unable to copy to the very end of the linear chromosome.
What does telomerase do?
Extends the 3’ end of parental strand of DNA and adds telomere sequences.
During DNA synthesis, the lagging strand is synthesised discontinuously, starting from an RNA primer.
After excision of the primer, there is no way of filling the gap, so chromosomes would shorten with each round of replication.
The enzymes telomerase compensates for this by adding multiple hexanucleotide repeats (copied from an RNA template) to the chromosome ends.
What cells is telomerase expressed in?
What does shortening of telomeres do?
Germ cells
Eventually limits cell lifespan, contributing to cell senescence (ageing). This enzyme is re-expressed in cancer cells.
Chromosomes in human cells:
Individual chromosomes cannot normally be seen, but become visible as a result of —-.
The number, size and shape of the chromosome complement (the —-) in characteristic of the species.
Classification of mitotic chromosomes is facilitated by special staining techniques which generate a characteristic banding pattern (due to differential condensation and different proteins attached). Can identify the chromosome based on length, where the telomeres are and the —- pattern.
Human somatic cells generally contain 46 chromosomes made up of 22 homologous pairs (autosomes) and one pair of sex chromosomes (X,Y).
condensation
karyotype
banding
