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
In a somatic cell:
One of each chromosome pair is of maternal origin, and the other of parental origin.
The chromosome pairs are separated into —- sets of 23 chromosomes by —- during the formation of gametes (egg, sperm).
—- of the male and female gametes at fertilisation restores the diploid chromosome complement of 46.
haploid
meiosis
Fusion
What are polyploid cells?
Cells that have more than the diploid chromosome number due to repeated rounds of DNA replication without any cell division
e.g. megakaryocytes
What are aneuploid cells?
What disease is this common in?
Cells that contain an abnormal chromosome number
Cancer
How long goes mitosis&cytokinesis last?
1 hour
What is the name for the time between one mitosis and the next?
Interphase
What are the 3 parts of interphase?
G1, S and G2
Where does regulation of the cell cycle occur primarily?
What does it depend on?
In G1.
Depends on stimulation by protein growth factors e.g. e.g. platelet-derived growth factor (PDGF), epidermal growth factor (EGF), fibroblast growth factor FGF, etc
Can a cell go back on its decision to divide?
What point is this?
No
Restriction point
What happens in the absence of growth factors?
Cell leaves the cell cycle in G1 and enters resting stage (G2)
Which phase is microtubule-dependant?
Which is actin-dependant?
Mitosis
Cytokinesis
What are 4 things that happen in prophase?
- Chromosomes condense
- Nucleolus disappears
- Cytoplasm’s microtubules break down
- Spindle begins to form
What 2 things happen in prometaphase?
- Nuclear envelope breaks down
- Chromosomes attach to spindle, via kenetochores.
What happens in metaphase?
Chromosomes line up on spindle in pairs (parent and daughter chromosome).
What happens in anaphase?
- Sister chromatids separate, assembly of actin filaments in the middle of the cell and around it creating a ring of actin
- Cytokinesis begins
What happens during telophase?
- Spindle disappears
- Nuclear envelope reforms
- Chromosomes decondense
- Nucleolus reforms
- Cytoplasmic microtubules reform
Spindle formation:
Depends on the centrosome, a —- organising centre (one on each pole of the nucleus).
Contains a pair of centrioles
Centrosome usually located —- to the nucleus in interphase
microtubule
adjacent
Centrioles:
Are cylindrical structures (approx 0.5 x 0.2um dia)
Wall made up of 9 triplets of —-
Centriole duplication begins in —-
Each cell has a pair of centrioles and you then get two pairs in replication, then one pair goes to each side of the nucleus (allows spindle to form between them).
Daughter centriole (precentriole) grows out at —- angles to base of a mother centriole
Begins in G2, by late G2, the procentriole is full-length, but still attached
At onset of mitosis, mother/daughter centriole pairs separate and spindle forms between them
—- of mother and daughter centrioles occurs after mitosis, in G1
microtubules
interphase
right
detachment
What causes entry into mitosis?
Mitosis Promoting Factor
What causes a interphase nucleus to enter mitosis prematurely?
Fusion of a mitotic cell with an interphase cell.
Shows that mitotic cells contain and inducer of mitosis dominant over all other phases of the cell cycle.
What two things does mitosis promoting factor consist of?
Cdk1 (catalytic subunit) and Cyclin B (regulatory subunit).
What happens to Cyclin B in the cell cycle?
Cyclin B accumulates steadily during interphase, peaking at M.
Degradation of Cyclin B is needed to exit M.
What is the S phase promoting factor?
Fusion of an S phase with a G1 cell causes the G1 nucleus to enter S phase prematurely
Shows that S phase cells contain a dominant inducer of S phase
SPF is also a cyclin-dependant kinase consisting of Cdk2 (catalytic subunit), Cyclin E or A (regulatory subunit)
Give brief details on meiosis
After DNA replication, homologous chromosomes pair up, forming —- (does this randomly as to which ends up in what cell)
Crossing over occurs, the —- chromosomes exchanging segments along their length
At anaphase of the first meiotic division, the homologous chromosomes are —- (sister chromatics remain attached)
At anaphase of 2nd meiotic division, the sister chromatids separate
Yields 4 genetically —- haploid cells (gametes)
Special type of cell division occurring in germ cells in which homologous chromosomes are separated to form haploid gametes.
bivalents
homologous
separated
different
Meiosis:
In the first meiotic division, the homologous chromosomes are held together at sites of crossing over (—-).
At anaphase 1, the —- between chromosomes at the chiasmata break down, allowing the homologues to —-.
At anaphase 1, the adhesions between chromosomes at the chiasmata break down, allowing the homologues to separate.
Meiosis 1 yields two cells with a haploid set of (duplicated) chromosomes.
After a brief return to —-, the cells enter the second meiotic division.
At anaphase II, the adhesions between sister chromatids at the —- breakdown, allowing the chromatids to separate.
chiasmata
adhesions
separate
interphase
centromere
What is the purpose of meiosis?
Random assortment of maternal and paternal chromosomes– In humans, 2^23 possible combinations = 8.4 x 10^6 different gametes (random as to if a maternal or paternal copy is inherited)
Recombination of maternal and paternal genes within a chromosome, through crossing over
Infinite number of possible combinations
Ensures that each gamete is genetically different
Give 3 differences between mitosis and meiosis:
Mitosis = 1 division giving 2 genetically identical diploid cells. Meiosis = 2 divisions to give 4 genetically different haploid cells
Mitosis: homologous chromosomes act independently
Meiosis = homologous chromosomes pair
Mitosis: no crossing over
Meiosis = crossing over occurs
