Marc Pilon - Cell division Flashcards
Four “events” in cell division
- Signal (from outside cell)
- Replication (of DNA)
- Segregation (of DNA copies)
- Cytokinesis (division of the cell into two cells)
Binary fission
In prokaryots.
1. Signal: environmental conditions, like nutrient availability.
2. DNA replication: single, circular chromosome. Replication complex (DNA pol and other proteins) start replicating DNA at Ori site. Replication ends at Ter site. Cell grows as replication progresses.
3. DNA segregation: ori regions of chromosome move to opposite ends of the cell.
4. Cytokinesis: protein ring (tubulin-like) pinches plasma membrane. The structure septum divides the mother cell just before it divides into two daughter cells.
Chromosomes eukaryotes
In the nucleus, loosely organized and not condensed.
Human karyotype
The ordered arrangement of metaphase chromosomes. The centromere is the constricted region of metaphase ch.
Organized chromosomes. Two of each, they are homologous. 23 pairs. 22 autosomes and 1 pair of the sex chromosomes, XX or XY. Displayed at metaphase. At this stage, the DNA has been replicated but not segregated, so in reality there are 92 chromosomes in the picture.
Human chromosomes
- Each somatic cell in humans have 46 ch. Diploid.
- Gametes have 23 ch. Haploid.
- The X and Y are the sex ch.
- Other ch are called autosomes.
- Somatic cells divide by mitosis.
Chromatid
One half of two identical copies of a replicated ch. Joined at centromere.
Chromatin
Complex of DNA and proteins that forms ch in eukaryotic cells.
Phases in cell cycle
The mitotic cell cycle:
- G1: gap 1. Cells that don’t divide usually arrest during G1 and enter G0. Can emerge from G0 from signal.
- S-phase: DNA is replicated.
- G2: gap 2.
- Mitosis phase: (of mitotic cell cycle). Nuclear division. Cytokinesis at the end.
Interphase: G1, S and G2.
Signal for cell division
Specific signal required in the cells of multicellular organisms. Only enough nutrients are not sufficient, otherwise cancer.
Quiescent
Cells in G0 that don’t divide. Arrest in G1. Cells can go back and forth from G0. 2C, 2n.
Interphase
G1, S-phase and G2.
G1
Gap 1. A lot of growth. Time can vary a lot, typically around eight hours. 2C, 2n.
S-phase
DNA synthesis and centrosome replication. Lasts about eight hours. 4C, 2n.
G2
Gap 2. Growth. Lasts around four hours. 4C, 2n.
M-phase
Mitosis or meiosis. Seperation of the chromosomes. Cytokinesis. Usually less than an hour. 2x(2C, 2n) in mitosis.
C and n
C: number of genome copies
n: number of haploid genomes
Regulation of progression through cell cycle
The S phase cell containt a substance that diffuses to the G1 nucleus and activates DNA replication. This can also effect cells in G2 in experiments.
What is cyclin and cdk
Many different types of both. Cdk is a cyclin dependent kinase. Cdk is always present, but its active site is not exposed. Cyclin protein is made only at a certain point in the cell cycle, and are then degraded. The levels of cyclin are cyclical through the cell cycle.
How does cdk and cyclin work?
When cyclin reaches high enough levels, it interracts with its Cdk.
1. Cyclin binding changes Cdk, exposing its active site.
2. A protein substrate and ATP bind to Cdk. The protein substrate is phosphorylated.
3. The phosphorylated protein regulates the cell cycle. Each Cdk has specific protein targets.
This allows progression through the cell cycle. ADP is biproduct.
Restriction points regulated by cyclin
- The G1 phase cyclin-Cdk regulates entry into the cell cycle at R.
- The s cyclin-Cdk regulates repairs of errors in DNA replication.
- The G2-M cyclin-Cdk regulates entry into M.
- The M cyclin-Cdk regulates progress through mitosis.
Restriction point
The G1 check point.
Cyclin regulation of the restriction point (G1)
Cdk is always present, but inactive. In G1, the cell recieves signal. The gene for the G1 cyclin is exposed and transcribed. It binds to Cdk, proteins become phosphorylated, and allows progression through the checkpoint. The cyclin is degraded in mitosis.
The four cyclins
Cyclin D: G1 (restriction point). High throughout the cell cycle, low in mitosis. Cdk 4 and 6. Signal to divide if there is no DNA damage.
Cyclin E: S (G1/S). Goes up in G1, down in early S. Cdk2. Checks for cell size and DNA damage.
Cyclin A and B: G2. Goes up in S phase. A persists in G2, B breaks at metaphase. Cdk 1, 2. Checks for DNA damage.
APC: M (spindle checkpoint). Activates seperase, degrades cyclin B. M-Cdk. Checks that all chromosomes are under tension (attached to spindle).
G1/S checkpoint, Rb, p21
Verifies absence of DNA damage before proceeding to S phase. Cyclin E.
Levels accumulate, interracts with Cdk2. Cdk2 phosphorylates the protein Rb (inhibitor). It is then not going to be able to stop the progression. Rb is a tumor supressor. It hinders progression if there is DNA damage. It is a supressor protein. Active Rb inhibits a transcription factor E2F, which is necessary to transcribe proteins required to move into S phase.
If there is damage, p21 is expressed and binds Cdk2, so that is can’t be attached to Cyclin E. Cell cycle arrest. p21 is activated by p53 (the guardian genom) when there is DNA damage. It detects damage, and leads to expression of downstream proteins like p21.
p53 and p21
Regulates at many checkpoints. Reuse the regulatory mechanisms. Prevents interaction between Cdk and their cyclins.
How DNA is packed into a mitotic chromosome
DNA double helix
Nucleosome: DNA wraps around histones.
Chromatin: nucleosomes pack into a coil that twists into another, larger coil, and so forth, producing supercoiled chromatin.
Scaffold-associated chromatin: the fibers folk to form loop domains attached to a protein scaffold.
Condensed heterochromatin: the looped structures firther condense in the interphase nucleus. Genes can’t be expressed.
Compacted chromosome: during mitosis, the loops coil even further.
Phases of the mitotic cycle
Interphase. Mitosis: prophase, prometaphase, metaphase, anaphase, telophase.
Interphase
The phase before mitosis DNA is replicated. 23x4 chromosomes. Centrosomes are replicated. Both are replicated in S. Chromatin uncoiled. 4C, 2n.
Prophase
The first subphase of mitosis. The centrosomes will move to opposite sides of the cell. Nuclear envelope breaks down. DNA begins to condense, begin to form visible ch. held together by cohesin.
Prometaphase
Second phase of mitosis. Spindles are taking shape. Microtubules emerge from centrosomes, forming spindle, attach to centromere of each ch. that are now two sister ch. Centromeres contain kinetochore where microtubules are attached, where they are called kinetochore microtubules. Condensation of DNA continues.
Metaphase
The third phase of mitosis. Condense ch. lines up on equatorial plate, held under tension. Microtubules try to pull. Cell senses when all ch. are under tension, the mitotic check point is passed, M check point.
Stage for karyotypes.
Anaphase
The fourth phase of mitosis. APC (anaphase promoting complex) activates seperase. Sister ch. disjoin to daughter ch. Migrate opposite ends.
Telophase
The fifth phase of mitosis. Haploid genomes in their furure cells, begin to relax, cytokinesis, septum divides cytoplasm, cell division completed. Ch. decondense. Nuclear envelope reforms.
Mitotic spindle
Kinetochore microtubules attach to kinetochores and to the spindle poles. Polar microtubules extend from each pole of the spindle, attach to centromeres.
Chromatid attachment and seperation
- During prophase, after DNA replication, the sister ch. are held together by cohesin at their entire length.
- In prometaphase, most of the cohesin is removed, except for some at the centromere.
- At the end of metaphase, a cyclin-Cdk complex activates the APC, which activates seperase, resulting in the removal (cleaving) of the remaining cohesin.
Cytokineses animal cell
The contractile ring seperates the cytoplasms of the two daughter cells, though the surfaces remain in contact. The cells then divide.
Cytokineses plants
They have a cell wall. A row of vesicles will fuse to form a cell plate between the two daughter cells, constructing a new cell wall.
Cohesin
A complex of SMC proteins. Enables post-replicative DNA repair of double stranded breaks. Shapes ch. in preparation to cell division. Holds sister ch. together until replaced by condensin, which compacts ch. further (cohesin remains at centromeres).
Haplontic life cycle
The mature organism is haploid and the zygote is the only diploid stage.
Diplontic life cycle
The organism is diploid and the gametes are the only haploid stage.
Alternation of generation (haplontic/diplontic)
The organism passes through haploid and diploid stages that are both multicellular.
Homologous chromosomes
Homologs. A set of one maternal and one paternal chromosome. They have the same genes in the same loci.
Tetrad
Prophase in meiosis I. Two copies of two homologs. Crossover occurs here.
Chiasma
The point of overcrossing of homologs. Overcrossing occurs in meiosis I. In prometaphase, the homologs are seperated except for the chiasma.
Metaphase meiosis I
All tetrads have to be on their tension on the equatorial plate. At least one overcrossing per tetrad. The sister ch. are still held together by their centromeres. Cohesin still holds sister ch. together, and therefore they cannot be pulled apart by the microtubules. Terminilization: cohesin moves toward ends of ch.
Prophase II
No interphase. Each daughter cell replicate the centromere, moves to poles. Ch. condense.
Metaphase II
Dyads gather at the equatorial plate, held under tension. Connected by centromeres.
Anaphase II
Monads are seperated.
Telophase II
Cytokinesis into two haploid cells.
Anaphase I
Pairs of sister ch. (dyads) seperate (disjunction) and move to opposite poles, held together by their centromeres.
Spermatogonium and oogonium
Undifferentiated diploid germ cells.
Spermatogenesis vs oogenesis
Spermatogenesis produces 4 sperm/spermatogonium.
Oogenesis produces 1 ovum/oogonium.
Special features of oogenesis
One polar body is discarded at each meosis I and II. Primary oocytes arrest in prophase I at the diplonete stage during embryogenesis. Starting at puberty, one oocyte matures at each menstrual cycle: completes meiosis I. Secondary oocytes arrest at metaphase II until fertilization.
Cohesins role in meiosis
Cohesin holds together sister ch. in tetrad along the length of the arms and at the centromeres. Kinetochore microtubules attach to centromeres, cohesin only at centromere. Meiosis II divides the monads.
Crossing over
During prophase I, homologous ch., each with a pair of sister chromatids, line up to form a tetrad. Adjacent chromatids of different homologs break and rejoin. Because there is still sister chromatid cohesion, a chiasma forms. The chiasma is resolved. Recombinant chromatids contain genetic material from different homologs.
Why crossing over?
Creates great diversity, and removing junk DNA through imprecise recombination, which can lead to small deletions. Facilitate the ability of natural selection to act independently on different genetic loci.
Nondisjunction meiosis
Nondisjunction can occur if, during anaphase of meiosis I, both homologs go to the same pole. In that case, two daughter cell gets an extra ch. (trisomy) and the other two misses one (monosomy). Both are called aneuploidy. Down syndrome is caused by an extra ch. 21. All other are lethal. This typically occurs during oogenis. Disjunction can happen both in meiosis I and II.
Aneuploidy
When one or more ch. is either lacking or present in excess in gametes.
Why are monosomies not viable?
- Gene dosage defects (some genes need both alleles to get enough expression). This is the main reason.
- Parent-of-origin imprinting (or parental or maternal alleles).
- No backup for lethal alleles.
Apoptosis
Programmed cell death.
1. External signals can bind to a receptor protein. Or, internal signals can bind to mitochondria, releasing other signals.
2. Inactive caspase changes its structure to become active.
3. Caspase hydrolyzes (chop) nuclear proteins, nucleosomes, etc., resulting in apoptosis.
Molecular changes in cancer cells
There need to be mutations that tell the cell to divide rapidly, and mutations that override checkpoints, and mutations in tumor supressor genes.
This could be an overexpression of receptors for growth factor, making the cell divide even though it shouldn’t. This is an oncogenic mutation.
In cervical cancer, a virus makes a protein (E7), that inactivates Rb, so the cell cycle can proceed, ignoring the ckeckpoint. This is a neutralized tumor supressor. Mutations can also cause this.
Viruses can also activate oncogenes.
How many mutations to get cancer?
More than one mutation is required for tumor cell formation. For example expression of both the Myc and Ras oncogenes.
Cancer treatment and the cell cycle
Some drugs block the function of the mitotic spindle. Mitosis.
Some drugs inhibit growth factor stimulation at the restriction point.
Some drugs block DNA replication. S phase.
Radiation damages DNA and causes apoptosis at the S and G2 ckeckpoints.
The problem is that these treatments are not specific, and therefore have a lot of side effects on healthy tissue.
