cell & molecular bio - cell division Flashcards
(23 cards)
Chromosomes vs chromatids vs chromatin vs sister chromatids vs centromere vs homologous chromosomes vs centrosomes
Chromatin: a condensed form of DNA that is wrapped around histones. DNA is stored as chromatin, but during cell division, it condenses even more into chromosomes.
Chromosomes: dense packaging of chromatin, existing during mitosis and meiosis. Chromosomes can be in duplicated or unduplicated forms.
- Chromatid: one half of a duplicated chromosome.
- Sister Chromatids: two identical, duplicated chromatids connected at the centromere.
- Centromere: region that connects two sister chromatids where kinetochores attach.
Homologous Chromosomes: pairs of chromosomes (one from each parent) found in diploid cells that are similar in length, gene position, and centromere position. Both chromosomes carry genes for the same traits but are non-identical.
Centrosomes are the Microtubule Organizing Centers (MTOCs) of animal cells.
* Spindle Fibers: microtubules that emerge from the centrosome and allow chromosomes and chromatids to be separated during cell division. - Spindle fibers do not attach directly to chromosomes – they attach to kinetochores (proteins that adhere to the chromosome centromere region).
* Plant cells do not have centrosomes.
Stages of mitosis characteristics
Prophase
*Chromatin condenses into chromosomes.
*Nucleolus disappears.
*Mitotic spindle begins to form. *Centrosomes begin to move towards opposite ends of the cell.
Prometaphase
*Nucleus disassembles.
*Chromosomes condense even further.
* Each chromatid is attached to a kinetochore. *Mitotic spindle further develops.
* Spindle fibers attach to kinetochores on chromosomes.
Metaphase
*Chromosomes line up at the center of the cell (metaphase plate). *Centrosomes are at opposite ends of the cell.
*Mitotic spindle is fully developed. *Microtubules are attached to kinetochores.
*Karyotyping is preformed during metaphase.
-Karyotyping = visualizing a cell’s chromosomes using a microscope.
-Used to identify chromosomal abnormalities (e.g. Trisomy 21 –> Down Syndrome).
Anaphase
*Microtubules shorten.
* Sister chromatids are pulled apart.
-Once separated, each sister chromatid is considered a chromosome. *Chromosomes are pulled to opposite ends of the cell.
Telophase
*Nucleoli redevelop
* Two nuclear envelopes develop (nuclei re-form).
*Chromosomes decondense back into chromatin.
* Spindle fibers disappear.
*Cytokinesis occurs during telophase.
Cytokinesis: division of the cytoplasm to form two cells.
- Cytokinesis is not part of mitosis (mitosis refers to nuclear division) but is required for cell division.
*Animal cells form a cleavage furrow: Actin and myosin ring contracts, separating the cell into two daughter cells.
* Plant cells form a cell plate: Cell wall forms between the two nuclei, fusing with the existing cell wall and separating the cell into two daughter cells.
Final result of mitosis (and cytokinesis):
two genetically identical, diploid daughter cells which have the same amount of DNA as the parent cell.
Meiosis
Meiosis (reductive division): Nuclear division that produces four haploid gametes that are not genetically identical to the original cell.
*Gametes = An organism’s reproductive cells (sperm in males, eggs in females). *Occurs in germ cells (specialized cells that produce gametes).
Meiosis is comprised of two stages
*Meiosis I: genetic recombination occurs and the cell separates homologous chromosomes.
*Meiosis II: sister chromatids separate to form gametes.
Meiosis I stages characteristics
Prophase I
*Nucleolus and nucleus disappear. *Chromatin condenses into chromosomes.
*Meiotic spindle begin to form. *Centrosomes begin to move towards opposite ends of the cell.
*Homologous chromosomes undergo synapsis and crossing over.
-Synapsis: Process by which homologous chromosomes pair up, forming tetrads.
‣Tetrad = structure formed from paired homologous chromosomes. Must be present for genetic recombination to occur.
-Crossing over: exchange of chromosome segments between paired-up chromosomes (genetic recombination). Produces genetic variation in gametes.
‣Chiasmata = Locations where homologous chromosomes meet to swap segments.
-Spindle fibers attach to kinetochores of homologous chromosomes.
Metaphase I
*Homologous pairs are lined up across the metaphase plate.
*Microtubules are attached to kinetochores.
*Meiotic spindle is fully developed.
Anaphase I
*Microtubules shorten.
*Homologous pairs uncouple and are pulled to opposite poles (disjunction).
Telophase I and Cytokinesis
* Two nuclear envelopes develop (nuclei re-form).
*Chromosomes decondense back into chromatin.
* Spindle fibers disappear.
*Cytokinesis splits the cell into two daughter cells.
Final result of meiosis I: Two daughter cells with half the number of chromosomes as the parent cell (2n parent cell n daughter cells). * Chromosomes are not genetically identical to the parent cell due to crossing over.
Meoisis II phases and characteristics
Meiosis II further separates two cells (the products of meiosis I) into four gametes.
* Starts and finishes with haploid cells. Meiosis II
Prophase II
*Nucleus disassembles.
*Chromatin condenses into chromosomes.
*Meiotic spindle begins to form. *Microtubules attach to kinetochores.
* Note: no crossing over occurs in Prophase II.
Metaphase II
*Chromosomes are lined up at the metaphase plate.
*Microtubules are attached to kinetochores on chromosomes. Anaphase II
*Microtubules shorten.
* Sister chromatids are pulled to opposite ends.
-Each sister chromatid is now considered an individual chromosome.
Telophase II and Cytokinesis
* Nucleoli redevelop.
* Two nuclear envelopes develop (nuclei re-form).
* Chromosomes decondense back into chromatin.
* Spindle fibers disappear.
* Cytokinesis splits the cell into two daughter cells.
Final result of meiosis: four haploid daughter cells, each with half the amount of DNA as the parent cell.
* The DNA of daughter cells is NOT identical to the parent cell due to genetic recombination in prophase I.
Three events contribute to genetic diversity in offspring:
Crossing over: genetic recombination that creates gametes with unique chromosomes.
- Crossing over plays the most significant role in increasing genetic diversity.
Independent assortment: each homologous chromosome or sister chromatid is randomly distributed into either daughter cell.
Random joining of gametes: different sperm can fertilize different eggs during fertilization.
Meoisis I vs meiosis II starting vs ending product
Meiosis I:
- the separation of homologous chromosomes, genetic recombination occurs ONLY here
1 diploid parent cell –> 2 haploid daughter cells
- Chromosomes are NOT genertically identical to parent cell due to recombination
Meiosis II:
- the separation of sister chromatids to opposite ends of the cell
2 haploid cells –> 4 haploid cells
At the end of meiosis:
There are 4 daughter cells
- each cell is haploid (n) - unlike the parent cell
- each contain 1/2 the DNA of the parent cell
- the DNA is NOT identical to parent cell (crossing over)
What are the three events that genetic recombination can occur during meiosis?
- Crossing over
- during prophase I - Independent assortment
- gametes that our parents produce arent indentical, overtime, the mix of which chromosome in each homologous pair gets separated to which end is random
- therefore chromosomes siblings get arent necessarily the same - Random joining of gametes
- which sperm fertilizes which egg, also random
Describe the steps of the cell cycle
- Interphase: stage of the cell cycle during which cells grow and prepare for cell division but are not actively dividing. - Ensures each daughter cell will have enough resources to survive.
The phases of interphase include:
- G1 (Gap 1) Phase: cell grows and synthesizes proteins needed for cell division.
‣ Usually the longest phase of the cell cycle.
- S (Synthesis) Phase: DNA and centrosomes are replicated. Sister chromatids are formed.
‣ Note: DNA replication occurs before mitosis. - G2 (Gap 2) Phase: cell continues to grow and organelles replicate. Cell checks that everything is ready (e.g. chromosomes are replicated) to proceed with mitosis.
- G0 Phase: resting phase. Cells are active but do not divide or prepare to divide.
‣ Senescent cells enter G0 permanently (e.g. nerve, muscle).
‣ Quiescent cells can enter G0 temporarily and then re-enter the cell cycle (e.g. memory T cells). - M (Mitotic) phase: mitosis and cytokinesis.
Why do cells have to divide?
Cells must divide because of functional limitations:
- Surface-to-Volume Ratio (S:V Ratio): as a cell grows, its volume grows faster than its surface area. A large cell must exchange more resources with the environment but has proportionately less surface area to do so.
- Large S:V = Efficient cellular exchange.
- Small S:V = Inefficient cellular exchange. - Genome-to-Volume Ratio (G:V Ratio): as a cell grows, its volume increases but its genome size (amount of DNA) remains constant –> G:V ratio decreases. A large cell must transcribe more gene products to support its cellular activity but has access to a limited number of genes.
- Small G:V = Cellular activity exceeds the production capacity of its genome.
- Skeletal muscle cells are senescent (nondividing) but still grow in response to exercise. To overcome limitations of large cells, skeletal muscle cells are multinucleated (↑G:V) and long/cylindrical (↑S:V)
What are the regulation of the cell cycle?
What is one type of cell that does not divide. How does its cell cycle work?
Skeletal muscle cells
- cannot divide (stay in G0)
- but via exercise they get larger (hypertrophy)
- multinucleation overcomes genome-to-volume limitation
- long and cylindrical cells improve the surface-to-volume ratio
What are the three types of cell specific regulation?
- Cell cycle checkpoints
- Density-dependent inhibition - phenomenom that prevents overcrowded cells from dividing (density reaches a max)
- Anchorage dependence - cells must be attached to something to divide
Describe the cell cycle checkpoints. What happens if a cell fails these?
Cell cycle checkpoints: ensure the cell is prepared to progress to the next stage of the cell cycle.
Restriction checkpoint (end of G1): ensures appropriate cell growth and intracellular conditions.
- If conditions are suitable, cell enters S phase.
DNA damage checkpoint (end of G2): ensures accurate and complete DNA replication.
- If DNA is replicated and undamaged, cell enters M phase.
M/Spindle checkpoint (during metaphase): Ensures sister chromatids are attached to spindle fibers.
- If spindle fibers are not attached to each sister chromatid, mitosis stops.
If a cell fails a cell cycle checkpoint, it enters G0 or dies (via apoptosis).
p53 gene
an important tumor suppressor gene that regulates cell division.
* p53 mutations lead to uncontrolled cell division and tumor formation.
Frequency of Cell Division for each of the following, and an example:
- Labile cells:
- Quiescent/stable cells:
- Fixed/permanent cells:
- Labile cells: continuously divide (e.g. skin cells).
- Quiescent/stable cells: do not usually divide, but can be stimulated to divide (e.g. liver cells).
- Fixed/permanent cells: little to no capacity for cell division (e.g. cardiac muscle cells).
What is the number of chromosomes in PMAT, and at the end of mitosis?
For cells where x = diploid number
P - x
M - x
A - 2x
T - 2x
End of mitosis (Seperated cells): x
What is the number of chromatids in PMAT, and at the end of mitosis?
For cells where x = diploid number
P - 2x
M - 2x
A - 2x
T - 2x
End of mitosis (seperated cells): x
What is the number of chromosomes in PMAT I, and at the end of meiosis I?
For cells where x = diploid number
P I: x
M I: x
A I: x
T I: x
End of meiosis I (separated cells): 1/2 x
What is the number of chromatids in PMAT I, and at the end of meiosis I?
For cells where x = diploid number
P I: 2x
M I: 2x
A I: 2x
T I: 2x
End of meiosis I (separated cells): x
What is the number of chromosomes in PMAT II, and at the end of meiosis II?
For cells where x = diploid number
P II: 1/2x
M II: 1/2x
A II: x
T II: x
End of meiosis II (separated cells): 1/2x
What is the number of chromatids in PMAT II, and at the end of meiosis II?
P II: x
M II: x
A II: x
T II: x
End of meiosis II (separated cells): 1/2 x
What is the diploid number in humans?
46
