THEME 4 MOD 1 Flashcards
Binary fission
cell division in prokaryotes where genetic material is divided equally between daughter cells resulting in asexual reproduction
Steps of binary fission
- DNA attached by proteins to the plasma membrane
- DNA replication occurs along region of replication
- cell elongates and newly synthesized dna is attached to plasma membrane as well
- cell elongates until two sites of replication are on opposite ends of the cell
- Cell is now double its size, replication of dna is complete, and the cell will begin to constrict along the cells midpoint
- as constriction occurs, synthesis of a new cell wall and membrane occurs resulting in two new identical daughter cells.
importance of cell division in eukaryotes
- fertilized egg develop into complex organism
- renewal and repair of cells in developed organism
Importance of stem cells in eukaryotic cell division
- in embryo, stem cells can reproduce indefinitely and differentiate into any cell type under specialized conditions and contribute to growth of the organism
- adult stem cells cannot differentiate into any cell type but can replace non-reproducing specialized cells
Example of stem cell function in adult human
- nondividing satellite stem cells in the basement membrane of skeletal muscle tissue can be activated and undergo proliferation, differentiation, and specialization into myoblasts to regenerate damaged skeletal muscle. once they become mature muscle cells in myofibers, they can no longer divide and exit the cell cycle, going into g) phase permanently
- these stem cells are important because skeletal muscle cells have very little turnover
what is different between eukaryotic and prokaryotic cell division?
eukaryotic:
- eukaryotic dna is larger, condensed into linear chromosomes and compact within a nucleus
- more regulated control for cell division as a part of a larger cell cycle
eukaryotic cell cycle stages and phases
- Interphase: S phase (dna synthesis in nucleus) and G1 and G2 growth phases (prepare cell for synthesis and mitosis respectively, overall increase in cell size)
- M phase (mitosis and cytokinesis): prophase, prometaphase, metaphase, anaphase, and telophase
G0 phase
A phase cells enter somewhere between the S and M phase and cells pause in the cell cycle. The pause can last from days to over a year, or some cells stay permanently in the g0 phase.
- cells with constant turnover(do not enter g0 phase): intestine or skin cells
- Cells in permanent g0 phase: cells in lens of our eyes, nerve cells, and mature muscle cells
quiescence
stem cells can reproduce indefinitely but may also enter periods of quiescence where they are in the dormant g0 phase
Who discovered the phases of mitosis which could be classified based on the position of chromosomes.
Walther Flemming in 1882.
he stained and observed developing salamander embryos to visualize chromosomes within dividing cells
after his work the 5 phases of mitosis were established.
S phase of interphase
- chromosomes in long thin chromatin fibres
- dna sequences are replicated from end to end of dna molecule
- newly synthesized dna associates with histones and other proteins to tightly compact
- centromere is also fully replicated the pair of centromeres are so tightly compact they appear fused
- duplicate copies of a chromosome are called sister chromatids
- note it isn’t until m-phase chromatids fully compact, and the sister chromatids separate into two new cells
Chromosome pair numbers and breakdown
23 pairs of chromosomes
- 22 homologous pairs: one maternal and paternal chromosome in orgin
- 23rd pair are our sex chromosomes
M stage beginning
- begins upon end of g2 phase of interphase
- duplicated chromosomes begin to condense into a shape visible under microscope.
Prophase (1st stage of M phase)
- chromosomes appear as sister chromatids joined by centromeres
- centrosomes (duplicated microtubule organizing centres) radiate along microtubules to form a mitotic spindle and will situate at opposite poles of the cell
prometaphase (2nd stage)
fragmentation of the nuclear envelope to allow microtubules of the mitotic spindle to be able to attach to kinetochores (specialized protein structures associated with one sister chromatid on one side on centromere) in centromeres of each chromosome to pull sister chromatids to opposite polls of the cell.
metaphase (3rd stage)
alignment of chromosomes at centre of cell known as the metaphase plate facilitated by kinetochore microtubules attached to kinetochores at centromere of sister chromatids
Anaphase (4th stage)
kinetochore microtubules shorten and sister chromatids separate into separate chromosomes and are pulled to opposite spindle polls. simultaneously polar microtubules push against each other to elongate the cell
telophase (5th and final stage of M phase)
two daughter nuclei form in the cell as the nuclear envelope reforms around the chromosomes at each end of the cell. chromosomes begin to decondense and spindle microtubules are depolymerized (broken down).
- there are now two identical nuclei, which marks the end of mitosis
cytokinesis
-underway before end of telophase, division of the cytoplasm and cell membrane into two distinct cells
- contractile ring of motor proteins around cell membrane that contract bundles of actin fibres along the midline of the cell
- cleavage furrow is established and separates the cell into two distinct daughter cells
Can cytokinesis be different is differing cell types and how?
yes, cytokinesis is different in plant and animal cells
plant cells will lay down a newly developed cell wall along a cell plate and when the new wall fuses with the existing cell wall, cytokinesis and cell division is complete
Tim Hunt’s Research
hypothesized in the 1970s, research by hunt in the 1980s discovered cyclin-CDK complexes and their regulatory role in the cell cycle.
- measured protein level changes in sea urchin embryos that were dividing.
- added radioactive methionine to sea urchin eggs to visualize newly synthesized proteins and measure protein changes
- took samples every 10 minutes to visualize protein changes using gel electrophoresis, most protein bands became darker as time progressed.
- one protein band was found to increase and decrease with each cell division cycle
- protein was named cyclin (due to cyclic nature)
- suspected the protein was involved with regulation of the cell cycle
- further research by hunt discovered the mitosis promoting factor was a cyclin protein and cyclin dependant kinase (CDK) protein that controlled progression of the cell cycle
explain the Cyclin-CDK mitosis promoting factor
- kinase is an enzyme that phosphorylates a target protein to activate or inactivate it with specific amino acids
- kinases that help regulate the cell cycle stay at a consistent volume in a cell, but are largely inactive until the cyclin protein bind to and activate it
- cyclin-CDK complexes activate proteins involved in cell division through phosphorylation
- cyclin dependant kinase activity rises and falls depending on the cyclin concentration
types of cyclin-CDK complexes
- G1/S Cycline-CDK complex: needed to transition from G1 to S phase of interphase and prepare cell for dna replication (increasing active histone proteins)
- S-Cyclin-CDK complex: initiates DNA synthesis
- M-Cyclin-CDK complex: initiates mitosis, phosphorylate proteins that break down the nuclear envelope, regulate proteins that synthesize microtubules in mitotic spindles
describe cell cycle checkpoints
-cellular surveillance that can inhibit cyclin-CDK complexes if something goes wrong during cell division processes
- pause cellular division until preperation for the next stage is complete
- opportunity for damage repair
