Chapter 7 Flashcards
What are cell cycles?
· The ability to produce more of their own kind - - to grow, divide and replicate - - is a characteristic feature of living organisms, and has a cellular basis
Cells typically do this in a coordinated manner
Examples of cell cycles?
Actively dividing cells provide for the increased population size of yeast (a), growth of skin (b), and expansion of conifer needles (c).
What is cell growth and division?
All cells originate from pre-existing cells through the process of cell division
· The cell division cycle is conceptually simple, but its regulation must be precise and complex
· All dividing cells must meet the challenge of closely coordinating their increase in size, DNA replication, and cell division in the face of a changing environment
What are cell growth and division important for?
- Increasing population size (single-celled organisms)
- Growing new tissues (e.g. new leaves during plant development)
- Asexual reproduction (bacteria, fungi, corals, and many others)
Replacement of lost cells (lining of the gut) and/or damaged cells (wound repair)
What is binary fission?
· Prokaryotic cells undergo a process of binary fission in order to divide (DNA replication occupies most of the cell cycle because of this)
· Involves coordinated cytoplasmic growth, DNA replication, and cell division
· Results in two daughter cells from an original single-parent cell
How are replicated chromosomes distributed actively to the daughter cells?
in binary fission
How does binary fission occur?
- As the cell elongates, the two DNA attachment sites move apart.
- When the cell is about twice its original size and the DNA molecules are well separated, a constriction forms at the midpoint of the cell.
A new membrane and cell wall are synthesized at the site of the constriction, resulting in two daughter cells, each the same as the parent cell.
What is the bacterial cell cycle?
· Replication of the bacterial chromosome consumes most of the time in the cell cycle
· It begins at a single site called the origin of replication (ori) through reactions catalyzed by enzymes located in the middle of the cell
· Once the ori is duplicated, the two origins actively migrate to either end of the cell
· Division of the cytoplasm then occurs to separate into two daughter cells:
· Inward growth of plasma membrane and cell wall
· Involves cytoskeletal proteins
The bacterial cell cycle. During the B period, from birth to the initiation of DNA replication, the cell grows in size. The chromosome is replicated and the resulting daughter chromosomes move to opposite ends during the C period. Then the cell divides by binary fission during the D period. In very fast growing cultures, the B period may be nonexistent; cells may be formed with chromosomes that are already partly replicated!
Under optimal conditions (in the lab) populations of E. coli can double every 20 minutes.
Bacterial growth curve?
How do mitosis and binary fission associate with each other?
· Believed that binary fission is the ancestral cell division process from which mitosis evolved from
· Variations of the mitotic apparatus in modern-day eukaryotes show possible intermediates in the evolutionary transition from binary fission to mitosis
· Note: chloroplasts and mitochondria divide independently of the host cell, using binary fission
Mitosis evolved from an early form of binary fission
What is mitotic cell division of eukaryotes ?
- Eukaryotic cells reproduce by mitotic cell division.
- To understand how cell division in eukaryotes differs from that in prokaryotes, it is important to first recall that the genome in eukaryotes is large and linear and the genetic material in a eukaryote is separated from the other cellular components because it is in the nucleus of the cell.
- Eukaryotic cell division requires first the breakdown and then the re-formation of the nuclear envelope, as well as mechanisms other than cell growth to separate replicated DNA.
The organelles also need to be distributed to daughter cells
Differences between eukaryotes and prokaryotes?
EUKARYOTES:
genome - Genome: multiple, large, linear chromosomes
DNA in nucleus
PROKARYOTES:
Genome - one small, circular chromosome
DNA in cytoplasm
How do mitosis and the eukaryotic cell cycle work together?
· Cell cycle: A period of growth followed by nuclear division and cytokinesis
· Mitosis divides replicated DNA equally and precisely
Ensures the two cells resulting from a cell division have the same genetic information as the parent cell entering division
What is mitosis the basis for?
□ Growth and maintenance of body mass in multicellular eukaryotes
□ The reproduction of many single-celled eukaryotes
What happened during mitotic cell division
· During mitotic cell division, DNA replication is followed by equal separation (segregation) of the replicated DNA molecules and their delivery to daughter cells
The process ensures that the two cell products of a division have the same genetic information as the parent cell entering division
What type of cell growth and division is cancer
uncontrolled
What is ploidy?
the number of chromosome sets
What are the types of ploidy?
- Haploid (n): a cell with one complete set of chromosomes
- Diploid (2n): a cell with two complete sets of chromosomes
- Humans are Diploid: n = 23, 2n = 46
What are chromosomes?
Nuclear units of genetic information are divided and distributed by mitotic cell division
What happens to eukaryotic chromosomes during mitosis?
In order for cell division to proceed, every chromosome in the parent cell must be duplicated so that each daughter cell receives a full set of chromosomes.
This duplication takes place during S phase.
Even though the DNA in each chromosome duplicates, the two identical copies, called sister chromatids, do not completely separate.
They stay side by side, held together at the centromere.
What are sister chromatids?
Replication of DNA of each individual chromosome creates two identical molecules called sister chromatids
Chromosome segregation: Equal distribution of daughter chromosomes to each of two cells resulting from cell division
What is interphase?
· The formation of a new daughter cell at the end of mitotic cell division marks the beginning of the cell cycle
· Interphase is the first stage of the cell cycle; it begins immediately after mitosis (& cytokinesis) and continues until the beginning of the next mitosis
· Interphase includes several sub-phases
- The initial period of cytoplasmic growth occurs during the G1 phase
DNA replication also takes place during interphase in preparation for mitosis (M phase)
What are the three stages of interphase?
g1, s and g2
G1 phase of interphase?
G1 phase: The cell carries out its function(s), and in some cases grows, during this initial “gap” phase. Interphase begins as daughter cell from previous division cycle enters initial period of cytoplasmic growth
G1, S and G2 phase in the cell cycle? What is G0
The length of G1 varies, but for a given cell type, the timing of S phase, G2 phase, and mitosis is usually relatively uniform. Cytokinesis (red segment) usually begins while mitosis is in progress and reaches completion as mitosis ends. Cells in a state of division arrest are considered to enter a side loop (or shunt) from G1 phase called “G0 phase.”
G0 phase represents that juts because a cell centers G1 doesn’t mean its automatically destined to go enter the S phase and replicate its DNA, its can re-enter and start to divide again.
Interphase diagram?
A chromosome in G1 becomes replicated during S phase and is then composed of two chromatids. Anaphase pulls the two chromatids to opposite poles. All the cells shown above contain one chromosome.
Figure 7.7: Chromosomes from the muntjac deer are individually “painted” with fluorescent stain. Note that there are two cells in this picture. The metaphase cell shows two copies of each of three long, condensed chromosomes. The interphase nucleus shows the DNA of different chromosomes organized in close proximity rather than randomly distributed.
Prophase?
· Chromosomes condense into compact, rod-like structures
· Spindle forms in the cytoplasm
What is DNA packaging around histones?
· DNA packaging around histones in eukaryotic chromosomes
Estimated that a chromosome contains approximately equal weight of DNA and protein
Prometaphase?
· Nuclear envelope breaks down
· Spindle enters the former nuclear area
· Microtubules from opposite spindle poles attach to two kinetochores of each chromosome
The nuclear envelope has broken down to allow the chromosomes to interact with the microtubules originating from two separate centrosomes.
What happens to spindles connections during metaphase?
Spindle connections are made by chromosomes at mitotic metaphase in typical animal cells. The two kinetochores of the chromosome connect to opposite spindle poles, ensuring that the chromatids are separated and moved to opposite spindle poles during anaphase.
Metaphase?
· Spindle is fully formed
· Chromosomes align at spindle midpoint
- Moved by spindle microtubules
All of the replicated chromosomes are aligned on the equator of the mature mitotic spindle.
What are karyotypes?
· The complete collection of metaphase chromosomes, arranged according to size and shape, forms the Karyotype of a given species. Karyotypes are often so distinctive, that they can be used as an identifying feature of a species.
Anaphase?
· Spindle separates sister chromatids and moves them to opposite spindle poles
· Chromosome segregation is complete and have decondensed to form two independent daughter nuclei. This cell has just begun cytokinesis.
Telophase?
· Chromosomes decondense
· Return to extended state typical of interphase
· New nuclear envelope forms around chromosomes
- two daughter cells
Mitosis diagram?
· Mitosis in the blood lily Haemanthus. The chromosomes are shown in purple; the spindle microtubules are shown in pink.
What is cytokinesis by furrowing?
- The furrow begins as an indentation running completely around the cell in the plane of the former spindle midpoint.
- The furrow deepens by contraction of the microfilaments, like a drawstring tightening around the cell.
Furrowing continues until the daughter nuclei are enclosed in separate cells.
- The furrow deepens by contraction of the microfilaments, like a drawstring tightening around the cell.
What is plant cytokinesis?
· Cell wall material is deposited along the plane of former spindle midpoint
· Deposition continues until continuous new wall (cell plate) separates daughter cells
· Cytokinesis by cell plate formation in plant cells.
1. A layer of vesicles containing wall material collects in the plane of the former spindle midpoint (arrow).
2. More vesicles are added to the layer until it extends across the cell.
3. The vesicles fuse together, dumping their contents into a gradually expanding wall between the daughter cells.
Centrosome and spindle formation diagram
What is a fully developed spindle?
Only microtubules connected to chromosomes have been caught in the plane of this section. One of the centrioles is visible in a cross-section in the centrosome at the top of the micrograph (arrow). (Original magnification × 14 000.)
What are spindle microtubules?
· Kinetochore microtubules
- Connect chromosomes to spindle poles
· No kinetochore microtubules
- Extend between spindle poles without connecting to chromosomes
At spindle midpoint, microtubules from one pole overlap with those from opposite pole
What is kinetochore movement?
· Chromosomes move by sliding over or along kinetochore microtubules
· Microtubules disassemble as kinetochore passes over them
Experimental Research: Movement of chromosomes during anaphase of mitosis. One hypothesis for how chromosomes move during anaphase of mitosis was that the kinetochore microtubules moved, pulling chromosomes to the poles. An alternative hypothesis was that chromosomes move by sliding over or along kinetochore microtubules. To test the hypotheses, G. J. Gorbsky and his colleagues made regions of the kinetochore microtubules visibly distinct.
Microtubule motor proteins at the kinetochore allowing a chromosome to “climb” along a microtubule diagram
What are CDKS and what are cyclin ?
► CDKs are only active when combined with the proper Cyclin
· CDKs
► Protein kinases, which phosphorylate, and regulate activity of, target proteins
► Activated when bound to cyclins
· Different cyclin:CDK combinations regulate cell cycle transitions at different “checkpoints”
The levels of cyclin rise and fall during the cell cycle.
R. Timothy Hunt was awarded the Nobel Prize for their discovery
Name cyclin reflects their pattern of increasing and decreasing concentration in the call -
Cyclin Levels Throughout the Human Cell Cycle?
What is the regulation of the cell cycle?
- Mitotic cell division occurs during the growth of a multicellular organism, wound healing, or in the maintenance of actively dividing tissues such as the skin or lining of the intestine.
- Cell division cannot occur all the time because uncontrolled division is dangerous and can lead to cancer.
- Progression through the cell cycle is controlled by proteins that appear and disappear in a cyclical fashion, and by several enzymes that become active and inactive, also in cycles.
- The proteins that appear and disappear cyclically are called cyclins.
- The cyclins activate the enzymes, which are called cyclin-dependent kinases (CDKs).
- Once the kinases are activated by cyclins, cell division is promoted.
Cyclin-dependent kinases (CDKs) are always present in the cell but are active only with the appropriate cyclin bond.
What is the relationship between cyclin and MPF?
Cyclin and MPF activity peak at M phase. Cyclin levels rise steadily starting at S phase.
What are cyclins?
· Maturation (or Mitosis)- promoting factor (MPF) is a cyclin-Cdk complex that participates in enzyme activation, nuclear envelope breakdown, and chromosome Condensation
Cyclins are proteins whose concentration varies over the course of the cell cycle. Cdks are Kinases whose activity is regulated by Cyclins. While the concentration of the Cyclins oscillates, the concentration of the Cdks is typically pretty stable. But their ACTIVITY varies together with the concentration of the Cyclin, upon which the activity of the CDK depends.
· **Cdk is present throughout the cell cycle - - it’s the cyclin that changes in concentration, thereby activating the Cdk (i.e. forming MPF) to allow cells to pass the M checkpoint.
What is contact inhibition?
· Prevents cells from growing if they become too crowded. Cell density is communicated by cell-cell interactions through cell surface receptors.
If cells are removed from culture surface, remaining cells will divide to fill the gap. Division stops once gap is filled.
Contact inhibition is lost in some cancer cells, which may produce excess growth factors or have failures in cell cycle regulation.
Contact inhibition involves cell surface proteins that interact with correponding proteins on adjacent cells when they come into contact with one another. Interaction of cell surface receptors triggers signal transduction pathway that prevents the cell cycle from proceeding. When contact with a neighbouring cell is lost, inhibition is released, cell cycle proceeds (ie. Cell division), until contact between cells occurs again.
Can cells divide indefinitely?
· Most cells in a multicellular body cannot divide indefinitely
· Cellular senescence: Loss of proliferative ability over time
· Two candidates for responsibility of cellular senescence
► DNA damage
► Telomere shortening
· As cells age, it is intuitive to think that damage to the DNA (or mutations) will accumulate. If this damage effects key cellular activities, it makes sense that the cell will eventually stop being able to divide.
