Cell cycle and death

Question 1

What are the three purposes of the cell cycle?

Answer 1

To replace lost or damages cells
To grow the multicellular organism
To maintain the number of cells in a adult multicellular organism

Question 2

How do prokaryotes divide?

Answer 2

Binary fission.
The nucleic acid binds to the plasma membrane and the cell enlarges and DNA duplicates.
Septum forms and cell divides forming two cells which separate.

Question 3

How does cytokineses in prokaryotes occur?

Answer 3

A protein known as ‘FtsZ’ is randomly dispersed in the cytoplasm of the prokaryote. When cytokineses needs to occur, FtsZ forms a ring at the division site. The FtsZ ring contracts allowing the two cells to form.

Question 4

How is circular DNA replicated?

Answer 4

At the ‘ori’ of the circular DNA, replication forks form allowing the DNA to be replicated in the same way that DNA in chromosomes. Replication is bilateral.

Question 5

How does the prokaryote overcome the issue of DNA replication being twice as long than cytokineses?

Answer 5

Multi-fork replication. Before the first round of DNA replication has even finished, the next round of replication occurs which means that the DNA replication can ‘keep up’ with he cell division.

Question 6

What is Interphase?

Answer 6

G1, S phase and G2.
G1 is the growth phase of the cell where every organelle doubles in mass. Additionally the enzymes required for DNA replication are synthesized
S phase is the replication of DNA
G2 is the preparation for mitosis

Question 7

What is significant about the sister chromatids at the end of S phase?

Answer 7

Cohesin is a protein that keeps the sister chromatids together. It is essential that they’re kept together because otherwise it would be difficult for the attachment to mitotic spindle to occur.

Cohesin has SMC 1 and SMC 3 (SMC stands for Structural Maintenance of Chromosomes)

Question 8

What events mark beginning of mitosis?

Answer 8

Condensation of chromosomes

Formation of mitotic spindles

Question 9

How is the chromosome ‘condensed’?

Answer 9

Condensin is a protein that allows the chromatids to condense for mitosis in the prophase stage.
DNA loops are encircled by the condensin to compact them.

Condensin contains SMC 2 and SMC 4 (SMC stands for Structural Maintenance of Chromosomes)

Question 10

What are Kinetochore?

Answer 10

They’re a protein complex attached to the centromere which bind to the microtubules of the spindle fibre.

Question 11

What is prophase…

metaphase…

Answer 11

.

Question 12

How are the sister chromatids separated?

Answer 12

Protease enzyme cleaves the cohesin by cleaving the kleisin subunit of the cohesin. This allows the two sister chromatids to separate.

Question 13

How does cytokineses of eukaryotic (animal) cells occur?

Answer 13

After mitosis has finished and the DNA is separated, actin and myosin II filaments along the contractile ring, contract to pinch the cell, forming two cells.

Question 14

How does cytokineses of plant cells occur?

Answer 14

Contracile ring does not fomr after mitosis has occured, instead, the cell wall just grows in the septum to divide the two cells.
New wall synthesis is guided by phragmoplast microtubules.

Question 15

How long does the cell cycle for the following take?

1) Mammalian intestinal epithelial cells
2) Mammalian fibroblast cells
3) Human liver cells
4) Yeast cells

Answer 15

1) around 12 hours
2) around 20 hours
3) around a year
4) 1.5 to 3 hours

Question 16

Why is early embryonic cell division so fast?

Answer 16

Usually, the cell needs to grow before it divides which takes time. Early embryonic cells simply just divide and don’t grow in size meaning that the division of these cells is around 20x faster than somatic cells.

Question 17

What is ‘closed mitosis’ and what is ‘open mitosis’?

Answer 17

Open mitosis is when the nuclear envelope breaks down for mitosis to occur and closed mitosis is when the nuclear envelope remains intact for the duration of mitosis.

Multicellular organisms operate under open mitosis as the SPB (Spindle Pole Body which is the site of mictotubule organisation) is outside of the nucleus, so the envelope needs to degrade so that the mitotic spindles may interact with the chromosomes.
Unicellular organisms (such as yeast) operate under closed mitosis as the SPB is embedded within the nuclear envelope so there's no need for the nuclear envelope to degrade.

Question 18

What are cell fate determinants?

Answer 18

These are factors (proteins, mRNA and other nucleic acids) from the mother cell given to one of the daughter cells to allow it to develop in a different pathway. This gives polarity to the daughter cells and enables the cells to not be identical after duplication.

Question 19

What is the purpose of ‘niche cells’ in terms of stem cells?

Answer 19

Stem cells are attached to stem cells. When division of the stem cell occurs, the niche cell allows asymmetric division of the stem cell, allowing one of the newly formed cells to remain a stem cell (by blocking differentiation) and the other newly formed cell to start to differentiate. The stem cell remains attached to the niche cell and the other cell is released and is allowed to differentiate.

Question 20

What is anchorage dependence?

Answer 20

Cells must be attached to the substratum in order to start dividing.

Question 21

What is density-dependent inhibition?

Answer 21

Cells stop dividing when they contact each other. Otherwise known as ‘contact inhibition’.

Question 22

What is necrosis?
What is apoptosis?
What is excitotocity?

Answer 22

Necrosis is when a cell dies due to excessive (irreparable) damage.
Apoptosis is programmed cell death (controlled)
Excitotoxcity is over-excitement of neuronal cell which cause death.

Question 23

What is blebbing?

Answer 23

When the cell membrane remains intact for the death of a cell. This ensures that there is no leakage of cell contents so that nearby cells aren’t damaged.

Question 24

What are some characteristics that separate necrosis from apoptosis?

Answer 24

Necrosis
Membrane is damaged (causing leakage of cell contents which may damage nearby cells or illicit an inflammatory response)
Energy levels for the cell rapidly deplete
Chromatin flocculation

Apoptosis
Membrane remains intact throughout cell death (preventing leakage of cell contents)
Energy level of cell reduce slowly over time
Chromatin condense

Question 25

What is the pre-determined pathway for cells undergoing apoptosis?

Answer 25

Chromatin condensation
Membrane blebbing
Cell fragmentation
Apoptotic bodies are engulfed by phagocytes

Question 26

Why is it important for cell membrane to remain intact during apoptosis?

Answer 26

If the membrane is damaged, there will be leakage of cell contents, which may damage nearby cells and will illicit a inflammatory response.

Question 27

Why do cells need to commit apoptosis?

there’s like 5 examples

Answer 27

Organisms that are metamorphic (such as frogs and flies) need apoptosis in order to develop into the next stage of their life cycle.
During fetal development, some cells purposes are simply to assist in the formation of body parts (such as the webbing on the paws/hands). The cells need to be destroyed before the animal is born so PCD occurs to get rid of them.
Cells infected by viruses kill themselves via apoptosis.
Cells that are malignant die via apoptosis.
Cells with excess DNA damage die via apoptosis.

Question 28

How do phagocytes know to engulf apoptotic cells?

Answer 28

Apoptotic cells released phosphatidylserine which attract phagocytes. Receptors on phagocytes bind to externalized phosphatidylserine causing anti-inflammatory cytokines to be released and the apoptotic cell to be engulfed.

Question 29

What are caspases?

Answer 29

They are protease enzymes with a Cys group in their active site which cleaves substrate at specific aspartate sites.

They are enzymes that drive apoptosis in multicellular organisms.

Question 30

How are caspases controlled?

Answer 30

Caspases are synthesized as inactive zymogens.

Highly evolved upstream regulatory pathways including the presence of endogenous inhibitors.

Question 31

What is the extrinsic pathway for apoptosis?

Answer 31

Extracellular signals that indicate that the cell is no longer needed for the well-being of the organism.
Involves transmembrane death receptors (which are members of the tumor necrosis factor receptor superfamily).
Members of this receptor family bind to extrinsic ligands and transduce intracellular signals that activate caspases.

Question 32

What is the intrinsic pathway for apoptosis?

Answer 32

Apoptotic stimuli cause mitochondrial membranes to
become leaky, leading to release of cytochrome c into
the cytoplasm. Cytochrome c activates a caspase.

Triggered by excessive DNA damage or cytotoxic drugs entering the cell.

Question 33

What types of UV light are there?

Which is more dangerous?

Answer 33

UV-C (180-190nm) which is the most dangerous but is not found in daylight because it is absorbed by the ozone layer.
UV-B (290-320nm) which is the major mutagenic fraction of sunlight. This is the UV light that causes dimers to form between adjacent thymines.

Question 34

Scientifically, describe ‘sunburn’.

Answer 34

The skin is unprotected and is damaged by UV-B. The DNA in the skin cells is damaged beyond repair (due to adjacent thymine dimer formation), so the skin cells undergo apoptosis, which is known as sunburn.

This occurs to prevent melanoma (skin cancer).

Question 35

Why do yeast cells undergo apoptosis?

Answer 35

Even though they’re unicellular, the death of a defective cell may be beneficial for the yeast colony as a whole.
Yes, the genome of the single yeast cell would be lost, bu it would protect the genome of the colony.

Question 36

What is the excitotoxixity process?

Answer 36

Glutamate binds to NDMA and AMPA receptors.
Activated AMPA receptors allow NA+ to enter the cell depolarising the plasma membrane.
This dislodges Mg2+ from the NMDA receptors, permitting entry of Ca2+.
Prolonged exposure to glutamate leads to prolonged entry of Ca2+.
The increased Ca2+ dependent enzymes involved breakdown of proteins, phospholipids and nucleic acids.

Question 37

What are the components of the cell cycle control system?

Answer 37

The cell cycle engine, which is the protein complex that drives the cycle.
Coordination, at any given point in the cell cycle, the cell knows where in the replication phase it is in
Checkpoints

Question 38

What is the cell cycle engine?

Answer 38

Phases of the cell cycle are driven by action of a protein kinase, cyclin dependent protein kinase (CDK).
Kinase levels remain constant throughout the cell cycle, and the activity of kinase activates the phases of the cycle.
As CDK is cyclin dependent, cyclins is the key to regulating the cell cycle.