cell cycle and cell division

Question 1

Describe the four events that characterize cell division.

Answer 1

1- cell division signals
- ordinate from either inside or outside cell

• prokaryote: environmental conditions, nutrients
• eukaryotes: neighbouring cells, secrete growth factor that serves as signal, cell division signals (mitogens)

2- DNA replication
- 2 new cells have full complement of genetic info

3- DNA segregation
- replicated DNA must be distributed appropriately

4- Cytokenisis
- cytoplasm divides to form 2 new cells

Question 2

Explain how replication and segregation co-occur during cell division in prokaryotes.

Answer 2

cell division signal:
- external environment, nutrient concentration

dna replication
- starts at ori ends at ter

dna segregation
- ori region move toward opposite end of cell during replication
- 2 ori parts taken to 2 opps ends of mother cells
- while dna is replicating, 2 new chromosomes are heading to daughter cells

cytokenesis
- starts immediately after end of chromosome replication

Question 3

Explain how binary fission results in identical daughter cells.

Answer 3

• 1 chromosome is replicated if favourable conditions
• asexual reproduction of entire single celled organism
• cell grows, replicates DNA, separates cytoplasm and DNA into 2 new daughter cells

Question 4

State the two ways in which DNA replication differs in prokaryotes and eukaryotes

Answer 4

EUKARYOTES
- many chromosomes with many more ori regions from which dna rep starts

• synthesis/replication of DNA is confined to a period (S phase) between cell divisions
• no overlap with chromosome segregation and replication, S phase, lapse of time, then segregation happens

PROKARYOTES
- 1 chromosome with 1 ori

• once division finished, cell immediately replicates dna and gets ready for next cell division

Question 5

Sketch and describe the four phases of the eukaryotic cell cycle.

Answer 5

see diagram

Question 6

Explain how the progress of a eukaryotic cell through the cell cycle is controlled by cyclin-dependent kinases.

Answer 6

• triggering of checkpoints rely on kinases
• cyclic dependent kinase (CDK)
• inactive unless bound to protein cyclin
• cyclin binds to CDK, changes conformation to make it active due to phosphorylation
• cyclin helps recognize target proteins and idk phosphorylates them
• mainly G1 to S and G2 to M
• G1 to S transition is called R point
• different cyclin - CDK complexes that act at different stages

in G1 to S –> S-CDK
- accumulates at beginning of S phase

in G2 to M –> M-CDK
- m cyclin reduced at G2, increase in concentration
- degradation half way through mitosis
- degradation of cyclin inactivates

accumulation of cyclins helps regulate activity of CDKs

increased concentration of relevant cyclin helps formation of cyclin-idk complex (more activity of complex)

concentration of complex changes but not of cdk
CDK ALWAYS PRESENT BECOME ACTIVE WHEN CYCLIN IS ACCUMULATED

accumulation of only cyclin that is needed

activity of CDK is not only regulated by production and degradation of cyclins

• degradation of cyclin marked by ubiquitin protein
• recognized by proteasomes and degrades it

also phosphorylation, if cyclin produced and bound, but dna damage occurs, cell can inactivate complex
- takes time to repair damage

-kinase phosphorylates complex and deactivates it

cell takes time and repairs damage

• remain inactive until phosphates removed by activating protein called phosphatase

inhibitors:
important in g1 to s
- activity of CDK can be blocked by an inhibitor protein
- binds to complex, prevents idk from phosphorylating target proteins

• dna damage arrests cell cycle in G1
• p53 when phosphorylated by kinase due to dna damage, stabilizes it
• no phosphorylation otherwise unstable and degrades
• accumulates and turns on genes, gene encoding for p21 which is inhibitor protein for CDK
• if no inhibitor CDK alway active so no signal of DNA damage, uncontrollable cell division

when CDK is active
- phosphorylates target protein: RB
- active: inhibits transcription factor, inhibits cell cycle at restriction point
- phosphorylation of RB changes conformation of protein and inactivates it
- release transcription factor turns on genes important for replication
- cell cycle proceeds

Question 7

Predict conditions under which external factors might stimulate the eukaryotic cell cycle.

Answer 7

• mitogens, growth factors,

Question 8

Describe DNA packaging and how it varies during the cell cycle.

Answer 8

interphase:
- chromosomes are thread like spread throughout nucleus

G1:
- 1 molecule of double strand DNA
-
S:
- 2 molecule of double strand DNA
- 1 chromosome becomes 2 sister chromatids remain together until mitosis

M:
- dna and proteins in each chromosome form compact structures

Question 9

List the five stages of mitosis and state what events occur in each.

Answer 9

interphase:
- in s phase, nucleus replicates DNA and centrosomes

prophase:
- chromsomes condense
- centrosomes move to opposite side of nucleus
- microtubules reorganize
- mitotic spindle forms

pro metaphase:
- nuclear envelope breaks down (phosphorylation of lamina)
- mitotic spindle is in place
- chromosomes begin to line up

metaphase (middle)
- chromosomes attach to microtubules and line up in middle

anaphase: (move up twds pole)
- sister chromatids separated
- move towards opposite poles

telophase: telo mean end
- 2 nuclei created, nuclear envelope reforms

cytokenesis:
- 2 daughter cells

Question 10

Explain how the spindle is involved in segregation of sister chromatids.

Answer 10

astral microtubules, kinetochore microtubules, and interpolar microtubules.

inter polar:
- have motor proteins that stabilize microtubules coming from 2 different centrosomes
- create spindle framework

• anaphase:
• cohesion protein attached at centromere
• digestion of these protein help separate chromatids and take them to opposite sides

Question 11

Describe the differences between cytokinesis in animals and plants.

Answer 11

animal cells:
- actin and myosin filaments, pinching of cytoplasm in cytokinesis

plant cells:
-inter polar microtubules important in construction of cell wall
- microtubules move vesicles full of carbs and proteins required to make cell wall in middle of cell
- vesicles deposited in middle, start to fuse and create ground for cell wall
-

Question 12

important genes

Answer 12

A few important tumor-suppressor genes:

loss of function:
- p53:a transcription factor that regulates cell division and cell death.

- for p21 which is inhibitor protein for CDK
- if no inhibitor CDK alway active so no signal of DNA damage, uncontrollable cell division

-Rb:alters the activity of transcription factors and therefore controls cell division.
- CDK-S phosphorylates Rb and inactivates it,
- loses activity, dna synthesis no longer inhibited

gain of function: