Chapter 11/12- Cell division Flashcards
(219 cards)
1
Q
Necessary for the growth of organisms, for wound healing, and to replace cells that are lost regularly, such as those in your skin and in the lining of your gut.
A
Cell Division
2
Q
All species of organisms __________ and ____________ to pass on the hereditary information
A
Grow; Reproduce
3
Q
Name 2 types of Cell Division
A
Bacterial Cell Division; Eukaryotic Cell Division
4
Q
Bacteria divide as a way of
A
reproducing themselves
5
Q
Although bacteria exchange DNA, they do not have a ______________ like eukaryotes
A
sexual cycle
6
Q
All growth in a bacterial population is due to ___________________
A
division to produce new cells
7
Q
reproduction of bacteria is
A
clonal
8
Q
Each cell produced by cell division is an identical copy of the original cell
A
Clonal reproduction
9
Q
Cell division in both bacterial and eukaryotic cells produces
A
two new cells with the same genetic information as the original
10
Q
In Cell division in both bacterial and eukaryotic cells, what are essentials of the process which are the same,
A
duplication and segregation of genetic information into daughter cells, and division of cellular contents
11
Q
___________have a genome made up of a single, circular DNA molecule found in the nucleoid
A
Bacteria
12
Q
Circular DNA of bacteria held together by
A
structural maintenance of chromosome , (SMC) proteins
13
Q
In eukaryotes the ________ and _________ proteins are SMC proteins
A
cohesin and condensin proteins are SMC proteins
14
Q
Causes replication of DNA and division of cell into 2 cells
A
Binary Fission
15
Q
A sexual reproduction by division of 1 cell or body into 2 equal or nearly equal parts
A
Binary fission
16
Q
Binary Fission occurs in
A
Bacteria
17
Q
produces two identical cells with the same genetic information as the original cell
A
Binary Fission
18
Q
What are the steps of Binary Fission
A
- DNA replication
- Cell Elongation
- Cell Division
19
Q
Binary Fission: DNA replication
Describe.
A
- Begins with the replication of the bacterial DNA at a specific site—the origin of replication (origin point)
- Enzymes for DNA duplication continue until they meet each other (finished replicating)
- Origin point of each DNA are in opposite directions: DNA proceeds both directions around the circular DNA to a specific site of termination
20
Q
Binary Fission: Cell Elongation
Describe.
A
- As DNA replication completes, the origin moves to opposite sides of the cell, causing elongation (growth) of the cell
- Nucleoids are assembled around each circular DNA
21
Q
Binary Fission: Cell Division
Describe.
A
- Once DNA is segregated, the midline is cleared of remaining nucleoids and cell division occurs
- production of septum (the wall) occurring at the midpoint of the cell forms the cell wall for the daughter cells
- The septum will pinch pinch off
22
Q
During replication, first the origin, then the rest of the newly replicated chromosomes are moved to opposite ends of the cell as two new nucleoids are assembled
The final event of replication is decatenation (untangling) of the final replication products.
After replication and segregation, the midcell region is cleared of daughter nucleoids, and division occurs.
The force behind chromosome segregation has been attributed to DNA replication itself, transcription, and the polymerization of actinlike molecules.
The cell’s other components are partitioned by the growth of new membrane and production of the septum (A wall between two cavities.) This process, termed septation, usually occurs at the midpoint of the cell
It begins with the formation of a ring composed of many copies of the protein FtsZ. Proteins assemble into ring and facilitate septation and cell division
A
chromosome segregation
23
Q
Chromosome number varies among species (T/F)
A
True
24
Q
Humans have _________ chromosomes and ________ pairs
A
46; 23
25
Eukaryotic DNA are packaged as ________________
chromosomes
26
Determines how a person’s body develops and functions
chromosomes
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Human embryos missing even one chromosome
monosomy
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Having an extra copy of any one chromosome
trisomy
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Chromosomes are composed of ____________, a complex of DNA and protein with a significant amount of RNA
chromatin
30
Chromosomes are the site of _____________
RNA synthesis
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Each chromosome contains a single _______________ that runs uninterrupted through the chromosome’s entire length
DNA molecule
32
Every 200 nucleotides =
DNA duplex (double strand)
33
DNA duplex (double strand) is coiled around a core of
eight histone proteins
34
Histones are ____________ charged because of an abundance of the basic amino acids _________ and ________. Thus, they are strongly attracted to the ___________charged phosphate groups of the DNA
positively ; arginine and lysine.
negatively
35
The complex of DNA duplex wound around a core of 8 histone proteins is termed
nucleosome
36
DNA is ________ charged while histones are ___________ charged
negatively
| positively
37
The DNA wrapped in nucleosomes is further coiled into an even more compact structure called
solenoid
38
Solenoids form looped domains that form
densely packaged DNA
39
During mitosis, proteins are assembled into a _________ that provides a framework for the final level of compaction. This gives chromosomes their familiar X-shaped structure, and facilitates separation by the mitotic machinery
scaffold
40
The particular array or morphology of chromosomes an individual organism possesses is called its
karyotype
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One complete set of chromosomes necessary to define an organism.
haploid (n)
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refers to n number of chromosomes; one set of chromosomes
haploid (n)
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For humans and many other species, the total number of chromosomes in a cell is called the ___________ number, which is twice the haploid number
diploid (2n)
44
For humans, the haploid number is ____ and the diploid number is ____.
23; 46
45
reflects the equal genetic contribution that each parent makes to offspring
Diploid chromosomes
46
Refers to similar structures that have the same evolutionary origin.
homologous
47
Refers to a pair of the same kind of chromosome in a diploid cell.
homologous
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In a diploid each chromosome has a ___________
homologue
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maternal and paternal chromosomes are referred to as _______________, and each 1 of the pair is termed ______________
homologous; homologue
50
Homologous chromosomes replicate themselves for ____________ and is held together by ___________
Cell division
| cohesins
51
A protein complex that holds sister chromatids together during cell division.
cohesins
52
The loss of _________ at the centromere allows the anaphase movement of chromosomes.
cohesins
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After replication, each chromosome is composed of two identical DNA molecules held together by a complex of proteins called
cohesins
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Refers to either of the two identical copies (chromatids) formed by the replication of a single chromosome ________________, with both (2) copies joined together by cohesions (proteins) via a region called_____________
sister chromatids ; centromere
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The region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division.
centromere
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Two replicas of a single chromosome held together at their centromeres by cohesin proteins after DNA replication.
sister chromatids
57
Composed of proteins found at the centromere that attach to microtubules during mitosis
kinetochore
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Eventually 2 sister chromatids separate, ONE in each ____________ cell.
daughter cell
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portion of DNA that is not expressed as RNA
Heterochromatin
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portion of DNA that is expressed as RNA
Euchromatin
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The repeating sequence of growth and division through which cells pass each generation, or growth and division of eukaryotic cells
requires the duplication of the genome, its accurate segregation, and the division of cellular contents.
Cell Cycle
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5 stages of Cell Cycle
```
– G1 (gap 1) phase
– S (synthesis) phase
– G2 (gap2) phase
– Mitosis
– Cytokinesis
```
63
The primary growth phase of the cell
G1 (gap 1) phase
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Longest phase of Cell Cycle
G1 (gap 1) phase
65
The phase in which the cell synthesizes a replica of the genome, or DNA is replicated
S (synthesis) phase
66
The second growth phase, where the cell is preparing for separation of the newly replicated genome, or DNA
G2 (gap2) phase
67
During this phase microtubules begin to reorganize to form a spindle for mitosis
G2 (gap2) phase
68
G1, S, and G2 together constitute _______________ , the portion of the cell cycle between cell divisions.
interphase
69
Phase of the cell cycle in which the SPINDLE apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart.
Mitosis
70
The essential step in the separation of the two daughter genomes
Mitosis
71
5 stages of mitosis
prophase, prometaphase, metaphase, anaphase, and telophase
72
phase of the cell cycle when the cytoplasm divides, creating two daughter cells
Cytokinesis
73
involves growth and preparation for DNA synthesis
G1 phase
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a copy of the genome is synthesized
S phase
75
prepares the cell for mitosis
G2 phase
76
replicated chromosomes are partitioned
mitosis
77
divides the cell into two cells with identical genomes.
cytokinesis
78
To distinguish the dividing phase from interphase, Mitosis and cytokinesis together are usually referred to collectively as
M phase
79
Cells often pause in G1 before DNA replication and enter a resting state called the
G0 phase
80
The stage of the cell cycle occupied by cells that are not actively dividing.
G0 phase
81
Most of the cells in an animal’s body are in this phase
Most cells are in this phase
G0 phase
82
All the events that happen during G1, S, and G2 phases
Interphase
83
Interphase prepares chromosomes for __________
mitosis
84
Interphase:
1) A point of constriction on the chromosome containing repeated DNA sequences that bind specific proteins.
2) These proteins make up a disklike structure called ______________
1) centromere
| 2) kinetochore
85
Interphase:
A disk-like protein structure bound to repeating DNA
sequences on the centromere
kinetochore
86
Interphase:
During mitosis, microtubules will attach at ___________ and pull the sister chromatids apart
kinetochore
87
Interphase:
In __________ phase, the replicated chromosomes will further condense ,
G2 phase
88
Interphase:
Chromosomes begin the process of condensation, coiling ever more tightly in __________ phase
G2 phase
89
Interphase:
In __________ phase, centrioles will replicate and tubulin will be synthesized (the two will form microtubules)
G2
90
Interphase:
Protein that forms microtubules.
Tubuiln
91
Interphase:
Involved in the rapid final condensation of the chromosomes that occurs early in mitosis
Special motor proteins
92
M phase: Mitosis (stages)
What am I?
- condensed chromosomes
- condensation process continues, or further condensation of chromosomes.
- Ribosomal RNA synthesis ceases when the portion of the chromosome bearing the rRNA genes is condensed.
- No further production of rRNA
Prophase
93
M phase: Mitosis (stages)
What am I?
- spindle apparatus will assemble
- two centriole pairs formed during G2 phase begin to move apart, forming between them an axis of microtubules (referred to as spindle fibers) until they reach opposite sides of the cell
Prophase
94
M phase: Mitosis----> Prophase
In animal cell mitosis, the centrioles extend a radial array of microtubules toward the nearby plasma membrane when they reach the poles of the cell. This arrangement of microtubules is called an___________
Aster
95
M phase: Mitosis (stages)
What am I?
The nuclear envelope begins to break down
Prophase
96
M phase: Mitosis (stages)
What am I?
During the formation of the spindle apparatus, the nuclear envelope breaks down competley and the endoplasmic reticulum reabsorbs its components
Condensed chromosomes will attach to (spindle)microtubules via kinetochores
Prometaphase
97
M phase: Mitosis (stages)
What am I?
Occurs following the disassembly of the nuclear envelope.
Prometaphase
98
The transitional phase between prophase and metaphase during which the spindle(microtubules) attaches to the kinetochores of sister chromatids.
Prometaphase
99
M phase: Mitosis (stages)
What am I?
- Condensed chromosomes become attached to the spindle by their kinetochores
- Each chromosome possesses two kinetochores, one attached to the centromere region of each sister chromatid
Prometaphase
100
M phase: Mitosis (stages)
What am I?
As _____________ continues, a second group of microtubules grow from the poles of the cell toward the centromeres.
These microtubules are captured by the kinetochores on each pair of sister chromatids. This results in the kinetochores of each sister chromatid being connected to opposite poles of the spindle.
Prometaphase
101
M phase: Mitosis (stages)
What am I?
- Second group of microtubules attach to kinetochore on centromere (opposite from the first site)
- Results in microtubules attached to each side of the sister chromatids
Prometaphase
102
M phase: Mitosis (stages)
What am I?
Microtubules will pull all of the chromosomes to the center of the cell
Prometaphase
103
M phase: Mitosis (stages)
What am I?
The alignment of the chromosomes in the center of the cell
metaphase
104
M phase: Mitosis (stages)
What am I?
Third stage of mitosis _______________
metaphase
105
M phase: Mitosis (stages)
What am I?
The alignment of the chromosomes in the center of the cell run along the ____________________
metaphase plate
106
M phase: Mitosis (stages)
What am I?
Not an actual structure, but rather an indication of the future axis of cell division.
metaphase plate
107
M phase: Mitosis (stages)
What am I?
Shortest phase
anaphase
108
M phase: Mitosis (stages)
What am I?
Begins when the cohesin proteins holding sister chromatids together at the centromere are removed
anaphase
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M phase: Mitosis (stages)
What am I?
Up to this point in mitosis, sister chromatids have been held together by cohesin proteins concentrated at the centromere
anaphase
110
M phase: Mitosis (stages)
What am I?
1. Kinetochores are pulled toward the poles as the microtubules that connect them to the poles shorten
2. Tubulin subunits are removed from the kinetochore ends of the microtubules, and progressively disassemble
3. Chromatids are pulled ever closer to the poles of the cell.
Anaphase A
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First stage of Anaphase
Anaphase A
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Second stage of Anaphase
Anaphase B
113
M phase: Mitosis (stages)
What am I?
Poles move apart as microtubular spindle fibers physically anchored to opposite poles slide past each other, away from the center of the cell
Anaphase B
114
M phase: Mitosis (stages)---> Anaphase
The kinetochores are pulled by shortening microtubules (disassembly of tubulin), resulting in _______________________, the poles then
separate as microtubules _____________________
sister chromatids being pulled in opposite directions (poles)
pull back to opposite poles of the cell
115
M phase: Mitosis (stages)
What am I?
The nucleus re-forms during ________________________
telophase
116
M phase: Mitosis (stages)
What am I?
- Spindle apparatus is diassembled
- Microtubules broken down to tubulin
- Nuclear envelop forms around chromosomes on both sides
telophase
117
In this phase chromosomes uncoil for gene expression and rRNA production, resulting in reappearance of the nucleolus
Telophase
118
During telophase cytoplasmic organelles are ___________________
sorted into both daughter cells
119
M phase: Mitosis (stages)
What am I?
Cytoplasmic organelles are sorted into both daughter cells
Telophase
120
M phase: Mitosis (stages)
What am I?
Achieved by means of a constricting belt of actin filaments
In animal cells
Cytokinesis
121
In cytokinesis, a belt of actin filaments slide towards each other, pinching the middle of the cell together, a structure called the ______________________
cleavage furrow
122
The constriction that forms during cytokinesis in animal cells that is responsible for dividing the cell into two daughter cells.
cleavage furrow
123
In cytokinesis, the __________________will deepen until the cells pinch off to form 2 daughter cells
cleavage furrow
124
Division of the cytoplasm of a cell after nuclear division.
cytokinesis
125
The structure that forms at the equator of the spindle during early telophase in the dividing cells of plants and a few green algae.
cell plate
126
M phase: Mitosis (stages) ----> Cytokinesis (plants)
During telophase, center of expanding cell forms a _______________
cell plate
127
M phase: Mitosis (stages) ----> Cytokinesis (plants)
What is the function of the cell plate?
Will fuse with the membrane and divide the cell into 2
128
Where is the cell cycle process checked for accuracy and halted if there are errors?
Check Points
129
Cell cycle can be put on hold at specific points called
Check Points
130
HIGH LEVEL OF CONTROL that ensures proper division and high accuracy of chromosome replication
Check Points
131
Proteins that start or stop each phase of the cell cycle
Check Points
132
What are the 3 cell cycle check points
G1/S checkpoint
G2/M checkpoint
Spindle checkpoint
133
Halts the cycle to assess internal state of cell and integrates external signals
check points
134
A positive regulator of cell-cycle progression in the cytoplasm of dividing cells
maturation-promoting factor (MPF )
135
Research on the activation of frog oocytes led to the discovery of a substance that was first called
maturation-promoting factor (MPF )
136
The first MPF was discovered during
oocytes experiments
137
What experiment is this?
Cytoplasm taken from a variety of actively dividing cells from the M phase / injected in G1 phase of cell / prematurely began mitosis, or induced cell division when injected into oocytes
oocytes experiments
138
____________ taken from a variety of actively dividing cells from the _________ / injected in _________of cell / prematurely began ________, or induced ________ when injected into __________
Cytoplasm
M phase
G1 phase
mitosis
cell division
oocytes
139
Oocyte experiment
____________ taken from a variety of actively dividing cells from the _________ / injected in _________of cell / prematurely began ________, or induced ________ when injected into __________
Cytoplasm
M phase
G1 phase
mitosis
cell division
oocytes
140
A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.
mitosis
141
True or false
Cytoplasm from M phase contains a positive regulator that causes a cell to enter mitosis
True
| MPF
142
True or False
MPF activity varied during the cell cycle
True
low in early G2,
rising throughout this phase,
and then peaking in mitosis
143
Two key aspects of MPF:
1) MPF activity varied during the cell cycle: ______ in early ____, _______ throughout this phase, and then ______ in mitosis
2) Enzymatic activity of MPF involved the __________________________
low
G2
rising
peaking
phosphorylation of proteins
144
MPF levels varied throughout the cell cycle this suggested _______________________ and that MPF probably had some sort of _______________ activity
Regulation of MPF
enzymatic (kinase)
145
MPF activity variation indicated _________
Phosphorylation of proteins in MPF indicated _____________
Regulation of MPF
enzymatic activity
146
Research in sea urchin embryos and surf clams uncovered several similar proteins named
cyclins
147
Any of a number of proteins that are produced in synchrony with the cell cycle and combine with certain protein kinases, at certain points during cell division.
cyclins
148
Do cyclins have kinase activity?
No
149
How are cyclins similar to MPF
Varied levels throughout cell cycle
150
Cyclins were found to bind to protein kinases called
cyclin-dependent kinase (Cdk)
151
Cyclin-dependent kinase, or CDK is expressed by what gene
CDC2 gene
152
cyclin plus cdc2 =
MPF
153
A protein kinase enzyme that is only active when complexed with cyclin.
cyclin-dependent kinase (Cdk)
154
composed of both a cyclin component and a kinase component.
MPF
155
cdc2 is a ___________
kinase, or protein
156
What is MPF composed of
cyclin and cdc2 (kinase)
157
The cyclin + cyclin-dependent kinase complex is
| one type of ___________ , originally discovered in the oocyte experiment
MPF
158
Cyclins + their kinase counterparts (cdc2/cdk1/p34… etc) =
maturation (mitosis) promoting factor
159
Several cyclins and kinases exist and comprise a large family of ___________ & ______________
proteins and kinases
160
What is the purpose of the cyclin + kinase complex?
phosphorylation of proteins associated at each check point so that the cell cycle can continue
161
True or False
The cell cycle is controlled at three checkpoints
True
G1/S checkpoint
G2 /M checkpoint
Spindle checkpoint (late metaphase)
162
Allow the cycle to be delayed or halted when necessary.
checkpoints
163
The cell uses ____________ to both assess its internal state and integrate external signals
checkpoints
164
Passage through these checkpoints is controlled by
Cdk enzymes
165
The primary control point at which a cell “decides” whether or not to divide. Also called START and the restriction point.
G1/S checkpoint
166
Primary point at which external signals can influence events of the cycle
G1/S checkpoint
167
The phase during which growth factors affect the cycle and also the phase that links cell division to cell growth and nutrition.
G1/S checkpoint
168
External signals may trigger or inhibit the cell cycle at this checkpoint
G1/S checkpoint
169
G2/M checkpoint
The second cell-division control point, at which division can be delayed if DNA has not been properly replicated or is damaged.
G2 /M checkpoint
170
Passage through this checkpoint represents the commitment to mitosis.
G2 /M checkpoint
171
This checkpoint assesses the success of DNA replication and can stall the cycle if DNA has not been accurately replicated.
G2 /M checkpoint
172
Cdks active at this checkpoint termed as M phase-promoting factor (MPF)
G2 /M checkpoint
173
G2 /M checkpoint contains active cdks termed
M phase-promoting factor (MPF)
174
A Cdk enzyme active at the G2/M checkpoint.
M phase-promoting factor (MPF)
175
The third cell-division checkpoint, at which all chromosomes must be attached to the spindle. Passage through this checkpoint commits the cell to anaphase.
Spindle checkpoint
176
Ensures that all of the chromosomes are attached to the spindle in preparation for anaphase.
Spindle checkpoint
177
At this checkpoint all chromosomes are attached to spindles for anaphase, at which point the process is IRREVERSIBLE
Spindle checkpoint
178
Enzymes that accomplish this phosphorylation
Cdks
179
At what checkpoint does the activation of cdc2 / G1 cyclin cause the cell to continue on to S-phase
G1/S
180
At what checkpoint does activation of cdc2 / mitotic cyclin cause the cell to continue on to M-phase
G2/M
181
At what checkpoint does activation of APC (anaphase promoting complex) causes the cell to continue on to anaphase
Spindle checkpoint
182
A protein complex that triggers anaphase; it initiates a series of reactions that ultimately degrades cohesin, the protein complex that holds the sister chromatids together. The sister chromatids are then released and move toward opposite poles in the cell.
anaphase-promoting complex (APC)
183
1) What protein does anaphase-promoting complex (APC) mark for DESTRUCTION by adding ubiquitin (protein marker for degradation) ?
2) T/F
APC /C acts as a ubiquitin ligase
securin
true
184
Acts as an inhibitor of another protease called separase
securin protein
185
Securin protein acts as an inhibitor of another protease called
separase
186
Separase destroys ______________
cohesin
187
Protease which normally degrades cohesin, or the proteins that hold the sister chromatids together
separase
188
Activates the PROTEASE that removes the cohesins holding sister chromatids together, and it is necessary for the DESTRUCTION of mitotic cyclins to drive the cell out of mitosis
anaphase-promoting complex (APC)
189
Cyclins and kinases, or CDKs themselves are
| activated via _______________________
phosphorylation
190
Growth factors act by triggering
intracellular signaling systems
191
______________ signals cause ______________ of Cdks
External
| phosphorylation
192
A receptor tyrosine kinase (RTK) that initiates/activates a MAP kinase cascade to stimulate cell division
platelet-derived growth factor (PDGF).
193
When a tissue is injured, a blood clot forms, and the release of ____________triggers neighboring cells to divide, helping to heal the wound.
PDGF
194
T/F
| PDGF is not a growth factor
False
| It is a growth factor
195
1) Over how many different proteins function as growth factors?
2) These growth factor receptors often initiate MAP kinase cascades in which the final kinase enters the nucleus and activates__________________ by phosphorylation
3) Stimulates the production of G1 cyclins and the proteins that are necessary for cell-cycle progression
50
transcription factors
transcription factors
196
Cells that are not exposed to growth factors stop at _________, checkpoint and enter _______ phase.
G1 checkpoint
G0 phase
197
Cells remain dormant at this phase
G0 phase
198
The unrestrained, uncontrolled growth of cells in humans leads to the disease called
cancer
199
A disease of cell division—a failure of cell division control.
cancer
200
What happens when cells ignore these
| checkpoints? (G1, G0)
Cancerous cells
201
The gene that produces the p53 protein
p53 gene
202
Monitors DNA integrity and halts cell division if DNA damage is detected.
p53 protein
203
Many types of cancer are associated with a damaged or absent
p53 gene
204
p53 gene is abundant during__________
G1 phase checkpoint
205
Determines chromosome integrity, checks for errors in copying DNA, DNA damage and mutations
p53 gene
206
Helps in cell repair mechanism
p53
207
p53 gene prevents the development of many mutated cells, and it is therefore considered a______________
tumor-suppressor gene
208
A gene that normally functions to inhibit cell division; mutated forms can lead to the unrestrained cell division of cancer, but only when both copies of the gene are mutant.
tumor-suppressor gene
209
p53 is also know as _________________
tumor-suppressor gene
210
Halts division of damaged cells, can also directs cell to kill itself
tumor-suppressor gene
211
p53 is entirely absent or damaged beyond use in the majority of __________________ cells
cancer
212
Causes cells to proliferate uncontrollably, leading to cancer
Abnormal P53 function or lack of P53 protein
213
Fails to stop cell division and repair DNA
Abnormal p53 protien
214
Genes that can, when introduced into a cell, cause it to become a cancer cell
oncogenes
215
A mutant form of a growth-regulating gene that is inappropriately “on,” causing unrestrained cell growth and division.
oncogene
216
Normal cellular genes that become oncogenes when mutated.
proto-oncogenes
Ex: PDGF receptor mutation causes receptor to
always be ‘on’ in the absence of ligand
217
The action of proto-oncogenes is often related to signaling by____________, and mutuation can lead to loss of _____________
Growth Factors
Growth Control
218
Encodes for proteins (Rb protein, p53 protein) that prevent uncontrollable cell division
Cause cancer when mutated
Tumor-suppressor genes
219
A gene that, when mutated, predisposes individuals to a rare form of cancer of the retina; one of the first tumor-suppressor genes discovered.
retinoblastoma susceptibility gene (Rb)
