Week 9-10 Flashcards
1
Q
Cell division -
A
the reproduction of cells
2
Q
Role of cell division in unicellular organisms:
A
Reproduction by cell division (e.g. binary fission)
3
Q
Role of cell division in multicellular organisms (3):
A
- Growth
- Development from a fertilized cell
- Repair of damaged tissues
4
Q
Mitosis (used for)
A
- production of somatic cells (diploid cells)
- conserves the chromosome number of the cells => production of 2 genetically identical cells that are also genetically identical to the parental cell
5
Q
Meiosis
A
- production of gametes (haploid cells)
- reduces the chromosome number in half => production of gametes in the gonads
=> Fertilization: A male and female gamete (haploid cells) fuse producing a zygote with a complete set of chromosomes (diploid)
6
Q
What is cancer?
A
abnormal growth of cells which tend to proliferate in an uncontrolled way and, in some cases, to metastasize (spread)
7
Q
Cell cycle -
A
functional process that a cell goes through until it is divided in 2 identical daughter cells
8
Q
Phases (stages) of the cell cycle (4):
A
INTERPHASE:
1. G1 (gap 1): preparation of the cell for DNA replication
2. S phase (synthesis): DNA replication
3. G2 (gap 2): preparation for cell division
M PHASE:
4. M phase (mitotic phase: prophase, (prometaphase), metaphase, anaphase, telophase): cell division (mitosis)
+ Cytokinesis
9
Q
G0 phase
A
Resting phase: non-dividing cells are resting at this phase
Differentiated cells enter from G0 to G1 after the action of growth factors
Cells exit G1 and enter G0 (G1 → G0) in order to differentiate
10
Q
Cell cycle control (2):
A
- Εxtracellular signals (e.g. presence of growth factors)
- Intracellular signals (e.g. cell size)
11
Q
Cell types according to their cell division potential (3):
A
- Post-mitotic cells
- Cells that divide upon appropriate stimulation (signal)
- Cells with high mitotic activity
12
Q
Post-mitotic cells:
A
terminally differentiated cells which have lost their ability to replicate => permanently arrested at G0 phase
Example: neural cells, cardiac muscle cells, red blood cells
13
Q
Cells that divide upon appropriate stimulation (signal):
A
most of the cells in our body only divide upon stimulation by growth factors or other signal
lymphocytes upon antigenic presentation
14
Q
Cells with high mitotic activity:
A
opposite of post-mitotic cells
germ cells, stem cells, epithelial cells
15
Q
Before cell division (during interphase):
A
- Cell components have to replicate (organelles, membranes, proteins)
- Chromosomes need to replicate in order for daughter cells to have the same genome as the parental cell
16
Q
Why is it important f/ genetic material to be replicated in order for daughter cells to have the same genome as the parental cell?
A
This ensures their survival
17
Q
Interphase can be divided into 3 sub-phases:
A
• G1 phase: preparation for DNA replication
- Protein synthesis, organelle production
- Duration: 5-6 h
• S phase: DNA synthesis (replication)
- Duration: 10-12 h
• G2 phase: preparation for cell division (mitosis)
- Protein synthesis, organelle production
- Duration: 4-6 h
18
Q
Typical cell cycle in eukaryotic cells has duration of
A
20-24 h
19
Q
Genome -
A
the complete set of genetic information (DNA) of a cell (all the genes)
20
Q
DNA molecules of a cell are packaged
A
into chromosomes
21
Q
Eukaryotic chromosomes consist of:
A
chromatin - a complex of DNA and proteins (histones)
22
Q
Each chromosome carries how many genes?
A
a few hundred to a few thousand genes
23
Q
2 types of cells according to their chromosomal content in humans:
A
Somatic cells: diploid cells (2n, n=23 chromosomes)
- have 2 sets of 23 chromosomes = 46 total => 23 chromosome pairs
- each homologous chromosome pair has 1 paternal and 1 maternal chromosome
Gametes (reproductive cells): haploid cells (n, n=23 chromosomes)
– have one set of chromosomes = 23 total
– Have only 1 chromosome (either paternal or maternal) from each homologous chromosome pair
24
Q
Homologous chromosomes -
A
carry the same genes at the same positions
25
Human Karyotype -
ordered display of chromosome pairs
26
Distribution of Chromosomes During Cell Cycle: Interphase
- chromosomes are not condensed
- **G1**: each chromosome consists of one chromatid (not replicated yet) (2n - diploid cells = 46 chromosomes)
- **S**: DNA is replicated (process of acquiring sister chromatids)
- **G2**: each duplicated chromosome has 2 sister chromatids (4n - tetraploid cells = 92 chromosomes)
27
Distribution of Chromosomes During Cell Cycle: Mitosis (cell division)
- the chromosomes condense => can be seen with a light microscope
- sister chromatids separate => each future daughter cell receives one chromatid
28
The Mitotic phase (M phase) consists of:
– Mitosis: division of the nucleus
– Cytokinesis: division of the cytoplasm
29
Mitosis consists of 5 phases:
1. Prophase
2. Prometaphase
3. Metaphase
4. Αnaphase
5. Τelophase
30
characteristic event that separates prophase and prometaphase -
degradation of nuclear envelope => mitotic spindle microtubules can invade nuclear region => bind to the chromosomes
31
Metaphase distinct event:
formation of metaphase plate = arrangement of duplicated chromosomes in the middle of the cells (equal distance from each pole)
32
Anaphase distinct event:
two sets of chromosomes start moving towards the diff pole
33
Telophase distinct events:
- reformation of nuclear envelope
- decondensation of chromosomes
34
Cytokinesis in animal cells:
cleavage furrow formation: membrane fuses to create 2 future daughter cells
35
Cytokinesis in plant cells:
cell plate formation
36
Centrosome replication happens when?
during S phase
37
3 types of mitotic spindle microtubules
1. **Αstral microtubules**: radial (star-like) structure *around the centrosome*; **fn**: *positioning of the spindle in the cell*
2. **Kinetochore (chromosomal) microtubules**: join the centrosome with the kinetochores on the centromeres of the chromosomes; **fn**: chromosomal movement
3. **Polar microtubules**: start from the centrosome but do not attach to the chromosomes, interact with other polar microtubules projecting from the other pole; **fn**: maintain the integrity of the spindle
38
The mitotic spindle -
an apparatus of microtubules that controls chromosome movement during mitosis
arises from centrosomes (animal cells, MTOC made by 2 centrioles) or other MTOC (plant cells since they lack centrioles at right angle to each other)
includes spindle microtubules (kinetochore and polar) and astral microtubules
39
Centrioles through cycles:
G1 phase - 1 centrosome consisting of 2 centrioles
S phase - 2 centrosomes, 2 centrioles each
mitosis - 1 centrosome consisting of 2 centrioles per each daughter cell
40
Prophase 4 key events:
1. chromosomal condensation
2. centrosomes move towards the opposite poles of the cell
3. mitotic spindle formation
4. nuclear envelope and organelle degradation
41
Prometaphase key events (2):
1. Chromosomes attach to the spindle microtubules (b/c the nuclear envelope is degraded completely)
2. Chromosomes move towards the cell center (metaphase plate)
42
Each chromosome is connected to both poles - how?
How are chromatids oriented?
each chromosome is duplicated => consists of 2 chromatids => microtubule from one centrosome at one pole is connected to kinetochore of one centromere AND microtubule from the other centrosome at the other pole is connected to kinetochore of the other centromere => one chromatid has orientation towards one pole and the other chromatid towards the other pole
43
how do chromosomes move towards the cell center
(metaphase plate)?
By polymerisation/depolymerisation of kinetochore microtubules (overall polymerisation from the side that is closer to the one pole and overall depolymerization from the side that is further away from the other pole)
44
Key event of metaphase:
Chromosomes align in the equatorial plane
(metaphase plate)
45
What happens if the chromosomes are not aligned correctly?
Cell cycle arrest signal
46
2 key events of anaphase:
1. sister chromatids separation due to inactivation of centromere proteins holding the two chromatids together
2. sister chromatids move along the kinetochore microtubules towards opposite ends of the cell
47
Anaphase is divided into:
Anaphase A
Anaphase B
48
What happens during the Αnaphase Α?
**depolymerisation of microtubules**: they need to become shorter to bring chromosomes to the opposite sides of the pole => **tubulin depolymerisation** => chromosomes start moving toward the poles of the spindle w/ help of **motor proteins**
49
What happens during Anaphase B?
separation of the 2 poles (spindle elongation): non-kinetochore microtubules from opposite poles
overlap and push against each other elongating the cell w/ the help of motor proteins
50
3 key events of telophase:
Genetically identical daughter nuclei form at the opposite poles of the cell:
1. Chromosomes transferred at the opposite poles of the cell (end of anaphase/beginning of telophase)
2. Reformation of the nuclear envelope upon vesicle fusion around the chromosomes
3. Reformation of ER and Golgi apparatus
51
Key event of cytokinesis:
The cytoplasm divides into 2 daughter cells
52
Cytokinesis in animal cells occurs by a process known as
cleavage, forming a cleavage furrow
contractile ring formation: a ring of actin and myosin microfilaments that contracts
53
Cytokinesis in cells w/ walls involves
cell plate formation: Vesicles containing cell wall materials arrive **from Golgi** to the metaphase plate => Cell plate formation from vesicle fusion => Cell plate elongates and fuses with the cell wall of the parental cell
54
By what type of cell division procaryotes reproduce by?
binary fission
55
How much time does binary fission take?
1-3 h
56
What happens during binary fission?
– The bacterial **chromosome** replicates
– The two daughter chromosomes actively move apart
- Cytokinesis w/ cell wall formation
57
multiple fission -
a type of cell division that seems intermediate between binary fission and mitosis carried out by most eukaryotic cells
multiple mitoses => multiple cytokineses
*replication of Plasmodium malariae*
58
apoptosis
controlled cell death
59
cancer -
uncontrolled cell growth: abnormal growth of cells which tend to proliferate in an uncontrolled way and, in some cases, to metastasize (spread)
60
Malignancy -
tumour property to invade nearby tissues and spread (metastasise) to other parts of the body
= more aggressive
61
Carcinogens -
and two major types
substances and exposures that can lead to cancer: mutations => uncontrolled cell proliferation and inhibition of apoptosis
2 types: Environmental, Genetic predisposition
62
Environmental carcinogens (3 types):
1. Chemical (*benzene, alkylating agents (chemotherapy)*)
2. Physical (*X-rays, UV light*)
3. Viral (*Hepatitis B, Human Papilloma*)
63
2 types of categories that most genes involved in carcinogenesis fall into:
1. oncogenes
2. tumor suppressor genes
64
Checkpoints:
- control the transition from one phase of the cell cycle to the next one
- ensure that certain processes have been completed (e.g. completion of DNA replication, presence of growth factors) before another phase starts
65
3 important Checkpoints:
– G1 Checkpoint
– G2 Checkpoint
– M Checkpoint
66
**G1 Checkpoint** or **G1/S checkpoint** or **Restriction point R** - when and does what?
+ 3 points from figure
When? - At the end of G1 phase (*e.g. checks for the **presence of growth factors***)
Does what? - Controls the transition from the G1 phase to the S phase (**DNA replication**)
1. Check extracellular environment: Growth factors present?
2. Check if the cell size OK (is the cell large enough to divide)?
3. Check DNA damage
67
G2 checkpoint:
2 points from the figure:
Controls the transition from the G2 phase to the Μ phase (mitosis):
1. Check DNA damage
2. Check completion of DNA replication: if there are 2 exact copies of DNA
68
M (metaphase) checkpoint:
Controls the transition **through the stages of mitosis**
(*e.g. correct chromosome alignment in the mitotic spindle during metaphase*)
69
Which molecules control and maintain cell cycle control?
Protein complexes which are composed of 2 subunits:
– Cyclin (**cyc**): the regulatory subunit
– Cyclin depended kinase (**cdk**): the catalytic subunit
70
Kinases - ?
enzymes that inactivate/activate other proteins by phosphorylation
71
Cdks are present in what concentration in the cell?
Mostly constant, they are inactive most of the time and become activated by binding to a particular cyclin
72
The concentration of cyclins in the cell
fluctuates
73
the activity of cdks is regulated by what?
by degradation of cyclins by the proteasome
cyclins tagged by ubiquitin w/ help of ubiquitin ligases (E3 ligases) (these are enzymes) => proteasome => proteolysis
74
MPF (Mitosis Promoting Factor (also called Maturation Promoting Factor)) -
signal that sends cells into mitosis
75
MPF consists of:
cyclin A or cyclin B + cdk-1
76
MPF does what and how?
- induces the progression from G2 to M phase
- by:
1) phosphorylation and inactivation of Anaphase promoting complex (APC) which is a E3 ubiquitin ligase (which targets S and M cyclins for degradation)
2) phosphorylation of proteins of the nuclear lamina→ fragmentation of the nuclear envelope
77
APC-MPF relationship through the cell cycle:
Interphase: APC active & MPF inactive
M phase: MPF active & APC inactive
78
ΜPF role in cell cycle regulation (what depends on it - basically whole mitosis) (6):
• Chromosomal condensation
• Nuclear envelope degradation
• Mitotic spindle formation
• Chromosome migration to opposite poles
• Organelle reformation
• Cytokinesis
79
Cell cycle regulation during interphase (4)
1. Mitogens (growth factors)
2. Expression of early response genes
3. Activation of G1 cyclin-cdk activity
4. Transcription of genes for DNA synthesis
80
What are mitogens (growth factors)?
External signals that stimulate cell division by triggering pathways that lead to cell cycle progression
81
Expression of early response genes:
1. G1 cyclins and cdks: cyc-D, cyc-E and cdks 2,4,6
2. Transcription factors: c-Fos, c-Jun and E2F => start transcribe genes that are involved in DNA replication (ex: DNA polymerase)
82
Activation of G1 cyclin-cdk activity:
– Cyclin D/CDK4/6 and cyc-E /cdk2 complexes become active in G1 and phosphorylate Rb leading to its inactivation
– Inactivation of Rb releases E2F, a transcription factor that drives the transcription of genes essential for DNA synthesis
83
Transcription of genes for DNA synthesis:
Activated E2F promotes transcription of DNA synthesis genes, including DNA polymerases and other factors needed for S phase entry
84
If the mitogen is removed:
– reduction in the cyclin-cdk levels
– the cell does not pass through the restriction point R
– the cell does not replicate
85
Cell-cycle control by cyc-cdks (which cyc-cdks are active at which stage)
G1 early: cyc-D / cdk-4/6
G1 late: cyc-E / cdk-2
S phase cyc-A / cdk-2 (required for DNA replication)
Mitotic cyc-cdks: cyc-A or cyc-B / cdk-1 (MPF)
86
2 tumor suppressor genes and proteins they produce:
RB1 => Rb
TP53 => p53
87
How do tumor suppressors work?
protein products of tumour suppressor genes inhibit cell division, thereby preventing the uncontrolled growth that contributes to cancer
88
G1 phase number of chromosomes & chromatids?
diploid set
2n chromosomes
2n chromatids
89
S phase number of chromosomes and chromatids?
after DNA replication tetraploid 4n set
2n chromosomes
4n chromatids
90
Metaphase cell number of chromatids and chromosomes?
2n chromosomes
4n chromatids
91
Anaphase cell chromosomes and chromatids?
Tetraploid
4n chromosomes
4n chromatids
92
Cytokinesis daughter cells
2n chromosomes
2n chromatids
93
Prophase - chromosomal condensation:
- interphase: chromosomes decondensed => not seen, f/ replication & translation
- **mitosis**: @ prophase condensation begins => cell stops doing anything else
94
sister chromatids -
identical DNA copies, replicated during S phase, joined by centromeres (consist of repetitive non-coding DNA sequences)
95
Prophase: centrosomes move towards the opposite poles of the cell
- found next to the nucleus (cell center)
- Centrosome replication: during S phase => 2 new centrosomes during mitosis => After mitosis: 1 centrosome per cell
96
Prophase - mitotic spindle formation:
- Mitotic spindle begins to form by the polymerisation of microtubules (tubulin, α/β-dimers)
- Microtubule polymerisation starts from the centrosome
- mitotic spindle microtubules attach to the kinetochores of the chromatids => move chromosomes towards the metaphase plate
97
uncontrolled cell division leads to
carcinogenesis
98
Carcinogenesis happens due to (2)
activation of oncogenes **and** inactivation of tumor suppressor genes, which are responsible for: uncontrolled cell division and defective pathway of apoptosis
99
Carcinogenesis is a multi-stage process involving multiple hits (4 stages from normal cells to malignant tumor):
ex:
1. Normal colon epithelial cells =>*loss of tumor-suppressor gene **APC** or other*=> 2. Small benign growth (**polyp**) =>*activation of **ras** oncogene* and *loss of tumor-suppressor gene **DCC***=> 3. Larger benign growth (**adenoma**) =>*loss of tumor-suppressor gene **p53** and additional mutations*=> Malignant tumor (**(adeno)carcinoma**)
100
The Cell Cycle Control System (G1, G2 and M Checkpoints):
• G1: checks for cell size, nutrients, growth factors, DNA damage
• G2: checks for DNA damage, DNA replication completion
• M: checks for chromosome alignment at mitotic spindle
101
Damage detected at Checkpoints
1. If DNA damage is detected at the checkpoints G1 and G2 this will lead to **cell cycle arrest** (aka **cell cycle block**)
2. This gives the opportunity to the cell to try to **repair this damage**
3. If this is not possible, this will lead to **apoptosis (programmed cell death)**
so:
1. Stop the cycle
2. Attempt DNA repair
3. Induce apoptosis
102
Which checkpoint is the most important f/ many cells?
G1, so that the cell doesn’t waste recources
103
Why is tight regulation of cdks very important?
Loss of cell cycle control can lead to unregulated cell proliferation => carcinogenesis
104
Retinoblastoma protein (Rb):
Tumour suppressor gene => codes for tumour suppressor protein => inhibits cell cycle progression
Isolates the transcription factor E2F => Inhibits E2F activation (*during G1*)
105
Regulation of Rb:
– *G1 phase*: Rb dephosphorylation by **PP-1** (protein phosphatase-1)
– At *the end of G1 phase* cyc-cdks **Cyclin D/cdk4/6** and **cyclin E/cdk2** phosphorylate Rb
– Phosphorylated Rb cannot sequester **E2F**
– **Ε2F** is released (activated) => *cell enters the S phase*
106
Rb and E2F through the cycle:
Early G1: Rb active (unphosphorylated), E2F inactive (bound to Rb)
Towards the end of G1: Phosphorylation of Rb (Rb inactive) and release of E2F (E2F active)
S, G2, M: Rb inactive, E2F active
107
What is Retinoblastoma?
Malignant tumour of the eye(s) that originates from the retina.
This disease is caused by a **mutation in the tumour suppressor gene RB1** which encodes for the Rb protein.
There are two forms of the disease: *familial (heritable)* and *sporadic (non-heritable)*.
108
Negative regulators (inhibitors) of the cell cycle (2 major categories)
Cdk inhibitors (CKIs): inhibit the activity of the cyclin cdk complexes. There are 2 major categories
- **INK4 family**: p15Ink4b, p16Ink4a, p18Ink4c, p19Ink4d - inhibit the activity of G1 cyclin cdks (e.g. cyc-D/cdk-4/6)
- **Cip/Kip family**: p21Cip1, p27Kip1, p57Kip2 - inhibit the activity of all other cyc-cdk complexes (late G1-M) (e.g. cyc-E/cdk-2/ and cyc-A/cdk-2); their expression is strongly stimulated by DNA damage => p53 activation
109
Activation of p53 in Response to DNA Damage
DNA =>*mutagen*=> DNA damage
p53 inactive => p53 destruction by proteasome
OR
kinase activation => p53 active (phosphorylated) => 1) Expression of CKIs: Cell cycle arrest, 2) DNA Repair e.g. Excision repair, 3) Repair not possible => apoptosis by p53
110
Internal and External Signals at the Checkpoints
Internal signals: cell size, incorrect alignment or separation of sister chromatids (at M phase checkpoint)
External signals: environmental conditions, presence of growth factors
111
External signals that cancer cells ignore (2):
**Density-dependent inhibition**: crowded cells stop dividing
**Anchorage dependence**: most animal cells must be attached to a substratum (support) in order to divide
112
process by which a normal cell becomes a cancer cell -
transformation
unlike **normal cells**, the **cancer cells** show no density inhibition, no anchorage dependence
113
Cancer cells do not respond normally to the body’s control mechanisms (4):
– make their own growth factors
– have signaling pathways **always ‘ON’**
– Exhibit abnormal cell cycle control
– form tumors: **Benign tumors**: not invasive, contained at a particular site; **Malignant tumors**: invasive, can spread to other organs
114
Stages of malignant tumors (4):
1. tumour grows from a single cancer cell
2. cancer cells invade neighboring tissues
3. cancer cells spread through lymph & blood vessels to other parts of the body
4. small percentage of cancer cells may survive and establish a new tumor in another part of the body - **metastatic tumor**
115
Cancer cells’ characteristics (6):
1. sustaining proliferative signaling
2. evading growth suppressors
3. activating invasion and metastases
4. enabling replicative immortality
5. inducing angiogenesis
6. resisting apoptosis
116
Summary of key steps of cell cycle regulation (6):
• Growth factors induce gene expression
• G1 cyclin-cdk complexes phosphorylate Rb
• E2F is released, stimulating expression of genes required for S-phase
• Cell replicates DNA (expression of S-phase cyclin-cdk complexes)
• G2/M cyclin-cdk complexes cause cell to enter mitosis
• If DNA damage- p53 initiates cell cycle arrest, DNA repair and if repair fails => apoptosis (programmed cell death)
117
Role of meiosis in sexual reproduction
- **gametes** are produced by meiosis
- sexual reproduction by fertilization: the union of gametes (the sperm and the egg) => fertilized egg - **zygote** has 1 set of chromosomes from each parent => produces somatic cells by mitosis and develops into an adult
118
Sets of Chromosomes in Human Cells (2 types of cells in humans):
Somatic cells: have two sets of 23 chromosomes = 46 total chromosomes => diploid cells (2n, n = 23 chromosomes)
Gametes (sperm and egg): have one set of chromosomes = 23 total => haploid cells (n, n =23 chromosomes)
119
Karyotype -
ordered display of the pairs of chromosomes from a cell
44 autosomes + 2 sex chromosomes
120
homologous chromosomes / homologs -
2 chromosomes (paternal & maternal) in each pair of the karyotype; they are the same length and shape and carry genes controlling the same inherited characters => same genes, but maybe diff alleles of those genes
121
Meiosis in animal sexual life cycles:
- **Gametes** are **the only haploid cells** and the only cell type produced by **meiosis**
- Gametes undergo no further cell division before fertilization
- Gametes fuse to form a **diploid zygote** that **divides by mitosis** to develop into a multicellular organism
122
Meiosis outlined:
• Production of haploid cells from diploid cells
- production of gametes (egg, sperm)
- reduction of chromosomal number by half
- each new cell has only 1 chromosome from each pair of homologous chromosomes
- 4 new haploid cells produced by 2 consecutive meiotic divisions from an initial diploid cell
• Includes pairing of homologous chromosomes and 2 meiotic divisions
123
Stages of Meiosis
1. Meiosis I (*reductional division*): pairing and **separation of homologous chromosomes** => Results in two haploid daughter cells with replicated chromosomes (2 chromatids per chromosome)
2. Meiosis II (*equational division*): **sister chromatids separation** => The result is four haploid daughter cells with unreplicated chromosomes (1 chromatid per chromosome)
124
recombination in Meiosis I…
may or may not occur
125
Chromosomal and DNA content during Meiosis I
• Meiosis I: results in two haploid daughter cells with replicated chromosomes (2 chromatids/chromosome)
Explanation:
- cells are haploid in terms of chromosome content as they have one chromosome from each pair of homologs
- However, they are diploid (2n) in terms of DNA content (chromatids) as they have replicated chromosomes (2 chromatids/chromosome)
Example: human cells have 23 chromosomes with 2 chromatids each => have 46 chromatids in total (2n)
=> Number of chromosomes = n (23 in humans)
=> Number of chromatids (DNA content) = 2n (46 in humans)
126
Chromosomal and DNA content during
Meiosis II
• Meiosis II: results in four haploid daughter cells with unreplicated chromosomes (1 chromatid/chromosome)
Explanation:
- cells are haploid both in terms of chromosome number and in terms of DNA content (chromatid number)
- Chromosomes only have 1 chromatid each
=> Number of chromosome = n (23 in humans)
=> Number of chromatids (DNA content)= n (23 in humans)
127
Meiosis I is preceded by…
interphase
• DNA replication during interphase => chromosomes duplicate to form sister chromatids
• The sister chromatids are genetically identical and joined at the centromere
• The single centrosome replicates during interphase, forming two centrosomes
