week 7 Flashcards
1
Q
What are the stages of Interphase split into?
A
G1
S phase
G2
2
Q
What happens in the G1 phase?
A
All the organelles and cytoplasmic components replicate.
Cell grows and contributes to
development, metabolism, behaviour, etc
Is the primary growth phase of the cell. The term gap phase refers to its filling the gap between cytokinesis and DNA synthesis. For most cells, this is the longest phase.
3
Q
What happens in the S phase?
A
The DNA replicates is the phase in which the cell synthesizes a replica of the genome.
4
Q
What happens in the G2 phase?
A
All the enzymes needed to aid the process of cell division are produced.
preparations for mitosis
Chromosomes not visible in the nucleus.
is the second growth phase, and preparation for separation of the newly replicated genome. This phase fills the gap between DNA synthesis and the beginning of mitosis. During this phase microtubules begin to reorganize to form a spindle.
5
Q
Do Eukaryotic cells spend more time in interphase or in the actual stages of mitosis?
A
Most eukaryotic cells spend a great deal of time in interphase and a very short period of time actually dividing.
6
Q
What happens in Prophase?
A
Chromosomes condense and become visible, appearing as two sister chromatids held together at the centromere. Cytoskeleton disassembles as the spindle begins to form.
7
Q
What occurs towards the end of prophase?
A
Chromosomes attach by proteins in their centromeres called Kinetochores to microtubules from each pole, moving chromosomes toward the equator of the cell.
8
Q
What happens during Metaphase?
A
All chromosomes are aligned at the equator of the cell called the metaphase plate.
9
Q
What happens during Anaphase?
A
begins with the degradation of proteins that hold sister chromatids together, freeing individual chromosomes. Free chromosomes are then pulled by their Kinetochores to opposite poles.
10
Q
What happens during Telophase?
A
a cleavage furrow forms in the center of the cell, indentation is made from a constricting belt of actin filaments surrounding the inside of the cells circumference. Chromosomes cluster at opposite poles and begin decondensing at the nuclear envelope reforms around them, the spindle apparatus disassembles as the microtubules are broken down into tubulin monomers that can be used to form the cytoskeleton of the daughter cells.
11
Q
In animal cells, how does cytokinesis complete cell division?
A
by extending the cleavage furrow to completely separate the newly formed daughter cells. Since plant cell walls cannot be constricted by actin fibers, vesicles form an expanding membrane partition called the cell plate.
12
Q
Why do plant cells use cytokinesis?
A
Like animal cells, plant cells use cytokinesis to finish the division of the contents of the cytoplasm between the two identical daughter cells.
13
Q
Why are checkpoints used in mitosis?
A
Certain checkpoints are used to maintain the mitosis cycle, if it is not, the cell will stop at the checkpoint and correct or possibly inhibit that cell from dividing.
14
Q
What happens during the first checkpoint in mitosis?
A
1st checkpoint- G1 and is considered the primary point at which the cell cycle continues or stops. External signals and growth factors can influence cell cycle and affect the progress at or before the checkpoint.
15
Q
What does the G2M checkpoint allow?
A
G2M checkpoint allows cells that have successfully completed all 3 stages of interphase to begin mitosis
16
Q
What does the last checkpoint allow?
A
Last checkpoint is the spindle checkpoint, ensuring that all chromosomes have attached to the spindle in preparation for anaphase.
17
Q
Name a contributing factor in the cell cycle regulation and what do they ensure?
A
Growth factors, the size of the cell and the nutritional state of the cell are contributing factors in the cell cycle regulation, ensuring that only certain cells divide at appropriate times.
18
Q
Give an example to suggest that mitosis happens at different frequencies?
A
skin there is a lot of wear and tear so go through mitosis very frequently whereas muscle cells rarely divide.
19
Q
What do all living organisms share?
A
Common ancestor or group of ancestors.
20
Q
How is information passed on in bacteria and archaea?
A
Binary fission
21
Q
What has been transmitted for billions of years?
A
Some info (DNA) has been transmitted successfully (with modifications).
22
Q
What do Eukaryotes have the ability to do during each generation of meiosis?
A
reshuffle information each generation during meiosis (= DNA recombination)
23
Q
How does the evolution of meiosis create an individual?
A
Offspring are different to either parent.
Greater genetic variety speeds up evolution.
24
Q
What are the two ways that most eukaryotes transfer genetic information?
A
Mitosis and Meiosis
25
What is Mitosis?
flow of information between cells (cell lines,
divisions in tissues and organs).
Genetic material is divided
• Cell divides into two
26
What is Meiosis?
Flow of information between genomes and generations
27
Why is DNA packing necessary?
```
Average length of DNA in human
chromosomes: 1.5 x 108 base pairs
• Length of one DNA molecule about 4 cm
• There are 46 DNA molecules in a human
cell
• 4cm * 46 = Total length of DNA 184 cm
Densely packing DNA into chromosomes allows error free separation of DNA molecules during mitosis:
Each daughter cell needs to receive one copy of each chromosome.
```
28
Why are Mitosis and Meiosis important?
Important mechanisms to pass on genetic
information (DNA) between cells and
generations of individuals.
29
What needs to be replicated before mitosis can take place?
DNA
30
What enables error free division of genetic material between daughter cells?
DNA needs to be tightly packed.
31
What is the longest stage of the cell cycle?
G1
32
What are the shortest stages of the cell cycle?
Mitosis and cytokinesis
33
What is the difference between interphase and mitosis?
The degree of packing or condensation of DNA. During interphase individual chromosomes can’t be distinguished within the nucleus. Whereas during mitosis, DNA is so densely packed into chromosomes that are easily visible with a simple microscope.
34
What helps DNA to be tightly packed?
DNA is tightly packed with the help of histones and other scaffolding proteins. So DNA double helix is wound around histones into nucleosomes.
35
What do these Nucleosomes do?
Nucleosomes then assemble into solenoids and other higher order structures until you arrive at the highly condensed structure of a mitotic chromosome. Tightly packed chromosomes enable this to happen error free.
36
When do the two sister chromatids form?
Only in G2 phase and until they are separated in M phase does each homologous chromosome consist of two pieces of DNA
37
What also occurs in gap phase one?
is the primary growth phase of the cell. The term gap phase refers to its filling the gap between cytokinesis and DNA synthesis. For most cells, this is the longest phase. involves growth and preparation for DNA synthesis.
38
What also happens in the synthesis phase?
is the phase in which the cell synthesizes a replica of the genome. A copy of the genome is synthesized.
39
What also happens in gap phase 2?
is the second growth phase, and preparation for separation of the newly replicated genome. This phase fills the gap between DNA synthesis and the beginning of mitosis. During this phase microtubules begin to reorganize to form a spindle. prepares the cell for mitosis.
40
What is cytokinesis?
is the phase of the cell cycle when the cytoplasm divides, creating two daughter cells. Cytokinesis divides the cell into two cells with identical genomes.
41
How do cells get pinched in two in animal cells?
In animal cells, the microtubule spindle helps position a contracting ring of actin that constricts like a drawstring to pinch the cell in two.
42
what happens between the dividing cells in cells with a cell wall?
In cells with a cell wall, such as plant cells, a plate forms between the dividing cells.
43
How is the cell cycle depicted?
As a circle
44
What happens to chromosomes during mitosis?
replicated chromosomes are partitioned.
45
What are mitosis and cytokinesis referred as together?
together are usually referred to collectively as M phase, to distinguish the dividing phase from interphase.
46
What happens during prophase?
• Homologous chromosomes pair and crossing over occurs in meiosis I
47
What happens during metaphase?
* Individual chromosomes (mitosis) or pairs of homologous chromosomes (meiosis I) line up at metaphase plate
* Kinetochores of sister chromatids are attached to microtubule fibers from opposing poles (mitosis) or from the same pole (meiosis I)
48
What is anaphase?
• Homologous chromosomes (meiosis I) or sister chromatids (mitosis, meiosis II) separate
49
What is the outcome of mitosis?
•Mitosis: 2 daughter cells that are diploid (2n)
50
What is the outcome of meiosis?
4 daughter cells (gametes) that are haploid (1n)
51
Compare the function of mitosis and meiosis.
```
Mitosis- Growth, maintenance and
repair of organism
Meiosis- Production of gametes
Introduction of genetic diversity
through recombination
```
52
Compare the homologous chromosomes of mitosis and meiosis.
Mitosis- Separated independently of each other (no crossing over)
Meiosis-Pair as bivalents/tetrads until anaphase I to facilitate recombination/crossing over
53
Compare the number of divisions mitosis and meiosis.
Mitosis-1
| Meiosis- 2 (meiosis I and II)
54
Compare the Chromosome number of daughter cells of mitosis and meiosis.
Mitosis- Identical to mother cell (diploid)
Meiosis- Reduced by half compared to
mother cell (haploid)
55
Compare the Genetic identity of daughter
| cells of mitosis and meiosis.
Mitosis- Identical to mother cell
Meiosis- New assortment of parental chromosomes
Chromatids not identical due to recombination
56
Compare the DNA replication of mitosis and meiosis.
Mitosis- After each cell division
| Meiosis- Not after meiosis I
57
Compare the Attachment of sister chromatid kinetochores
| of mitosis and meiosis.
Mitosis- To microtubule fibres from
opposing poles
Meiosis- To microtubule fibres from the
same (meiosis I) or opposing
(meiosis II) poles
58
Compare the Time needed to complete of mitosis and meiosis.
Mitosis- ~90 minutes (actively dividing
cells not in G0)
Meiosis- ~24 days (human male)
> 10 years (human female)
59
The cell cycle of most mammalian cells is controlled by two external cues:
* Density-dependent inhibition
| * Anchorage-dependent inhibition
60
What is Density-dependent inhibition?
cells stop dividing when they perceive
| that they are surrounded by other cells.
61
What is Anchorage-dependent inhibition?
cells will only divide if they are attached to a matrix
62
What happens when we loss both Density-dependent inhibition and Anchorage-dependent inhibition?
Loss of those two inhibition mechanisms can lead to cancers.
63
What are some Anti-cancer drugs inhibit the cell cycle directly or indirectly?
* Taxol (paclitaxel) blocks spindle function
* Herceptin blocks HER (receptor kinase, promotes cell division)
* Tamoxifen blocks ER (estrogen receptor, promotes cell division)
64
Proto-oncogenes and tumor suppressor
| genes regulate what?
cell cycle activity
65
What are examples of Proto-oncogenes?
* Examples: Growth factor receptors, components of the growth factor signal transduction cascade
* Normal function: promote cell division
* Gain-of-function mutants of proto-oncogenes cause cancer.
66
What are some examples of Tumor suppressor genes?
• Examples: Retinoblastoma (Rb) gene, p53
• Normal function: stop the cell cycle if errors occur
(for example DNA damage)
• Loss-of-function mutants of tumor suppressor
genes cause cancer.
67
Transformation of of Chlamydomonas with
| Gonium Rb leads to what?
colony formation
68
A life cycle with sexual reproduction requires a
a haploid phase
69
What is reduced in animals during meiosis?
In animals, meiosis reduces diploid cells to haploid gametes
70
What is formed during fertilisation?
Two gametes fuse (fertilization) to form a
| diploid zygote
71
What is the Lifecycle of Saccharomyces cerevisiae
| (Brewer’s yeast)?
* Unicellular organism, haploid for majority of life cycle
* Haploid cells can form diploid zygote (mating) if mating types (a, α) match
* Diploid zygote undergoes meiosis to produce four haploid spores
72
What is the lifecycle of bryophytes?
• Cells are haploid for majority of life cycle
• Spores germinate and grow filamentous tissue (protonema)
• Gametophytes develop on protonema, male and female gametes are produced by mitosis
• After fertilization, diploid sporophyte develops on gametophyte
• Meiosis in sporophyte leads to the
production of haploid spores
73
What is the Sporophyte?
– diploid tissue
74
What is the Spores/Protonema/Gametophyte?
Haploid tissue
75
How does chromatid cohesion as maintained by cohesin proteins differ in mitosis and meiosis?
Cohesion between sister chromatids is maintained during anaphase I of meiosis but not during anaphase of mitosis.
76
Suppose you have a multicellular eukaryotic organism
| that is composed entirely of haploid cells. This organism would produce gametes by the process of
mitosis.
77
If a somatic cell from a frog contains 20 picograms of DNA during G2 of interphase, how many picograms of DNA would be present in each gamete produced by this species?
5 picograms
20 / 2 = 10 meiosis I
10 / 2 = 5 meiosis II
78
If a germ-line cell from a frog contains 10 picograms of
DNA during G1 of interphase, then how many picograms of DNA
would be present in each cell during prophase II of meiosis?
10 picograms
10 * 2 = 20 prophase I of meiosis
20 / 2 = 10 after meiosis I
No DNA replication after meiosis I!
79
Centromeres divide in
mitosis and meiosis II.
80
In germline cells, when does DNA replicate?
During S phase
81
How can you compare the amount of DNA in an interphase (G1) diploid cell and a cell that has just completed meiosis I?
They have the same amount of DNA, but the meiotic cell has half as many chromosomes.
82
How do chromosomes move to the spindle poles?
* Microtubules from opposite poles attach to kinetochores in prometaphase
* Sister chromatids are held together by cohesins at the centromere
* Chromosomes arranged in metaphase plate
* Cohesins are removed in anaphase, allowing the separation of sister chromatids
83
How are sister chromatids held together?
Sister chromatids are held
| together by cohesins at the centromere
84
How are tubulin subunits removed?
Tubulin subunits are removed from the microtubule fibers at the kinetochore.
85
What does the removal of tubulin subunits lead to?
Leads to shorter microtubule fiber and movement towards the pole
86
Where does tubulin depolymerize?
Tubulin depolymerizes at the kinetochore
87
What do the motor proteins do?
Motor proteins in the kinetochore move the chromosome along the microtubule
88
What three mechanisms move chromosomes apart?
* Tubulin depolymerizes at the kinetochore
* Motor proteins in the kinetochore move the chromosome along the microtubule
* Spindle fibers (microtubules) move past each other
89
What do the Kinetochore protein complex also contain?
contains motor proteins that can walk along a microtubule fibre.
90
What happens in Anaphase B?
Pull the chromosomes to the pole but you also have microtubule fibres sliding away from each other-
91
Animal and plant cells use different mechanisms for
cytokinesis
92
What do animal cells have that plant cells do not?
centrioles
93
What role do centrioles play in animal cells?
these organise the microtubule fibres during mitosis
94
What are unique to plant cells?
Unique to plant cells are chloroplasts and a rigid cell wall made out of cellulose and other carbohydrates.
95
What do the rigid cell walls provide plant cell walls?
Plant cells are pressurised and the rigid cell wall provides the counterforce so that this pressure doesn't explode the cell.
96
Why can you easily change the shape of animal cells?
Animal cells are usually at the same pressure as their surroundings, probably why you can change the shape easily.
97
There are other ways of doing
cell division and cytokinesis
98
The decision to divide or not can be depend on
on how big the cell is, whether there are enough nutrients available and on the neighbouring cells and no major DNA damage in order to pass the G1/S checkpoint.
99
Explain what cytokinesis in animals?
* Ring of fibres made of the protein actin forms around the centre of the cell.
* Fibres contract, pinching the cell into two daughter cells, each with one nucleus
100
Explain what cytokinesis in plants?
* Rigid cell wall prevents migration of cells or division by cleavage of plant cells
* Golgi-derived vesicles move along microtubules towards middle of cell.
* Vesicle coalesce to form cell plate (new compartment surrounded by membrane)
* Cell wall forms inside cell plate
101
There are ______ checkpoints that control
| progression through the cell cycle
Three
102
Why is regulation of the cell cycle important?
is important to prevent
| uncontrolled divisions.
103
What do checkpoints allow?
Checkpoints allow integration of internal and external signals
• Checkpoints make sure that cell cycle/mitosis happens mostly error free.
104
Why are checkpoints Particularly important in multicellular organisms?
where unchecked divisions can lead to cancer.
105
What does Cell cycle control use?
cyclins and
| cyclin-dependent kinases
106
What are Cyclin-dependent kinase (CDK)?
has kinase activity to phosphorylate (activate) target proteins, is only active when bound to cyclin
107
What are Cyclins?
activate CDKs through phosphorylation and are regulated by protein abundance – usually sharp decline at particular point in cell cycle through proteolysis
108
What is the role of CDK and cyclin together?
Mitosis-promoting factor (MPF)
109
What is the decision to pass the G1/S checkpoint controlled by?
controlled/influenced by
| environment, nutrient status and DNA damage.
110
What happens when the cell passes the G1 checkpoint?
it will start DNA replication and go through
| at least one more cell division.
111
What happens if there is blockage at the G1 checkpoint?
Blockage at G1 checkpoint leads to differentiation of cell (G0). Decision to enter G0 can be reversed in some cases.
112
What is the movement through the G1/S controlled by?
• Controlled by cyclin-dependent kinase/cyclin complex.
113
What is the decision to pass the G2/M checkpoint controlled by?
• Decision to pass G2/M checkpoint depends on completion of DNA replication and whether there is DNA damage.
114
Why do cells need pass the G2 checkpoint?
• Passing the G2 checkpoint is required for cells to enter M phase/commits cell to mitosis/division.
115
What is the movement through the G2/M controlled by?
Controlled by cyclin-dependent
| kinase/cyclin complex.
116
What are the three checkpoints?
G1/S
G2/M
Spindle Checkpoint
117
What is the decision to pass the spindle checkpoint dependent on?
• Decision to pass spindle checkpoint depends on presence of correct number of chromosomes at the metaphase plate and their attachment to microtubules linked to opposite poles.
118
What is the decision to pass the spindle checkpoint controlled by?
```
• Controlled by anaphase
promoting complex (APC). APC targets cohesins and mitotic cyclins for degradation.
```
119
What is chromosome movement facilitated by?
by microtubule depolymerization,
| kinetochore motor proteins and the movement of microtubule fibers.
120
Cytokinesis is different in
animals and plant cells
121
The cell cycle has _____ checkpoints
three
122
What do the cell cycle checkpoints control?
The cell cycle has three checkpoints that control progression through the
anaphase promoting complex or cyclindependent kinases and cyclins.
123
Why are Mitosis and meiosis important mechanisms?
to pass on genetic information (DNA) between cells and generations of individuals.
124
DNA needs to be
be replicated before mitosis can take place
125
DNA needs to be tightly packed for
for error free division of genetic material between daughter cells.
126
Chromosome movement is facilitated by
microtubule depolymerization, kinetochore motor proteins and the movement of microtubule fibers
127
What is meiosis?
is the process by which haploid cells are produced from a diploid cell- for this to occur the chromosomes must be correctly sorted and distributed in a manner to create genetically unique cells with half the no of chromosomes as the original cell.
128
Where does meiosis occur?
Happens in germ cells within the gonads of males and females. Two rounds of division are necessary.
129
What does mitosis and meiosis result in?
Mitosis results in 2 diploid daughter cells and meiosis in 4 haploid cells.
130
When does the first difference in the process of mitosis and meiosis occur?
Original stages look similar to mitosis but the first difference occurs in prophase I
131
What occurs in Prophase I?
when homologous pairs of sister chromatids lie side-by-side in a process called synapsis, forming a tetrad(bivalent). The homologous chromosomes share similar but not necessarily identical genes.
132
What happens after prophase I?
Once this is complete crossing over can occur during crossing over a physical exchange between chromosomes segments of non sister chromatids occurs increasing genetic diversity.
133
When does Prophase one conclude?
Prophase one concludes with the nuclear envelope as the duplicated central pairs move to opposite poles of the cell.
134
What happens when the central pairs move?
As they moved, the centrioles extend spindle fibres forming the mitotic spindle in pro metaphase one with the paired centrioles in place. The sister chromatids attached to the spindle fibres by their kinetochore.
135
What happens because of crossing over and synapsis?
homologous chromosomes remain aligned so that a pair of sister chromatids is attached to only one pole by the kinetochore microtubules
136
Where are the sister chromatids attached in mitosis?
In mitosis, each sister chromatid is attached to a spindle fibre, originating from opposite poles.
137
What happens during metaphase 1?
During metaphase 1 they randomly align along the metaphase plate due to independent assortment- this is random and adds to genetic diversity.
138
What happens during anaphase 1?
In anaphase one the homologous chromosomes separate and move toward opposite poles.
139
When does meiosis end?
Meiosis one ends with teleophase one when the chromosomes decondense and nuclear envelope reforms, cytokinesis separates the cytoplasmic material and the 2 daughter cells are separated by a cleavage furrow.
140
What does meiosis two begin without?
Meiosis 2 begins without the chromosomes going through another round of DNA replication. Centrioles again duplicate and begin moving to opposite poles of each cell.
141
What happens in prophase two?
the sister chromatids condense while the spindles start to form as the nuclear envelope disappears in pro metaphase the sister chromatids attached to the spindle by kinetochore microtubules with sister chromatids attached to opposite poles. The spindle aligns the sister chromatids along the metaphase plate.
142
What happens during anaphase 2?
During anaphase 2, 2 sister chromatids separate and individual chromosomes move toward the poles
143
Entire process ends with
telophase two
144
What happens in telophase 2?
chromosome decondense and nuclear envelope reforms. 4 haploid daughter cells form. These cells specialise into gametes, either sperm or egg. These fuse in fertilization to form a zygote which will grow into a child, child receiving half its chromosomes from its mother and half from its father.
145
Where does diversity come from?
Diversity comes from several sources, in prophase one, non sister chromatids can exchange DNA through crossing over, increases the genetic diversity. Independent assortment produces 4 genetically distinct haploid gametes
146
A life cycle with sexual reproduction requires
a haploid phase
147
In animals, meiosis reduces
diploid cells to haploid
| gametes
148
• Two gametes fuse
| (fertilization) to form a
diploid zygote
149
Sexual reproduction generates
individuals
150
What are differences between sexual and asexual reproduction?
Asexual- • Duplication using only mitosis (like a cell line)
• Genetically identical offspring
• Offspring are clones (exact copies of the diploid parent)
• Offspring identical;diversity from mutations only.
Sexual- • Haploid gametes formed using meiosis; two fuse to form zygote
• Genetically unique offspring
• 50% contribution from either parent
• Chromosome segregation and recombination
151
What are the costs of not having sex?
```
• “Twiggy” phylogenetic distribution of
asexuality: asexual lineages seem to
often go extinct
• Many potentially asexual organisms still
have occasionally sex, e.g. aphids,
despite high cost of sexual organs
```
152
What are the costs of having sex?
• Sexual reproduction is actually very costly in terms of
short term fitness
• Risk of predation, time, energy etc.
• Each allele only has a 50% chance of being passed
on…
• Need a two fold advantage per generation of
generating offspring very different from parents
153
```
Homologous chromosomes (not sister chromatids) are
separated in
```
meiosis I
154
Homologous chromosomes form
pairs, recombination
| occurs between them (crossing over).
155
• Meiosis shares many of the _______ ________ of mitosis (and is probably evolved from mitosis).
biochemical feature
156
What occurs in Crossing over and synapsis (Prophase I)?
• Two chromatids of a homologous pair align
• Corresponding alleles are aligned
• DNA of two non-sister chromatids is cut at same locations
• Synaptonemal complex stabilises non-sister chromatid alignment
• DNA breaks are repaired, joining ends from nonsister chromatids
• Synaptonemal complex dissolves
• Sister chromatids are held together by cohesins, homologous chromosomes
are held together by chiasmata
157
Meiosis and sexual reproduction are crucial
| for
producing the genetic variety necessary
| for evolution.
158
Genetic variety is increased during
meiosis by crossing over of DNA molecules between
| homologous chromosomes.
159
What are some features of asexual reproduction?
there are not really any differences in the DNA between the progenitor and the generations of offspring. Changes can be introduced in the DNA through mutations but the combination of alleles or the genetic variety remains mostly the same.
160
What does meiosis reduce?
not only reduced no of chromosomes and the gametes, but also leads to DNA recombination
161
What is DNA recombination?
means that this combination of alleles can be mixed up in the gametes, have chromosomes with a large A, large B or a small a and small b.
162
when does recombination happen?
happens in all individuals, benefit is that you have offspring that are genetically different from the parental generation but also from each other.
163
what do you have to rely on without meiosis?
so without meiosis, you have to rely on spontaneously arising mutations to achieve this change. Have to happen in the same organism. In a species that uses meiosis and sexual reproduction you can combine beneficial alleles, that usually happens a lot quicker and through asexual reproduction.
164
What three mechanisms contribute to genetic variation produced by meiosis and sexual reproduction?
1. Independent assortment of chromosomes
Number of possible combinations is 2n,
e.g. in humans 223 = 8,388,608
2. Recombination (Crossing over) In humans, ~ 1-3 crossovers per chromosome arm during
prophase I
3. Random fertilization
In humans, fusion of male and female gamete creates one of 70 trillion (223 x 223) chromosome combinations
165
What are the problems with meiosis?
• Complex process, more prone to errors than mitosis, especially during separation
of tetrads in anaphase I.
• But high error rate is less critical than in mitosis as most bad gametes won’t achieve fertilization.
166
What may result due to an error in meiosis?
```
• Nondisjunction - homologues don't
separate properly in meiosis I
• Aneuploidy = gain or loss of chromosomes
• Often embryo lethal
• Some mono- or trisomies are viable:
• Down syndrome: trisomy 21
• Turner syndrome: monosomy X
• Klinefelter syndrome: XXY
```
167
What is another example of a meiosis error?
Translocation and deletion: transfer of a piece of one
chromosome to another, or loss of fragment of a chromosome
• Some parts of chromosome more likely to break and misalign
than others…
168
Why does evolution by natural
| selection happen?
1. Overproduction (finite resources)
2. Variation between individuals in ability to use
these resources
3. (Some of) this variation can be passed between
generations
169
Why are some meiotic errors not bad?
```
Gene duplication
• Central to generation of new genes
• Unduplicated genes are constrained by
selection pressure
• Gene duplication events lead to more
genetic material not under selection
pressure. Sub- or neofunctionalisation of
duplicated gene(s) can lead to new gene
function.
```
170
Why is it beneficial to have natural selection and the flow of genetic information?
New combinations of alleles (genotypes) are generated
by mutation and recombination (meiosis)
Differences in the transmission rate of these alleles leads
to evolution by natural selection
Adaptation = changes in allele (and therefore genotype)
frequencies resulting from this process
171
Understanding heredity is important for
| understanding
evolution
172
What did Darwin not understand?
```
• But: Darwin didn’t understand
heredity (or know about DNA)
• His initially favoured theory of
blending inheritance was wrong
(and lethal to his theory)
```
173
What was Darwins mistake?
With just a few genes acting on a trait (e.g. seed colour), particulate (Mendelian) inheritance produces almost continuous variation.
Looks like “blending inheritance” Discovery of particulate inheritance was critical to the acceptance of natural selection in the 20th century
174
What creates genetic variety important for evolution?
Independent assortment of chromosomes, recombination and random fertilization
175
Where do more mistakes happen?
More (uncorrected) errors happen during meiosis than mitosis, but very serious
defects are usually not propagated.
176
Why is it important to understand meiosis?
Understanding meiosis (and laws of
inheritance) is necessary to understand
evolution.
177
Meiosis and sexual reproduction are crucial for
producing the genetic variety necessary for evolution.
178
Genetic variety is increased during
meiosis by crossing over of DNA molecules between homologous chromosomes
179
Independent assortment of chromosomes, recombination and random fertilization create
genetic variety important for evolution.
180
What happens to allele combinations in next generations?
Certain allele combinations will be transmitted to the next generation with higher frequencies and this is evolution by NS.
181
What is Cytokinesis ?
is the phase of the cell cycle when the cytoplasm divides, creating two daughter cells. In animal cells, the microtubule spindle helps position a contracting ring of actin that constricts like a drawstring to pinch the cell in two. In cells with a cell wall, such as plant cells, a plate forms between the dividing cells.
182
What is the cell cycle?
The cell cycle is depicted as a circle. The first gap phase, G1, involves growth and preparation for DNA synthesis. During S phase, a copy of the genome is synthesized. The second gap phase, G2, prepares the cell for mitosis. During mitosis, replicated chromosomes are partitioned. Cytokinesis divides the cell into two cells with identical genomes.
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What is the M phase?
Mitosis and cytokinesis together are usually referred to collectively as M phase, to distinguish the dividing phase from interphase.
