Lecture 14 mitosis and meiosis Flashcards
(192 cards)
1
Q
What is chromatin?
A
A dense material made of double stranded DNA molecules and proteins
2
Q
What are the two double stranded DNA molecules formed after DNA replicates during S phase called?
A
Sister chromatids
3
Q
How are sister chromatids held together?
A
By a protein complex called cohesin along most of their length
4
Q
When is chohesin removed?
A
Mitosis, most is removed, except at the centromere
5
Q
What is the centromere?
A
The point at which sister chromatids are held together
6
Q
After DNA replication, how is DNA further compacted?
A
Condensins coat the DNA molecule
7
Q
What is the diameter of the nucleus?
A
5 micrometers
8
Q
How long is the typical DNA in a human cell, end to end?
A
2m
9
Q
What proteins are contained within chromosomes?
A
Histones
10
Q
How many classes of histones are there?
A
5
11
Q
What charge do histones have?
A
Positive at cellular pH levels
12
Q
Why are histones positively charged?
A
High content of basic amino acids lysine and arginine
13
Q
What do positive charges on histones do?
A
Attract negative phosphate groups on DNA
14
Q
What interactions result in nucleosomes?
A
DNA- Histone interactions
Histone-histone interactions
15
Q
What are nucleosomes like?
A
Bead like
16
Q
How many histone molecules are their in each nucleosome?
A
Eight, 2 each of 4 of the histone classes
17
Q
How are histones arranged within a nucleosome?
A
United to arrange a core or spool
18
Q
How many base pairs are there in a nucleosome?
A
146
19
Q
How many turns of base pairs are wrapped around each histone spool?
A
1.65 turns
20
Q
What is the final component of a nucleosome?
A
Histone H1
21
Q
What does H1 do?
A
Found on the outside of the DNA
clamps it to the histone core
22
Q
What runs between nucleosomes?
A
A variable amount of non-nucleosomal linker DNA
23
Q
How is DNA arranged during interphase?
A
A single DNA molecule running between chromosomes
24
Q
What is DNA exposed to during interphase?
A
Nuclear environment
Including proteins involved in replication and regulation of expression
25
What happens to chromatin during mitosis and meiosis?
It becomes more tightly coiled and condensed
26
Up until what point does further coiling of chromatin take place?
Until chromatids begin to move apart
27
What is mitosis?
A single nucleus gives rise to two nuclei that are genetically identical to each other and to the parent nucleus
28
What are the subdivisions of mitosis, in order?
```
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
```
29
What is the centrosome?
An organelle in the cytoplasm near the nucleus
30
What happens to the centrosome during S phase?
It doubles, forming a pair of centrosomes
31
What does a centrosome consist of?
A pair of centrioles- each one a hollow tube lined with nine micro tubules. The tubes are at right angles to each other.
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When do the two centrosomes separate from each other?
At the G2 to M transition
33
Where do the centrosomes go after they have separated?
To opposite ends of the nuclear envelope
34
What does orientation of the centrosomes correlate with?
The plane at which the cell will divide
35
What is the consequence of determining the plane at which the cell divides?
It determines the spatial relationship of the two new cells to the parent cell- important for making body tissue
36
What surrounds the centrioles?
A high concentration of tubulin dimers
37
What do tubulin dimers do?
Initiate the formation of microtubules which orchestrate chromosomal movement
38
Plants lack centrosomes. How is chromosomal movement controlled?
A distinct microtubule organizing center at either end of the cell plays the same role
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What does formation of microtubules lead to?
The formation of the spindle structures that segregate chromosomes
40
When do chromatids become visible?
prophase
41
When does the spindle form?
Prophase
42
What is visible under a light microscope during interphase?
Nuclear envelope
Nucleoli
Tangle oc chromatin
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What happens during prophase to cohesin?
It is removed (not removed at centromere)- chromatids become visible
44
What structure develops late in prophase?
Specialized three-layered structures called kinetochores
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Where do kinetochores develop?
In the centromere region, one on each chromatid
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What are kinetochores important for?
Chromosome movement
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What do the two centromeres act as?
A mitotic center, or pole, towards which the chromosomes move
48
How does the spindle form?
Microtubules form between each pole and the chromosomes
49
What is the purpose of the spindles?
A structure to which chromosomes attach and as a frame work for keeping the poles apart
50
What is the behavior of the spindles when they first form?
Initially unstable- they constantly form and fall apart
51
When do spindles become more stable?
When they contact microtubules from the other half spindle
52
What is the structure of the spindle?
Made up of 2 half spindles- each microtubule runs from one pole to the middle of the spindle, where it overlaps with the microtubules from the other half spindle
53
What are the two types of microtubule in the spindle?
Polar microtubules
| Kinetochore microtubules
54
What are polar microtubules?
Microtubules that form the framework of the spindle
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What is the structure of polar microtubules?
Abundant tubulin around centrioles
56
How are polar microtubules made up?
Tubulin dimers aggregate to form long fibers that extend to the middle region of the cell
57
What are kinetochore microtubules?
Microtubules that attach to the kinetochores on the chromosomes
58
When do kinetochore microtubules form?
Later than the polar microtubules
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How do sister chromatids become attached to the kinetochore microtubules?
Sister chromatids in each chromosome pair become attached by their kinetochore to kinetochore microtubules in opposite halves of the spindle
60
What does the attachment of kinetochores to kinetochore microtubules ensure?
That one chromatid of the pair will move to one pole and the other chromatid to the opposite pole
61
What marks prometaphase?
The disappearance of the nuclear envelope and the nucleoli
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What happens to the material that composes the nuclear envelope and nucleoli when it disappears during prometaphase?
It remains in the cytoplasm to be reassembled when daughter nuclei form
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What begins to happen in prometaphase but is countered?
The movement of chromosomes towards the poles
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What counters the movement of chromosomes towards the poles during prometaphase?
- Repulsive force from the poles pushes the chromosomes towards the middle region (equatorial plate) of the cell
- The two chromatids are still held at the centromere by cohesin
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What is the middle region of the cell called?
The equatorial plate, or metaphase plate
66
What does the movement and countermovement of chromosomes during prometaphase do?
Makes the chromosomes appear to move aimlessly back and forth between poles and the middle as they approach the equatorial plate
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What marks the start of metaphase?
When the centromeres arrive at the equatorial plate
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When is the best time to see the sizes and shapes of the chromosomes and why?
Metaphase
| Because they are maximally condensed
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What happens by the end of metaphase?
All of the chromatid pairs separate simultaneously.
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What marks the beginning of anaphase?
Separation of the chromatids
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What happens during anaphase?
The sister chromatids move to opposite ends of the spindle
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What does each chromatid contain during anaphase?
One double stranded DNA molecule called the daughter chromosome
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Why does separation of the chromatids occur during anaphase?
One subunit of cohesin holding sister chromatids together is hydrolyzed
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What type of molecule hydrolyzes the subunit of cohesin during anaphase?
A protease called separase
75
How is separase present in a cell?
It is present but inactive until anaphase, bound to an inhibitory subunit called securin
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What happens to securin once all of the chromatids are connected to the spindle?
Securin in hydrolyzed
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What effect does the hydrolysis of securin during anaphase have?
Separase catalyzes the cohesin breakdown
78
What is the process of hydrolyzing securin in order for separase to break down cohesin called?
The spindle checkpoint
79
What does the spindle checkpoint sense?
Whether there are any unattached kinetochores- if so, securin breakdown is blocked.
80
What two things move chromosomes along?
- Molecular motors
| - Shortening microtubules
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What act as molecular motors?
Proteins at the kinetochore
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What are the names of the proteins at the kinetochore that act as molecular motors?
Cytoplasmic dynein
83
How do cytoplasmic dynein's work?
They hydrolyze ATP to ADP
| This releases energy to move chromosomes along microtubules towards the poles
84
What % of the force of motion do molecular motors account for?
75%
85
What % of force does the shortening of microtubules towards the poles account for?
25%
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What happens to the poles during anaphase?
They are pushed further apart, doubling the distance between them
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Why does distance between poles increase during anaphase?
Overlapping polar microtubules extending from opposite ends contain motor proteins that cause them to slide past each other
88
What happens to make the cell enter telophase?
The chromosomes stop moving at the end of anaphase
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What is the situation at the start of telophase?
Two sets of identical daughter chromosomes are at opposite ends of the spindle
90
What happens to the spindle in telophase?
It starts to break down
91
What happens to the chromosomes in telophase?
They start to uncoil and become diffuse tangle of chromatin
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What happens to the nuclear envelope and nucleoli in telophase?
They coalesce and re-form their respective structures
93
What does mitosis only refer to? (Hint: what separates it from cytokinesis?)
The division of the nucleus
| Division of cytoplasm is cytokinesis
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What effects how cytokinesis occurs?
The type of organism- for example, plants and animals are very different in their cytokinesis
95
How do animal cells divide during cytokinesis?
By furrowing of the plasma membrane as if an invisible thread was cinching the cytoplasm between poles
96
What is the invisible thread between poles that separates the cells during cytokinesis?
Microfilaments of actin and myosin in a contractile ring beneath the plasma membrane
97
How does actin and myosin form a contractile ring?
They interact like they do during muscle contraction to produce contraction
98
What do the microfilaments used to form the contractile ring assemble from?
Actin monomers that are present in the interphase cytoskeleton
99
What controls the assembly of microfilaments during cytokinesis?
Calcium ions released from storage sites in the center of the cell
100
Why do plant cells divide differently to animal cells?
Because of the presence of the cell wall
101
What happens after the spindle breaks down in mitosis?
Membranous vesicles derived from the golgi apparatus appear at the equatorial plate.
102
What happens after membranous vesicles appear at the equatorial plate of a plant cell during cytokinesis?
They are propelled along microtubules by motor protein kinesin and fuse to form new plasma membrane
103
What happens to the contents of the vesicles used to form a new plasma membrane in plant cells during cytokinesis?
Contribute vesicle contents to the cell plate
104
What is the cell plate?
The beginning of a new cell wall
105
What is not distributed precisely during mitosis?
Organelles such as ribosomes, mitochondria and chloroplasts
106
Why are organelles not precisely distributed?
Because it doesn't matter as long as some of each is present
107
Why is unequal distribution of cytoplasmic components important?
It can have functional significance for the two new cells
108
What is the product of meiosis?
4 daughter cells that cannot undergo further duplications
109
What type of nuclear division is involved in reproduction?
Mitosis and meiosis, however, they have different reproductive roles.
110
What are the off spring produced in asexual reproduction called?
Clones- if the offspring are genetically identical
111
What can cause variation among asexually produces offspring?
Mutations
112
What is an advantage to asexual reproduction?
It is rapid and effective
113
What is an advantage to sexual reproduction?
It results in genetic diversity
114
How does meiosis produce some variation?
Gametes are different to each other and from their parent
115
What are cells not specialized for reproduction called?
Somatic cells
116
What are a set of chromosomes found in somatic cells called?
Homologous chromosomes
117
What are the characteristics of chromosomes found in homologous pairs?
They are similar size and appearance (except some sex chromosomes)
118
What genetic information do homologs have?
Similar but not usually identical genetic information
119
What type of cell are gametes?
Haploid
120
What is it called when two haploids fuse? What is the name of this process?
Zygote
| Fertilization
121
What are the three hall marks of all sexual life cycles?
- Two parents, each provides chromosomes in the form of gametes produced by meiosis
- Each gamete is haploid
- Two gametes fuse to form a single cell (zygote) (diploid)
122
What are the three types of sexual life cycle that can be observed after zygote formation?
Haplontic
Alternation of generations
Diplontic
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What organisms are haplontic?
Most protists, many fungi
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What is a haplontic organism?
One where the zygote is the only diploid cell in the life cycle
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What happens to the zygote in a haplontic organism?
It undergoes meiosis to produce haploid cells (spores)
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What do spores of haplontic organisms do?
Form a new organism (single or multicellular) by mitosis
127
How do mature, haploid organisms produce gametes?
By mitosis
128
What organisms have alternation of generation sexual life cycles?
Plants, some protists
129
What does meiosis give rise to in alternation of generations?
Not gametes
| Haploid spores
130
What happens to the haploid spores formed during alternation of generations?
Divide by mitosis for form an alternate, haploid life stage called gametophyte
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What does the gametophyte produce?
Gametes, by mitosis
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What happens to the gametes produced by haploid gametophytes?
Fuse to form a zygote
133
What happens to the zygote produced during alternation of generations?
It divides by mitosis to become the diploid sporophyte
134
What organisms are diplontic?
Animals, some plants
135
What are the only haploid cells in the life cycle of diplontic organisms?
The gametes
136
How are diplontic organisms zygotes produced?
Meiosis
137
What is able to be observed during metaphase of mitosis and hence counted and characterized?
Individual chromosomes
138
What is made to be able to observe an entire set of chromosomes?
A photomicrograph
139
What happens to the photomicrograph?
Images are arranged to reveal number, shape and sizes of the chromosomes
140
What do the number, shape and sizes of the chromosomes constitute?
The karyotype
141
What can the chromosomes of a karyotype be divided into?
22 pairs of autosomes
| 1 pair of sex chromosomes
142
How many nuclear divisions are there during meiosis?
2
143
How many times is the DNA replicated during meiosis?
once
144
What are the names of the two mitotic divisions?
Meiosis 1 and meiosis 2
145
What is a unique feature of meiosis 1 that separates it from mitosis?
Homologous chromosomes come together to pair along their entire lengths
146
What happens during metaphase 1 of meiosis?
The homologous chromosomes separate
147
When does crossing over occur?
Late stage prophase 1- prometaphase
148
What does crossing over result in?
exchange of genetic material
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How is prophase 1 in meiosis 1 different?
It is long,
| Chromosomes change markedly
150
How do chromosomes change during prophase 1 in meiosis 1?
The homologous chromosomes adhere along their length in a process called synapsis
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When does the pairing process of synapsis start and end?
Starts during prophase 1
| ends at the end of metaphase 1
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Where does the joining of homologous chromosomes begin?
At the telomeres
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How is joining of homologous chromosomes mediated?
Recognition of homologous DNA sequences on homologous chromosomes
154
What else helps to mediate joining of homologous chromosomes during meiosis 1?
A special group of proteins form a scaffold called synaptonemal complex
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What is the structure of the synaptonemal complex?
Runs lengthwise along the homologous chromosomes ti join them together.
156
What are the four chromatids of each pair of homologous chromosomes called?
tetrad, or bivalent
157
How many chromatids are there in prophase 1 of meiosis 1 in humans?
96
158
During meiosis 1, when homologous chromosomes seem to repel each other, how are they held together?
By physical attachments mediated by cohesins
| different to cohesins holding sister chromatids
159
What are regions of homologous chromosomes held together shaped like and called?
X-shaped
| Chiasmata
160
What does a chiasma reflect?
An exchange of genetic material between non-sister chromatids- called crossing over
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When do chromosomes start exchanging genetic material during meiosis 1?
Shortly after synapsis begins
| not visible until repelling begins
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How long does it take for prophase I to occur in the human male testes? What about for the entire meiotic cycle?
1 week, 1 month for entire meiotic cycle
163
In meiosis I, what attaches to the same half spindle?
The kinetochores of both chromatids, so the entire chromosome moves to the pole
164
How are homolgous chromosomes sorted?
The member attached is random, so sorting to each pole is random
165
What do the daughter nuclei at the end of meiosis I contain?
One set of chromosomes consisting of two chromatids
166
What happens in some organisms following anaphase I of meiosis I?
Telophase I
167
What follows telphase I of meiosis I?
Interphase called interkinesis
168
What happens during interkinesis?
Chromatin is partially uncoiled,
| No replication of genetic material
169
What is the purpose of the second meiotic division?
To separate the chromatids
170
What are the three major difference between mitosis and meiosis II?
- DNA replicates before mitosis
- Chromatids making up chromosome are identical in mitosis
- Number of chromosomes on equatorial plate is 1/2 for meiosis II then mitosis
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What is the result of meiosis II?
4 haploid nuclei
172
Why are the products of meiosis I genetically diverse?
- Synapsis during prophase I creates recombinant chromatids (crossing over)
- Independent assortment
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What is independent assortment?
Homologous pairs line up randomly at metaphase I, paternal chromatids and maternal chromatids assort independently of each other
174
What is it called when homologous chromosomes fail to separate during meiosis I, or sister chromatids fail to separate during meiosis II?
Nondisjunction
175
How else may a meiotic error occur?
when chromosomes fail to remain together
176
What is the result of a meiotic error?
Aneuploid cells --> aneuploidy
177
What is aneuploidy?
A condition in which one or more chromosomes are either lacking or present in excess
178
What can cause aneuploidy?
Lack of cohesins
179
Why does a lack of cohesins cause aneuploidy?
Without them, homologs line up randomly at metaphase I, 50% chance both will go to the same pole.
180
What is a zygote with three copies of a particular chromosome called?
Trisomic
181
What causes Down syndrome?
Trisomy of chromosome 21
182
What are some symptoms of Down syndrome?
Impaired intelligence
Characteristic abnormalities of the hands, tongue, eyelids
Increased susceptibility to cardiac abnormalities and leukemia
183
What is a zygote with only one copy of a chromosome called?
Monosomic for that chromosome
184
Name another type of abnormal chromosomal event that can occur?
Translocation
185
What is translocation?
A piece of chromosome breaks off and becomes attached to another chromosome
186
How common are trisomies and monosomies in human zygotes?
10-30% of all conceptions
187
What happens to most embryo's that develop from aneuploidy zygotes?
Do not survive to birth, or die before age 1
188
How many pregnancies are spontaneously terminated (miscarried) during the first 2 months, largely due to trisomies/monosomies?
1/5th
189
What occurs when organisms have complete extra sets of chromosomes?
Polyploidy
190
Why can polyploid cells still under go mitosis?
Because chromosomes behave independently of each other
191
Why can't polyploid cells undergo normal meiosis?
Homologous chromosomes cannot synapse to begin division
192
Where is polyploidy common?
Modern agriculture:
| -modern bread, wheat plants are hexaploids
