Cell and Molecular Bio- Meiosis and Mitosis (11) Flashcards
1
Q
Recap question: Explain cell theory:
A
Modern cell theory
All known living things are made up of cells
-The cells is the structural and functional unit of all living things
-All cells come from pre-existing cells by division
2
Q
The cell cycle and cell division, All cells come from existing cells.
What are the 2 important processes?
A
- Growth
- cell duplicates its contents (including DNA and organelles) - Cell division
- parent cell contents divide into two daughter cells
3
Q
What is The cell cycle?
A
The process of cell growth and division
4
Q
Why and how do cells divide?
A
The life cycle of an organism is linked to cell division
Unicellular organisms use cell division primarily for reproduction
Multicellular organisms, cell division is important in growth and repair of tissues
- Reproduction
- Growth
- Repair of tissues
5
Q
What are the 4 events that must occur for cell division?
A
- Reproductive signal: to initiate cell division
- Replication: DNA to be duplicated
- Segregation: Distribution of the DNA into the 2 new cells
- Cytokinesis: Separation of the 2 new cells
6
Q
Reproductive signal examples:
A
- Growth factors- external chemical signals that stimulate these cells to divide
- Platelet-derived growth factor that initiate blood clotting, stimulates skin cells to divide and heal wounds
- some white blood cells produce interleukins that promote cell division in other white cells (important for defense against pathogens) - Erythropoietin produced in the kidneys stimulates division of bone marrow cells and production of red blood cells
7
Q
Why and How do prokaryotes undergo asexual reproduction?
A
Binary fission results in 2 new cells
Reproductive signals include external factors such as nutrient concentration and environmental conditions that initial cell division
- For many bacteria, abundant food supplies speed up the division of the cycle
- Most prokaryotes have 1 chromosome and usually circular
- Replication begins at the ori site and moves towards the ter site (where replication ends)
8
Q
Why and How do Eukaryotes divide?
A
Asexual and Sexual reproduction
Cell division is significant in growth and repair of tissues
Signals for cell division are related to the needs
Have many chromosomes, the processes of replication and the segregation are more intricate
DNA replication usually occurs between cell divisions
Sister Chromatids-newly replicated chromosomes are closely associated
9
Q
How do Eukaryotic cells divide?
Explain Meiosis and Mitosis
A
Meiosis is nuclear division in cells involved in sexual reproduction
-resulting cells are not identical to the parent cells
Mitosis separates them into 2 new nuclei, identical to the parent cell
10
Q
Role of cell division in a Asexual Life Cycle:
What is asexual reproduction?
A
Asexual reproduction is based on mitotic division of the nucleus
- A unicellular organism may reproduce itself
- Cells of multicellular organisms break off to form a new individual
- The offspring are clones- genetically identical to the parent
11
Q
Role of cell division in a Sexual Life Cycle:
What is sexual reproduction?
A
Sexual reproduction: the offspring are not identical to the parents
It requires gametes created by meiosis, two parents each contribute one gamete to an offspring
Gametes and offspring differ genetically from each other and from the parents
12
Q
What role of cell division play in a Sexual Life Cycle?
A
Sexual reproduction generates diversity among individual organisms
It allows the random selection of half the diploid chromosome set-this forms a haploid gamete that fuses with another to make a diploid cell
-thus no 2 individuals have the same genetic make up
13
Q
What are the differences between sexual reproduction and asexual reproduction?
A
Sexual- involves sex cells and fertilization
Asexual- Does not involve sex cells and fertilization
-Only 1 parent plant
Advantages/Disadvantages of Sexual
- Offspring are not genetically identical to one another
- There is variation in the offspring
Advantages/Disadvantages of Asexual
- Offspring have no variation
- Offspring are genetically identical to one another
14
Q
Define the term Ploidy:
A
the number of sets of chromosomes in a cell
15
Q
Define the term diploid and haploid:
A
Diploid (2n)-cells with 2 sets of chromosomes. Human cells have 23 pairs of chromosomes
Haploid(n)- cells with one set of chromosomes. Gametes have 1 set of chromosomes or total of 23 chromosomes
16
Q
What are Somatic cells?
A
Body cells that are not specialized for reproduction
All cells other than sexual gametes cells
Each somatic cell contains homologous pairs (diploid 2n) of chromosomes with corresponding genes
Undergo Mitosis
Daughter cells are genetically the same
17
Q
Characteristics of Gamete cells
A
contain only 1 set of chromosomes (haploid) (23 chromosomes not pairs)
Used in Fertilization: 2 haploid gametes (female egg and male sperm) fuse to form a diploid zygote (2n)
Diplontic life cycle
18
Q
Diplontic life cycle:
A
Human, mammals and some plants: gametes are the only haploid stage
Mature organism is diploid and produces gametes by meiosis
Gametes fuse to form diploid zygote
Zygote divides by mitosis to form mature organism
19
Q
Haplontic Life cycle:
A
In protist, fungi and some algae-only zygote is diploid
After zygote forms it undergoes meiosis to form haploid spores, which germinate to form a new organism
Organism is haploid, produces gametes by mitosis- cells fuse to form zygote
(spend most of their time as haploids)
20
Q
What are the phases of the Cell cycle?
A
- Interphase
- non dividing stage - Mitosis
- nuclear division - Cytokinesis
- cytoplasm division
21
Q
What occurs during the Interphase:
A
- DNA has replicated, but has not formed the condensed structure of chromosome. They remain as loosely coiled chromatin
- The nuclear membrane is still intact to protect the DNA molecules from undergoing mutation
- 3 stages
- G1-Gap 1/ Growth
- S- Synthesis of DNA
- G2-Gap 2/ Growth
22
Q
What occurs during the Interphase-G1?
What is the purpose of this stage?
A
G1 phase- Stage before DNA replication
- Decision to divide (move to S phase)(if there are any signals if not they don’t) or not to divide (move to G0 phase)
- Doubling of organelles
- Accumulation of materials for DNA synthesis
- some cells, such as nerve, typically do not complete the cell cycle and are permanently arrested in G1.
- they won’t ever continue to the S and G2 phases, they are instead said to be in a G0 stage
-Embryonic cells spend very little time in G1 and complete the cell cycle in a few hours (they undergo cell division for fetus formation)
23
Q
What occurs during the Interphase-S phase?
What is its function?
A
S phase-DNA replication/synthesis
- Semi-conservative replication
- results in each chromosome being composed of two sister chromatids
24
Q
What occurs during the Interphase-G2?
A
G2 phase- stage after DNA replication
- synthesis of proteins needed for cell division
- this is the preparation stage for meiosis
25
Chromatids and Chromosomes:
A chromatid is a very specific term for a newly-replicated chromosome that is still attached to it's sibling chromosome
One chromosome replicates into 2 "sister chromatids"
Once replication and cell division is over, the chromatid reverts to being a plain old chromosome
26
Define the term Centrosome:
the microtubule organizing center of the cell. It replicates in S phase
centrosomes is composed of 2 centrioles surrounded by pericentriolar material
27
Function of centrosomes:
Key to the division of cells
-Before cell division, the centrosome duplicates and then, as division begins, the two centrosomes move to opposite ends of the cell.
28
What is Aster?
microtubules radiating from a centrosome
29
What are centrioles?
Cannot be seen when the cell is not dividing
The centrioles also help in the formation of the spindle fibers that separate the chromosomes during cell division...
-Plant cells do not have centrioles. The plant cells have the ability to reproduce even without the centrioles
30
What is the centromere and kinetochore?
The sister chromatids are held together at a region called centromere
The kinetochore is the protein complex assembled at centromere
specifies the attachments between the chromosomes and microtubules of the spindle and is thus essential for accurate chromosome segregation.
31
What are spindle fibres?
Long microtubules extend from the centrioles in all possible directions, forming what is called a spindle
Some of the spindle fibers attach to the chromosomes by connecting to kinetochores
32
Summary:
| Phases of the Cell cycle and their checkpoints
G1 checkpoint
Cell cycle main checkpoint. If DNA is damaged, apoptosis (death of cells) will occur.
Otherwise, the cell is committed to divide when growth signals are present and nutrients are available
G2 checkpoint
Mitosis checkpoint, Mitosis will occur if DNA has replicated properly
Apoptosis will occur if the DNA is damaged and cannot be repaired
M checkpoint
Spindle assembly checkpoint. Mitosis will not continue if chromosomes are not properly aligned
33
Shorter summary of Checkpoint in Eukaryotic cells- progression can be arrested to prevent uncontrolled cell division
G1/S Boundary: G1
- lack of growth factors
- DNA damage
G2/M boundary: G2
- DNA damage
- Un replicated DNA
Metaphase/Anaphase boundary
-spindle checkpoint-kinetochore attachment
34
What are the 2 ways cell death can occur?
1. Necrosis (accidental death of cells)
Necrosis is not always related to an underlying condition. It may be caused by:
```
Injury (physical damage to tissues)
Extremely high or low temperature
Radiation
Chemicals
-cells start to swell
-may lead to inflammation
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2. Apoptosis-programmed cell death
not due to accidental death of cells
natural for cells to die and generate new cells such as in the skin
-cells don't swell like necrosis but shrink -no inflammatory response
35
Cell death cycle is controlled by signals such as....
1. Lack of a mitotic signal (growth factor)
2. Recognition of damaged DNA
3. External signals cause membrane proteins to change shape and activate enzymes called caspases- hydrolyze proteins of membranes
36
How does unregulated cell division lead to cancer?
Cancer cells differ from original cells in several ways:
- cancer cells lose control over cell division
- cell evade programmed cell death
- they can migrate to other parts of the body (metastasis)
37
G1 checkpoint:
- determines whether all conditions are favorable for cell division to proceed to progress to the S phase (DNA synthesis)
- also called restriction point, point at which the cell irreversibly commits to the cell division process
- External influences such as growth factors, play a large role in carrying the cell past the G1 checkpoint
- check for DNA damage, check integrity of DNA
- If cell cycle stops at this point if DNA is damaged and sent to to cell G0 to be repaired
-If DNA cannot be repaired it may undergo apoptosis (programmed cell death)
38
G2 checkpoint:
The cell cycle halts here until the cell verifies that DNA has replicated properly
-prevents the initiation of the M stage unless the chromosomes are duplicated
-If DNA is damaged, arresting the cell cycle allows time for the damage to be repaired so that is not passed on to daughter cells
39
Mitotic checkpoint:
aka Spindle assembly checkpoint
- Occurs between metaphase and anaphase to make sure that chromosomes are properly attached to the spindle so can be distributed accurately to the daughter cells
- Mitosis will not continue unless chromosomes are not properly aligned
40
Mitosis and Cytokinesis:
Following interphase is M phase:
M phase has 2 stages:
Mitosis stage and cytokinesis
Mitosis is a type of nuclear division. Also referred to as duplication division since each new nucleus contains identical chromosomes as the former cell
Cytokinesis-division of the cytoplasm
41
What are the Characteristics and Function of Mitosis?
- cell division
- forms identical chromosomes from parent cell
Asexual reproduction
- Replace and repair body cells
- Increase in cell number during growth
42
What happens during mitosis?
During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells
Critical process- carefully controlled number of genes
When mitosis is not regulated correctly, health problems such as cancer can result
43
What are the 6 stages and 4 phases of Mitosis?
1. Interphase
2. Prophase
3. Metaphase
4. Anaphase
5. Telophase
6. Cytokinesis
44
What occurs in the Early Prophase?
The DNA molecules progressively shorten and condense by coiling, to form chromosomes
The centrioles has migrate to opposite poles of the cell
The nuclear membrane and nucleolus begin to breakdown
45
What occurs in the Late prophase stage?
Mitotic spindle (made from microtubule protein) if formed
It attaches to kinetochores located at the centromere of the chromosome
46
What occurs during the metaphase
(lines up at the center, takes one spindle fibre)
Chromosomes are moved to the central or metaphase plate of the cell by the spindle microtubules
Sister chromatids line up in the middle of the cell
Each chromatid takes one spindle fibre which comes from the opposite poles
47
What occurs in the Anaphase stage?
(splits and pulled to the ends)
The microtubules attached to the kinetochores of each chromosome shorten and the centromere splits, and separated sister chromatids are pulled to the opposite end of the cells
Each chromatid becomes an individual chromosome (identical to the original parent chromosome)
The unattached microtubules elongates stretching the cells
48
What occurs in the telophase?
Begins when chromosomes arrive at the poles
Chromosomes become indistinct chromatin again
chromosomes uncoil
The spindle disappears
The nuclear envelope reappears
Characterized by the presence of 2 daughter nuclei
49
What occurs during cytokinesis?
Division of the cytoplasm and organelles
An indentation called a cleavage furrow passes around the circumference of the cell
Actin filaments form a contractile ring; as the ring becomes smaller, the cleavage furrow pinches the cell in half
Cytokinesis happens at the same time as telophase in many cells, so they are often presented together
Each daughter cell contains the same number and same quality of chromosome
50
What happens during Mitosis for plant cells?
Plant cells have cell walls, so cytokinesis cannot proceed with a cleavage furrow.
During telophase a cell plate forms across the cell in the location of the old metaphase plate
Vesicles from the Golgi apparatus migrate to the plane of cell division (metaphase plate). These fuse to form a new plasma membrane
Contents of vesicles form the cell plate- the beginning of the new cell wall
51
What happens during Meiosis?
Consists of 2 nuclear divisions but DNA is replicated only once. The function of meiosis is to:
reduce the chromosome number from diploid to haploid and produce 4 gamete cells
Ensure that each haploid has a complete set of chromosomes
Generate genetic diversity in the gamete cells
52
What is the importance of meiosis?
It is required to produce egg and sperm cells for sexual reproduction
During reproduction, when the sperm and egg unite to form a single cell, the number of chromosomes is restored in the offspring
Generate genetic diversity in the gamete cells
53
Meiosis I and II:
The process is split into meiosis I and meiosis II
Meiosis I is a type of cell division unique to germ cells, while meiosis II is similar to mitosis
54
Define the term Homologous chromosomes:
or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis
Have the same genes in the same loci where it enables a pair of chromosomes to align correctly with each other before separating during meiosis
55
What are the two main purposes of Meiosis I?
- reduction division
- genetic recombination
1. It is the reduction division, so it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid)
2. Most genetic recombination occurs here
56
What is Synapsis?
The pairing of homologous chromosomes
Crossing over (homologues exchange pieces of chromosomes)
57
Alignment of chromosomes differs between Meiosis 1 and Mitosis
What is the difference between Meiosis 1 and Mitosis alignment?
Meiosis 1
-Sister chromatids do not split but fuse together to the opposite ends of Anaphase 1
How it goes about:
Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and function as one
Microtubules can attach to only one side of the centromere
Microtubules pull the homologous chromosomes apart but sister chromatids are held together
Mitosis
Homologues do not pair
Kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere
Microtubules pull sister chromatids apart
58
What occurs in the Pre-meiotic interphase?
Prior to meiosis, all chromosomes are duplicated (process similar to duplication prior to mitosis)
-2 centrosomes are produced by the duplication of a single centrosome during pre- meiotic interphase
59
What occurs during Prophase 1 in Meiosis 1?
During prophase 1, the complex of DNA and protein known as chromatin condenses to form chromosomes
The pairs of replicated chromosomes (chromatids), remain joined at a central point called the centromere
A large structure called the meiotic spindle also forms microtubules (long proteins)
Short:
- chromatin condenses to form chromosomes
- chromatids remain joined at the centromere
- meiotic spindle forms microtubules
keep in mind:
During anaphase, they do not split apart like in mitosis because there are no spindle fibers attached to each of the sister chromatids (just one sent binded to the kinetochore)
60
What occurs in Prophase 1 after the meiotic spindle forms microtubules?
At this point, each homologous chromosome pair is visible as a bivalent (tetrad), tight grouping of 2 chromosomes, each consisting of 2 sister chromatids
crossing over occurs as chromosomes condense
Nuclear envelope breaks down
Centrosomes travels to opposite poles of the cell to form the spindle
61
S phase of premeiotic interphase:
chromosome appear as sister chromatids
Meiosis begins following one round of DNA replication in cells within the male or female sex organs
62
Bivalent (Tetrad):
paired maternal and paternal homologous chromosomes with 2 chromatids each
63
Explain the continuation of prophase 1 after genetic crossing occurs:
nucleolus disappears during prophase 1
in the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the 2 pairs of centrioles as they migrate to opposite poles of the cell
nuclear envelope disappears at the end of prophase 1, allowing the spindle to enter the nucleus
64
What occurs during Metaphase 1 in Meiosis 1?
The centrioles are at opposite poles of the cell
Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids)
Homologous chromosomes pair up and align in the middle of the cell
Independent assortment: homologous chromosome pairs line up in random orientations at the middle of the cell:
-a maternal or paternal chromosome goes to either pole
65
What occurs during Anaphase 1 during meiosis 1?
Anaphase 1 begins when the 2 homologous chromosomes separate and start moving toward opposite poles of the cell as a result of the action of the spindle
The sister chromatids remain attached at their centromeres and move together toward the poles
Key difference between mitosis and meiosis is that the sister chromatids remain joined after metaphase in meiosis 1 where in mitosis they separate
66
What occurs during Telophase 1 in Meiosis 1?
The homologous chromosome pairs complete their migration to the 2 poles as a result of the action of the spindle
Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids
67
Telophase 1 in Meiosis 1:
| What happens in telophase 1 when cells have to undergo rapid meiosis?
When cells have to undergo rapid meiosis they do not decondense chromosomes to become chromatins in telophase 1
other cells do exhibit chromosome de-condensation at this time; the chromosomes then re-condense in prophase II
68
What occurs during Meiosis II-Prophase II?
Meiosis begins without any further replication of the chromosomes
Chromosomes condense (shorten and thicken)
The nuclear envelope breaks down
The centrioles duplicate, travel to the opposite poles of the cells
The spindle apparatus forms
69
What occurs during Meiosis II-Metaphase II?
Chromosomes line up in the middle of the cell
Each chromatid attached to 1 spindle fibre that comes from the opposite spindle poles
70
What occurs during Meiosis II-Anaphase II?
Sister chromatids break apart at the centromere and move to opposite poles
Each chromatid becomes an individual chromosome
71
What occurs during Meiosis II-Telophase and Cytokinesis II?
Chromosomes have reached their respective ends/ poles and decondense
Nuclear envelope re-form
Cytokinesis takes place to divide the cells
(haploid daughter cells forming)
Four haploid daughter cells are genetically different from each other and from the parent cell
72
Mitosis vs Meiosis table summary:
Mitosis
2 daughter cells
Identical
1 nuclear division
Ends with Diploid
Homologous
chromosomes do not pair up
In anaphase- sister chromatids separate
its function is growth and asexual reproduction
Occurs in the body cells
Meiosis
4 daughter cells are made
Non-identical and genetically different daughter cells
2 Nuclear divisions
Ends with Haploid
Homologous chromosomes pair up
In Anaphase 1: homologous chromosomes separate
Anaphase II: sister chromatids separate
Function is to make gametes (sex cells)
Occurs in the Gonads
73
Meiosis and Gamete formation:
Spermatogenesis (gamete formation in males)
- lots of sperm cells can be made
- diploid sperm cell
Oogenesis (gamete formation in the female)
- diploid egg cell
- limited number of egg cells (must be big) to contain enough nutrients for it to undergo many cell divisions and provide nutrients to the fetus through the umbilical cord
74
Abnormal chromosome inheritance:
Nondisjunction may occur during meiosis I (usually during the checkpoint before the M stage the cell ensure the spindle fibers are attached and pulled apart properly) sometimes this does not occur properly and result in extra chromosomes
-Homologous chromosomes during separation do not separate properly and the spindle fibres are not attached properly they do not separate but move towards the opposite ends together
- Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This results in an abnormal number of chromosomes of gametes
- can happen in meiosis I or II leading to genetic disease
- about 10% of fertilized human eggs result in an embryo with an abnormality in chromosome numbers
-50% of all spontaneous abortions are due to this abnormality
remaining 50 % will survive and develop despite the abnormalities usually in sex chromosomes
75
Abnormal chromosome inheritance examples:
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Trisomy
21
Down syndrome
Mental impairment
Abnormal pattern of palm creases, slanted eyes, flattened face, short stature
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Trisomy
18
Edward syndrome
Mental and Physical impairment
Facial abnormalities
Extreme mucle tone
Early death
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Trisomy
13
Mental and physical impairment
Early death
Wide variety of organ defects
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