2.6. Cell Division, Cell Diversity and Cellular Organisation Flashcards
(42 cards)
1
Q
What is the cell cycle and what are the names of its stages?
For cells that undergo mitosis
A
- A cycle of around 24 hours that all dividing cells experience
- It results in cell division
Its stages are:
* Interphase - split into G₁, S and G₂
* Mitotic phase - split into mitosis and cytokinesis
The G in G₁ and G₂ stands for growth
2
Q
What happens in the interphase of the cell cycle?
A
- The cell carries out its normal function in the interphase, which is the longest stage of the cell cycle
- In G₁, the cell grows, proteins are synthesised and organelles like mitochondria proliferate
- In S, DNA replication is carried out
- In G₂, the cell grows, energy stores are increased and the newly replicated DNA is checked for errors
3
Q
What is the name given to the phase cells are in when they exit the cell cycle and what are the causes of such an exit?
A
- G₀
- Cells can enter G₀ temporarily or permanently
- Specialised cells are usually unable to divide as they lack certain organelles, for example erythrocytes lack nuclei, though there are some exceptions
- DNA damage may be suffered or there may be errors in DNA replication; the cell can detect these and voluntarily exit the cell cycle
- Cells can become senescent and lose the machinery required to divide, causing them to permanently enter G₀
Some cells in G₀, like lymphocytes, can re-enter the cell cycle when they are required to proliferate
4
Q
How is the cell cycle regulated?
A
- Through checkpoints that ensure the fidelity of the two cells produced by mitosis
- There is a checkpoint at the end of G₁ to ensure that the cell is ready for DNA replication; the cell checks cell size, DNA integrity and nutrients
- Another checkpoint at the end of G₂ ensures the cell is ready for cell division; it mostly involves checking for errors in DNA replication
- Finally, there is a checkpoint in the metaphase of mitosis called the spindle assembly checkpoint, which checks whether chromosomes are aligned properly and attached to spindles before the anaphase
- If the checks are not passed, the cell cycle cannot proceed; in the case of the G₁ and G₂ checkpoints, G₀ may be entered
5
Q
What does mitosis produce?
A
Two genetically-identical diploid cells
- Mitosis technically only refers to nuclear division; cytokinesis is then required to split the cell in two
- As it is nuclear division, mitosis only takes place in eukaryotic cells (bacteria divide by binary fission)
6
Q
Why is mitosis significant for life?
A
- It is necessary for the growth and repair of tissues
- It is necessary for asexual reproduction, reproduction that produces genetically identical offspring from one parent, in animals, fungi and some protists
7
Q
What are the names of the four stages of mitosis?
A
- Prophase
- Metaphase
- Anaphase
- Telophase
8
Q
What is the structure of replicated DNA?
A
- Each chromosome (single DNA molecule) duplicates in DNA replication
- This results in an X-shaped molecule, which is still referred to as a single chromosome
- It is composed of two identical sister chromatids, which are connected at their region of intersection called the centromere
- This is the state chromosomes are in at the beginning of mitosis
9
Q
What happens in the prophase of mitosis?
A
- The chromatin fibres that make up chromosomes coil and condense, forming visible X-shaped chromosomes able to take up stain
- The nuclear envelope disassembles and the nucleolus disappears
- The two centrioles replicate to form two centrosomes, which migrate to either pole of the cell
- Microtubules form spindles, which prepare to attach to chromosomes
In meristem cells, tubulin threads are used instead of centrioles
10
Q
What happens in the metaphase of mitosis?
A
- Spindle fibres attach to the centromeres of chromosomes
- They are aligned, one after the other, in a plane along the centre of the cell called the metaphase plate and held in position
11
Q
What happens in the anaphase of mitosis?
A
- The centromeres holding sister chromatids together divide
- One sister chromatid from each chromosome is pulled to either pole of the cell by spindle fibres, dragged by its centromere through the cytosol
- At this stage, each side of the cell will have identical genetic material as sister chromatids are idential to one anothother
12
Q
What happens in the telophase of mitosis?
A
- The chromatids at either pole are now called chromosomes
- They assemble and two nuclear envelopes form around each group
- The chromosomes uncoil to form chromatin and reform the nucleolus
- Cytokinesis begins
13
Q
What is the process of cytokinesis in animal cells?
A
- A cleavage furrow forms in the middle of the cell
- The cytoskeleton pulls the cell surface membrane on either side of the cleavage furrow along it until it fuses to form a membrane separating the two halves of the cell
- The cell then splits into two along the newly formed central membrane
14
Q
What is the process of cytokinesis in plant cells?
A
- The cell wall prevents the formation of a cleavage furrow
- Instead, vesicles from the golgi apparatus assemble along where the metaphase plate was
- The vesicles fuse with one another and the cell surface membrane to form the cell plate, which pinches the cell in two
- New cell walls form around the outside of the new sections of membrane
15
Q
What does meiosis produce?
A
- Four genetically-varied haploid cells
- It is therefore known as reduction division
16
Q
Why is meiosis important for life?
A
- It is necessary for sexual reproduction and intraspecific diversity
- This is because it produces genetically varied cells
Certain cells are predetermined to undergo meiosis instead of mitosis at the end of the cell cycle
17
Q
What are homologous chromosomes?
A
- Pairs of chromosomes containing genes that influence the same traits and have the same loci on the chromosomes
- One chromosome in a homologous pair will come from the father and the other from the mother
- Homogolous chromosomes are identical in size and shape; the only difference comes from genes for which each homologous chromosome in a pair has a different allele
There are 23 homologous pairs in human diploid cells
18
Q
What is each stage of meiosis called?
A
Meiosis I:
* Prophase I
* Metaphase I
* Anaphase I
* Telophase I
Meiosis II:
* Prophase 2
* Metaphase 2
* Anaphase 2
* Telophase 2
19
Q
What happens during prophase I of meiosis?
A
- Homologous X-shaped chromosomes coil, condense and pair up, forming bivalents (also known as tetrads)
- As the homologous chromosomes are brought together, non-sister chromatids entangle, sharing sections of DNA with one another at points called chiasmata
- This process is known as crossing over or recombination, and results in each homologous pair consisting of genetically different chromosomes with genetically different chromatids
- All the other mitotic prophase processes occur in preparation for metaphase I
Crossing over, which only occurs during prophase I, can involve the swapping of entire alleles or parts of alleles
20
Q
What happens during metaphase I of meiosis?
A
- Bivalents are aligned along the metaphase plate
- The pole each homologous chromosome within each bivalent faces towards is random and independent of other bivalents, although the chromosomes in each bivalent always face in opposite directions
- The orientation of a chromosome determines which side of the cell it ends up on after anaphase I
- This is significant as the chromosomes within each homologous pair are genetically different, so this process, which is called independent assortment, results in genetic variation
21
Q
What happens during anaphase I of meiosis?
A
- Spindle fibres pull the chromosomes within each bivalent by their centromeres to opposite poles, with sister chromatids staying together
- In human cells, this results in two groups of 23 chromosomes on either end of the cell
22
Q
What happens during telophase I of meiosis?
A
- It is the same process as telophase in mitosis
- Cytokinesis occurs after, producing two cells with the haploid number of X-shaped chromosomes
23
Q
How does meiosis II differ from mitosis?
Meiosis II takes place directly after meiosis I in both cells produced by meiosis I
A
- Prophase 2 is the same as mitotic prophase, except there are half as many X-shaped chromosomes
- The process of metaphase 2 is similar to mitotic metaphse, but in metaphase 2, more variation is introduced from random orientations because the sister chromatids are non-idential after crossing over in prophase I
- Anaphase 2 is the same as mitotic anaphase
- Telophase 2 and cytokinesis are the same as in mitosis, though four cells are produced as meiosis II occurs in two cells (and these cells are haploid)
24
Q
What are the sources of genetic variation?
A
- Independent assortment (in humans cells, there are 2²³ combinations of orientations in metaphase I)
- Crossing over
- Random fertilisation after genetic variation produced by independent assortment and crossing over
- Mutations
- Epigenetics
25
What are the adaptations of erythrocytes?
* Flattened biconcave shape to maximise surface area to volume ratio and confer flexibility
* Haemoglobin to carry oxygen and carbon dioxide
* No nucleus to carry more haemoglobin
26
What are the adaptations of neutrophils?
* Multi-lobed nucleus providing manoeuverability
* Granular cytoplasm containing lysosomes
27
What are the adaptations of sperm cells?
* Flagella with many mitochondria for motility
* Acrosome on head with digestive enzymes to burrow through the wall of the ovum
* Haploid nucleus to ensure the diploid number is restored upon fertilisation
28
What are the adaptations of palisade cells?
* Rectangular shaped to allow close packing and maximise light absorption
* Thin cell walls to increase rate of diffusion
* Large vacuole to maintain turgor pressure
* Allow for the intracellular movement of chloroplasts to maximise light absorption
29
What are the adaptations of root hair cells?
Long, cytoplasmic extensions called root hairs to maximise surface area for water and mineral uptake
30
What are the adaptations of guard cells?
* Have a thicker cell wall on one side to ensure it does not change shape symmetrically when gaining or losing water
* This allows each guard cell on either side of a stoma to contribute to closing it when flaccid and opening it when turgid
31
What are the adaptations of squamous epithelium?
* Very thin tissue made of squamous epithelial cells that lines areas like the lungs
* It is only one cell thick to facilitate rapid diffusion
32
What are the adaptations of ciliated epithelium?
* Epithelial tissue that lines areas of the body like the trachea
* Ciliated epithelial cells have many cilia that move rhythmically to waft substances through vessels
* In tracheal ciliated epithelium, goblet cells will be present to produce mucus
33
What are the adaptations of cartilage?
* It consists of chondrocyte cells with fibres of elastin and collagen
* This makes it flexible and elastic
* It functions as a connective tissue in the ears and nose and prevents the ends of bones from rubbing together
34
What are the adaptations of muscle tissue?
* Skeletal muscle must be able to shorten
* It therefore contains fibres called myofibrils with many contractile proteins
* These myofibrils are joined by connective tissue
35
What are the adaptations of plant epidermis?
* It is a single layer of closely packed cells covering the surface of plants
* The upper epidermis in leaves is covered by a waxy cuticle to prevent water loss
* The lower epidermis has stomata and guard cells to allow for efficient gas exchange and regulate water loss
36
What is the process of differentiation?
* Stem cells, which are generally spherical, uniform bodies, proliferate by mitosis or meiosis
* Some stem cells change gene expression through upregulation and down regulation to become specialised, while others keep dividing
* This results in specialised cells with physiological adaptations, most of which are in the G₀ phase of the cell cycle
37
What are the different levels of stem cell potency?
* **Totipotent** - able to differentiate into any cell, including cells found in embryonic or extra-embyronic tissue
* The only totipotent stem cells in animals are the zygote and the first 16 cells after mitotic division begins
* **Pluripotent** - able to differentiate into any cell type found in adult tissue, but unable to form whloe organisms
* **Multipotent** - able to differentiate into a few different types of cells
* **Unipotent** - able to differentiate into one type of cell
38
Which type of stem cell produces blood cells like erthyrocytes and neutrophils?
* Haematopoetic stem cells, which constantly divide and differentiate to replace dead or lost blood cells
* They are an example of multipotent stem cells
39
What are the sources of stem cells in animals?
* Totipotent embyronic stem cells are present for a few divisions after fertilisation; after this, all stem cells in the fetus are pluripotent until birth
* Therefore, stem cells can be extracted from embryos
* Stem cells can alos be harvested from the umbilical chord
* In adult animals, stem cells are found in various places in the body including the bone marrow and muscle tissue
40
Where are stem cells found in plants?
* In meristematic tissue
* Apical meristem is found at the tips of roots and shoots
* Vascular cambium, another type of meristem, is found in between xylem and phloem and differentiates to form both xylem vessels and sieve tube elements
41
How can stem cells be used in medicine?
* To repair damaged tissue, such as replacing the damaged insulin-producing tissue in type 1 diabetic patients
* To treat neurodegenerative diseases like Alzheimer's by replacing damaged nervous tissue
* To provide a vital window into how multicellular organisms develop into adults from embyros (developmental biology)
42
What are the ethical issues associated with stem cell treatments?
* Embryos created for the purpose of stem cell treatments currently must be killed before harvesting can take place, which many view as murder
* There is no consensus on who owns the embyros and whether it has rights
* Harvesting from umbilical chords or adult cells solves some of these problems, but these stem cells are not pluripotent
* To overcome anti-innovation and anti-science religious/moral views, a method is being developed that induces pluripotency in adult stem cells through genetic modification
## Footnote
Methods of harvesting without killing the embyro are in development
