2.1.6 Cell division, cell diversity and cellular organisation

Question 1

Identify the functions of cell division by mitosis

Answer 1

1. mitosis (nuclear duplication and separation) and the resulting cell division, or cytokinesis (separation of organelles and cytoplasm) has a number of functions:
2. They produce genetically identical cells required for the growth of the multicellular organism
3. As old and damaged cells undergo cell death, cell division by mitosis allows repair and maintenance of tissues (identical cells with identical functions)
4. In asexually reproducing organisms, mitosis is also a means by which offspring genetically identical to the parent organism can be produced

Question 2

Summarise the key stages of the cell cycle

Answer 2

1. The cycle cycle has two main phases, interphase and mitotic phase
2. Interphase consists of G₁ (first growth phase), S phase and G₂ (second growth phase)
3. Mitotic phase consists of mitosis (nuclear division), and then cytokinesis (cytoplasmic division)
4. Once cytokinesis has occurred, cells enter interphase, G1 again
5. Cells that have exited the cell cycle are in G₀

Question 3

Describe the events taking place during G1 of interphase

Answer 3

1. Proteins and lipids required for the formation of organelles are synthesised (gene expression, the Endoplasmic reticula and the Golgi apparatus are highly involved in this)
2. Mitochondria and chloroplasts undergo fission (much like prokaryotic cell division) and increase in number
3. The cell size increases as its contents increase
4. There is still only one copy of all the original chromosomes

Question 4

Describe the events of S phase of interphase

Question 5

Describe the events of G₂ of interphase

Question 6

State what G₀ is and describe its purpose

Question 7

Describe how checkpoints control the progression of the cell cycle

Question 8

Describe the key events occurring in the cell before mitosis begins

Question 9

Name the stages of mitosis

Question 10

Detail the key events that occur during prophase

Answer 10

1. DNA condenses to form chromosomes (these are visible in a microscope when stained)
2. The nucleolus disappears (as synthesis of ribosomes stops)
3. The nuclear membrane breaks down (so that spindle fibres can attach to chromosomes)
4. Centrioles move to opposite poles of the cell
5. Spindle fibres emanating from centrioles attach to the centromeres of chromosomes

Question 11

Detail the key events that occur during metaphase

Answer 11

1. Spindle fibres from opposite poles are attached to each chromosome
2. Spindle fibres from opposite poles of the cell begin to shorten, pulling on the chromosomes
3. This results in chromosomes aligning across the cell equator
4. This is called the metaphase plate

Question 12

Detail the events that occur during anaphase

Answer 12

1. The pulling of the spindle fibres at the centromeres continues until the sister chromatids are separated from each other
2. The sister chromatids are pulled towards each of the spindle poles

Question 13

Detail the events that occur during telophase

Answer 13

1. Chromatids reach the poles of the cells and are now called chromosomes (again)
2. The nuclear envelope reforms
3. Chromosomes decondense
4. Nucleolus reforms

Question 14

Describe cytokinesis in animal and plant cells

Answer 14

1. In animal cells, the actin cytoskeleton (microfilaments) causes the membrane at the cell equator to pull inwards
2. A cleavage furrow is formed
3. When the cleavage furrow around the cell meets in the middle the cytoplasms of the two cells are separated
4. In plant cells, golgi vesicles fuse at the metaphase plate
5. Forming the new cell membrane boundaries of the daughter cells
6. Cellulose is released to form new cell wall

Question 15

State the key features of meiosis as well as its role in organisms

Answer 15

1. Meiosis produces haploid cells (one of each type of chromosome) from diploid cells (two of each type of chromosome)
2. The haploid cells produced are called gametes
3. Gametes from male and female organisms fuse to produce the zygote during fertilisation
4. Meiosis introduces genetic variation in the gametes through independent assortment and crossing over

Question 16

List the key events that occur during the two stages of meiosis

Answer 16

1. Each of the chromosomes in a diploid cell is duplicated by the process of DNA replication
2. The duplicated DNA joins at centromeres to form chromosomes with identical sister chromatids
3. In meiosis I, homologous pairs of chromosomes are separated into two daughter cells
4. In meiosis II, the identical sister chromatids are separated into two daughter cells
5. This results the production of cells that have only one of each chromosome

Question 17

Describe the events of prophase 1

Answer 17

1. Chromosomes condense
2. Nuclear envelope breaks down
3. Nucleolus disappears
4. Spindle forms
5. Duplicated homologous chromosomes form bivalents
6. Crossing over (formation of chiasmata) can occur at this stage

Question 18

Describe the events of Metaphase 1

Answer 18

1. Homologous pairs of chromosomes (bivalents) align on the cell equator due to spindle fibres
2. The arrangement of maternal and paternal chromosomes occurs by chance and results in independent assortment

Question 19

Describe the events of Anaphase 1

Answer 19

1. Spindle fibres shorten
2. Non-sister homologous chromosomes are separated
3. pulled to opposite poles of the cell

Question 20

Describe the events of Telophase 1

Answer 20

1. Chromosomes reach the poles of the cell
2. Chromosomes decondense
3. The nuclear membrane reforms

Question 21

Describe the events of cytokinesis

Answer 21

1. Actin cytoskeleton is involved in pulling the cell membrane inwards at the equator
2. The cleavage furrows become narrower until two daughter cells are formed

Question 22

Describe the events of Prophase 2

Answer 22

1. Chromosomes condense
2. Each chromosome consists of two sister chromatids
3. Spindle forms
4. Nuclear membrane breaks down

Question 23

Describe the events of Metaphase 2

Answer 23

1. Individual chromosomes (sister chromatids)
2. align on the equator due to the action of attached spindle fibres
3. centromes are attached to spindle fibres

Question 24

Describe the events of Anaphase 2

Answer 24

1. Sister chromatids are pulled apart by the spindle fibres
2. sister chromatids move towards opposite poles of the cell

Question 25

Describe the events of Telophase 2

Answer 25

1. Chromatids arrive at the poles of the cell
2. Begin to decondense
3. Nuclear membrane reforms
4. Nucleolus reappears

Question 26

Explain how independent assortment increases genetic variation in gametes

Answer 26

1. During metaphase 1 of meiosis bivalents (duplicated homologous chromosome pairs) align on the cell equator independently of other chromosomal pairs
2. The allocation of maternal and paternal chromosomes is affected by this arrangement
3. In different cells undergoing meiosis, the arrangement of the bivalents can be different
4. Resulting in a number of different types of gametes possible, each with a different combination of maternal and paternal chromosomes
5. (even though one cell undergoing meiosis could produce only four genetically different gametes, remember that other combinations are possible in other cells also undergoing meiosis)
6. The number of genetically different gametes possible is related to the number of chromosome pairs (n): 2n

Question 27

Explain how crossing over increases genetic variation in gametes

Answer 27

1. During prophase 1, the chromosomes of non-sister homologous chromosomes (same genes, but possible different alleles) can intertwine and swap over sections
2. This causes sister chromatids to no longer be identical and generates new allele combinations within chromosomes
3. Which produce gametes with new allele combinations when chromosomes are separated during anaphase 2

Question 28

Describe the role of specialised cells in multicellular organisms

Answer 28

1. Many organisms are multicellular
2. But the cells of these organisms are not identical in structure and function
3. Groups of cells differentiate (expressing different genes) to become specialised
4. So that they can perform specific functions better
5. These groups of specialised cells form tissues that perform a specific function
6. And tissues may work together form an organ with a particular function
7. A number of organs may work together for a collective purpose in an organ system
8. Such adaptations all work to give organisms a better chance of survival and reproduction

Question 29

Describe the specialisations of the following cell types in animals: erythrocytes (red blood cells)

Answer 29

1. Flattened, biconcave shape increases surface area to volume ratio to exchange gases by diffusion
2. No nucleus, ER or mitochondria: maximises space available to store haemoglobin
3. Contain haemoglobin which is used to transport oxygen, and to some extent, carbon dioxide
4. Cells are flexible, which allows them to squeeze through narrow capillaries, minimising distances for the diffusion of gases (maximising rate of diffusion)

Question 30

Describe the specialisations of the following cell types in animals: neutrophils (phagocyte)

Answer 30

1. Multilobed nucleus to help cells migrate through small gaps to sites of infection/damage
2. Cytoplasm contains many lysosomes, each filled with hydrolytic enzymes which can break down pathogens

Question 31

Describe the specialisations of the following cell types in animals: sperm cells

Answer 31

1. Flagellum allows sperm to move towards an egg to fertilise it
2. Numerous mitochondria are contained in the mid section to provide energy for flagellar movement
3. The acrosome in the sperm head is a modified lysosome, containing hydrolytic enzymes needed to penetrate the substances surrounding the egg
4. There is very little cytoplasm contained in the sperm, which helps sperms cells to encounter less resistance/be more streamlined

Question 32

Describe the specialisations of the following cell types in plants: Palisade cells

Answer 32

1. Numerous chloroplasts to maximise absorbance of light for photosynthesis
2. Cells have a regular box-like shape, minimising gaps and maximising the number of cells in the tissue (making light absorbance more efficient)
3. Thin cell walls to maximise the rate of diffusion into/out of cells
4. Large vacuole allows turgor pressure to maintain the cell shape
5. Chloroplasts can be repositioned in the cytoplasm to maximise light absorbance

Question 33

Describe the specialisations of the following cell types in plants: Root hair cells

Answer 33

1. Have long thin extensions called root hairs, which increase the surface area of the cell (increasing the rate of osmosis and diffusion)

Question 34

Describe the specialisations of the following cell types in plants: Guard cells (forming stomata)

Answer 34

1. Their shape allows the formation of a pore through which carbon dioxide can enter the leaf for photosynthesis
2. Water loss causes guard cells to change shape, closing the stomatal pore, reducing further water loss
3. This is possible because the cell wall is thicker on the inner side of the pores than the outside

Question 35

Describe the specialisations of the following animal tissues: squamous epithelium (alveoli)

Answer 35

1. Cells form a continuous layer to be effective barriers
2. Cells have flat shape to reduce diffusion distance
3. Layers are one cell thick to minimise diffusion distance
4. For example the alveoli of the lungs

Question 36

Describe the specialisations of the following animal tissues: ciliated epithelium (lung airways)

Answer 36

1. Cells form a continuous layer to form effective barriers
2. Some cells are further specialised to produce mucus (goblet cells), which traps inhaled pathogens
3. Most cells are ciliated: these hair-like structures move mucus out of the airways
4. For example in the trachea and bronchioles

Question 37

Describe the specialisations of the following animal tissues: cartilage (e.g. trachea/bronchioles)

Answer 37

1. Connective tissue
2. Chondrocyte cells embedded in lots of extracellular matrix
3. Extracellular matrix is made of fibrous proteins such collagen and elastin
4. Cartilage is firm and flexible
5. For example outer ear, nose and at the end of bones

Question 38

Describe the specialisations of the following animal tissues: muscle (e.g. smooth muscle of airways)

Answer 38

1. Muscle cells (and so muscle tissue) is contractile
2. The cytoskeleton of muscle cells is specialised to form a contractile structure made of the proteins actin and myosin called a myofibril
3. Connective tissue exists between muscle cells

Question 39

Describe the specialisations of the following plant tissues: Epidermis

Answer 39

1. A layer of cells one cell thick (to reduce diffusion distances) covering the surface of plants
2. Has a waxy cuticle to reduce water loss
3. Contains stomata to allow and control movement of water and gases contained in air (carbon dioxide and oxygen)

Question 40

Describe the specialisations of the following plant tissues: Xylem

Answer 40

1. Functions to transport water and mineral ions and support the plant
2. Composed of elongated, dead cells joined end-to-end to form tubes (to allow movement of water)
3. The walls of the cells are strengthened with waterproof lignin (support function)
4. The walls of cells have pits, that allow water to move into and out of xylem vessels

Question 41

Describe the specialisations of the following plant tissues: Phloem

Answer 41

1. Functions to transport organic molecules (for example sucrose)
2. Usually from leaves towards stem and roots
3. Living cells, joined end-to-end
4. Sieve tubes have no organelles which aids movement of substances
5. Perforated sieve plates allow fluid to move from one cell to the next
6. Companion cells carry out functions for the sieve tube elements
7. Companion cells have many mitochondria which provide energy for active transport of substances

Question 42

Summarise the role of stem cells in organisms

Answer 42

1. Stem cells are unspecialised, undifferentiated cells which have unlimited proliferative potential (they can divide many times, have not exited the cell cycle into G0)
2. They can divide by mitosis, and undergo the process of differentiation
3. To become specialised cells, with particular structures and function
4. But once specialised, they cannot divide again, or revert back to being stem cells, they have exited the cell cycle into G0
5. Stem cells can divide to produce more stem cells
6. They are a way to generate, regenerate or repair tissues within the organism
7. Lack of stem cells is associated with ageing
8. Overproduction of stem cells is associated with cancer

Question 43

Discuss (with examples) the different types of stem cell potency

Question 44

Use blood cells to exemplify the role of stem cells in animals

Answer 44

1. There are various types of specialised (differentiated) cells in the blood.
2. These include red blood cells (oxygen/carbon dioxide transport), neutrophils and macrophages (phagocytes, inflammation) and lymphocytes (immune response)
3. They have a limited and varied functioning lifespan in the body, for example red blood cells last for about 120 days, neutrophils for 6 hours and lymphocytes up to a few years
4. Haematopoietic stem cells in the bone marrow undergo division and differentiation to regenerate these different types of cells to maintain their number (and function) in the body
5. Stem cell populations must also be maintained so that this process can continue for the lifespan of the organism

Question 45

Use vascular tissue (xylem and phloem) to exemplify the role of stem cells in plants

Answer 45

1. Xylem and phloem have similar but distinct functions
2. Xylem transports water and mineral ions, phloem transports organic molecules (amino acids, sucrose)
3. They have different structures in order to perform these different functions
4. Between the two tissues lies the cambium, which is made up of meristem tissue (stem cells)
5. These stem cells can divide and differentiate into the cells that form xylem and those that form phloem
6. As the plant grows, new vascular tissue must be produced, for example in the shoots and root tips.

Question 46

State the sources of human stem cells

Answer 46

1. Embryonic stem cells
2. That are totipotent can be acquired from embryos to the 16 cell stage
3. After that point, cells from the embryo are pluripotent
4. These can be acquired from embryos/fetus after the 16 cell stage until birth
5. Adult stem cells (tissue stem cells)
6. Are multipotent
7. And can be acquired from particular parts of the body (for example hematopoietic stem cells from bone marrow. Stems cells can also be acquired from skin and blood vessels)

Question 47

Suggest how stem cells can be used to repair damaged or degenerated tissue

Answer 47

1. Stem cells can be isolated
2. They can be placed into the affected tissue
3. Stem cells are exposed to chemical signals from neighbouring cells
4. Which causes them to differentiate into the affected cell type
5. The presence of new cells may restore the function of the affected area/tissue

Question 48

Give examples of the use of stem cells in medicine

Answer 48

1. Repair damaged heart tissue (cardiac muscle cells)
2. Treat type I diabetes, regenerate insulin-producing cells in the pancreas, removing the need for insulin injections
3. Treat Parkinson’s disease, regenerate dopamine-producing cells in the brain
4. Treat Alzheimer’s disease, regenerate brain cells destroyed by build-up of abnormal proteins
5. Treating macular degeneration: loss of vision is caused by the degeneration of cells in the retina.
6. Reversing birth defects
7. Restoring spinal cord function after injury
8. Source of plant-based medicines: plant stem cells can be cultured and used as an unlimited source of possible plant-based medicinal substances

Question 49

Give examples of the use of stem cells in medical research

Answer 49

1. Stems cells are an unlimited source (due to their proliferative potential) of human cells.
2. These cells can be cultured in vitro
3. And used for experimentation
4. for example, to test the effect of drugs, before clinical trials
5. Or model human tissues and organs reducing the need for animal experimentation