Topic 1: Cell Biology

Question 1

What are the 3 principles of cell theory?

Answer 1

1. All living things are composed of cells (or cell products)
2. The cell is the smallest unit of life
3. Cells only arise from pre-existing cells

Question 2

What are the 3 exceptions to the cell theory?

Answer 2

Striated muscle
- challenges the idea that a cell has one nucleus
- muscle cells have more than one nucleus per cell
- muscle cells called fibers can be very long (300 mm)
- they are surrounded by a single plasma membrane but they are multi-nucleated (many nuclei)
- this does not conform to the standard view of small single nuclei within a cell

Asepated fungal hyphae
- challenges the idea that a cell is a single unit
- fungal hyphae are again very large with many nuclei and a continuous cytoplasm
- the tubular system of hyphae forms dense networks called mycelium
- like muscle cells they are multi-nucleotide
- they have cell walls composed of chitin
- the cytoplasm is continuous along the hyphae with no end cell wall or membrane

Gianted algae
- single-celled organisms that challenges both the idea that cells must be simple in structure and small in size
- gigantic in size (5-100 mm)
- complex in form, it consists of three anatomical parts: bottom rhizoid (resembles a set of short roots), long stalk, top umbrella of branches that may fuse into a cap
- the single nucleus is located in the rhizoid

Question 3

What are the functions of life that cells carry out?

Answer 3

Metabolism: the web of all the enzyme-catalyzed reactions in a cell or organism e.g. respiration

Response: living things can respond to and interact with the environment

Homeostasis: the maintenance and regulation of internal cell conditions, e.g. water and pH

Growth: living things can grow or change size/shape

Excretion: the removal of metabolic waste

Reproduction: living things produce offspring

Nutrition: feeding by either the synthesis of organic molecules (e.g. photosynthesis) or the absorption of organic matter

Question 4

How are stem cells used to treat Stragardt’s disease and leukaemia?

Answer 4

Stragardt’s disease affects around 1 in 10,000 children:
THE PROBLEM
- recessive genetic (inherited) condition
- the mutation causes an active transport protein on photoreceptor cells to malfunction
- the photoreceptor cells degenerate
- the production of a dysfunctional protein that can’t perform energy transport
- that causes progressive and eventually total, loss of central vision
THE TREATMENT
- embryonic stem cells are treated to divide and differentiate to become retinal cells
- the retinal cells are injected into the retina
- the retinal cells attach to the retina and become functional
- central vision improves as a result of more functional retinal cells
THE FUTURE
- this treatment is still in the stage of limited clinical trials, but will likely be in usage in the future

LEUKEMIA
THE PROBLEM
- cancer of the blood or bone marrow, resulting in abnormally high levels of poorly functioning white blood cells
THE TREATMENT
- hematopoietic stem cells (HSCs) are harvested from bone marrow, peripheral blood or umbilical cord blood
- Chemotherapy and radiotherapy used to destroy the diseased white blood cells
- new white blood cells need to be replaced with healthy cells
- HSCs are transplanted back into the bone marrow
- HSCs differentiate to form new healthy white blood cells
THE BENEFIT
- the use of a patient’s own HSCs means there is far less risk of immune rejection than with a traditional bone marrow transplant

Question 5

Calculate the magnification of a specimen when given a scale bar.

Answer 5

scale bar = um, so convert ruler to um
1mm = 1,000um so 20mm = 20,000 um

scale bar measurement 20,000 um
———————————- = —————–
scale bar label 10 um

magnification = 2,000 times

Question 6

Compare the functions of life in Paramecium and Chlorella

Answer 6

PARAMECIUM
M (Metabolism): Metabolic reactions such as respiration and digestion are constantly taking place in the cytoplasm.

R (Reproduction): Generally asexual. After nuclear division (mitosis) occurs the two nuclei formed are separated by the construction of the cytoplasm.

H (Homeostasis): The contractile vacuole fills up with water and then expels the water through the plasma membrane to maintain a constant osmotic potential.

G (Growth): After obtaining nutrition and assimilating the nutrients, the organisms increase in size until it divides.

R (Response): The beating of the cilia moves the Paramecium through the water in response to changes in the environment.

E (Excretion): Waste products (e.g. carbon dioxide) are expelled or diffused out through the plasma membrane.

N (Nutrition): Food particles that are swept into the oral groove are packaged into food vacuoles. After the enzymes, contained within the vacuoles, digest the particles the nutrients are absorbed into the cytoplasm.

CHLORELLA
M (Metabolism): Metabolic reactions such as respiration and digestion are constantly taking place in the cytoplasm.

R (Reproduction): Nuclear division (mitosis) produces autospores that are released when the parent cell wall breaks down

H (Homeostasis): Extra glucose is stored as starch, in pyrenoids, to maintain the osmotic potential of the cell

G (Growth): After obtaining nutrition and assimilating the nutrients, the organisms increase in size until it divides.

R (Response): Chlorophyll pigments located in the chloroplast absorb light

E (Excretion): Metabolic waste products (e.g. oxygen) diffuse out of the cell through the plasma membrane

N (Nutrition): Synthesis of carbohydrates through photosynthesis.

Question 7

Why is the surface area to volume ratio important in determining the size of cells and organisms?

Answer 7

For cells to survive, metabolic reactions must be occurring, these reactions depend on:
- Materials constantly being exchanged across the plasma membrane
- The volume or mass of cytoplasm (as this is where the reactions take place)

As organisms increase in size their SA: V ratio decreases
- There is less surface area for the absorption of nutrients and gases and secretion of waste products
- The greater volume results in a longer diffusion distance to the cells and tissue of the organism

• Thus the rate at which substances (e.g. oxygen and heat) are exchanged across the plasma membrane is dependent on the surface area (the larger the surface area the more substances are exchanged)
• The rate at which a cell metabolizes is dependent on the mass or volume of the cytoplasm (the larger the mass or volume the longer it takes for metabolic reactions to occur)
• Single-celled organisms have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion
• The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products
• The small volume means the diffusion distance to all organelles is short

Question 8

What is cell differentiation?

Answer 8

Cell differentiation is when an unspecialized cell begins to perform a specific function. By becoming specialized, the cells in a tissue can carry out their role more efficiently than if they had many different roles. E.g., red blood cells carry oxygen, and a rod cell in the retina of the eye is able to absorb light and transmit impulses to the brain.

In complex multicellular organisms, eukaryotic cells become specialized for specific functions. This can also be referred to as the division of labor. Specialization enables the cells in a tissue to function more efficiently as they develop specific adaptations for that role. The development of these distinct specialized cells occurs through differentiation. These specialized eukaryotic cells have specific adaptations to help them carry out their functions. For example, the structure of a cell is adapted to help it carry out its function (this is why specialized eukaryotic cells can look extremely different from each other)
Structural adaptations include:
- The shape of the cell
- The organelles the cell contains (or doesn’t contain)
For example:
- Cells that make large amounts of proteins will be adapted for this function by containing many ribosomes (the organelle responsible for protein production)

Question 9

What are stem cells? How are embryonic stem cells different from adult stem cells?

Answer 9

A stem cell is a cell that can divide (by mitosis) an unlimited number of times. Each new cell (produced when a stem cell divides) has the potential to remain in a stem cell or to develop into a specialized cell such as a blood cell or a muscle cell (by a process known as differentiation). This ability of stem cells to differentiate into more specialized cell types is known as potency.

Embryonic stem cells are totipotent (if taken in the first 3-4 days after fertilization) and pluripotent (if taken on day 5) hence giving patients a higher chance of living a healthy life and adult stem cells are multipotent. Embryonic stem cells have the ability to differentiate into multiple cell types, stem cells have huge potential in the therapeutic treatment of disease. Embryonic stem cells also have less of a chance of genetic damage, due to an accumulation of mutations. However, have a higher chance of tumors, and involve the creation and destruction of embryos.

Adult stem cells however are less controversial because the donor is able to give permission (like bone marrow). There is also a lower chance of rejection since the patient is using their own stem cells and there is a lower chance of tumors. But they are difficult and painful to obtain as they are buried deep in the tissue. They also need to be a very close match in blood type and other body antigens to be a donor.

Question 10

Calculate the actual size of a specimen when given the magnification

Answer 10

To calculate the actual size of a magnified specimen, the equation is simply rearranged: Actual Size = Image size (with a ruler) ÷ Magnification. (Remember AIM)

Question 11

How are eukaryotes different from prokaryotes?

Answer 11

Prokaryotic cells differ in a number of key features, including:
DNA (composition and structure)
Organelles (types present and relative sizes)
Reproduction (mode differs according to chromosome structure)
Average size (exceptions may exist)

Prokaryotes:
- DNA is naked
- DNA is circular
- usually no introns
- no nucleus
- no membrane-bound
- 70S ribosomes
- binary fission
- single chromosome (haploid)
- smaller (~1-5 um)

Eukaryotes
- DNA bound to protein
- DNA is linear
- usually has introns
- has a nucleus
- membrane-bound
- 80S ribosomes
- mitosis and meiosis
- chromosomes paired (diploid or more)
- larger (~10-100 um)

Question 12

How are electron microscopes different from light microscopes in terms of understanding the
cell structure?

Answer 12

Electron microscopes have 2 key advantages when compared to light microscopes:
- they have a much higher range of magnification (can detect smaller structures)
- they have much higher resolution (can provide clearer and more detailed images)

Light microscopes are used for specimens above 200 nm. Light microscopes shine light through the specimen, this light is then passed through an objective lens, which magnifies the specimen to give an image that can be seen by the naked eye. The specimen can be living or dead. Light microscopes are useful for looking at whole cells, small plant and animal organisms, and tissues within organs such as leaves or skin.

Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm. They fire a beam of electrons at the specimen either a broad static beam (transmission) or a small beam that moves across the specimen (scanning). Due to the higher frequency of electron waves (a much shorter wavelength) compared to visible light, the magnification and resolution of an electron microscope is much better than a light microscope. Electron microscopy requires the specimen to be dead however this can provide a snapshot in time of what is occurring in a cell eg. DNA can be seen replicating and chromosome positions within the stage of mitosis are visible.

Question 13

How are membranes constructed including the arrangement of the phospholipid bilayer and the fluid mosaic model which explains this? Draw the fluid mosaic model.

Answer 13

1920s Gorter and Grendel
- The Gorter and Grendel model showed that the phospholipids in the membrane of cells were arranged into a bilayer
- Evidence: the number of phospholipids extracted from red blood cell membranes was double the area of the plasma membrane if it was arranged as a monolayer.
- Problem: their model didn’t explain the location of the proteins or how molecules that were insoluble in lipids moved into and out of the cell

1930s Davson and Danielli
- Suggested that the proteins were arranged in layers above and below the phospholipid bilayer
- Evidence: membranes were effective at controlling the movement of substances in and out of cells. Electron micrographs showed the membrane had two dark lines with a lighter band between. In electron micrographs, proteins appear darker than phospholipids.
- Problem: freeze-etched electron micrographs of the center of the membrane showed globular structures scattered throughout. Improvements in technology used to analyze the proteins in the membranes showed that proteins were globular, varied in size, and had parts that were hydrophobic. These problems suggested it was unlikely that the proteins would form continuous layers.

1970s Singer and Nicolson
- Proposed the fluid mosaic model which stated that membranes were fluid and that the globular proteins were both peripheral and integral (with some crossing both membranes) and dispersed throughout the membrane.
- Evidence: analysis of freeze-etched electron micrographs showed proteins extending into the center of membranes. Biochemical analysis of the plasma membrane components.

Membranes are vital structures found in all cells. The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment. Intracellular membranes (internal membranes) form compartments within the cell, such as passing through them; they are partially permeable. Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles, and also within organelles. Substances can cross membranes by diffusion, facilitated diffusion, osmosis, and active transport. Membranes play a role in cell signaling by acting as an interface for communication between cells. The fluid mosaic model of membranes includes 4 main components:
- phospholipids
- cholesterol
- glycoproteins and glycolipids
- transport proteins

Question 14

What are the functions of proteins in cell membranes?

Answer 14

Proteins may be either integral (transmembrane) or peripheral and serve a variety of roles: JETRAT

Junctions- serve to connect and join two cells together
Enzymes- fixing to membranes localizes metabolic pathways
Transport- responsible for facilitated diffusion and active transport
Recognition- may function as a marker for cellular identification
Anchorage- attachment points for cytoskeleton and extracellular matrix
Transduction- functions as receptors for peptide hormones

Question 15

What is the function of cholesterol in cell membranes?

Answer 15

Cholesterol interacts with the fatty acid tails of phospholipids to moderate the properties of the membrane:
- Cholesterol functions to immobilize the outer surface of the membrane reducing fluidity
- It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
- It functions to separate phospholipid tails and so prevent crystallization of the membrane
- It helps secure peripheral proteins by forming high-density lipid rafts capable of anchoring the protein

Question 16

What is the function of phospholipids in cell membranes?

Answer 16

Phospholipids form the basic structure of the membrane (the phospholipid bilayer). They are formed by a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails. As phospholipids have a hydrophobic and hydrophilic part they are known as amphipathic. The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water. The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble in water.

The basic cellular structure acts as a barrier to protect the cell against various environmental insults and more importantly, enables multiple cellular processes to occur in subcellular compartments.

Question 17

How are materials taken into the cell by endocytosis?

Answer 17

Endocytosis involves cells taking in substances from outside the cell by engulfing them in a vesicle derived from the cell membrane. It is the bulk transport in eukaryotes. Endocytosis occurs when a portion of the cell membrane folds in on itself, encircling extracellular fluid and various molecules or microorganisms. The resulting vesicle breaks off and is transported within the cell.

Endocytosis serves many purposes, including:
- Taking in nutrients for cellular growth, function, and repair: Cells need materials like proteins and lipids to function.
- Capturing pathogens or other unknown substances that may endanger the organism:
- When pathogens like bacteria are identified by the immune system, they are engulfed by immune cells to be destroyed.
- Disposing of old or damaged cells: Cells must be safely disposed of when they stop functioning properly to prevent damage to other cells. These cells are eliminated through endocytosis.

There are two types of endocytosis: phagocytosis and pinocytosis:

1. A particle or substance binds to receptors on the cell’s surface, stimulating the release of pseudopodia (extensions of the plasma membrane filled with cytoplasm).
2. Pseudopodia surround the object until their membranes fuse, forming a phagocytic vesicle.
3. The phagocytic vesicle pinches off from the cell membrane, entering the cell.
4. The phagocytic vesicle fuses with lysosomes,
   which recycle or destroy the vesicle’s contents.
5. Molecules bind to receptors located along the surface of the cellular membrane.
6. The plasma membrane folds in, forming a pinocytic vesicle that contains the molecules and the extracellular fluid.
7. The pinocytic vesicle detaches from the cell membrane inside the cell.
8. The vesicle fuses with early endosomes where the contents found within are sorted.

Question 18

How do materials leave the cell by exocytosis?

Answer 18

Exocytosis is where the cells shift materials, such as waste products, from inside the cell to the extracellular space. The materials are engulfed in a vesicle, again derived from the cell membrane. It is the bulk transport in eukaryotes.

Exocytosis serves the following purposes:
- Removing toxins or waste products from the cell’s interior: Cells create waste or toxins that must be removed from the cell to maintain homeostasis. For instance, in aerobic respiration, cells produce the waste products carbon dioxide and water during ATP formation. Carbon dioxide and water are removed from these cells via exocytosis.
- Facilitating cellular communication: Cells create signaling molecules like hormones and neurotransmitters. They are delivered to other cells following their release from the cell through exocytosis.
- Facilitating cellular membrane growth, repair, signaling and migration: When cells absorb materials from outside the cell during endocytosis, they use lipids and proteins from the plasma membrane to create vesicles. When certain exocytotic vesicles fuse with the cellular membrane, they replenish the cell membrane with these materials.

Below is an outline of the basic steps of exocytosis:
1. A vesicle is formed, typically within the endoplasmic reticulum and the Golgi apparatus or early endosomes.
2. The vesicle travels to the cell membrane.
The vesicle fuses to the plasma membrane, during which the two bilayers merge.
3. The vesicle’s contents are released into the extracellular space.
4. The vesicle either fuses with or separates from the cell membrane.

Question 19

What is the purpose of vesicles inside a cell?

Answer 19

Vesicles help transport materials that an organism needs to survive and recycle waste materials. They can also absorb and destroy toxic substances and pathogens to prevent cell damage and infection. Vesicles are involved in metabolism, transport, buoyancy control, and enzyme storage.

Question 20

Why is it important to maintain osmolarity for tissues and organs used in medical procedures?
Use terms such as hypotonic, isotonic, and hypertonic.

Answer 20

Animal cells can lose and gain water as a result of osmosis. If an animals cells is placed in a solution with a lower water potential than the cell, water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up. This is crenation (the cell has become crenated), by which is usually fatal for the cell. Crenation occurs when the cel, is in a hypertonic environment (the solution outside of the cell has a higher solute concentration than the inside of the cell). If an animals cell is placed in pure water or a dilute solution, water will enter the cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential. The cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis), as it hs no cell wall to withstand the increased pressure created. Lysis occurs when the cell is in a hypnotic environment (the solution outside if the cell has the same solute concentration as the inside of the cell). If an animal cell is in an isotonic environment (the solution outside of the cell has the same solute concentration as the inside of the cell). The movement of water molecules in and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells.

Tissue and organs that a re to be used in medical procedures must be kept in solution to prevent damage to the cells. The osmolarity of the solution is key. The osmolarity of a solution measures the number of solute particles (that can form bonds with water) per 1 L of solvent. Osmolarity is expressed as osmoses or milliosmoles per litre of solution (Osm/L or mOsm/L). Human tissue is normally 306 mOsm/L. A solution with the same osmolarity = isotonic. A solution with a higher osmolarity = hypertonic and a solution with a lower osmolarity = hypotonic. Isotonic sodium chloride solutions (normal saline) are generally used as they can be:
- frozen to create a slush used to pack donor organs for transportation
- injected into a patient’s blood system
- used to sterilize wounds
- used as eye drops

Question 21

Review the cell theory and use Pasteur’s evidence from his experiments to explain how cells come from pre-existing cells.

Answer 21

Cell theory states that all living organisms are made of one or more cells, cells are the basic functioning unit in living organisms, and new cells are produced from pre-existing cells. Louis Pasteur’s experiments were designed to verify the principle that cells can only come from pre-existing cells. He used swan neck flasks (S-shaped flasks) which trapped the microorganisms in the bend of the neck. Pasteur added nutrient broth and then boiled them to sterilize them. He broke off some of the necks and noticed that the broth in the flasks with the snapped necks had gone cloudy whereas the broth in the swan neck flasks remained clear. Thus Pasteur had shown that the swan neck prevented microorganisms in the air from entering the broth and that no organisms appeared spontaneously.

Question 22

What is the endosymbiosis theory and how does the origin of eukaryotic cells depend on it? Notable organelles are the mitochondria and chloroplasts.

Answer 22

The endosymbiotic theory - an explanation for the evolution of eukaryotic cells

Endosymbiosis is where one organism lives within another. If the relationship is beneficial to both organisms the engulfed organism is not digested. For endosymbiosis to occur one organism must have engulfed the other by the process of endocytosis.

The endosymbiotic theory is used to explain the origin of eukaryotic cells. The evidence provided for this theory comes from the structure of the mitochondria and chloroplasts. Scientists have suggested that ancestral prokaryote cells evolved into ancestral heterotrophic and autotrophic cells through the following steps:
Heterotrophic cells:
- To overcome a small SA: V ratio ancestral prokaryote cells developed folds in their membrane. From these infoldings organelles such as the nucleus and rough endoplasmic reticulum formed
- A larger anaerobically respiring prokaryote engulfed a smaller aerobically respiring prokaryote (which is not digested)
This gave the larger prokaryote a competitive advantage as it had a ready supply of ATP and gradually the cell evolved into the heterotrophic eukaryotes with mitochondria that are present today

Autotrophic cells:
- At some stage in their evolution, the heterotrophic eukaryotic cell engulfed a smaller photosynthetic prokaryote. This cell provided a competitive advantage as it supplied the heterotropic cell with an alternative source of energy, carbohydrates
- Over time the photosynthetic prokaryote evolved into chloroplasts and the heterotrophic cells into autotrophic eukaryotic cells

The evidence to support the endosymbiotic theory arises from the features that the mitochondria and chloroplasts have in common with prokaryotes:
- Both reproduce by binary fission
- Both contain their own circular, non-membrane-bound DNA
- They both transcribe mRNA from their DNA
- They both have 70S ribosomes to synthesize their own proteins
- They both have double membranes

Question 23

What is the cell cycle? What are the stages in the cell cycle?

Answer 23

The cell cycle is the regulated sequence of events that occur between one cell division and the next. The cell cycle has three phases:
- interphase
- nuclear division (mitosis)
- cell division (cytokinesis)
The movement from one phase to another is triggered by chemical signs called cyclins.

INTERPHASE (3 phases : G1, S, G2)
- Interphase is the longest and most active phase of the cell cycle.
- During interphase, the cell increases in mass and size. It carries out many cellular functions in the nucleus and cytoplasm eg. synthesizing proteins and replicating its DNA ready for mitosis (these only occur during interphase)
- The number of mitochondria and chloroplast increases
-G1 phase stands for growth. The cells make the RNA, enzymes, and other proteins required for growth during the G1 phase.
- Signal during the G1 phase tells the cell to divide again. After the G1 phase of interphase the cell enters the next phase of the cell cycle, the S phase stands for synthesis (of DNA)
- The S phase is relatively short. The DNA in the nucleus replicates, resulting in each chromosome consisting of two identical sister chromatids.
- Between the G2 phase, the cell continues to grow and the new DNA that has been synthesized is checked and any errors are usually repaired. Other preparations for cell division are made (e.g. production of tubulin protein, which is used to make microtubules for the miotic spindle)

The cell cycle is controlled by cyclins (D, E, A & B)
- D: present first, triggers cells to move from G1 to S phase
- E: higher concentration at the start of the S phase, prepares the cell for DNA replication during the S phase
- A: highest concentration in the G2 phase but activates two different kinases that trigger two processes: in the S phase it activates DNA replication, and in the G2 phase, it prepares the cell for mitosis.
- B: highest concentration at the beginning of mitosis, promotes the formation of the mitotic spindle

Question 24

Define mitosis. What are the stages of mitosis and what happens in cytokinesis. Recognize the stages of mitosis using electron micrographs.

Answer 24

Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell). Mitosis is divided into 4 main stages or phases:
- prophase
- metaphase
- anaphase
- telophase

PROPHASE
- chromosomes condense and are now visible when stained
- the chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
- the 2 centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
- spindle fibers (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)
- the nuclear envelope (nuclear membrane) breaks down into small vesicles
- the nucleolus disappears

METAPHASE
- centrosomes reach opposite poles
- spindle fibers (protein microtubules) continue to extend from centrosomes
- chromosomes line up at the equator of the spindle (aka the metaphase plate) so they are equidistant to the two centrosome poles
- spindle fibers (protein microtubules) reach the chromosomes and attach to the centromeres (involves specific proteins called kinetochores)
- Each sister chromatid is attached to a spindle fiber originating from opposite poles

ANAPHASE
- the sister chromatids separate at the centromere (divides in 2)
- spindle fibers (protein microtubules) begin to shorten
- the separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibers (protein microtubules)

TELOPHASE
- chromosomes arrive at opposite poles and begin to decondense
- nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
- the spindle fibers break down
- new nucleoli form within each nucleus

CYTOKINESIS
- follows the nuclear division (mitosis) phase
- once the nucleaus has divided into two genetically identical nuclei, the cell divides in two with one nucleus moving into each cell to create two genetically identical daughter cells.

• in animals cells: a ‘cleavage furrow’ forms and separates the daughter cells
• the cleavage furrow forms when actin and myosin proteins form a contractile ring just under the plasma membrane
• this ring is formed at the equator (centre) of the cell
• as the proteins contract, they pull the plasma membrane towards the centre eventually separating the cell into two daughter cells
• in plant cells: a ‘cell plate; forms at the equator. Once the cell plate reaches the cell walls of the parent cell, new cell walls are produced, separating the new daughter cells
• the cell plate is formed from vesicles carrying carbohydrates, lipids and proteins fusing together to create the plasma membranes
• after this other vesicles, carrying pectin and cellulose, deposit these substances by exocytosis in the gap between the two new membranes leading to the formation of new cell walls.

Question 25

How do chromosomes condense and become visible during mitosis?

Answer 25

DNA molecules are very long molecules (human DNA can be more than 50, 000 um) that need to fit within much smaller nuclei (human nuclei average less than 5 um). Prior to mitosis, the DNA molecules are loosely coiled (around histones in eukaryotic cells) to form a complex called chromatin. During prophase, the chromatin gets condensed by supercoiling to form chromosomes. Condensation occurs by the repeated coiling of the DNA molecule (supercoiling). This supercoiling is aided in eukaryotic cells by the presence of histone proteins and enzymes.

Question 26

What is the mitotic index? What is it used for? How is it calculated?

Answer 26

The miotic index is the proportion of cells (in a group of cells or a sample of tissue) that are undergoing mitosis. The miotic index can be calculated using the formula below:

Miotic index =
number of cells with visible chromosomes
——————————————————————
total number of cells

Miotic index =
(prophase + metaphase + anaphase + telophase)
——————————————————————-
total number of cells

(you can multiply the answer by 100 if you need to give the miotic index as a percentage

Question 27

How is the cell cycle controlled? Use terms: cyclins and cyclin-dependent kinase.

Answer 27

Cell-cycle control depends exclusively on post-transcriptional mechanisms that involve the regulation of cyclin-dependent kinase activity by phosphorylation and the binding of regulatory proteins such as cyclins, which are themselves regulated by proteolysis.

Question 28

What happens when cells divide uncontrollably? Define mutagen, oncogene, primary tumour, secondary tumour, benign vs. malignant tumour, and metastasis.

Answer 28

Cancer is a disease caused when cells divide uncontrollably and spread into surrounding tissues.

mutagen: anything that causes a mutation (a change in the DNA of a cell

oncogene: a mutated gene that has the potential to cause cancer

primary tumor: a term used to describe the original, or first tumor in the body

secondary tumor: the same type of cancer as the original (primary) cancer. For example, cancer cells may spread from the breast (primary) to form new tumors in the lung (secondary)

benign tumors: are those that stay in their primary location without invading other sites of the body
malignant tumor: have cells that grow uncontrollably and spread locally and/or to distant sites. They are cancerous.

metastasis: the spread of cancer cells from the place where they first formed to another part of the body. Cancer cells break from the primary tumor and travel through the blood or lymph system to form a new tumor.

Question 29

What is an emergent property?

Answer 29

Multicellular organisms can undertake functions that unicellular organisms cannot. This is a result of properties emerging when individual cells organize and interact to produce living organisms. In multicellular organisms, specialized cells of the same type group together to form tissues. A tissue is a group of cells that work together to perform a particular function. For example, epithelial tissue (the function of which, in the small intestine, is to absorb food). Different tissues also work together to form organs. For example, the heart is made up of many different tissues (including cardiac muscle tissue, blood vessel tissues, connective tissue, etc.)

Question 30

Explain Gene Expression

Answer 30

Every nucleus within the cells of a multicellular organism contains the same genes, that is, all cells of an organism have the identical genome
Despite cells having the same genome, they have a diverse range of functions because during differentiation certain genes are expressed (‘switched’ on). Controlling gene expression is the key to development as the cells differentiate due to the different genes being expressed. Once certain genes are expressed the specialization of the cell is usually fixed so the cell cannot adapt to a new function

Question 31

The use of a sodium-potassium pump in active transport.

Answer 31

Active transport is the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

Sodium-potassium carrier pump proteins are integral proteins that enable an electrochemical gradient (resting membrane potential) to be maintained between the inside and outside of the axon. Nerve impulses that travel along axons are dependent on sodium and potassium ions being moved across the axon membrane to create this gradient. the sodium-potassium pumps move three sodium ions out of the axon and two potassium ions into the axon using one ATP molecule per cycle. The pumps are always moving the ions against their concentration gradient via active transport. The cycle continues until the resting membrane potential is reached. The steps to the cycle are:
- Three sodium ions from the inside of axon bind to the pump
- ATP attaches to the pump and transfers a phosphate to the pump (phosphorylation) causing it to change shape, resulting in the pump opening to the outside of the axon
- the three sodium ions are released out of the axon
- two potassium ions from outside the axon enter and bind to their sites
- the attached phosphate is released altering the shape of the pump again
- the change in shape causes the potassium ions to be released inside the axon.