Cell Structure, Cell Cycle and Microscopy

Question 1

Which organelles are found in an animal cell?

Answer 1

A typical animal cell has:
- cell surface (plasma) membrane
- rough endoplasmkc reticulum
- smooth endoplasmkc reticulum
- nucleus
- mitochondria
- cytoplasm
- ribosomes
- Golgi apparatus
- lysosomes

Question 2

Which organelles are found in a plant cell?

Answer 2

A typical plant cell has:
- cell surface membrane
- cellulose cell wall
- chloroplasts
- rough endoplasmic reticulum
- smooth endoplasmic reticulum
- plasmodesmata
- mitochondria
- Golgi apparatus
- permanent vacuole
- ribosomes
- nucleus
- cytoplasm

Question 3

What is the structural difference between a plant cell and an algal cell?

Answer 3

Algal cells have all the same organelles as plant cells and have cellulose cell walls, but they often have one large chloroplast instead of many smaller chloroplasts. Algal cells also tend to be more circular in shape compared to a plant cell.

Question 4

What is the structural difference between a fungal cell and a plant cell?

Answer 4

Fungal cells are similar to plant cells, except that their cell walls are made of chitin, not cellulose and they don’t have chloroplasts because they don’t photosynthesise.

Question 5

What is the difference between eukaryotic and prokaryotic cells?

Answer 5

Eukaryotic cells have a nucleus, whereas prokaryotic cells do not have a nucleus.

Question 6

What is the ultrastructure of a cell?

Answer 6

It is the detailed structure of the cell that can only be seen at high magnification (it can only be viewed with an electron microscope).

Question 7

What are the main components of a mitochondrion?

Answer 7

• outer membrane
• inner membrane
• inter-membrane space
• cristae
• matrix
• ribosomes
• DNA

Question 8

What is the function of the double membrane and the inter-membrane space?

Answer 8

The double membrane controls the entry and exit of substances into and out of the mitochondrion. The inner membrane is folded to form cristae (extensions). The double membrane forms two aqueous compartments called the inter-membrane space and the matrix.

Question 9

What is the function of cristae?

Answer 9

Cristae are extensions of the inner membrane, which provide a large surface area for the attachment of enzymes and other proteins involved in respiration.

Question 10

What is the function of the matrix?

Answer 10

The matrix is the inner aqueous compartment of the mitochondrion, which contains proteins, lipids, ribosomes and DNA that allows the mitochondria to control the production of their own proteins. Many enzymes involved in respiration are found in the matrix.

Question 11

What is the function of mitochondria?

Answer 11

Mitochondria are the sites of the aerobic stages of respiration (Krebs cycle and the oxidative phosphorylation pathway). They are responsible for the production of ATP, from respiratory substances such as glucose. Therefore, there is a high number of mitochondria in metabolically active cells, which require lots of ATP, such as muscle cells.

Question 12

What is the structure of a ribosome?

Answer 12

Ribosomes are small organelles which consist of a large and a small subunit. They are made up of proteins and RNA and are not surrounded by a membrane.

Question 13

What is the function of ribosomes?

Answer 13

Ribosomes may float freely in the cytoplasm or be associated with the RER. There are two types (80S - found in eukaryotic cells and 70S - found in prokaryotic cells, mitochondria and chloroplasts). 70S are smaller. Ribosomes are the site of protein synthesis.

Question 14

What is the structure and function of the rough endoplasmic reticulum?

Answer 14

The RER is a system of membranes enclosing a fluid-filled space, which has ribosomes on the surface. Its functions are to provide a large surface area for the synthesis of proteins and glycoproteins and to provide a pathway for the transport of materials, especially proteins, throughout the cell.

Question 15

What is the structure and functions of the smooth endoplasmic reticulum?

Answer 15

The SER is a system of membranes enclosing a fluid- filled space (similar to RER but without ribosomes). The functions of the SER are to synthesise, store, and transport lipids and carbohydrates.

Question 16

What is the structure and function of the Golgi apparatus?

Answer 16

The Golgi apparatus consists of a stack of membranes that make up flattened sacs called cisternae, with small, hollow, rounded structures called vesicles. Proteins and lipids produced by the ER are passed through the Golgi apparatus in sequence. The Golgi modifies the proteins, often adding non-protein components, such as carbohydrates, to them. Proteins and lipids are also sorted and transported to vesicles which are pinched off from the ends of the Golgi cisternae.

Question 17

What are the functions of the Golgi apparatus and vesicles?

Answer 17

• add carbohydrate to proteins to form glycoproteins
• produce secretory enzymes, such as those secreted by the pancreas
• secrete carbohydrates, such as those used to make call walls
• transport, modify and store lipids
• form lysosomes
  Main function of apparatus is to process and package lipids and proteins. Main function of vesicles of to store proteins and lipids modified by the Golgi apparatus and transport them out of the cell.

Question 18

How are lysosomes formed?

Answer 18

Lysosomes are formed from the budding off of the Golgi body, and as such, contains proteases and lipases found in the Golgi, which were originally made in the endoplasmic reticulum. They also contain lysozymes, which are enzymes which hydrolyse the cell walls of certain bacteria.

Question 19

What are the functions of lysosomes?

Answer 19

Lysosomes are responsible for:
- hydrolysing material ingested by phagocytic cells, such as white blood cells and bacteria
- releasing enzymes to the outside of the cell (exocytosis), in order to destroy material around the cell.
- digest worn-out organelles so that the useful chemicals they are made out of can be reused.
- completely break down cells after they have died (autolysis)

Question 20

What are centrioles?

Answer 20

Centrioles are paired barrel-shaped organelles located in the cytoplasm of animal cells near the nuclear envelope. Most animal cells have two centrioles, which can only be seen with an electron microscope.

Question 21

What is the function of centrioles?

Answer 21

The functions of centrioles are to:
- play a role in facilitating the reproduction of cells
- engage in the arrangement of mitotic spindles during cell division (ensuring equal separation of chromosomes between daughter cells)
- aids in cytokinesis
- organises microtubules in the cell’s cytoplasm (which acts as a part of the cytoskeleton and keeps all organelles in place).

Question 22

What are the main components of chloroplasts?

Answer 22

The components of chloroplasts include:
- chloroplast envelope
- grana and thylakoids (lamellae)
- stroma
- 70S ribosomes and circular DNA

Question 23

What is the structure and function of the chloroplast envelope?

Answer 23

The chloroplast envelope is a double plasma membrane that surrounds the organelle. It is highly selective about what it allows to enter and leave the chloroplast, and as such, controls the movement of substances.

Question 24

What is the structure and function of grana and lamellae?

Answer 24

Grana are stacks of disc-like structures called thylakoids, within which is the photosynthetic pigment chlorophyll. Some grana have tubular extensions called lamellae (singular lamella), which join up with adjacent grana. The grana is where light absorption for photosynthesis occurs.

Question 25

What is the structure and function of the stroma?

Answer 25

The stroma is a fluid-filled matrix within which the second stage of photosynthesis takes place (synthesis of sugars). A number of other structures can be found within the stroma, such as starch grains.

Question 26

What is the function of 70S ribosomes and circular DNA within chloroplasts?

Answer 26

The circular DNA within chloroplasts contains the basic genes for producing chloroplasts (thus allowing their replication). 70S ribosomes produce rubisco (important in the light-independent part of photosynthesis) and thylakoids (which contain chlorophyll).

Question 27

What is the tonoplast and what are its functions?

Answer 27

The tonoplast is the membrane surrounding the permanent vacuole in plant cells. It separates the vacuolar components from the cell’s cytoplasm, which allows for the isolation of potential harmful materials within the vacuole. The tonoplast also regulates the movement of ions within the cell.

Question 28

What is the vacuole and what are its functions?

Answer 28

The vacuole is a membrane-bound organelle, which contains a solution of mineral salts, sugars, amino acids, wastes and pigments like anthocyanins. The functions of the vacuole include the storage of the listed materials, the isolation of metabolic waste that might be harmful to the cell, the maintenance of turgor pressure (keeping the cell turgid), which helps maintain the rigidity of the cell and the plant. The sugars and amino acids may act as a temporary food store, while the pigments may colour petals to attract pollinating insects.

Question 29

What are the components of a plant cell wall?

Answer 29

Plant cell walls consist of microfibrils of cellulose, embedded in a matrix. There is a thin layer, called the middle lamella, which marks the boundary between adjacent cell walls and joins them together.

Question 30

What are the functions of the cell wall in plants?

Answer 30

The cell wall:
- provides mechanical strength to prevent the cell bursting under the pressure created by the osmotic entry of water
- gives mechanical strength to the plant as a whole
- allows water to pass along it so contributes to the movement of water throughout the plant

Question 31

Which materials make up the cell walls of plants, algae and fungi?

Answer 31

• plants - cellulose
• algae - either cellulose, glycoproteins, or a mixture of both
• fungi - chitin (nitrogen-containing polysaccharide), glycan (polysaccharide) and glycoproteins.

Question 32

What are the main components of the nucleus?

Answer 32

The components of the nucleus include:
- nuclear envelope
- nuclear pore
- nucleoplasm
- chromosomes
- chromatin
- nucleolus

Question 33

What is the structure and function of the nuclear envelope?

Answer 33

The nuclear envelope is a double membrane surrounding the nucleus. Its outer membrane is continuous with the ER of the cell and often has ribosomes on its surface. It controls the entry and exit of materials into and out of the nucleus and contains the reactions taking place within it.

Question 34

What are nuclear pores and what is their function?

Answer 34

Nuclear pores are channels in the nuclear envelope, which allow the passage of large molecules, such as mRNA, out of the nucleus. There are around 3000 pores per nucleus, each around 40-100 nm in diameter.

Question 35

What is the nucleoplasm and what is its function?

Answer 35

The nucleoplasm is the granular, jelly-like substance which makes up the bulk of the nucleus. It is a suspension substance for the organelles inside the nucleus and helps to maintain the shape and structure of the nucleus. It plays an important role in the transportation of materials which are vital to cell metabolism and function. It also helps facilitate processes such as DNA replication and transcription.

Question 36

What are chromosomes and what is their function?

Answer 36

Chromosomes are thread- like structures found in the nucleus of plant and animal cells, consisting of a DNA molecule and its associated proteins. Chromosomes carry hereditary genetic information from one generation of cells to the next (ensuring accurate cell replication), and carry the DNA that codes for proteins.

Question 37

What is the structure and function of chromatin?

Answer 37

Chromatin is a complex of DNA and protein found in the nucleus of eukaryotic cells (it is a chain of nucleosomes), and is responsible for the packaging and condensing of long sections of DNA into more compact, denser structures, which are able to fit into the tight space of the nucleus. Chromatin also facilitates DNA replication, repair and transcription.

Question 38

What is the structure and function of the nucleolus?

Answer 38

The nucleolus is a spherical structure found in the cell’s nucleus, which manufactures ribosomal RNA and assembles ribosomes. There may be more than one nucleolus within a nucleus.

Question 39

What are the functions of the nucleus as a whole?

Answer 39

The nucleus:
- acts as a control centre of the cell through the processes of mRNA and tRNA production, and hence, protein synthesis
- retains the genetic material of the cell in the form of DNA and chromosomes
- manufacture ribosomal RNA and ribosomes

Question 40

How do you prepare a temporary mount (optical microscope slide)?

Answer 40

1. pipette a small drop of water onto the centre of the glass slide (suspends the specimen between the slide and the coverslip and makes the image clearer)
2. use forceps to place a thin section of specimen on top of the water (must be thin to let light through)
3. add a drop of a stain to highlight certain organelles/substances (e.g. eosin stains cytoplasm, iodine in potassium iodide stains starch grains)
4. use a mounted needle to lower a coverslip onto the specimen slowly (this ensures no air bubbles form, which would obstruct the view of the specimen). The coverslip ensures the specimen doesn’t move and protects it from contamination from the environment. It also protects the lens of the microscope, to stop it coming into contact with the specimen.

Question 41

How do you prepare a slide for an electron microscope?

Answer 41

To prepare a slide for an electron microscope, a sample must undergo primary and secondary fixation, dehydration, resin infiltration and embedding and sectioning and mounting sections on specimen grids. Glass or diamond knives are needed to cut the specimen thin enough. Don’t need to know specific detail, just that electron microscopy is much more time-consuming and difficult to prepare for, due to the necessity for much thinner specimens.

Question 42

What are microscope artefacts?

Answer 42

Artefacts are things that you can see looking down a microscope that aren’t part of the cell or specimen you’re looking at. They can be dust, air bubbles or fingerprints etc. and are created during the preparation of the slide.

Question 43

Which type of microscopy produces the most artefacts?

Answer 43

Electron microscopy produces more artefacts than light microscopy as the slide preparation is so much more complex, which is a limitation of its use.

Question 44

What are the two types of electron microscope?

Answer 44

Transmission electron microscopes (TEM) and scanning electron microscopes (SEM)

Question 45

How do light (optical) microscopes work?

Answer 45

Optical microscopes use light to form an image. A light source emits light, which is passed through a condenser. This creates a thin but concentrated ray of light which passes through the specimen. The light spreads out again but is focused by the glass objective lens ad again by the ocular lens, which produces an image for the viewer to see.

Question 46

How do electron microscopes (TEM) work?

Answer 46

An electron gun emits pulses of electrons, which are focused by electromagnets onto the specimen. In TEM, the electrons are passed through the specimen and focused by more electromagnets through a glass objective lens, allowing the viewer to see the image. Denser parts of the specimen absorb more electrons, so appear darker.

Question 47

How do electron microscopes (SEM) work?

Answer 47

SEM is similar to TEM except that the electrons don’t pass through the specimen, they scan the surface of the specimen. The way SEM works is that a beam of electrons is scanned across the specimen, which knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image.

Question 48

What is magnification?

Answer 48

Magnification is the number of times larger an image appears compared to its actual size.

Question 49

What is resolution?

Answer 49

Resolution is the ability to distinguish between two points (smallest distance between two points that can still be viewed as two separate points).

Question 50

What are the differences between TEM and SEM?

Answer 50

• TEM transmits electrons through the specimen, while SEM scans the surface of the specimen with electrons.
• TEM forms an image by focusing the beam of electrons using electromagnets, while SEM collects electrons which have been knocked off the specimen in a cathode ray tube to form an image
• In TEM, denser parts of the specimen appear darker as they absorb more electrons, while in SEM, the density of various parts of the specimen is not shown
• TEM gives 2D images while SEM gives 3D images
• TEM shows the ultrastructure of cells and organelles but SEM shows the overall 3D shape
• TEM has a higher resolution than SEM, so shows smaller objects.
• TEM requires thinner specimens than SEM
• TEM images are black and white, while SEM can show colour (it may be false colour)

Question 51

What are the differences between light and electron microscopes?

Answer 51

• optical uses light to form an image, electron uses electrons to form an image
• optical uses glass lenses to focus light, electron uses electromagnets to focus electrons
• it is easier to prepare slides for optical microscopy than electron microscopy, and as such electron microscope images are more likely to contain artefacts.
• electron requires much thinner specimens than optical
• optical microscopes have a maximum resolution of 0.2 micrometers, while electron has 0.0002 micrometres.
• maximum useful magnification of optical is x1500, electron is x1500000
• therefore, electron microscopes can be used to view smaller organelles in greater detail, such as ribosomes, centrioles and lysosomes
• optical shows colour while electron usually doesn’t (if it does it is often false colour)
• optical can be used on living specimens, while electron can only be used on dead/dehydrated specimens due to the use of a vacuum.

Question 52

What is the equation for magnification?

Answer 52

magnification = image size/actual size (assuming the units are the same for both)

Question 53

What is an eyepiece graticule/eyepiece micrometer?

Answer 53

It is a piece of glass with a scale consisting of 100 divisions on it, which can be placed in the eyepiece of a microscope to measure the size of a specimen. Eyepiece units are not a true scale, as they change their value depending on magnification.

Question 54

What is a stage micrometer?

Answer 54

A stage micrometer is a microscope slide with an accurate scale on it. It is often 10mm in length, with each division representing 0.1mm. When aligned with the eyepiece graticule, it can be used to calculate the actual value of 1 epu at a certain magnification, which can be used to calculate the actual size of specimens.

Question 55

What is the formula for finding the value of 1 epu (eyepiece unit) at a particular magnification?

Answer 55

1 epu = stage micrometer units (smu) / eyepiece units (epu)

Question 56

What is the conversion from mm to micrometres?

Answer 56

1mm = 0.001 um

Question 57

How would you calibrate a microscope and calculate an actual size of a specimen using a microscope?

Answer 57

1. place the micrometer eyepiece into the eyepiece of the microscope
2. place the stage micrometer slide onto the stage of the microscope at the desired objective lens
3. align the eyepiece graticule and stage micrometer units to calculate the value of 1 epu at that objective lens
4. remove the stage micrometer from the stage of the microscope and add the specimen slide that you want to measure to the stage
5. measure epu of the specimen and calculate actual size by multiplying the number of epu by the calculated actual value of 1 epu.

Question 58

How do you calculate actual size of a specimen from an image of it?

Answer 58

Measure the scale bar in mm, then convert to um. Divide image size of scale bar (measured length) by actual size of the scale bar (as stated in the image) to get the magnification factor. Measure the length/width of the structure you are asked to measure then divide this image size by the magnification factor to get the actual size of the structure.

Question 59

How do you use an optical microscope?

Answer 59

• clip the slide to the stage
• select the lowest-power objective lens
• use the coarse focusing knob to bring the stage up to just below the objective lens
• use the coarse focusing knob to move the stage downwards until the image is roughly in focus
• adjust the focus with the fine adjustment knob until you get a clear image
• to get a greater magnification, swap to a higher power objective lens and refocus

Question 60

How do you do a scientific drawing?

Answer 60

Draw what you see in the microscope without using shading, and use continuous lines (no feathering). Include a scale bar and labels, but don’t cross over label lines.

Question 61

What is cell fractionation?

Answer 61

Cell fractionation is a technique which separates organelles according to their density, enabling different organelles to be viewed under a microscope and their ultrastructure to be studied.

Question 62

What are the main steps of cell fractionation?

Answer 62

1. Homogenisation (breaking up the cells)
2. Filtration (removing large pieces of debris)
3. Ultracentrifugation (separating organelles)

Question 63

What is homogenisation and how can it be achieved?

Answer 63

Homogenisation is the process by which cells are broken up. It can be done by vibrating the cells or by grinding them in a blender, which breaks up the plasma membrane and releases the organelles into solution. The solution must be ice cold, to reduce the activity of enzymes that may damage the organelles. The solution should be isotonic (same concentration of chemicals as outside the cell), which prevents damage to the organelles by osmosis. A buffer solution is added to maintain a constant pH.

Question 64

What is filtration (in the context of cell fractionation) and what does it achieve?

Answer 64

Filtration is when the homogenised cell solution is filtered through a gauze to separate large cell debris, such as connective tissue, from the organelles. The organelles are much smaller than the debris, so can pass through the gauze.

Question 65

What is ultracentrifugation and what does it achieve?

Answer 65

Ultracentrifugation is when the solution of organelles is poured into a tube and spun at low speed in a centrifuge. The heaviest organelles, like nuclei, are flung to the sides and form a sediment at the bottom of the pellet. The rest of the organelles stay suspended in the fluid above the sediment - the supernatant. The supernatant is drained off and poured into another tube. It is spun in the centrifuge again at higher speed. The next heaviest organelles form a pellet at the bottom of the tube (chloroplasts, mitochondria). The supernatant is drained again and the process repeated at higher speeds to remove lighter organelles.

Question 66

Which order are organelles removed in during ultracentrifugation?

Answer 66

Organelles are removed from heaviest to lightest, which is nuclei, chloroplasts, mitochondria, lysosomes, endoplasmic reticulum, ribosomes.

Question 67

What is the supernatant in ultracentrifugation?

Answer 67

The supernatant is the solution above the sediment, containing lighter organelles than those removed.

Question 68

What is the pellet in ultracentrifugation?

Answer 68

The pellet is the sediment at the bottom of the centrifuge tube containing heavy organelles removed by the spinning of the tube.

Question 69

What are the biggest differences between eukaryotic and prokaryotic cells?

Answer 69

Prokaryotic cells are much smaller than eukaryotic cells (often 10-100 times smaller). Prokaryotic cells have circular DNA which lies free in the cytoplasm and extra plasmids, while eukaryotic cells have linear DNA which is contained in a nucleus. P cells have ‘naked’ DNA (not associated to his tones or RNA to form chromosomes), while E cells have DNA which is associated with histones to form chromosomes. P cells have 70S ribosomes (smaller) while E cells have 80S ribosomes (larger). P cells have few organelles and none which are envelope-bound. E cells have many organelles and many which are envelope-bound (e.g. nucleus, mitochondria, chloroplasts). P cells have rigid cell walls made of murein (a glycoprotein), while E cells have cellulose cell walls (plants), chitin cell walls (fungi). P have mesosomes (extensions of the cell membrane) to provide a surface for respiration, while E have mitochondria for aerobic respiration.

Question 70

What are the main components of a prokaryotic cell?

Answer 70

Always present:
- cell wall containing murein
- cell surface membrane
- cytoplasm
- circular DNA
- 70S ribosomes

Sometimes present:
- flagellum
- capsule
- mesosomes
- plasmids
- pili

Question 71

What is the function of the cell wall in prokaryotic cells?

Answer 71

The cell wall supports the cell and prevents it from changing shape. It’s made of murein, which is a glycoprotein, which is strong.

Question 72

What is the function of the cell surface membrane in prokaryotic cells?

Answer 72

The cell surface membrane is mainly made of lipids and proteins. It controls the movement of substances into and out of the cell.

Question 73

What is the function of the cytoplasm in a prokaryotic cell?

Answer 73

The cytoplasm is where many metabolic processes of the cell occur, and maintains cell shape. It also suspends the organelles.

Question 74

What is the function of ribosomes in prokaryotic cells?

Answer 74

The ribosomes in prokaryotic cells are 70S ribosomes, so are smaller than those found in eukaryotic cells. They do still carry out protein synthesis.

Question 75

What is the function of the circular DNA in prokaryotic cells?

Answer 75

The circular DNA in prokaryotic cells is free in the cytoplasm and not associated to histones, so doesn’t form chromosomes. The circular DNA stores genetic material and ensures continuity between generations of cells through DNA replication and codes for proteins.

Question 76

What is the function of plasmids in prokaryotic cells?

Answer 76

Plasmids are small loops of DNA that are separated from the main circular DNA molecule. They contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance). They are not always present.

Question 77

What is the function of the capsule in prokaryotic cells?

Answer 77

The capsule is made of secreted slime and helps protect bacteria from drying out and from attack by cells of the immune system of the host organism. Not always present.

Question 78

What is the purpose of the flagellum in prokaryotic cells?

Answer 78

The flagellum is a long, hair-like structure that rotates, enabling the prokaryote to move. Some have more than one but they are not always present.

Question 79

What is the function of the pili in prokaryotic cells?

Answer 79

The pili are used for the attachment of the cell to other cells or surfaces and can be involved in sexual reproduction. Not always present

Question 80

What is the function of mesosomes in prokaryotic cells?

Answer 80

Mesosomes are infoldings of the cell surface membrane which may allow photosynthesis or to carry out nitrogen fixation. Not always present.

Question 81

What is an example of organelles working together within cells?

Answer 81

The exocytosis and production of enzymes and the production of cell surface glycoproteins are both examples of organelles working together within the acinar cells of the pancreas.

Question 82

What is exocytosis?

Answer 82

Exocytosis is the fusion of the secretory vesicles to the cell surface membrane, to release substances from inside the cell to the outside of the cell.

Question 83

Which organelles have to work together during the production and exocytosis of enzymes in the pancreas?

Answer 83

• RER (ribosomes)
• Golgi apparatus
• Golgi vesicles
• lysosomes
• cell surface plasma membrane

Question 84

How are enzymes produced and secreted by a cell?

Answer 84

Amino acids pass from the blood into the cell via the plasma membrane. The amino acids are transported to ribosomes on the Rough Endoplasmic Reticulum (RER) by transfer RNA (tRNA). In the nucleus, transcription occurs, producing an mRNA strand which codes for the enzyme to be produced. The mRNA binds to ribosomes on the RER, initiating protein synthesis from amino acids. The newly synthesised polypeptide/protein enters the RER. Small transfer vesicles bud off the RER, carrying proteins and fuse with the Golgi apparatus. Proteins may be modified e.g. addition of carbohydrate groups and packaged as they move through the Golgi apparatus. Secretory vesicles e.g. lysosomes or Golgi vesicles bud off the Golgi apparatus. Proteins are now in the final structures e.g. glycoproteins or enzymes. Secretory vesicles move towards the membrane, sometimes they are stored. The secretory vesicles fuse to the plasma membrane to release enzymes into the digestive system (exocytosis).

Question 85

How do bacteria reproduce?

Answer 85

Bacteria reproduce via binary fission, which involves the replication of circular DNA and plasmids (if present) and the division of the cytoplasm to produce two daughter cells.

Question 86

Bacteria multiply exponentially - what does this mean?

Answer 86

This means that if a culture of bacteria increases by a factor of 2 in one generation, then the formula to find the number of cells (N) after one generation is N=N0 x 2^n, where N0 is the initial number at time zero, and n is the number of divisions or generations.

Question 87

What does it mean that viruses are acellular?

Answer 87

It means that viruses are not made up of cells, and as such, do not contain many organelles that cells often have.

Question 88

What does it mean that viruses are non-living?

Answer 88

Viruses are non-living because they can’t reproduce on their own, they need to take over the processes of a host cell in order to do this. Therefore, they are not classified as alive.

Question 89

What are the three main structures in a virus particle?

Answer 89

• capsid (protein coat)
• core of nucleic acids (DNA or RNA)
• attachment proteins (e.g. glycoproteins)

Question 90

What does the capsid do?

Answer 90

Protects the nucleic acids from enzymes when outside host. Binds to host cell surface and assists in penetration of host and introduction of nucleic acid.

Question 91

What does the core of nucleic acids do in a virus particle?

Answer 91

The core of nucleic acids carries genetic information for the particle and allows viral proteins and nucleic acids to be replicated, enabling the production of more virus particles.

Question 92

What do virus particles sometimes have?

Answer 92

They sometimes have an external envelope derived from host plasma membrane. If it contains glycoproteins, spikes will form, allowing attachment to host. Some viruses also have enzymes within them (e.g. HIV has the enzyme reverse transcriptase within it).

Question 93

What is the classic shape of a bacteriophage?

Answer 93

Bacteriophages are viruses which infect bacteria. They have a polyhedral head and helical tail. They work by injecting their DNA into the host cell and getting the host cell to replicate them. The cell lyses (which kills it) and releases more virus particles to infect other cells.

Question 94

How are viruses classified?

Answer 94

Virus structures are classified as either helical, polyhedral or complex (polyhedral head, helical tail). The nucleic acid type is either DNA or RNA and the plasma membranes of host cells can be enveloped or naked.

Question 95

What are logarithm scales?

Answer 95

Logarithm scales show orders of magnitude in a linear fashion in which zero can never be reached. An example of a logarithmic scale would be 1 10 1000 10000 all equally spaced with graduation within them representing either one, ten, 100 or 1000 depending on what the section began with. The graduation get closer together as you move from right to left (towards the higher number).

Question 96

In multicellular organisms, can all cells continue to divide throughout their lifetimes?

Answer 96

No, only some cells retain their ability to divide throughout their lifetimes. Those than can divide show a cell cycle.

Question 97

What are the main stages of the cell cycle?

Answer 97

The main stages of the cell cycle in the order that they occur are:
- interphase
- mitosis (nuclear division)
- cell division (cytokinesis)

Question 98

What are the stages of interphase and what happens in each one?

Answer 98

In the order in which they occur:
- gap phase 1 - cell grows and new organelles and proteins are made
- synthesis - cell replicates its DNA, ready to divide by mitosis
- gap phase 2 - cell keeps growing and proteins needed for cell division are made
During interphase, normal cell activity occurs, but the cell’s DNA is unravelled and replicated, organelles are replicated and the ATP content is increased (as it is needed to provide energy for cell division)

Question 99

What is mitosis?

Answer 99

Mitosis is the stage of the cell cycle in which a eukaryotic cell divides to produce two identical daughter cells, each with identical copies of DNA produced by the parent cell during DNA replication.

Question 100

What is the purpose of mitosis?

Answer 100

Mitosis is needed for the growth of multicellular organisms and for repairing damaged tissues.

Question 101

Describe the structure of chromosomes.

Answer 101

When mitosis begins, the chromosomes are made of two strands of DNA called chromatids held together by a centromere. Two strands on the same chromosome are sister chromatids. Two identical chromosomes which code for the same characteristics are homologous chromosomes.

Question 102

What are the stages of mitosis in order?

Answer 102

• Prophase
• Metaphase
• Anaphase
• Telophase

Question 103

What happens during prophase?

Answer 103

Chromosomes shorten and thicken by condensation, becoming visible as a result. Centrioles move to opposite poles of the cell. Protein microtubules radiate from each centriole, making the spindle fibres. Towards the end of prophase, the nuclear envelope disintegrates and the nucleolus disappears. Each chromosome exists as a pair of sister chromatids, joined by a centromere.

Question 104

What happens in metaphase?

Answer 104

In metaphase, the chromosomes attach to the spindle fibres at their centromeres and they align on the equator of the cell (spindle equator).

Question 105

What happens in anaphase?

Answer 105

The centromeres separate and divide into two. The spindle fibres shorten, due to the contraction of microtubules, which in turn, pull the chromatids to the poles, where the centromere leads first.

Question 106

What happens in telophase?

Answer 106

In telophase, the chromatids have reached opposite poles of the cell, so are now referred to as chromosomes. The chromosomes uncoil and lengthen, so are no longer visible. The spindle fibres break down and the nuclear envelope reforms. The nucleolus reappears.

Question 107

What happens during cell division (cytokinesis)?

Answer 107

Cytokinesis is the final stage of the cell cycle, in which the cytoplasm of the cell divides, forming two separate, genetically identical daughter cells. In reality, cytokinesis doesn’t happen after mitosis but it actually occurs during anaphase and telophase.

Question 108

What can cause cancer?

Answer 108

Cancer can be caused by uncontrolled mitosis. This may be a result of a genetic mutation. Many treatments of cancer focus on disrupting the cell cycle as a result, as this will stop the tumour from growing. Some drugs target G1 phase of interphase, as this prevents the synthesis of enzymes needed for DNA replication, meaning the cell can’t enter S phase and is forced to kill itself. Some drugs and radiation damage DNA, which causes the cell to kill itself.