AS1 Chapter 3 - Cells Flashcards
1
Q
Describe the structure of a bacteriophage.
A
- Capsid protein head
- DNA bounded by a protein coat
- Protein contractile sheath surrounding tubular core
- Tail fibres (bind to bacterial cell surface)
- Base plate (made of protein)
2
Q
Describe the structure of the HIV virus.
A
- Two strands of RNA
- Two molecules of reverse transcriptase enzyme
- Capsid protein surrounding RNA core
- Another protein coat surrounding the initial capsid protein (known as the membrane-associated protein shell)
- Phospholipid bilayer containing glycoprotein
3
Q
What does HIV stand for?
A
Human immunodeficiency virus
4
Q
What does AIDS stand for?
A
Acquired immune deficiency syndrome
5
Q
HIV is known as a …
A
Retrovirus
6
Q
How does the HIV virus synthesise new viruses after infecting the host cell?
A
HIV viruses deliver the RNA into the host cell together with the enzyme reverse transcriptase. The reverse transcriptase catalysed the synthesis of a strand of DNA; double stranded DNA is then created with DNA polymerase activity. The viral DNA is then integrated into the host DNA where it ensures that viral protein (new protein coats) and viral RNA for new viruses are made.
7
Q
What is a retrovirus?
A
The viral RNA is used as a template to make DNA. This is the reverse of the normal transcription process, where DNA is used to make mRNA as part of protein synthesis.
8
Q
In humans, HIV invades …
A
A type of lymphocyte called the helper T-cells
9
Q
What type of organism do phages invade?
How do they replicate and destroy the host cell?
A
Bacteria
Inside their host cells (bacteria) the viral DNA codes for the production of new protein - for new protein coats. The DNA itself replicates to make copies that are then packaged within the protein coats, forming new viruses. In due course, the bacterial cell is destroyed as it ruptures and many new viruses are released.
10
Q
How does HIV weaken the immune system of those infected?
A
In humans, HIV invades a type of lymphocyte called helper T-cells. These T-cells are very important in the immune system when protecting against disease. As progressively more T-cells are destroyed, the immune system becomes critically compromised and the medical condition AIDS can develop.
11
Q
What is a eukaryotic cell?
A
Eukaryotic cells are those with a membrane-bound nucleus, chromosomes (helical DNA with a histone protein coat), mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus, vesicles, lysosomes and microtubules.
12
Q
What is a prokaryotic cell?
A
Prokaryotic cells are those without nuclei, mitochondria or endoplasmic reticulum, and possess naked, circular DNA, small ribosomes, possibly plasmids, and a cell wall.
13
Q
What organelles are present in a eukaryotic cell?
A
- Membrane bound nucleus
- Chromosomes (helical DNA with a histone protein coat)
- Mitochondria
- Rough and soft endoplasmic reticulum
- Ribosomes
- Golgi apparatus
- Vesicles
- Lysosomes
- Microtubules
14
Q
What organelles are present in a prokaryotic cell?
A
- Naked, circular DNA
- Small ribosomes
- Plasmids
- Cell wall
- No nuclei, mitochondria or endoplasmic reticulum
15
Q
Give some examples of eukaryotic cells.
A
Animal cells
Plant cells
Fungal cells
16
Q
What are the two types of electron microscope?
A
SEM (Scanning Electron Microscope)
TEM (Transmission Electron Microscope)
17
Q
What is cell ultrastructure?
A
The detail of a cell when viewed through an electron microscope is known as its ultrastructure.
18
Q
Give an example of a prokaryotic cell.
A
Bacterial cell
19
Q
What is the current model for membrane structure in animal cells?
A
Fluid mosaic model
20
Q
Describe the structure of an animal cell membrane
A
- Phospholipid bilayer
- Intrinsic and extrinsic proteins
- Transmembrane proteins (intrinsic)
- Glycoproteins
- Glycolipids
- Cholesterol (not found in plant cells)
- Carbohydrate glycocalyx
21
Q
What is the function of the phospholipid bilayer?
A
Selectively permeable, allowing specific substances to pass through and blocking the entry of others.
22
Q
What is the function of cholesterol in the cell membrane?
A
Increases membrane stability by restricting the sideways movement of phospholipid molecules at high temperatures. At low temperatures the cholesterol helps to maintain membrane fluidity by acting as a wedge between adjacent phospholipid molecules and stopping adjacent molecules sticking together. In effect, the cholesterol makes the membrane less fluid at high temperatures and more fluid at low temperatures.
23
Q
What is the function of proteins in the cell membrane?
A
- They help provide stability and support as they help to ‘anchor’ the phospholipid molecules.
- They may act as enzymes.
- Some membrane proteins act as adhesion sites - areas where adjacent cells are held together.
- Proteins are also involved in cell recognition and as receptors or antigens.
- Proteins are particularly important in transporting substances across the bilayer. They aid transport by acting as:
- Channels: These proteins span the membrane and work by creating a hydrophilic channel that allows polar molecules to bypass the hydrophobic centre of the bilayer.
- Carriers: These proteins carry specific ions and molecules across the membrane.
24
Q
What is the glycocalyx?
A
The outer phospholipid layer of the membrane is the glycocalyx. The glycocalyx contains polysaccharides that are bound to membrane proteins (glycoprotein) or to the phospholipids (glycolipids).
25
What is the function of membranes in eukaryotic cells?
They define the boundaries of organelles within the cytoplasm.
26
What is the structure of a mitochondrion?
- Sausage shape
- Surrounded by an envelope, i.e. a double membrane comprised of an inner and outer membrane
- The inner membrane is folded to form cristae, which increases the surface area of the inner membrane, therefore increasing the number of enzymes that can be embedded within the membrane.
- Fluid filled matrix
27
What is the function of mitochondria?
Synthesis of ATP by aerobic respiration.
28
What is the structure of chloroplasts?
- They are bounded by an envelope (double membrane)
- Stroma (inner fluid)
- Thylakoid membranes are stacked into grana
- Thylakoid membranes contain chlorphyll
- Starch grains and lipid droplets
- Intergranal lamellae (made up of thylakoid membranes) connect adjacent grana
29
What is the function of chloroplasts?
Site of photosynthesis; chlorophyll molecules are attached to the lamellae
30
What is the structure of rough endoplasmic reticulum?
A membrane system that is attached to the nuclear envelope which extends throughout the cytoplasm. The rough ER's 3-D membrane structure has enclosed sacs called cisternae. The rough ER has ribosomes attached to its outer surface.
31
What is the function of rough endoplasmic reticulum?
Polypeptides (proteins) made on the ribosomes are folded (i.e. develop their secondary and tertiary structure after the formation of their primary structure by the ribosomes) and accumulate in the rough ER and are passed on, in vesicles, to the Golgi apparatus.
32
What is the structure of smooth endoplasmic reticulum?
A membrane system that is attached to the rough ER which extends throughout the cytoplasm. The smooth ER's 3-D membrane structure has enclosed sacs called cisternae. The smooth ER does not have ribosomes attached to its outer surface.
33
What is the function of smooth endoplasmic reticulum?
The SER has many roles including the synthesis of lipids (for example, phospholipids), the detoxification of drugs and poisons, and carbohydrate metabolism.
34
What is the structure of ribosomes?
Small bodies of protein and RNA free in the cytoplasm or attached to the rough endoplasmic reticulum. Each ribosome is formed of a large and a small sub-unit, and is made of protein and ribosomal RNA (rRNA). They frequently occur in groups called polyribosomes, creating protein synthesis 'hotspots'.
35
What is the function of ribosomes?
Site of polypeptide (protein) synthesis; free ribosomes produce proteins that will function within the cytoplasm.
36
What is the structure of the Golgi apparatus?
A stack of membrane bound sacs (cisternae). Forming (convex) face has vesicles from the rough ER joining it. Mature (concave) face has vesicles pinching off.
37
What is the function of the Golgi apparatus?
Dynamic structure in which polypeptides are combined (forming quaternary proteins) and/or modified (with the addition of a carbohydrate to form a glycoprotein or a lipid to form a lipoprotein or a prosthetic group or cofactor). Finished protein is labelled, packaged and sorted into vesicles either for secretion by exocytosis or for delivery elsewhere in the cell.
38
What is the structure of lysosomes?
Vesicles produced by the Golgi apparatus, which have a relatively thick membrane, that contain hydrolytic enzymes.
39
What is the function of lysosomes?
Lysosomes combine with membrane-bound degenerate organelles or ingested particles (e.g. bacteria) to form secondary lysosomes; hydrolytic enzymes digest the contents.
40
What is the structure of microtubules?
Microtubules are hollow cylinders (fibres) formed from the protein tubulin. They form the cytoskeleton, the network of fibres that maintains cell shape and keeps organelles anchored in place. They also provide a network aiding the movement of structures within the cell.
The spindle fibres, important in the movement of chromosomes during mitosis and meiosis, are formed of microtubules.
Centrioles, involved in the assembly of spindle fibres during cell division are formed of microtubules. Animal and fungal cells contain a pair of centrioles.
41
What is the function of microtubules?
Centrioles form the spindle fibres during cell division of animal and fungal cells. (Centrioles are formed of microtubules).
Spindle fibres are important in the movement of chromosomes during mitosis and meiosis. (Spindle fibres are formed of microtubules).
Microtubules also form part of the cytoskeleton, which allows the movement of cell organelles.
42
What is the structure of the plasmodesmata?
Strands of cytoplasm between neighbouring plant cells that pass through pores in the walls.
43
What is the function of the plasmodesmata?
Facilitate transport of materials between adjacent cells in plants.
44
What is the singular term for plasmodesmata?
Plasmodesma
45
Name the components of the nucleus
Chromosomes (in the form of chromatin, euchromatin/heterochromatin)
Nucleolus
Nuclear envelope (nuclear double-membrane)
Nuclear pores
Ribosomes encrusted on the outside of the nuclear envelope (site of origin of rough ER)
46
What is the structure of the nucleus?
Enclosed within an envelope (double membrane).
Contains chromatin (DNA wound round a histone protein coat.
Perforated nuclear double membrane (i.e. Nuclear pores).
May contain one or more nucleoli (singular: nucleolus).
Heterochromatin (darker areas indicating a higher density of chromatin).
Euchromatin (lighter areas indicating a lower density of chromatin).
47
What is the function of the nucleus?
DNA codes for the synthesis of polypeptides in the cytoplasm; pores in the envelope allow large molecules in (e.g. enzymes) and out (e.g. RNA); nucleolus synthesises ribosomal RNA and manufactures ribosomes.
48
What is the nuclear envelope?
A perforated double membrane
49
What are plant cells?
Plant cells are protoplasts bordered by an extracellular cellulose cell wall and possess chloroplasts and a large permanent vacuole bounded by a tonoplast membrane.
50
How are plant cells joined together?
Neighbouring cell walls are adhered by a middle lamella (a sticky material composed of calcium pectate).
51
What are fungal cells?
Fungal cells are protoplasm (often multinucleate) bounded by an extracellular wall of chitin
52
What are animal cells?
Animal cells are eukaryotic and lack chloroplasts and a cell wall. Animal cells possess centrioles.
53
What are the two different types of electron microscope?
Scanning Electron Microscope (SEM)
| Transmission Electron Microscope (TEM)
54
What is magnification?
Magnification is a measure of how many times bigger the image is than the object.
55
What is the formula for magnification?
Magnification = Size of image/ Actual size of object
56
What is resolution?
Resolution is a measure of a microscopes ability to distinguish between two separate points.
57
Name two commonly used stains for cells to prepare them for microscopic viewing
Methylene blue - stains nuclei blue
| Iodine
58
Practical Work
What is homogenisation?
How can homogenisation be carried out?
Homogenisation (cell fractionation) is the breaking up and mixing of the material to give a uniform preparation. Homogenisation can be carried out using a mortar and pestle or a blender.
59
Practical Work
What is centrifugation?
Give an example.
Centrifugation involves placing the preparation in centrifuge tubes and spinning them in a centrifuge at high speed. The forces involved will cause the larger (heavier) particles to sediment (move to the bottom of the centrifuge tube) with the smaller (lighter) particles remaining at the top in the supernatant.
For example, if a homogenate of liver tissue is centrifuged at high speed, nuclei will sediment at the bottom (in the pellet) and mitochondria will settle in a higher zone with ribosomes near the top.
60
Practical Work
| Note two things students should take into consideration when setting up a centrifuge.
Note 1: When setting up a centrifuge, it is important that each centrifuge tube is counterbalanced by a tube containing the same volume/mass of material.
Note 2: By altering the speed of rotation and/or the time of centrifugation, it is possible to isolate and sediment particular organelles or cell components. If the process is repeated, progressively lighter organelles will sediment in sequence.
61
Past Paper Question June 2014 AS1 (Legacy Material)
| Q3 c) Outline how the thylakoid membrane system increases the amount of light energy absorbed. [2]
Q3 c) Large surface area of thylakoid membranes means more chlorophyll molecules can be accommodated for.
62
Past Paper Question June 2014 AS1 (Legacy Material)
| Q3 d) Suggest a reason why a nucleus is not visible in the photograph. [1]
Q3 d)
• Specimens for TEM are thin, not all structures are evident.
Or
• This section does not transect the nucleus (i.e. This is a cross section of a plant cell, nucleus may not be present on this cross section).
63
Past Paper Question June 2015 AS1 (Legacy Material)
Q6 b) & c) In plants, nitrates are absorbed from the soil and travel to the leaf in the xylem. In the leaf cells, they are used to make enzymes and other macromolecules that are needed by the leaf.
The diagram below shows some of the organelles in a leaf cell that have nitrogen-containing molecules within them.
```
A = Rough endoplasmic reticulum
B = (ER) Vesicles [The vesicles came from the ER]
C = Golgi apparatus
D = (Golgi Apparatus) Vesicles [Heading towards cell surface membrane]
```
Scientists used radioactive nitrogen (15N) to trace the pathway of nitrogen through the organelles in a leaf cell. They introduced nitrates containing radioactive nitrogen into the xylem. When this reached the leaf cells, the times at which radioactive nitrogen was detected in different locations were recorded.
b) After two minutes, molecules with radioactive nitrogen (15N) were detected in both the cytoplasm and the nucleus. Suggest the identity of the first molecules containing radioactive nitrogen in each location. [2]
c) Explain concisely how each of the organelles labelled A-D are involved in the production and transport of the proteins which are eventually found in the cell surface membrane. [4]
b) Answer = Cytoplasm - Amino acids
Nucleus - Organic (nitrogenous) bases or nucleotides
c) In A (RER and ribosomes) are used to make the protein/polypeptide chains; polypeptides/proteins are transported in B (ER vesicles) to C (Golgi body); and modified into the final protein/glycoprotein in C (Golgi);
proteins are transported to the cell membrane in D which fuses with the cell membrane;
64
Past Paper Question June 2013 AS1
Proteins have many roles in cells. For example, enzymes are involved in both extracellular and intracellular digestion, while conjugated proteins provide essential receptors and recognition sites on cell surface membranes.
Any thirteen points from
Nucleus and rough ER
• the nucleolus produces ribosomal RNA/ribosomes
• chromosomes within the nucleus contain the genetic code/DNA has a nucleotide sequence
• that determines the sequence of amino acids in a polypeptide chain
• mRNA carries a copy of this code out of the nucleus/through the nuclear pores
• onto the rough ER/ribosomes
• ribosomes (on RER) are the site of protein/polypeptide synthesis
Golgi body
• ER vesicles containing the newly synthesised (primary) protein bud off RER
• and carry it to the forming face of the Golgi apparatus
• within Golgi the primary protein is modified into the final enzyme
• carbohydrate/lipid may be added to make glycoprotein/lipoprotein
• polypeptides brought together to form quaternary proteins
• prosthetic group may be added
Vesicles and lysosomes
• vesicles then bud off the mature face of Golgi
• secretory vesicles will carry enzymes to the cell-surface membrane
• where they fuse with it/release their enzymes
• other vesicles may carry glyco/lipoproteins to the surface membrane
• where they can become incorporated into the membrane as receptors/ recognition sites
• lysosomes are also budded off Golgi
• they fuse with worn-out organelles that have been enclosed in a
membrane/phagocytosed material
• the hydrolytic enzymes (in the lysosome) then digest the old organelle/ phagocytosed material/carry out autolysis [13]
65
Past Paper Question January 2013 AS1
08 a) Describe the similarities and differences in the structure of a bacteriophage virus and the human immunodeficiency virus (HIV). [5]
Give five points, with a maximum of four points from either part.
Similarities
• both non-cellular/lacking typical cellular structure
• both possess a head/capsid composed of protein
• genetic material (DNA/RNA) is located inside capsid/protein coat
• both very small (up to 200 nm approx.)/visible only with electron
microscope
Differences
• HIV roughly spherical, while bacteriophage (capsid) is icosahedral (or drawn/described)
• HIV capsid is within a lipid envelope (absent in bacteriophage)
• HIV genome is made of RNA, while bacteriophage genome is made
of DNA
• HIV contains the enzyme reverse transcriptase
• bacteriophage has a tail section/(consisting of) a protein sheath around
a core/(and) tail fibres attached to a base plate (absent in HIV)
• different proteins on exterior allow each virus to invade different cells
• HIV has glycoproteins, while bacteriophages do not [5]
66
Past Paper Question January 2013 AS1
08 b) Describe the similarities and differences in the structure of a bacterial and an animal cell. [8]
Quality of written communication [2]
Eight marks, with a maximum of six differences
Similarities
• both possess a cell membrane/cytoplasm (cytosol)
• cell membrane has fluid mosaic structure/composed of phospholipids and proteins
• both possess ribosomes
• both possess DNA
• both may contain glycogen
Differences
• bacterial cell is much smaller (up to 10 μm) than an animal cell (up to 100μm)
• a cell wall surrounds a bacterial cell, but not an animal cell
• bacterial cell may also have a capsule/pili
• cell membrane of an animal cell contains cholesterol (absent in
bacterial cell membrane)
• ribosomes in a bacterial cell are smaller than those in an animal cell
• DNA linear in an animal cell while bacterial DNA is circular
• DNA associated with histone/proteins in animal chromosomes while
bacterial DNA lacks protein (is naked)
• bacterial cells may contain plasmids, while animal cells do not
• genetic material is enclosed in a nuclear membrane in animal cells/they possess a nucleus
• bacterial cell contains no membrane-bound organelles/membrane
systems, while animal cell does (e.g. mitochondria/endoplasmic reticulum/Golgi body/lysosomes) [not chloroplasts]/bacteria are prokaryotic while animal cells are eukaryotic
• centrioles/mitotic spindle/microtubules present in animal cells, but not in bacterial cells [8]
67
Past Paper Question June 2017 Q8 a) (Revised Spec.)
Discuss the advantages and disadvantages of the light microscope (LM), the transmission electron microscope (TEM), and the scanning electron microscope (SEM) to observe biological structures. [6]
Q8 a) Minimum of seven points for 5-6 marks
• LM allows live/whole specimens to be viewed/EM allows only dead/thin sections to be viewed.
• LM has lower resolving power/ TEM has much greater resolving power.
• Hence LM allows only macrostructure/ TEM allows ultrastructure to be viewed (smallest organelles can be seen).
• LM is much cheaper/ EM is expensive.
• LM more portable/ preparation of specimens much easier than for EM
• SEM allows surface of structures to be viewed (at high resolution)
• Resolution is not as high in SEM image
• LM allows colour to be seen/ TEM image is black and white.
• Preparation of specimen for EM may introduce artefacts
• Other appropriate responses
68
Past Paper Question June 2017 Q3 b)i) (Revised Spec.)
3. Human Immunodefciency Virus (HIV) is a retrovirus which invades and destroys lymphocytes. HIV can lead to Acquired Immunodefciency Syndrome (AIDS). The number of viral particles (viral load) detected in a sample of blood can contribute to a diagnosis of AIDS.
AZT is a drug which can be used to treat HIV, either on its own or in combination with other drugs. After drug treatment, measuring viral load can determine the drug’s effectiveness for an infected individual. The aim of the treatment is to reduce the viral load to below 50 particles in one cubic centimetre of blood.
The blood of an infected individual was tested monthly after beginning AZT treatment. The results are shown in the table below.
Time/months. Viral load/cm−3
0 150 000
1 97 573
2 10 320
3 15 720
4 16 666
b)i) Describe the effect of AZT treatment on viral load and evaluate its effectiveness for this individual. [3]
b)i) Initially AZT caused a dramatic reduction in viral load;
Viral load begins to increase after month 2;
AZT alone is not an effective treatment/ the dose is not effective as viral load never falls as low as 50cm-3.
69
Past Paper Question June 2017 Q3 b)ii) (Revised Spec.)
3. Human Immunodefciency Virus (HIV) is a retrovirus which invades and destroys lymphocytes. HIV can lead to Acquired Immunodefciency Syndrome (AIDS). The number of viral particles (viral load) detected in a sample of blood can contribute to a diagnosis of AIDS.
AZT is a drug which can be used to treat HIV, either on its own or in combination with other drugs. After drug treatment, measuring viral load can determine the drug’s effectiveness for an infected individual. The aim of the treatment is to reduce the viral load to below 50 particles in one cubic centimetre of blood.
The blood of an infected individual was tested monthly after beginning AZT treatment. The results are shown in the table below.
Time/months. Viral load/cm−3
0 150 000
1 97 573
2 10 320
3 15 720
4 16 666
b)ii) Using the information provided, suggest a future treatment plan for this individual. [1]
AZT may be used in combination with other HIV drugs
Or
AZT may be replaced by another HIV drug
Or
An increased dosage of AZT may be used
70
Past Paper Question June 2017 Q3 c) (Revised Spec.)
3. Human Immunodefciency Virus (HIV) is a retrovirus which invades and destroys lymphocytes. HIV can lead to Acquired Immunodefciency Syndrome (AIDS). The number of viral particles (viral load) detected in a sample of blood can contribute to a diagnosis of AIDS.
AZT is a drug which can be used to treat HIV, either on its own or in combination with other drugs. After drug treatment, measuring viral load can determine the drug’s effectiveness for an infected individual. The aim of the treatment is to reduce the viral load to below 50 particles in one cubic centimetre of blood.
The blood of an infected individual was tested monthly after beginning AZT treatment. The results are shown in the table below.
Time/months. Viral load/cm−3
0 150 000
1 97 573
2 10 320
3 15 720
4 16 666
c) Suggest how scientists in a laboratory could ensure reliability and validity when testing blood samples.
Reliability - ______________________
Validity - _________________________
c) Reliability: take repeat counts from the same sample
Validity: Use the same method to count the viral particles/ keep blood in the same conditions before counting/ avoid sample contamination.
71
Past Paper Question June 2017 Q3 d) (Revised Spec.)
3. Human Immunodefciency Virus (HIV) is a retrovirus which invades and destroys lymphocytes. HIV can lead to Acquired Immunodefciency Syndrome (AIDS). The number of viral particles (viral load) detected in a sample of blood can contribute to a diagnosis of AIDS.
AZT is a drug which can be used to treat HIV, either on its own or in combination with other drugs. After drug treatment, measuring viral load can determine the drug’s effectiveness for an infected individual. The aim of the treatment is to reduce the viral load to below 50 particles in one cubic centimetre of blood.
The blood of an infected individual was tested monthly after beginning AZT treatment. The results are shown in the table below.
Time/months. Viral load/cm−3
0 150 000
1 97 573
2 10 320
3 15 720
4 16 666
d) AZT can also be used to treat other conditions caused by retroviruses. Using this information and your knowledge of the genetic make-up of retroviruses, suggest how AZT works. [1]
Target reverse transcriptase action/ prevents conversion of RNA to DNA (so incorporation into host DNA is prevented)
72
Past Paper Question June 2017 Q5 c)i) (Revised Spec. + Legacy Material)
The tobacco mosaic virus (TMV) is a virus which infects a wide range of plants, including the tobacco plant. The virus usually enters through an open wound in the plant. Once it enters, it multiplies rapidly and spreads quickly from cell to cell.
c)i) The virus cannot pass through plant cell walls. Suggest how it spreads from cell to cell. [1]
Plasmodesmata
73
Past Paper Question June 2017 Q5 c)ii) (Revised Spec. + Legacy Material)
The tobacco mosaic virus (TMV) is a virus which infects a wide range of plants, including the tobacco plant. The virus usually enters through an open wound in the plant. Once it enters, it multiplies rapidly and spreads quickly from cell to cell.
c)ii) Plants that are infected with TMV often have chloroplasts in the palisade layer that have an unusual shape. The chloroplasts appear to contain a large vacuole-like structure within the stroma.
Suggest and explain how the presence of this structure might affect the functioning of the chloroplast. [2]
c)ii) The thylakoids/grana may be destroyed/pushed to one side;
Less light can be absorbed (for photosynthesis)
74
Past Paper Question June 2017 Q5 d) (Revised Spec. + Legacy Material)
The tobacco mosaic virus (TMV) is a virus which infects a wide range of plants, including the tobacco plant. The virus usually enters through an open wound in the plant. Once it enters, it multiplies rapidly and spreads quickly from cell to cell.
d) Plant cells contain mitochondria as well as chloroplasts. State two similarities between the structures of mitochondria and chloroplasts. [2]
d) Any two from:
- Both have a double membrane;
- Large internal surface area/ many internal membranes;
- Both contain DNA;
- Both contain ribosomes;
75
Past Paper Question - June 2017 AS1 (Revised Spec.)
Q1 a) Cells can be categorised as prokaryotic or eukaryotic.
Complete the following table by inserting a (tick) if the component is present and a (X) if it is absent. (All boxes must be completed). [3]
```
Component Prokaryotic Cell Eukaryotic Cell
Endoplasmic Reticulum
DNA
Histone protein
Ribosome
Golgi apparatus
Plasmid
```
Q1 a)
Component Prokaryotic Cell Eukaryotic Cell
Endoplasmic Reticulum X Yes
DNA Yes Yes
Histone protein X Yes
Ribosome Yes Yes
Golgi apparatus X Yes
Plasmid Yes X
76
Past Paper Question - June 2017 AS1 (Revised Spec.)
| Q1 b) State one structure that contains microtubules in a eukaryotic cell. [1]
Q1 b) Spindle fibres
Cilia
Flagella
Cell cytoskeleton
77
Past Paper Question June 2017 Q3 a) (Revised Spec.)
3. Human Immunodefciency Virus (HIV) is a retrovirus which invades and destroys lymphocytes. HIV can lead to Acquired Immunodefciency Syndrome (AIDS). The number of viral particles (viral load) detected in a sample of blood can contribute to a diagnosis of AIDS.
AZT is a drug which can be used to treat HIV, either on its own or in combination with other drugs. After drug treatment, measuring viral load can determine the drug’s effectiveness for an infected individual. The aim of the treatment is to reduce the viral load to below 50 particles in one cubic centimetre of blood.
The blood of an infected individual was tested monthly after beginning AZT treatment. The results are shown in the table below.
Time/months. Viral load/cm−3
0 150 000
1 97 573
2 10 320
3 15 720
4 16 666
a) Calculate the percentage reduction in viral load between the beginning of treatment with AZT and the lowest viral load. [2]
Q3 a) 150,000 - 10,320 = 139,680
139,680
———— x100 = 93.12% 150,000
78
Past Paper Question - June 2016 Q4 a)ii) AS1 (Legacy Material)
Q4 In order to complete its life cycle, a virus must enter a host cell via the cell surface membrane. Several types of molecules are found in the cell membranes of animal cells, including cholesterol and glycoproteins.
a)ii) State the role of cholesterol in the cell membrane. [1]
Q4 a)ii) Regulates fluidity/stability of the cell membrane/provides support; [1]
79
Past Paper Question - June 2015 Q2 b) AS1 (Legacy Material)
Q2 Cell surface membranes are composed mainly of phospholipids and proteins.
b) Suggest why the cell surface membrane is described as having a 'fluid mosaic' structure. [2]
Q2 b) The phospholipids can move from side to side;
proteins are irregularly arranged throughout the membrane; [2]
80
Past Paper Question - June 2015 Q2 c) AS1 (Legacy Material)
Q2 Cell surface membranes are composed mainly of phospholipids and proteins.
c) Cholesterol is another molecule found in the cell surface membrane of animal cells.
Describe the function of cholesterol in the membrane. [1]
Q2 c) Cholesterol provides support to the membrane/affects uidity; [1]
81
Past Paper Question - January 2014 Q7 a) AS1 (Legacy Material)
Q7 An experiment was carried out to investigate the effect of cyanide poison on cell organelles extracted from a sample of liver tissue.
Organelles can be separated from the rest of the cell contents using a centrifuge. Ground-up tissue is placed in a tube, which the centrifuge then spins at high speeds. The cell contents are separated into ‘fractions’, depending on their size. The smaller the organelle, the higher the speed required to separate it from the other cell contents.
(a) Circle the name of the organelle fraction which would require the slowest spinning speed in order to separate it from the rest of the cell contents.
mitochondria ribosomes nuclei [1]
Q7 a) Nuclei; [1]
82
Past Paper Question - January 2014 Q7 b)i) AS1 (Legacy Material)
Q7 An experiment was carried out to investigate the effect of cyanide poison on cell organelles extracted from a sample of liver tissue.
Organelles can be separated from the rest of the cell contents using a centrifuge. Ground-up tissue is placed in a tube, which the centrifuge then spins at high speeds. The cell contents are separated into ‘fractions’, depending on their size. The smaller the organelle, the higher the speed required to separate it from the other cell contents.
b) A fraction of mitochondria was obtained using the technique outlined above. These mitochondria were placed in a buffer which was isotonic to the liver tissue (i.e. with the same water potential as the liver tissue).
i) Explain fully the purpose of the isotonic buffer. [2]
Q7 b)i) The buffer resists changes in pH;
and prevents loss/gain of water by the organelles; [2]
83
Past Paper Question - January 2014 Q7 b)ii) AS1 (Legacy Material)
Q7 An experiment was carried out to investigate the effect of cyanide poison on cell organelles extracted from a sample of liver tissue.
Organelles can be separated from the rest of the cell contents using a centrifuge. Ground-up tissue is placed in a tube, which the centrifuge then spins at high speeds. The cell contents are separated into ‘fractions’, depending on their size. The smaller the organelle, the higher the speed required to separate it from the other cell contents.
Two test tubes (A and B) were then prepared as follows:
* 2cm^3 of mitochondria in buffer solution was added to each test tube
* 1cm^3 of cyanide solution was added to test tube A
* 1cm^3 of the isotonic buffer was added to tube B
ii) Suggest why an extra 1cm^3 of buffer was added to tube B, as detailed above. [1]
Q7 b)ii) To ensure equal volumes in each test tube; [1]
84
Past Paper Question - January 2014 Q7 b)iii) AS1 (Legacy Material)
Q7 An experiment was carried out to investigate the effect of cyanide poison on cell organelles extracted from a sample of liver tissue.
Organelles can be separated from the rest of the cell contents using a centrifuge. Ground-up tissue is placed in a tube, which the centrifuge then spins at high speeds. The cell contents are separated into ‘fractions’, depending on their size. The smaller the organelle, the higher the speed required to separate it from the other cell contents.
A fraction of mitochondria was obtained using the technique outlined above. These mitochondria were placed in a buffer which was isotonic to the liver tissue (i.e. with the same water potential as the liver tissue).
Two test tubes (A and B) were then prepared as follows:
* 2cm^3 of mitochondria in buffer solution was added to each test tube
* 1cm^3 of cyanide solution was added to test tube A
* 1cm^3 of the isotonic buffer was added to tube B
The concentration of oxygen in each tube was then investigated using an oxygen probe. Readings were taken from each tube initially and then every minute for 10 minutes. Oxygen concentration was measured in micromoles (μM).
In tube A, the concentration of oxygen initially was 520. Subsequent readings were 511, 505, 500, 497, 495 and then five readings of 493.
In tube B, the concentration of oxygen initially was 505. Subsequent readings were 475, 444, 415, 386, 355, 324, 304, 297, 292 and 290.
b)iii) Construct a table of these results in the space below.
Your table should have a caption and should include appropriate
column headings, units and all the data. [4]
Q7 b)iii) Caption refers to oxygen concentration, mitochondria and cyanide present/absent (and time);
data organised in columns/rows;
appropriate column headings (cyanide present/absent must be clear, not just ‘tube A/tube B’);
units included (min and μM); [4]
Sample Caption: Oxygen concentration over time in a suspension of mitochondria with and without cyanide added
Time/min Oxygen concentration/μM
with cyanide without cyanide
0 520 505
1 511 475
2 505 444
3 500 415
4 497 386
5 495 355
6 493 324
7 493 304
8 493 297
9 493 292
10 493 290
85
Past Paper Question - January 2014 Q7 b)iv) AS1 (Legacy Material)
Q7 An experiment was carried out to investigate the effect of cyanide poison on cell organelles extracted from a sample of liver tissue.
Organelles can be separated from the rest of the cell contents using a centrifuge. Ground-up tissue is placed in a tube, which the centrifuge then spins at high speeds. The cell contents are separated into ‘fractions’, depending on their size. The smaller the organelle, the higher the speed required to separate it from the other cell contents.
A fraction of mitochondria was obtained using the technique outlined above. These mitochondria were placed in a buffer which was isotonic to the liver tissue (i.e. with the same water potential as the liver tissue).
Two test tubes (A and B) were then prepared as follows:
* 2cm^3 of mitochondria in buffer solution was added to each test tube
* 1cm^3 of cyanide solution was added to test tube A
* 1cm^3 of the isotonic buffer was added to tube B
The concentration of oxygen in each tube was then investigated using an oxygen probe. Readings were taken from each tube initially and then every minute for 10 minutes. Oxygen concentration was measured in micromoles (μM).
In tube A, the concentration of oxygen initially was 520. Subsequent readings were 511, 505, 500, 497, 495 and then five readings of 493.
In tube B, the concentration of oxygen initially was 505. Subsequent readings were 475, 444, 415, 386, 355, 324, 304, 297, 292 and 290.
b)iv) Describe the results of the experiment. [2]
Q7 b)iv) Any two from:
• in both tubes oxygen concentration falls over time
• the oxygen concentration falls much faster when mitochondria
are not treated with cyanide (in tube B)
• with cyanide present (tube A), oxygen concentration in the
suspension stops decreasing (after 6 minutes)/with cyanide absent, oxygen concentration continues to decrease [2]
86
Past Paper Question - January 2014 Q7 b)v) AS1 (Legacy Material)
Q7 An experiment was carried out to investigate the effect of cyanide poison on cell organelles extracted from a sample of liver tissue.
Organelles can be separated from the rest of the cell contents using a centrifuge. Ground-up tissue is placed in a tube, which the centrifuge then spins at high speeds. The cell contents are separated into ‘fractions’, depending on their size. The smaller the organelle, the higher the speed required to separate it from the other cell contents.
A fraction of mitochondria was obtained using the technique outlined above. These mitochondria were placed in a buffer which was isotonic to the liver tissue (i.e. with the same water potential as the liver tissue).
Two test tubes (A and B) were then prepared as follows:
* 2cm^3 of mitochondria in buffer solution was added to each test tube
* 1cm^3 of cyanide solution was added to test tube A
* 1cm^3 of the isotonic buffer was added to tube B
The concentration of oxygen in each tube was then investigated using an oxygen probe. Readings were taken from each tube initially and then every minute for 10 minutes. Oxygen concentration was measured in micromoles (μM).
In tube A, the concentration of oxygen initially was 520. Subsequent readings were 511, 505, 500, 497, 495 and then five readings of 493.
In tube B, the concentration of oxygen initially was 505. Subsequent readings were 475, 444, 415, 386, 355, 324, 304, 297, 292 and 290.
b)iv) Describe the results of the experiment. [2] (Do not answer)
b)v) Suggest the effect of cyanide on cell function. [1]
Q7 b)v) cyanide inhibits respiration/ATP production. [1]
87
Past Paper Question - January 2014 Q7 b)vi) AS1 (Legacy Material)
Q7 An experiment was carried out to investigate the effect of cyanide poison on cell organelles extracted from a sample of liver tissue.
Organelles can be separated from the rest of the cell contents using a centrifuge. Ground-up tissue is placed in a tube, which the centrifuge then spins at high speeds. The cell contents are separated into ‘fractions’, depending on their size. The smaller the organelle, the higher the speed required to separate it from the other cell contents.
A fraction of mitochondria was obtained using the technique outlined above. These mitochondria were placed in a buffer which was isotonic to the liver tissue (i.e. with the same water potential as the liver tissue).
Two test tubes (A and B) were then prepared as follows:
* 2cm^3 of mitochondria in buffer solution was added to each test tube
* 1cm^3 of cyanide solution was added to test tube A
* 1cm^3 of the isotonic buffer was added to tube B
The concentration of oxygen in each tube was then investigated using an oxygen probe. Readings were taken from each tube initially and then every minute for 10 minutes. Oxygen concentration was measured in micromoles (μM).
In tube A, the concentration of oxygen initially was 520. Subsequent readings were 511, 505, 500, 497, 495 and then five readings of 493.
In tube B, the concentration of oxygen initially was 505. Subsequent readings were 475, 444, 415, 386, 355, 324, 304, 297, 292 and 290.
b)vi) Between 7 and 10 minutes, the rate of oxygen consumption in tube B slows significantly. Suggest a reason for this. [1]
Q7 b)vi) Rate of respiration/oxygen consumption is limited by availability of respiratory substrates/mitochondria are running out of e.g. glucose; [1]
88
Past Paper Question - June 2014 Q3 a)+b) AS1 (Legacy Material)
Q3 Photograph 1.3 is an electronmicrograph of a plant cell, with some parts of surrounding cells also visible. Some structures in the photograph have already been labelled.
a) Identify the structures labelled A to C.
A ___________________
B ___________________
C ___________________ [3]
b) The magnification of this photograph is ×7500. Calculate the width of the cell in μm along the line X–X.
(Show your working.) [3]
(Go do this past paper question)
Q3 a) A: starch grain;
B: tonoplast;
C: middle lamella; [3]
| (b) Magnifed length = 115mm; = 115 000 μm;
115 000 ÷ 7500 = 15.33 μm; [3]
89
Past Paper Question - June 2014 Q6 c) AS1 (Legacy Material)
Q6 Proteins comprise a large group of organic molecules with a wide variety of functions. The specific function of a protein depends on its shape which is determined by its sequence of amino acids.
c) Several cell structures play a role in protein synthesis. Describe the role of each of the following structures in the synthesis of a functional protein.
• Ribosomes
_____________________________________________________
_____________________________________________________
• Rough endoplasmic reticulum
_____________________________________________________
_____________________________________________________
• Golgi body
_____________________________________________________
_____________________________________________________ [3]
Q6 c)
• Ribosomes are the site of production of a polypeptide chain;
• Rough endoplasmic reticulum provides support for the ribosomes/ transports the polypeptide through the cell/is involved in folding the polypeptide into its tertiary structure;
• Golgi body may add a prosthetic group (or by example)/packages proteins for transport or export; [3]
90
Past Paper Question - June 2014 Q7 a) AS1 (Legacy Material)
Q7 During the summer of 2013, it was reported that large numbers of oysters had died in Carlingford Lough in County Down. Tests on the dead oysters showed the presence of a virus called Ostreid Herpes Virus (OsHV). It is thought that the extended period of warm weather triggered increased infection rates in oyster populations.
The diagram below shows the structure of a Herpes Virus, similar to that which infects oysters.
a) Using the information in the diagram, state one way in which the structure of this virus is similar to the structure of:
* HIV _________________________________________
* a bacteriophage _________________________________ [2]
(Go do this past paper)
Q7 a) Lipid envelope/envelope proteins/spherical shape;
genetic material is DNA/nucleic acid protected by capsid (protein coat)/ hexagonal capsid; [2]
91
Past Paper Question - January 2013 Q1 AS1 (Legacy Material)
Q1 Identify the cell structure associated with each of the following processes.
• Protein synthesis
____________________
• Formation of secretory vesicles
____________________
• Destruction of worn out organelles
____________________
• Direct movement of substances between adjacent plant cells
____________________
• Formation of ribosomes
____________________ [5]
Q1 Ribosomes/rough endoplasmic reticulum;
| Golgi apparatus;
lysosomes;
plasmodesmata;
nucleolus; [5]
92
Past Paper Question - January 2013 Q4 a)+b)+c) AS1 (Legacy Material)
Q4 Photograph 1.4 is a colour-enhanced, transmission electron micrograph of part of a villus in the ileum.
(a) Identify the cells labelled A and B, and structures C and D.
A ______________________
B ______________________
C ______________________
D ______________________ [4]
(b) The magnification of this photograph is 32050. Calculate the actual length in micrometres (μm) of the cell along the line indicated by X–Y. (Show your working.)
Answer ______________ μm [3]
There are other structures visible in the cytoplasm of cell A. However, they are not distinct in this particular photograph and it is not possible to identify them.
(c) Suggest two organelles that you would expect to be present in cell A. For each organelle, explain its role in cell A.
Organelle 1 ____________________________________
_________________________________________________
Organelle 2 ____________________________________
_________________________________________________ [4]
(Go do this past paper question)
Q4 a) A: Columnar epithelium cell;
B: goblet cell/mucus secreting cell;
C: microvilli/brushborder;
D: nucleus/nucleoplasm/chromatin; [4]
b) Magnified length = 82mm;
= 82000μm;
82000 ÷ 2050 = 40μm; [3]
c) Two appropriate organelles with relevant explanation
Mitochondria;
to provide ATP for active transport [insist on ATP, not energy];
or
ribosomes/rough ER;
for manufacture of digestive enzymes/carrier proteins/channel proteins/ glycoproteins;
or
Golgi body;
to produce vesicles carrying enzymes/carrier proteins/channel proteins/ glycoproteins;
or
vesicles;
containing small proteins absorbed by pinocytosis/proteins produced
by rough ER (or Golgi body);
or
lysosomes;
to digest proteins/bacteria taken in (by endocytosis); [4]
93
Past Paper Question - January 2012 Q5 a)i)+ii)+iii) AS1 (Legacy Material)
Q5 a) Photograph 1.5 is a colour-enhanced, transmission electron micrograph of part of a mammalian pancreas cell. This cell produces large amounts of protein.
(i) Identify the structures labelled A to D.
A _______________________
B _______________________
C _______________________
D _______________________ [4]
(ii) The actual width of the organelle along the line XY is 2μm. Calculate the magnification of the photograph. (Show your working.)
Answer _____________ [3]
(iii) Identify two pieces of evidence from the photograph which indicate that there is a high level of protein production and secretion from this cell.
• evidence for protein production _______________________________
__________________________________________________________________
• evidence for protein secretion _________________________________
__________________________________________________________________ [2]
(Go do this past paper question)
Q5 a)i) A: Vesicle/lysosome;
B: RER/rough endoplasmic reticulum/ribosome;
C: (mitochondrial) matrix/mitochondrion;
D: cristae; [4]
ii) XY on photograph = 70 mm;
70 × 1000 = 70000 μm;
70000 ÷ 2 = 35000 [not 35000μm]; [3]
iii) Large numbers of ribosomes/large amounts of RER;
high numbers of vesicles; [2]
94
Past Paper Question - June 2012 Q1 AS1 (Legacy Material)
Q1 Identify the cell structures described by the following statements.
• A rod-shaped organelle with a double membrane, the inner of which is highly folded.
_______________________________________________________________________
• An organelle which contains genetic material and is surrounded by an envelope that contains pores.
_______________________________________________________________________
• Finger-like extensions of the cell-surface membrane, which increase the surface area.
_______________________________________________________________________
• A network of membrane-bound tubules and cisternae, responsible for synthesis and transport of lipid molecules.
_______________________________________________________________________
• Layer between two plant cell walls that holds them together.
______________________________________________________________________ [5]
Q1 Mitochondrion;
| nucleus;
microvilli;
smooth endoplasmic reticulum/smooth ER;
middle lamella; [5]
95
Past Paper Question - June 2012 Q6 d) AS1 (Legacy Material)
| Q6 d) State two ways in which fungal cells differ from plant cells. [2]
Q6 d) Any two from
• fungal cells lack chloroplasts/chlorophyll
• fungal cell walls made of chitin instead of cellulose
• fungal cells are multinucleate
• fungal cells store glycogen instead of starch
• fungal cells lack a large (central/sap) vacuole
• fungal cells have lysosomes
• fungal cells lack plasmodesmata
• fungal cells are heterotrophic/lysotrophic/not autotrophic
96
Past Paper Question - June 2011 Q2 a)+b) AS1 (Legacy Material)
Q2 Fungi are composed of eukaryotic cells, similar to both plant and animal cells. The cellular structure of a fungus is represented in the diagram.
a) Identify which structures labelled in the diagram above are also found
• in both plant and animal cells
__________________________________________________________
__________________________________________________________
• in plant cells but not in animal cells
__________________________________________________________
• in animal cells but not in plant cells
__________________________________________________________ [3]
b) Identify one feature which is unique to the cells of fungi.
___________________________________________________________ [1]
(Go do this past paper question)
Q2 a) In both: cytoplasm, mitochondria, endoplasmic reticulum, nucleus [Accept any 3];
in plant only: vacuole, cell wall;
in animal only: glycogen granules; [3]
b) The cell wall is made from chitin/the cell is multinucleated
(lacks dividing membranes)/cells contained within hyphae; [1]
97
Past Paper Question - June 2011 Q9 Section B AS1 (Legacy Material)
Q9 Give an account of the structure of proteins and their roles in the cell-surface membrane. [13]
Quality of written communication [2]
Section B
Q9 Thirteen points, with at least six from each section
Structure:
• the primary structure is the sequence of amino acids in a
polypeptide chain
• amino acids are linked by peptide bonds/condensation reactions
(between -NH2 and -COOH groups)
• the secondary structure is when the polypeptide chain winds into a
alpha helix or beta pleated sheet
• (the helix is) held in shape by hydrogen bonds (between -NH and
-C=O groups)
• the tertiary structure is when the helix folds further into a globule
(making it more compact)
• this is held in shape by bonds between neighbouring R-groups
• such as hydrogen bonds, ionic bonds, disulphide bridges and
hydrophobic interactions [any two]
• the quaternary structure is when more than one polypeptide is
present in the protein
• fibrous protein lack a tertiary structure
• conjugated proteins also have a non-protein part/prosthetic group
[appropriate example]
Role in cell-surface membrane:
• proteins stabilize membrane structure
• may act as hydrophilic channels
• that allow polar molecules (e.g. ions) to diffuse through the
membrane
• aquaporins are specific protein channels through which water can
travel
• some transmembrane proteins may act as carriers that can change
shape
• each carrier/channel is specific to one substance (only fits one
substance)
• some are used for facilitated diffusion
• others are used for active transport which requires energy
expenditure
• membrane bound enzymes
• glycoproteins/lipoproteins act as recognition sites/antigens on the
outer surface of a cell
• act as receptors
• act to anchor the cytoskeleton
• other appropriate role
[13]
98
Past Paper Question - January 2011 Q1 AS1 (Legacy Material)
Q1 The statements in the table below refer to some of the features found in prokaryotic and eukaryotic cells. Some features may be found only in one type of cell or in both cell types.
If a feature is present, place a tick (✓) in the appropriate box and, if the feature is absent, place a cross (✗) in the appropriate box. (Do not leave any boxes empty.)
Feature Prokaryotic cell Eukaryotic cell
Plasmid
Ribosome
Golgi apparatus
[3]
Q1 Plasmid ✓ ✗;
Ribosomes ✓ ✓;
| Golgi apparatus ✗ ✓; [3]
99
Past Paper Question - January 2011 Q4 a)i+ii)+b) AS1 (Legacy Material)
Q4 Photograph 1.4 is an electromicrograph of parts of two adjacent spongy mesophyll cells found in the leaf of a geranium plant.
(a) (i) Identify the structures labelled A to D.
A _________________________________
B _________________________________
C _________________________________
D _________________________________ [4]
(ii) Explain the position of the nuclei in these mesophyll cells. [1]
___________________________________________________________________
___________________________________________________________________
(b) Use the scale bar to calculate the magnification of this electronmicrograph. (Show your working) [3]
(Go do this past paper question)
Q4 a)i) A Granum/thylakoids/lamellae;
B cell wall;
C nucleolus;
D nuclear envelope; [4]
ii) Nuclei are against the cell wall as these cells have a large central vacuole/pushed against the cell wall/cell is turgid; [1]
b) Scale bar measured as 30mm;
equal to 30000μm;
magnification = 30000 / 0.5 = 60000; [3]
100
Past Paper Question - January 2011 Q5 a)i)+ii) AS1 (Legacy Material)
Q5 a) The diagram below represents the fluid mosaic model of the cell surface membrane.
i) Identify the structures labelled A to D.
A __________________________
B __________________________
C __________________________
D __________________________ [4]
ii) Place an X on the diagram to indicate the outer surface of the membrane. Give a reason for your answer. [1]
(Go do this past paper question)
Q5 a)i) A: carbohydrate/glycoprotein/glycocalyx;
B: polar/hydrophilic head/glycerol-phosphate (not just head);
C: non-polar/hydrophobic tails/fatty acid tails/hydrocarbon chains (not just tails);
D: protein; [4]
ii) X placed on top of diagram since the glycocalyx (structure A) is on the outer surface; [1]
101
Past Paper Question - June 2018 Q6 b) AS1 (Revised Spec.)
Q6 b) DNA holds the code for protein synthesis. It is copied into a type of RNA which is able to leave the cell nucleus through nuclear pores.
Suggest why DNA is not able to leave the nucleus. [2]
Q6 b) DNA is larger/longer molecule/DNA is millions of bases long; associated with histone proteins; [2]
[Teachers comment: i.e. the DNA molecule is too large (relative to RNA) to leave through nuclear pores]
