dc2 textbook

Question 1

nucleotide strucutre

Answer 1

• a pentose sugar (so called because it has five carbon atoms) * a phosphate group * a nitrogen-containing organic base. These are: cytosine C, thymine T , Uracil U , adenine A and guanine G.

Question 2

how are the components of a nucleotide joined

Answer 2

condensation reactions

Question 3

what id dna

Answer 3

sequence of bases

Question 4

what is protein

Answer 4

sequence of amino axids

Question 5

what is RNA

Answer 5

-polymer made up of nucleotides
-single,short,polynucleotide chain
-ribose
-trasnfers genetic info from dna to ribosomes
-ribosomes themselves are made up of proteins and another type of rna
-involved in protein synthesis

Question 6

dna structure

Answer 6

pemose sugar is deoxyribose and the organic bases are adenine, thymine, guanine and cytosine. DNA is made up of two uracilstrands of nucleotides (polynucleotides). Each of the two strands is extremely long, and they are joined together by hydrogen bonds formed between certain bases

Question 7

base pairings

Answer 7

Adenine always pairs with thymine * Guanine always pairs with cyt0sine
adenine and thymine in DNA are always the same, a n d so a re the quantities o f guanine and cytosine

Question 8

stability of dna

Answer 8

phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
Hydrogen bonds link the organic base pa irs forming bridges (rungs) between the phosphodiester uprights. As there are three hydrogen bonds between cystosine and guanine, the higher the proportion of C-G pairings. the more stable the DNA molecule.

Question 9

function of dna

Answer 9

passing genetic information from cell to cell and generation to generation.

It is a very stable structure whic h no rmally p asses fro m generation to generation without change.

ts two separate strands are joined only with hydrogen bonds, which allow them to separate during DNA replication

large molecule and therefore carries an immense amount of genetic information.

Base pairing leads to DNA being able to replicate and to transfer information as mRNA.

Question 10

5’ and 3’

Answer 10

The 5’ carbon has an attached phosphate group, while the 3’ has a hydroxyl group.
When nucleotides are organised into the double strands of a DNA molecule, one strand runs in the 5’ to 3’ direction while th e other runs the opposite way - in the 3’ to 5’ direction. The two strands are therefore said to be antiparallel.

Question 11

why dna must be replicated

Answer 11

ensure that all the daughter cells have the genetic information to produce the enzymes and other proteins that they need.

Question 12

semi conservative replication

Answer 12

The enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of DNA. * As a result the double helix separates into its two strands and unwinds. * Each exposed polynucleotide strand then acts as a template to which complementary free nucleotides bind by specific base pairing * Nucleotides are joined together in a condensation reaction by the enzyme DNA polymerase to form the ‘missing’ polynucleotide strand on each of the two original polynuclcotidc strands of DNA. * Each of the new DNA molecules contains one of the original DNA strands. that is, half the original ONA has been saved and built into each of the new DNA molecules

Question 13

conservative model

Answer 13

suggested that the original DNA molecule remained intact and that a separate daughter DNA copy was built up from new molecules of deoxyribose, phosphate and organic bases. Of the two molecules produced, one would be made of entirely new material while the other would be entirely original material

Question 14

semi conservative model

Answer 14

proposed that the original DNA molecule split into two separate strands, each of which then replicated its mirror image [i.e. the missing half). Each of the two new molecules would therefore have one strand of new material and one strand of original material

Question 15

three facts that meselson and stahl based their work on

Answer 15

• All the bases in ONA contain nitrogen. * Nitrogen has two forms: the lighter nitrogen 14Nand the isotope 15N, which is heavier. * Bacteria will incorporate nitrogen from their growing medium into any new DNA that they make

Question 16

structure of atp

Answer 16

adenine - a nitrogen-containing organic base * ribose -a sugar molecule with a 5-carbon ring structure (pentose sugar) that acts as the backbone t0 which the other parts are attached * phosphates -a chain of three phosphate groups.

Question 17

how atp stores energy

Answer 17

The bonds between these phosphate groups are unstable and so have a low a ctivat io n energy, which means they are easily broken. When they do break they release a considerable amount of energy.

Question 18

synthesis of atp

Answer 18

ATP to ADP is a reversjble reaction
can be used to add an inorganic phosphate to ADP to re-form
catalysed by the enzyme ATP synthase
water is removed in this process, the reaction is known as a condensation reaction.

Question 19

three ways atp from adp

Answer 19

• in chlorophyll-containing plant cells during photosynthesis (photophosphorylation) * in plant and animal cells during respiration (oxidative phosphorylation) * in plant and animal cells when phosphate groups are transferred from donor molecules to ADP

Question 20

roles of atp

Answer 20

metabolic processes
movement
active transpor
secretion
activation of molecules

Question 21

metabolic processes

Answer 21

ATP provides the energy needed to build up macromolecules from their basic units. For example, making starch from glucose or polypeptides from amino acids.

Question 22

movement

Answer 22

ATP provides the energy for muscle contraction. In muscle contraction, ATP provides the energy for the filaments of muscle to slide past one another and therefore shorten the overall length of a muscle fibre.

Question 23

active transport

Answer 23

TP provides the energy to change the shape of carrier proteins in plasma membranes. This allows molecules or ions to be moved against a concentration gradient.

Question 24

secretion

Answer 24

ATP is needed to form the lysosomes necessary for the secretion of cell products.

Question 25

activation of molecules

Answer 25

The inorganic phosphate released during the hydrolysis of ATP can be used to phosphorylate other compounds in order to make them more reactive, thus lowering the activation energy in enzyme-catalysed reactions. For example ­the addition of phosphate to glucose molecules at the start of glycolysis.

Question 26

water properites

Answer 26

dipolar
spec heat cap
latent heat of vapouiraation
cohesion and surface tension
metabolism
solvent

Question 27

spec heat cap

Answer 27

it takes more energy to heat a given mass of water. that is water has a high specific heat capacity. Water therefore acts as a buffer against sudden temperature variations, making the aquatic environment a temperature-stable one. As organisms are mostly water, it also buffers them agains t sudden temperature changes especially in terrestrial environments.

Question 28

latent heat cap

Answer 28

Hydrogen bonding between water molecules means that it requires a lot of energy to evaporate l gram of water. This energy is called the latent heat of vaporisation. Evaporation o f wacer such as sweat in mammals is therefore a very effective means of cooling because body heac is used to evaporate the water.

Question 29

cohesion and tension

Answer 29

The tendency of molecules to stick together is known a s cohesion. With its hydrogen bonding, water has large cohesive forces and these allow it tO be pulled up through a tube, such as a xylem vessel in plan ts. In the same way, where water molecules meet air they tend
to be pulled back into the body of water rather than escaping from it. This force is called surface tension and means that the water surface acts like a skin and is strong enough to support small organisms such as pond skaters

Question 30

metabolism

Answer 30

Water is used to break down many complex molecules by hydrolysis, for example, proteins to amino acids. Water is also produced in condensation reactions. * Chemical reactions take place in an aqueous medium. * Water is a major raw material in photosynthesis.

Question 31

solvent

Answer 31

gases such as oxygen and carbon dioxide * wastes such as ammonia and urea * inorganic ions and small hydrophilic molecules such as amino acids, monosaccharides and ATP * enzymes, whose reactions take place in solution.

Question 32

iron ions

Answer 32

found in haemoglobin where they play a role in the transport of oxygen

Question 33

phosphate ions

Answer 33

that form a structural role in DNA molecules (Topic 2.1) and a role in storing energy in ATP molecules

Question 34

hydrogen ions

Answer 34

important in determining the pH of solutions and therefore the functioning of enzymes

Question 35

sodium ions

Answer 35

important in the transport of glucose and amino acids across plasma membranes.

Question 36

These responses involve a type of white blood cell called a lymphocyte and take two forms:

Answer 36

cell-mecliared responses involving T lymphocytes * hum oral respon ses in volving B lymphocytes.

Question 37

How lymphocytes recognise cells belonging to the body

Answer 37

• There are probably around ten million different lymphocytes present at any time, each capable of recognising a different chemical shape. * In the fetus, these lymphocytes are constantly colliding with other cells. * Infection in the fetus is rare because it is protected from the outside world by the mother and, in particular, the placenta. * Lymphocytes will therefore co llide almost exclusively with the body’s own material (self). * Some of the lymphocytes will have receptors that exactly fit those of the body’s own cells. * These lymphocytes eirher die or are suppressed. * The only remaining lymphocytes are those that might fit foreign material (non-self), and cherefore only respond to foreign material. * In adults, lymphocytes produced in the bone marrow initially only encounter self-antigens. * Any lymphocytes that show an immune response to these self­antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes. * No clones of these anti-seH lymphocytes will appear in the blood, leaving only those that might respond to non-sell antigens.

Question 38

phagocytosis

Answer 38

• Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen (e.g., a bacterium) . * Phagocytes have several rccepwrs on their cell-surface membrane that recognise, and attach to, chemicals on the surface of the pathogen. * They engulf the pathogen to form a vesicle, known as a phagosome. * Lysosomes move towards the vesicle and ruse with it. * Enzymes called lysozymes are present within the lysosome. These lysozymes destroy ingested bacteria by hydrolysis of their cell walls. The process is the same as that for the digestion of food in the intestines. namely the hydrolysis or larger, insoluble molecules into smaller. soluble ones. * The soluble products [rom the breakdown ol’ the pathogen are absorbed into the cytoplasm o[ the phagocyte .

Question 39

antigens

Answer 39

recognised as non-self (foreign) by the immune system and stimulates an immune response. Antigens are usually proteins that are part of the cell-surface membranes or cell walls of invading cells, such as m icroorganisms, or a bnormal body cells, such as cancer cells.

Question 40

lymphocytes

Answer 40

B lymphocytes (B cells)
T lymphocytes (T cells)

Question 41

B lymphocytes (B cells

Answer 41

mature in the bone marrow. They are associated with humoral immunity, that is, imm u ni ty in volvin g antibodies that arc present in body fluids, or ‘ humour’ such as blood plasma

Question 42

t lymphocytes

Answer 42

mature in the thymus gland. They are associated with cell-mediated immunity, tha t is immunity involving body cells.

Question 43

cell mediated immunity

Answer 43

l Pathogens invade body cells or are taken in by phagocytes. 2 The phagocyte places antigens from the pathogen on its cell­surface membrane. 3 Receptors on a specific helper T cell (TH cell) fit exactly onto these antigens. 4 This attachment activates the T cell to divide rapidly by mitosis and form a clone o f gen etically identical cells. 5 The clon ed T cells: a develop into memory cells that enable a rapid response to future infectio ns by the same pathogen b stimulate phagocytes to engulf pathogens by phagocytosis c stimulate B cells to divide and secrete their antibody d activate cyto toxic T cells (Tc cells).

Question 44

humoral immunity

Answer 44

I The surface antigens of an invading pathogen are taken up by a B cell. 2 The B cell processes the antigens and presents them on its surface. 3 Helper T cells (activated in the process described in Topic 5.3) attach to the processed antigens on the B cell thereby activating the B cell. 4 The B cell is now activated to divide by mitosis to give a clone of plasma cells. 5 The cloned plasma cells produce and secrete the specific antibody that exactly tits the antigen on the pathogen’s surface. 6 The antibody attaches to antigens on th e pathogen and destroys them (see Topic 5.5). 7 Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies. This is the secondary immune response.

Question 45

Targeting medication to specific cell types by attaching a therapeutic drug to an antibody

Answer 45

• Monoclonal antibodies are produced that are specific to antigens on cancer cells. * These antibodies are given to a patient and auach themselves to the receptors on their cancer cells.
• They attach to the surface of their cancer cells and block the chemical signals that stimulate their uncontrolled growth

Question 46

ethical use of monolconal antibodies

Answer 46

Production of monoclonal antibodies involves the use of mice
Monoclonal anribodies have been used successfully to rreat a number of diseases. including cancer and diabetes
muhiple organ failure

Question 47

passive immunity

Answer 47

introduction of antibodies into individuals from an outside source. No direct contact with the pathogen or its antigen is necessary LO induce immunity. Immunity is acquired immediately.

Question 48

active immunity

Answer 48

produced by stimulating the production of antibodies by th e individuals’ own immune system. Direct contact with th e pathogen or its antigen is necessary. immunity takes time to develop. It is generally long-lasting

Question 49

herd immunity

Answer 49

when a sufficiently large proportion of the population has been vaccinated to make i.t difficult for a pathogen to spread within that population. The concepl i s based on the idea that pathogens are passed from individual to individual when in close contact. Where the vast majority or the population is immune, it is highly improbable that a susceptible individual will come in contact with an infected person. 1n this way those individuals who are nor immune to the disease are nevertheless protected.

Question 50

Why vaccination may not eliminate a disease

Answer 50

The pathogen may mutate frequently, so that its antigens change suddenly rather than gradually. This means that vaccines suddenly become ineffective because the new antigens on the pathogen are no longer recognised by the immune system. As a result the immune system does not produce the antibodies to destroy the pathogen. This antigenic variability happens with the influenza virus, which drnnges its amigens frequently. Immunity is therefore short-lived and individuals may develop repeated bouts of influenza during their lifetime .

Question 51

The ethics of using vaccines

Answer 51

use or animals.
side-effects
unknown healrh risks

Question 52

Why antibiotics are ineffective against viral diseases like AIDS

Question 53

Structure ofthe human immunodeficiency virus

Answer 53

lipid envelope
attachment proteins.
protein layer called the capsid that encloses two single strands of RNA
reverse transcriptase