Topic 2: Genes and Health

Question 1

What are DNA and RNA polymers made up of?

Answer 1

Many repeating units called nucleotides.

Question 2

What is a nucleotide formed from?

Answer 2

• A pentose sugar (a sugar with 5 carbon atoms)
• A nitrogen-containing base
• A phosphate group

Question 3

What is the difference in structure between a DNA nucleotide and a RNA nucleotide?

Answer 3

DNA nucleotide consist of:
- A deoxyribose with hydrogen at the 2’ position.
- A phosphate group
- One of the four nitrogen bases - adenine (A), cytosine (C), guanine (G) or thymine (T)

RNA nucleotides consist of:
-A ribose sugar with a hydroxyl (OH) group at the 2’ position.
- A phosphate group
- One of the four nitrogenous bases - adenine (A), cytosine (C), guanine (G), or Uracil (U).

Question 4

What is the name of the sugar found in an DNA nucleotide?

Answer 4

A deoxyribose sugar
(It contains a hydrogen at the 2’ positions)

Question 5

What is the name of the sugar in a RNA nucleotide?

Answer 5

A ribose sugar
(it contains a hydroxyl (OH) group at the 2’ position)

Question 6

What four bases can be attached to a RNA nucleotide?

Answer 6

adenine (A), cytosine (C), Guanine (G), or Uracil (U)

Question 7

What four bases can be attached to a DNA nucleotide?

Answer 7

adenine (A), cytosine (C), Guanine (G), or thymine (T)

Question 8

Purine bases

Answer 8

The bases adenine and guanine are purines - they have a double ring structure.

Question 9

pyrimidine bases

Answer 9

The bases cytosine, thymine and uracil are pyrimidines - they have a single ring structure

Question 10

What is the name of the reaction that occurs when two separate nucleotides join together?

Answer 10

A condensation reaction

Question 11

Polynucleotide

Answer 11

A polymer composed of several nucleotides linked together (e.g. DNA and RNA)

Question 12

What is the name of the bond that forms between the phosphate group of one nucleotide and the pentose sugar of another nucleotide in a polynucleotide?

Answer 12

A phosphodiester bond

Question 13

Describe the structure of DNA

Answer 13

• Made of two antiparallel polynucleotides joined together by hydrogen bonding between the bases.
• The hydrogen bonds between the bases keep the strands coiled together in a double-helix shape.

Question 14

Name the bases of RNA

Answer 14

adenine (A), Uracil (U), Cytosine (C) and Guanine (G)

Question 15

Describe how mononucleotides are joined together in DNA

Answer 15

• The mononucleotides are joined through condensation reactions between the phosphate of one mononucleotide and the sugar group of another. As in all condensation reactions, water is a by-product.

Question 16

Describe how two single polynucleotide strands are joined to make a DNA double helix

Answer 16

• Two DNA polynucleotide strands join together by hydrogen bonding between the bases.
• Each base pairs with another specific base. This is known as complementary base pairing.
• Adenine pairs with thymine (A-T) forming two hydrogen bonds and cytosine pairs with guanine (C-G) forming three hydrogen bonds.
• The two antiparallel polynucleotide strands twist to form the DNA double-helix.

Question 17

What is gene?

Answer 17

A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain.

Question 18

What is the nature of the genetic code?

Answer 18

• The genetic code is non-overlapping (each base triplet is read in sequence, separate from the triplet before it and after it. Base triplet don’t share their bases).
• The genetic code is degenerate (there are more possible combinations of triplets than there are amino acids, meaning that some amino acids are coded for by more than one base triplet.) e.g. tyrosine can be coded for by UAU or UAC.

Question 19

What are the two stages of protein synthesis called?

Answer 19

Transcription and translation

Question 20

Transcription

Answer 20

1) Happens in the nucleus
2) The enzyme RNA polymerase attaches to the start codon of a gene in the DNA strand. The enzyme DNA helicase breaks the hydrogen bonds between the two DNA strands in the gene, separating them. One of the strands is then used as a template strand (the antisense strand).
2) Free RNA mononucleotides line up along the template strand via complementary base pairing.
3) Once the RNA mononucleotides have paired up with their specific bases, the RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand.
4) Once the RNA polymerase reaches a stop codon it detaches from the DNA and the mRNA moves out of the nucleus through a nuclear pore.
5) The hydrogen bonds reform and the DNA winds back up into a double helix.

Question 21

Translation

Answer 21

1) Takes place in the CYTOPLASM
2) The mRNA attaches to a RIBOSOME and TRANSFER RNA (tRNA) molecules carry AMINO ACIDS to the ribosome.
3) A tRNA molecule, with an ANTICODON that’s complementary to the base codon to the START CODON on the mRNA, attaches itself to the mRNA by COMPLEMENTARY BASE PAIRING.
3) The second tRNA molecule attaches itself to the next codon on the mRNA in the same way.
4) The two amino acids attached to the tRNA molecules are then joined together by a PEPTIDE BOND. The first TRNA moves away, leaving it’s amino acid behind.
5) The ribosome moves along to it’s next codon.
6) The process continues, producing a chain of linked amino acids (A POLYPEPTIDE CHAIN), until there’s a STOP CODON on the mRNA molecule.
7) The polypeptide chain MOVES AWAY from the ribosome and translation is complete.

Question 22

What are the monomers of proteins?

Answer 22

Amino acids

Question 23

Dipeptide

Answer 23

A dipeptide is formed when two amino acids monomers join together.

Question 24

Polypeptide

Answer 24

A polypeptide is formed when more than two amino acids join together.

Question 25

Proteins

Answer 25

Proteins are made up of one or more polypeptides

Question 26

2.9i) Describe the general structure of an amino acid

Answer 26

-Amino acids contain the elements carbon, hydrogen, nitrogen and oxygen.
- They consist of:
- Central carbon atom (a alpha carbon)
- NH2 (an amine group)
- COOH (carboxyl group)
- H (a hydrogen atom)
- ‘R’ (variable/side group).

Question 27

2.9ii) Explain the formation polypeptides

Answer 27

Amino acid monomers are linked together by condensation reactions to form polypeptides. A molecules of water is released during the reaction. The bonds formed between amino acids are called peptide bonds and they form between the carboxyl group on one amino acid and the amine group on another.

Question 28

2.9iii) Levels of protein structure

Answer 28

1) Primary structure
2) Secondary structure
3) tertiary (three-dimensional) structure
4) Quaternary structure

Question 29

2.9iiii) Primary Structure

Answer 29

• This is the specific order of amino acids in a polypeptide chain.
• The primary structure is important because it helps to determine the final 3 dimensional shape of the protein molecule and the shape of a protein is critical for its function.

Question 30

2.9iiii) Secondary structure

Answer 30

• Hydrogen bonds form between the amino acids in the polypeptide chain and these hydrogen bonds cause the chain to coil in a alpha helix or fold into a beta pleated sheet.
• The secondary structure is made more stable by hydrogen bonds, as even though they’re weak, there are many of them.

Question 31

2.1i) How are gas exchange surfaces in living organisms adapted for efficient diffusions?

Answer 31

• They give gas exchange organs (like the lungs) a large surface area to volume ratio.
• They’re thin (often just one layer of epithelial cells) - this provides a short diffusion pathway across the gas exchange surface.
• The organism also maintains a steep concentration gradient of gases across the exchange surface.

All of these features increase the rate of diffusion.

Question 32

Fick’s law of diffusion

Answer 32

Rate of diffusion is proportional to (area of diffusion surface x difference in concentration) / thickness of diffusion surface

Question 33

A section of alveolar epithelium has a surface area of 2.2um^2 and is 1.0um thick. The permeability constant of the alveolar epithelium for oxygen is 0.012s^-1. The concentration of oxygen on the one side of the epithelium (C1) is 2.3x10^-16 mol um^-3 and the concentration of oxygen on the other side (C2) is 9.0x10^-17 mol um^-3.
Calculate the rate of diffusion of oxygen across the alveolar epithelium.

Rate = p x A x ((C1-C2)/T)

Answer 33

Rate = P x A x ((C1-C2)/T)
P=permeability constant
A=Surface Area
(C1-C2) = difference in concentration
T = thickness of the exchange surface

Rate of diffusion:
0.012 x 2.2 x ((2.3x10^-16 - 9.0x 10^-17)/1.0)
= 3.7 x 10^-18 mol um^-2s^-1

Question 34

2.1ii) How is the structure of the mammalian lung adapted for rapid gas exchange?

Answer 34

The mammalian lungs have the following features, which all help to increase the rate of gas exchange:
- Lots of alveoli means there is a large surface area for diffusion to occur across.
- The alveolar epithelium and capillary endothelium (a type of epithelium that form the capillary wall) are each only one cell thick, giving a short diffusion pathway.
- All the alveoli have a good blood supply from capillaries - they constantly take away oxygen and bring more carbon dioxide, maintaining the concentration gradient.
- Breathing in and out refreshes the air in the alveoli, keeping the concentration gradient high.

Question 35

2.2i) Describe the structure and properties of cell membranes

Answer 35

• Cell membranes have a a ‘fluid mosaic’ structure.
• Phospholipids molecules form a bilayer. The bilayer is ‘fluid’ because the
  phospholipids are constantly moving.
• The membrane is partially permeable - fat soluble/non-polar molecules can move through the gaps between the phospholipids, but large molecules, like ions, can only pass through via channel proteins.
• Phospholipid molecules arrange themselves into a bilayer with the hydrophilic heads facing out towards the water on either side of the membrane (because hydrophilic attracts water). The hydrophobic tails are on the inside (because they repel water), making the centre of the bilayer hydrophobic. This means that the membranes don’t allow water-soluble substances (like ions) through it.

The phospholipid bilayer contains other components:
- PROTEINS are scattered through the bilayer and can move around within it. They are involved in cell transport and communication and can be intrinsic (embedded) or extrinsic (on the surface).
- CHOLESTEROL - fits in between the phospholipids and regulates membrane fluidity. They increase the fluidity at low temps by stopping the phospholipids from packing too closely together and decrease fluidity at high temperatures by forming bonds with the tails of phospholipids, stabilising them and causing them to pack more closely together. Cholesterol increases the mechanical strength and stability of membranes.
- GLYCOLIPIDS AND GLYCOPROTIENS are present on the surface of the cell, where they aid cell-to-cell communication. Glycoproteins are proteins are proteins with carbohydrates attached,, while glycolipids are lipids with carbohydrates attached. These bind with substances at the cells surface, e.g. hormones.

Question 36

Explain how the fluid mosaic model is based on scientific evidence

Answer 36

1) Before the 1970s, most scientists believed cell membranes composed of a PHOSPHOLIPID LAYER between TWO CONTINOUS LAYERS OF PROTEIN. This was because ELECTRON MICROSCOPE (EM) IMAGES appeared to show THREE LAYERS in a cell membrane.
2) In time, IMPROVED EM techniques showed a BILAYER of phospholipids, and NEW METHODS for ANALYZING PROTEIN showed that they were RANDOMLY DISTRIBUTED in cell membranes, not in a continuous layer.
3) Scientists also carried out experiments that proved the cell membrane was FLUID - e.g. they fused a MOUSE CELL with a HUMAN CELL, and found that the mouse and human MEMBRANE PROTIENS completely INTERMIXED throughout the cell membrane - the proteins could only MIX like this if the membrane was fluid.
4) All of this new evidence led to the FLUID MOSAIC MODEL.

Question 37

Osmosis

Answer 37

• Osmosis is the diffusion of free water molecules across a PARTIALLY PERMEABLE MEMBRANE from an area of HIGHER CONCENTRATION of water molecules to an area of LOWER CONCENTRATION of water molecules.
• (Water molecules will diffuse both ways through the membrane, but the net movement will be to the side with the lower concentration of water molecules.)

Question 38

Facilitated Diffusion

Answer 38

Facilitated diffusion usually occurs when a molecule cannot pass through the plasma membrane because of their polarity, charge or size. Proteins that aid in facilitated diffusion shield these molecules from the hydrophobic core of the membrane, allowing molecules to cross the membrane through a pore or channel.
- Facilitated diffusion moves particles DOWN A CONCENTRATION GRADIENT and is a PASSIVE PROCESS (It doesn’t require energy).
- Uses carrier and channel protiens

Question 39

What are the two proteins used in facilitated diffusion?

Answer 39

Carrier proteins and channel proteins

Question 40

Channel proteins

Answer 40

Channel proteins are TRANSMEMBRANE PROTEINS involved in facilitated diffusion. They form HYDROPHILIC tunnels across the membrane. These channels are HIGHLY SELECTIVE of the molecules they allow across, and will usually only aid in the transport of one molecule or a group of similar molecules. Channel proteins act like PORES in the membrane that let water molecules or small ions through quickly. They transfer CHARGED PARTICLES across the semipermeable biological membrane, down their concentration gradient.

Question 41

Carrier Proteins

Answer 41

Carrier proteins are proteins that are able to CHANGE THEIR SHAPE to allow LARGE MOLECULES to diffuse across the cell membrane, down their concentration gradient, but they do not actively push them through. Carrier proteins are VERY SELECTIVE and unique to each type of molecule that needs to be transported across a membrane.

Question 42

Active Transport

Answer 42

Active transport uses ENERGY to move MOLECULES and IONS across plasma membranes, AGAINST a CONCENTRATION GRADIENT. This process involve CARRIER PROTIENS.

Question 43

Where does the energy used in active transport come from?

Answer 43

The energy used in active transport comes from ATP:
ATP is produced by RESPIRATION and acts as IMMEDIATE ENERGY SOURCE in the cell. When ATP is HYDROLYSED (broken down) in the cell, energy is RELEASED. This energy is used to move the molecule against it’s concentration gradient.

Question 44

Endocytosis

Answer 44

Endocytosis is the process of ACTIVELY TRANSPORTING molecules, which are too large to be taken into the cell by carrier proteins, into the cell by ENGULFING it with its membrane.
(The cell surrounds a substance with a section of its cell membrane. The membrane then pinches off to form a VESICLE inside the cell containing the ingested substance).
(uses ATP)

Question 45

Exocytosis

Answer 45

• Exocytosis is the process by which a cell SECRETES LARGE SUBSTANCES (such as digestive enzymes or hormones) that have been produced inside the cell. New lipids and proteins produced inside the cell are processed in an organelle called the GOLGI APPARATUS. The Golgi Apparatus exists as a series of flattened sacs of membrane.
• VESICLES containing these substances PINCH OFF from the sacs of the GOLGI APPARATUS and MOVE TOWARDS the cell membrane. The vesicles FUSE with the CELL MEMBRANE and RELEASE their contents OUTSIDE the cell.
• Uses ATP as an energy source.

Question 46

Phagocytes

Answer 46

An example of a white blood cell that uses endocytosis to take in things like microorganisms and dead cells so that they can destroy them.

Question 47

2.9 iv) Globular proteins

Answer 47

Globular proteins are round structures. Like their name, globular proteins have a round, spherical formation. This is because the hydrophobic parts of the protein fold inwards while the hydrophilic parts become arranged around the external surface. Globular proteins are water soluble.

Question 48

2.9 iv) How does the structure of haemoglobin relate to its function?

Answer 48

• Haemoglobin is a GLOBULAR PROTEIN made of FOUR polypeptide chains. It CARRIES OXYGEN around the body in the blood.
• IT’s SOLUBLE, so can be EASILY TRANSPORTED in the blood (this is due to their hydrophobic R-groups facing inwards (helping preserve their 3 dimensional spherical shape) and their hydrophilic R-groups facing outwards (helping maintain it’s solubility)).
• Each subunit has an IRON-containing PROSTHETIC HAEM GROUP that is able to BIND TO OXYGEN.

Question 49

2.9 iv) Fibrous proteins

Answer 49

• Made of LONG, INSOLUBLE POLYPEPTIDE CHAINS that are TIGHTLY COILED round each other to form a ROPE SHAPE.
• They are INSOLUBLE in water due to their large number of hydrophobic R groups.
• The chains are held together by LOTS OF BONDS (e.g. disulfide and hydrogen bonds), which make the proteins STRONG. Because of their strength they are often found in SUPPORTIVE TISSUE.

Question 50

2.9) iii) How does a protein’s primary structure determine it’s 3D structure and properties?

Answer 50

The amino acid sequence of a protein determines what bonds will form and how the protein will fold up into it’s 3D structure. The 3D structure of a protein determines its properties. Its properties relate to its function in the body.

Question 51

2.9 iv) How is the structure of collagen related to its function?

Answer 51

Collagen is a STRONG, FIBROUS PROTEIN that form connective tissue in ANIMALS.
- The presence of the MANY HYDROGEN BONDS within the TRIPLE HELIX STRUCTURE of collagen results in GREAT TENSILE STRENGTH. This enables collagen to be able to withstand large pulling forces without stretching or breaking

Question 52

2.9 iii) Tertiary Structure

Answer 52

• Interactions between R groups (the side chains on amino acids) creates the complex 3D tertiary structure of the protein.
• The 3D structure is usually coiled or folded.
  -There are many weak and strong interactions that determine the final 3D shape of the protein:
• hydrogen bonds
• ionic bonds
• disulphide bonds
• hydrophobic and hydrophilic interactions.
• For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure.

Question 53

Ionic bonds

Answer 53

An ionic bond is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions.

Question 54

Disulfide bonds

Answer 54

Whenever two molecules of the amino acid cysteine come close together, the sulfur atom in one cysteine bonds to the sulfur in the other cysteine, forming a disulfide bond. (S-S bonds)

Question 55

Hydrophobic and hydrophilic interactions in proteins

Answer 55

When hydrophobic (water-repelling) groups are close together in the protein, they tend to clump together. This means that hydrophilic (water-attracting) groups are more likely to be on the outside, which affects how it folds up in into it’s final structure.

Question 56

hydrogen bond

Answer 56

A weak bond of attraction between a hydrogen atom and another atom.
Hydrogen bonds are relatively weak, but when there are many the overall stability of the tertiary structure increases.

Question 57

Quaternary structure

Answer 57

Some proteins are made of several different polypeptide chains held together by bonds. The quaternary structure is the way these polypeptide chains are assembled together.
(A protein consisting of more than one amino acid chain)
The quaternary structure can be influenced by all the bonds involved in the tertiary structure.

Question 58

A conjugated protein

Answer 58

A conjugated protein is a specific type of globular protein that contains a prosthetic group

Question 59

Prosthetic group

Answer 59

A prosthetic group is a non-protein part of a protein. in haemoglobin this is the heme group.

Question 60

Enzyme models

Answer 60

• The lock and key model - The substrate fits into the enzyme’s active site in the same way that a key fits into a lock, forming an enzyme-substrate concentration and then producing products. The enzyme is unchanged after the reaction.
• The induced fit model - As the substrate binds, the active site changes shape slightly. This helps to explain why enzymes are so specific and only bond to one particular substrate.

Question 61

Enzyme’s

Answer 61

Enzymes are biological catalysts that speed up the rate of chemical reactions without being used up or undergoing permanent change. They are made up of proteins and are essential for many cellular processes, such as metabolism and energy production. Enzymes lower the activation energy required for a reaction to occur, allowing reactions to happen at a lower temperature so they can take place at body temperature. Enzymes are usually specific to only one reaction and have an active site that acts as a catalyst.

Question 62

How do enzyme’s reduce the activation energy required for reactions?

Answer 62

When a substrate fits into the enzyme’s active site it forms an enzyme-substrate complex - it’s this that lowers the activation energy. Here’s two reasons why:
1) If two substrate molecules need to be JOINED, being attached to the enzyme holds them CLOSE TOGETHER, REDUCING any REPULSION between the molecules so they can bond more easily.
2) If the enzyme is catalysing a BREAKDOWN REACTION, fitting into the active site puts a STRAIN on bonds in the substrate, so the substrate molecule BREAKS UP more easily.

Question 63

Enzyme specifity and structure

Answer 63

• Enzymes are PROTEINS. Their properties relate to their TERTAIRY STRUCTURE.
• Enzymes are VERY SPECIFIC - they usually only catalyse ONE reaction.
• This is because ONLY ONE COMPLEMENTARY SUBSTRATE WILL FIT into the active site.
• The active site’s SHAPE is determined by the enzyme’s TERTAIRY STRUCTURE (which is determined by the enzyme’s primary structure)
• Each different enzyme has a different tertiary structure and so a different shaped active site, if the substrate shape doesn’t match the active site, an enzyme-substrate complex won’t be formed and the reaction won’t be catalysed.
• If the tertiary structure is ALTERED in any way, the SHAPE of the active site will CHANGE. This means the SUBSTRATE WON’T FIT into the active site, an enzyme-substrate complex WON’T be formed and the enzyme will no longer be able to carry out it’s function.
• The tertiary structure of an enzyme may be ALTERED by changes in pH or TEMPERATURE.
• The PRIMARY STRUCTURE of a protein is determined by a GENE. if a MUTATION occurs in that gene, it could change the structure of the enzyme PRODUCED.

Question 64

Intracellular enzymes

Answer 64

Catalyse reactions inside cells

Question 65

Extracellular enzymes

Answer 65

Produced and secreted by cells to catalyse reactions outside cells.

Question 66

Semi-conservative replication

Answer 66

DNA copies itself before cell division so that each new cell has the full amount of DNA. The method is called semi-conservative replication because half of the strands in each new DNA molecule are from the original DNA molecule. This means that there’s genetic continuity between generations of cells.

Question 67

2.11 i) Describe the process of DNA replication, including the role of DNA polymerase.

Answer 67

DNA replicates by semi-conservative replication:
Stage 1: The enzyme DNA HELICASE BREAKS the HYDORGEN BONDS between bases on the two POLYNUCLEOTIDE DNA strands. This makes the helix UNWIND to form two single strands.
Stage 2: Each ORIGINAL single strand acts as a TEMPLATE for a new strand. COMPLEMENTARY BASE PAIRING means that FREE-FLOATING DNA NUCLEOTIDES are attracted to their complementary EXPOSED BASES on each original template strand - A with T and C with G.
Stage 3: CONDENSATION REACTIONS join the nucleotides of the new strands together - catalysed by the enzyme DNA POLYMERASE. Hydrogen bonds FORM between the bases on the original and new strands.
Stage 4: Each new DNA molecule contains ONE STRAND from the ORIGINAL DNA molecule and one NEW STRAND - semi-conservative replication.

Question 68

The role of DNA polymerase in DNA replication

Answer 68

• In the nucleus there are** free nucleotides** (also known as nucleoside triphosphates or ‘activated nucleotides’) which contain three phosphate groups.
• The bases of the free nucleoside triphosphates align with their complementary bases on each of the template DNA strands.
• The enzyme DNA polymerase synthesises new DNA strands from the two template strands.
• It does this by catalysing condensation reactions between the deoxyribose sugar and the phosphate groups of the adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
• DNA polymerase **cleaves (breaks off) the two extra phosphates **and uses the energy released to created the phosphodiester bonds (between adjacent nucleotides).
• Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands.

Question 69

what would happen if DNA replication was conservative?

Answer 69

If the method was Conservative, the original DNA strands would stay together and the new DNA molecules would contain two new strands.

Question 70

Meselson and Stahl’s experiment

Answer 70

1) Two samples of bacteria were grown - one in a nutrient broth containing light nitrogen (14N), and one in a broth with heavy nitrogen (15N). As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA. So the nitrogen gradually became part of the bacteria’s DNA.
2) A sample of DNA was taken from each batch of bacteria, and spun in a centrifuge. The DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube than the DNA from the light nitrogen bacteria - because it’s heavier.
3) Then bacteria grown in the** heavy** nitrogen broth were** taken out and put in a broth containing only light nitrogen. The bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge.
4) If the replication was conservative, the original heavy DNA, which would still be together, would settle at the bottom **and the **new light DNA would settle at the top.
5) If replication was semi-conservative, the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one strand of new DNA containing light nitrogen. So the DNA would settle out in the middle.
6) The DNA did settle out in the middle**, showing that the DNA molecules contained a mixture of heavy and light nitrogen. The bacterial DNA had **replicated semi-conservatively **in the light nitrogen.

Question 71

2.12.i) Genetic Mutations

Answer 71

• Mutations are changes to the base sequence of DNA. They can be caused by errors during DNA replication.
• The order of DNA bases in a gene determines the order of amino acids in a particular protein. If a mutation in a gene, the primary structure of the protein it odes for could be altered: This could change the final 3D shape of the protein so it doesn’t work properly.
• If a mutation occurs in a gene it can cause a gentic disroder, which is then passed on e.g. Cystic fibrosis.

Question 72

2.12.i) Gene mutations: What are the type of errors that can occur?

Answer 72

• Substitution - One base is substituted/swapped with another.
• Deletion - One base is randomly deleted (frameshift mutation)
• Insertion - One extra base is added (frameshift mutation)
• Duplication - One or more bases are repeated (can be a frameshift mutation).
• Inversion - A sequence of bases is reversed.

Question 73

Genetic disorders

Answer 73

Genetic disorder are often caused by a mutation in a gene that results in a differently functioning or non-functioning protein that alters the phenotype of the individual.
Some genetic disorders can be caused by lots of different mutations.

Question 74

Cystic Fibrosis

Answer 74

• A genetic disorder caused by a recessive allele, so a person will only have the disorder if they’re homozygous for the allele - they must inherit a recessive allele from each parent.

Question 75

Why does cystic fibrosis cause the production of thick sticky mucus?

Answer 75

1) Cystic Fibrosis is caused my a mutation in the gene that codes for the CFTR protein.
2) CFTR is a channel protein. It transports chloride ions out of cells into mucus - this causes water to move into the mucus by osmosis, which makes mucus watery.
3) Mutant CFTR protein is much ** less efficient** at transporting chloride ions **out ** of the cell, so less water moves out by osmosis. This makes the mucus of people with CF abnormally thick and sticky.
5) This thick and sticky mucus causes problems in the respiratory, digestive and reproductive systems.

There are many different mutations in the CFTR gene that can lead to cystic fibrosis.

Question 76

Effects of cystic fibrosis on the respiratory system

Answer 76

1) The Celia are unable to move the mucus towards the throat because it’s so thick and sticky.
2) This means that mucus builds up in the airways.
3) Some airways can become completely blocked by the mucus - gas exchange can’t take place in the area bellow the blockage.
4) This means that the surface area available for gas exchange is reduced, causing breathing difficulties.
5) People with CF are also more prone to lung infections as mucus containing microorganisms can’t be removed.

Question 77

Effects of cystic fibrosis on the digestive system

Answer 77

1) The tube that connects the pancreas to the small intestine can become blocked with mucus - preventing digestive enzymes produced from the pancreas from reaching the small intestines. This reduces the ability of someone with CF to digest food and so fewer nutrients can be absorbed.
2) The mucus can cause cysts (growths) to form in the pancreas. These inhibit the production of enzymes, which also reduces the ability to digest food and absorb nutrients.
3) The mucus lining the small intestines is abnormally thick - this inhibits the absorption of nutrients.

Question 78

The effects of cystic fibrosis on the reproductive system

Answer 78

1) In some men with CF, the tubes connecting the testicles (where sperm are produced) to the penis are absent and can become blocked by the thick mucus in others. So, any sperm produced can’t reach the penis.
2) In women, thickened cervical mucus can prevent the sperm from reaching the egg. The sperm has to travel through this mucus to reach the egg - thick mucus reduces the motility of the sperm, reducing it’s chances of making it to the egg.

Question 79

Gene

Answer 79

A sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic e.g. the gene for eye colour.

Question 80

Allele

Answer 80

Alleles are different forms/version of genes that code for different versions of the same characteristics. They are found in the same place on the chromosome and determine the phenotype (expressed characteristics) of an organism.

Question 81

Genotype

Answer 81

The alleles a person has, e.g. BB, Bb, or bb for eye colour.

Question 82

Phenotype

Answer 82

The characteristics displayed by an organism, e.g. brown eyes.

Question 83

Recessive

Answer 83

An allele whose characteristics only appear in the phenotype if two copies are present. e.g. bb

Question 84

Dominant

Answer 84

An allele whose characteristic appears in the phenotype even when there’s only one copy. e.g. Bb

Question 85

Incomplete dominance

Answer 85

When the trait from a dominant allele isn’t completely shown over the trait produced by the recessive allele, so both alleles influence the phenotype.
Some flowers show incomplete dominance e.g. snapdragons can have alleles for red flowers (RR), white flower (rr) or pink flowers (Rr).

Question 86

heterozygote

Answer 86

An organism that carries two different alleles for a certain characteristic, e.g. Bb.

Question 87

Homozygote

Answer 87

An organism that carries two coppies of the same allele for a certain characteristic, e.g. BB or bb.

Question 88

Uses Of Genetic Screening

Answer 88

Genetic screening involves analysing DNA to see if it contains alleles for genetic disorders.
The three main uses are:
1) Identification of carriers
2) Preimplantation Genetic Diagnosis (PGD)
3) Prenatal Testing

Question 89

Identification of carriers

Answer 89

carrier testing is offered to people with a family history of genetic disorders to see if they carry an allele that causes a disorder. Couples can be tested before having children to determine the chances of any future children having the disorder.

Benefits:
- Carrier testing allows people to make informed decisions about things like whether to have children and whether to carry out prenatal testing if the women is pregnant.

Social and ethical issues:
- Finding out you’re a carrier may cause emotional distress or affect your ability to find a partner
- Test aren’t always 100% accurate - they could give a false result, which means poor decisions could be made based on incorrect information.
- Other genetic abnormalities may be found, which could cause further stress.
- concerns that results of genetic tests could be used by employers or life insurance companies - resulting in genetic discrimination.

Question 90

Preimplantation Genetic Diagnosis (PGD)

Answer 90

Involves screening embryos produced in IVF for genetic disorders before they’re implanted into the woman. Only embryos without the genetic disorder will be implanted.

Advantages:
- Reduces the chance of having a baby with a genetic disorder.
- Performed before implantation so avoids the issue of abortion.

Social and ethical issues:
- It can be used to find out other characteristic (e.g. gender, eye-colour) - leading to concerns that in the future, embryos may be selected for other characteristics (designer babies).
- False results could provide incorrect information and lead to poor decisions being made about the fate of embryo.

Question 91

Prenatal testing

Answer 91

Involves screening fetuses for genetic disorders and are offered to pregnant women with a family history of genetic disease.
Two types:
- Amniocentesis
- Chorionic villus sampling (CVS)

Question 92

Amniocentesis

Answer 92

• Usually carried out at 15-20 weeks of pregnancy.
• A sample of amniotic fluid is obtained via the abdomen using a very fine needle. This fluid contains fetal cells. The cells contain DNA, which can be analysed.
• 1% risk of micarraige.
• Results aren’t available for 2-3 weeks after sample is taken. Rapid test results, which only look at few common disorders, are available in 3-4 days.

Question 93

Chorionic Villus Sampling (CVS)

Answer 93

• Usually performed at 11-14 weeks of pregnancy - an early decision can be made, meaning that the procedure is less physically traumatic than amniocentesis.
• A sample of cells is taken from the chorionic villi/placenta (part of the fetus that connects it to it’s mother). The cells contains fetal DNA which can be analysed. This procedure is done via either the abdomen using a fine needle, or the vagina, using a catheter.
• CVS has a 1-2% risk of miscarriage, which is greater than Amniocentesis.
• Initial results available in a few days but more detailed results can take two weeks or more.

Question 94

Benefits and Implications of parental genetic screening

Answer 94

Benefits:
- Allows parents to make informed decisions.
- Helps parents prepare for the future care of the child - any medical treatment can be started immediately after birth.

Social and ethical Issues:
- Slightly increases the risk of miscarriage.
- False results could provide incorrect information.
- Some people considered it unethical to abort a fetus because it has a genetic disorder.

Question 95

Calorimeter

Answer 95

A machine that passes light of specific wavelength through a liquid and measure how much of that light is absorbed.

Question 96

Core practical 3: Investigating Cell membrane structure
Method for investigating how temperature affects beetroot membrane permeability

Answer 96

Beetroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane the the more pigment leaks out the cell.

Soak some beetroot cubes in water at different temperatures:
1) Use a cork borer to cut beetroot into 5 identical cylinders. Then using a scalpel, carefully cut 5 equally sized discs of beetroot on a cutting board.
2) Rinse the beetroot to removes any pigment released during cutting
3) Use a measuring cylinder or pipette to measure 5cm^3 of water into five different test tubes.
4) Place the test tubes into water baths at different temps, e.g. 10, 20, 30, 40 , 50 DC, for around 5 minutes.
5) Then place the 5 pieces of beetroot into the 5 different test tubes, for the same length of time (use stopwatch).
6) Remove the pieces of beetroot, leaving just the coloured liquid in the test tubes.

Use a colorimeter to measure the absorbance of the coloured liquid:
1) allow colorimeter 5 mins to stabilise, then set it up using a blue filter. Use pure distilled water to calibrate machine to zero.
2) Use pipette to transfer a sample of the liquid from the first beetroot test tubes to a clean cuvette - it should be about 3/4 full.
3) Put cuvette in colorimeter and record absorbance of the coloured solution.
4) Repeated steps 2-3 for the liquids in the remaining test tubes (using a clean pipette and cuvette each time).
5) Analyse results - the higher the absorbance reading, the less light is passing through the solution. This means more pigment has been released, so the higher the permeability of the membrane.

Question 97

Core practical 3: How does increasing the temperature affect membrane permeability

Answer 97

As the temperature increases, the permeability of the cell-surface membrane also increases. This is because the proteins in the membrane denature as the heat damages the bonds in their tertiary structure. This creates gaps in the membrane so it is easier for molecules to pass through it.

At low temperatures, phospholipids have little energy and are packed closely together to make the membrane rigid. This causes a decrease in permeability and restricts molecules from crossing the membrane.
NB: At very low temperatures, ice crystals can form which pierce the cell membrane and increase the permeability

Question 98

Core practical 3: What is the affect of alcohol concentration on membrane permeability

Answer 98

Increasing the alcohol concentration (i.e. ethanol) increases membrane permeability. This is because alcohol dissolves the lipids in the cell membrane, so the membrane loses it’s structure.
Lipids in the cell membrane are soluble in detergents.

Question 99

Biological catalysts

Answer 99

Proteins/enzymes that are made in organisms/cells and speed up the rate of chemical reactions in the body by lowering the activation energy needed for reactions to take place.

Question 100

Core Practical 4: Investigate the effect of enzyme and substrate concentrations on the initial rates of reaction

Answer 100

• Trypsin Enzyme (A protease)
• Casein - milk powder solution - substrate (protein)
• Milk powder will clear as trypsin breaks down the protein in milk (casein)

Variables:
IV: Trypsin conc. (0.1%-0.5%)
DV: measure of absorbance (using calorimeter)
CV’s: Same stock of enzyme and milk powder, vol. of milk solution, agitation (stirred up), temp, pH.

Method:
1) Make up a range of concentrations of trypsin (0.1%-0.5%)
2) Have a beaker of milk powder solution (set conc. and vol.)
3) . Make a control by adding 2cm^3 of trypsin solution and 2cm^3 of distilled water. Use this to set the colorimeter absorbance to zero Calibrate to red absorbency.
4) Add 2cm^3 of milk powder solution to cuvette, add 2cm^3 of chosen trypsin conc. to this. mix for 10 seconds
5) Place in colorimeter and take a reading.
6) Take reading every 20 seconds for 5 minutes.
7) Repeat 3x for each conc. of trypsin, calc. mean, exclude anomalous results.
8) Plot data on graph measure of absorbance against time and draw tangent to work out initial rate.

Conclusion:
Increasing the conc. of trypsin enzyme, increases initial rate of reaction as there are more enzyme molecules (trypsin) for the substrate molecules (milk) to bind with. This is until the substrate becomes a limiting factor.

To work out the effect of substrate concentration on initial rate of reaction:
Keep trypsin concentration the same and vary the concentration of the milk solution.

Question 101

Give two differences between endocytosis and exocytosis

Answer 101

• Exocytosis involves (molecules/substances) leaving the cell whereas endocytosis involves (molecules/substances) entering the cell. (1)
• Exocytosis involves vesicles fusing with cell surface membranes whereas endocytosis involves the formation of vesicles (from the cell surface membrane). (1)

Question 102

Compare the processes if active transport and facilitated diffusion

Answer 102

• Active transport only involves carrier proteins.
• However, facilitated diffusion involves both carrier and channel proteins.
• Facilitated diffusion is a passive process, whereas active transport requires energy in the form of ATP.
• Facilitated diffusion occurs down a concentration gradient, whereas active transport is against the concentration gradient.