Chapter 2 Biological Molecules

Question 1

How are the following ions used in the body?
a) Calcium ions
b) sodium ions
c) potassium ions
d) hydrogen ions
e) ammonium ions

Answer 1

a) Nerve impulse transmission and muscle contraction.
b) Nerve impulse transmission and kidney function.
c) Nerve impulse transmission and stomatal opening.
d) Catalyst of reactions and pH determination.
e) Production of nitrate ions by bacteria.

Question 2

How are the following ions used in the body?
a) Nitrate ions
b) Hydrogen carbonate ions
c) Chloride ions
d) Phosphate ions
e) Hydroxide ions

Answer 2

a) Nitrogen to supply plants for amino acids and protein formation.

b) Maintenance of blood pH.

c) Balance positive charge of sodium and potassium ions in cells.

d) Cell membrane formation, nucleic acid and ATP formation, bone formation.

e) Catalysis of reactions and pH determination.

Question 3

Why is the oxygen in water delta negative, and the hydrogens in water delta positive?

Hence, why are hydrogen bonds able to form between water molecules?

Answer 3

This is because water is a polar molecule, meaning the electrons in the covalent O-H bonds are shared unevenly, giving water slightly positive hydrogens and slightly negative oxygen.

Hydrogen bonds form between positive and negative regions- forms between the delta positive hydrogen on one water molecule and the delta negative oxygen on another water molecule.

Question 4

Water is a simple covalent molecule. Other simple covalent molecules, like oxygen and carbon dioxide, is a gas at room temperature. Why is water a liquid at room temperature ?

Answer 4

Between water molecules, there are many hydrogen bonds. Thus, so much energy (a high temperature) is needed to break these hydrogen bonds and convert water to a gas. Hence, water has a high specific heat capacity (energy needed to increase 1 kg of a substance by 1 degrees Celsius). It also has a high specific latent heat capacity (energy needed to change the state of 1 kg of a substance).

Question 5

Why is it beneficial that water is a liquid at room temperature ?

Answer 5

> Provides habitats for living things in rivers, lakes and sea.
Forms a major component of tissue in living things.
Provides a reaction medium for chemical reactions.
Provides effective transport medium, like in blood and xylem of plants.

Question 6

Why is ice able to float on the top of liquid water?

Answer 6

> As liquid water starts to freeze, the hydrogen bonding allows the water molecules to rearrange themselves so the overall structure of ice is less dense than liquid water, and therefore floats.

Question 7

As ice is less dense than liquid water, there are some benefits. What are they?

Answer 7

> Aquatic organisms have a stable environment to live in through the winter.

> Layers of ice provides a habitat for animals like polar bears.

> Ice insulates the body of water underneath, so protects animals from extreme cold.

Question 8

How does water act as a solvent?

Answer 8

As water is polar, it has negative and positive parts. These negative and positive parts are attracted to the positive and negative part of solutes. The solutes cluster around the water molecules and dissolve so a solution is formed.

Question 9

Water can act as a solvent. Why is this beneficial?

Answer 9

Many chemical reactions takes place in the cytoplasm of cells, and 70% of these cells are made of water. Molecules and ions can be transported around whilst being dissolved in water, like the solution transported in the xylem of plants or that a large amount of blood plasma is made of water.

Question 10

When a water droplet is on a flat surface, it looks spherical and does not spread out. Why is this?

Answer 10

Hydrogen bonding pulls the water molecules inwards to form a spherical shape. This is called cohesion.

Question 11

What is surface tension?

Answer 11

This is when water molecules at the surface of a body of water, is more attracted to the water molecules underneath, rather than the air molecules above. There is more attraction to water molecules beneath due to hydrogen bonding. This gives the body of water the property to resist some force that has been applied to the surface.

Question 12

Why is surface tension and cohesion beneficial?

Answer 12

Water is able to move up the xylem vessel in plants due to cohesion and surface tension; it helps with transportation of water around a plant. Insects, like pond skaters, are able to walk on the surface of water, so the surface of water becomes a habitat.

Question 13

Water has a high specific heat capacity and high latent heat of vaporisation. Why is this beneficial?

Answer 13

Having a high specific heat capacity, means water needs a lot of energy to heat up or cool down. Hence, this keeps living things at a fairly constant temperature and allows enzyme controlled reactions to take place (a temperature too high causes them to denature). This also means a body of water will be kept at a fairly constant temperature, creating a stable habitat. High specific latent heat of vaporisation means that when liquid water evaporates to a gas, it releases a lot of energy. This is helpful with the sweating mechanism in animals; when sweat evaporates, it cools down the animal (this can also be applied to plants).

Question 14

What are three types of carbohydrates?

Answer 14

Monosaccharides, disaccharides and polysaccharides.

Question 15

What are three functions of carbohydrates?

Answer 15

They act as a source of energy (glucose), store of energy (glycogen and starch), or as structural units (cellulose in plant cell walls).

Question 16

1. Why are monosaccharides well suited to its roles as
   a source of energy?
2. Describe the solubility of monosaccharides.
3. In what forms can monosaccharides exist in?
4. What are isomers?
5. How do alpha glucose and beta glucose differ to one
   another?

Answer 16

1. Monosaccharides are a good source of energy as it
   has a large number of carbon-hydrogen bonds.
2. Monosaccharides are soluble in water, but insoluble
   in non-polar solvents.
3. They can exist in ring/ cyclic forms or straight chains.
4. Isomers are molecules with the same chemical
   formula but different structures.
5. Alpha glucose molecules has the hydrogen at the top
   of carbon 1, and hydroxyl at the bottom. In beta
   glucose, it is the other way.

Question 17

1. What are disaccharides?
2. How are disaccharide formed?
3. Give some examples of disaccharides.

Answer 17

1. Disaccharides are made of two monosaccharides.
2. Two monosaccharides undergo a condensation
   reaction, in which a molecule of water is lost to form
   a glycosidic bond (an oxygen bond) between the
   monosaccharides. This is the end product, called the
   disaccharide.
3. Maltose, sucrose, fructose, lactose. Sucrose is a non-
   reducing sugar, while maltose and lactose are
   reducing.

Question 18

1. What type of sugar is ribose?
2. What is the chemical formula for ribose?
3. What is the role of ribose in the body?

Answer 18

1. Ribose is a pentose sugar.
2. Ribose has chemical formula C5 H10 O5.
3. Ribose is a component of RNA, ATP and NAD.

Question 19

1. What type of sugar is deoxyribose?
2. What is the chemical formula of deoxyribose?
3. What is the role of deoxyribose in the body?

Answer 19

1. Deoxyribose is a pentose sugar.
2. Deoxyribose has chemical formula C5 H10 O4.
3. Deoxyribose is a component of DNA.

Question 20

How do we change from a disaccharide to two monosaccharides ?

Answer 20

To go from a disaccharide to two monosaccharides, a hydrolysis reaction must occur, by adding water.

Question 21

1. Give examples of disaccharides.
2. Which are reducing and which are non-reducing?

Answer 21

1. Maltose, sucrose (fructose + glucose) and
   lactose (glucose + galactose).
2. Sucrose is a non-reducing sugar. Lactose and
   maltose are reducing sugars.

Question 22

How is glucose used in living things?

Answer 22

It is used in respiration to form ATP ( as well as carbon dioxide and water).

Question 23

1. In what form do plants store energy? Where is this stored?
2. In what form do animals store energy? Where is this stored?

Answer 23

1. Plants store energy as starch, in chloroplast and membrane- bound starch grains.
2. Animals store energy as glycogen, in muscle and liver cells.

Question 24

Why are polysaccharides, like starch and glycogen, good energy stores?

Answer 24

> Polysaccharides are compact, so they do not take
up a large amount of space.
These polysaccharides are in the form of chains, so
glucose molecules on the end can be hydrolysed to
be used in respiration.
Some polysaccharides are unbranched (amylose),
while others like amylopectin and glycogen are
branched. Branched polysaccharides have a higher
rate of hydrolysis of glycose molecules, as there are
more branches, which gives more opportunity for
glucose molecules to be snipped of the ends. As
more glucose molecules can be obtains, there
would be a higher rate of respiration. Also, being
more branched means it is more compact.

Question 25

Are polysaccharides soluble or insoluble in water? Is this beneficial or not ?

Answer 25

Polysaccharides are insoluble in water due to their large size (in comparison to monosaccharides). This is beneficial because it means they will not affect the water potential, and influence the movement of water via osmosis.

Question 26

Amylose is a polysaccharide. What monomer makes up amylose? What bonds are found in amylose? What form does amylose take?

Answer 26

Amylose is made of an alpha glucose molecules, which are joined together by 1-4 glycosidic bonds. Amylose takes the shape of a spiral that is held together by hydrogen bonds. Found in plants. makes up energy store starch.

Question 27

Amylopectin is a polysaccharide. What monomer makes up amylopectin? What bonds are found in amylopectin? What form does amylopectin take?

Answer 27

Amylopectin is made of alpha glucose monomers, joined together by 1-4 and 1-6 glycosidic bonds. Amylopectin takes the form of a branched spiral, held together by hydrogen bonds. Found in plants, makes up energy store starch.

Question 28

Glycogen is a polysaccharide. What monomer makes up glycogen?

Answer 28

Glycogen is made of alpha glucose molecules, joined by 1-4 and 1-6 glycosidic bonds. Takes the form of a branched spiral, held by hydrogen bonds. It is different to amylopectin as the 1-4 bonds are shorter, giving it less tendency to coil. It also has more branching so it is more compact. Found in animals, acts as an energy store.

Question 29

What is the difference between a homopolysaccharide and heteropolysaccharide?

Answer 29

A homolpolysaccharide is a polysaccharide made of only one type of monomer. A heteropolysaccharide means the chain is made of more than one type of monomer.

Question 30

What monomer is polysaccharide cellulose made from?

Answer 30

Beta glucose molecules.

Question 31

What type of bonds form in the cellulose polysaccharide?

Answer 31

Beta 1-4 glycosidic bonds. (between carbon 1 of one molecule and carbon 4 of one molecule).

Question 32

Cellulose exists in straight chains rather than spiralling. Why is this?

Answer 32

Cellulose exists in straight chains because every other glucose molecule is rotated by 180 degrees so that the carbon 1 on one molecule and carbon 4 on the other molecule lies side by side, to make it easier for the hydroxyl groups to bond and form a glycosidic bond.

Question 33

Other than rotating beta glucose molecules, how else is additional strength gives to cellulose chains?

Answer 33

Additional strength and support is given by the hydrogen bonding between beta glucose molecules within a chain.
Due to the rotation of beta glucose molecules, there is also hydrogen bonding between beta glucose molecules on different chains.

Question 34

How many cellulose chains make up a microfibril?
How many microfibrils make up a macrofibril?
How does the structure of macrofibrils give additional strength to cell wall?

Answer 34

60-70 cellulose chains makes up a microfibril.
400 microfibrils make up a macrofibril.
Macrofibrils give additional strength as they are embedded with pectin (like glue). Macrofibrils also has a structure of running in all directions, criss-crossing for extra strength.

Question 35

Outline all the components of a plant cell wall that makes it strong.

Answer 35

Alternating beta glucose molecules gives B 1-4 glycosidic bonds, to form a straight chain rather than spiralling.
Hydrogen bonding exists between beta glucoses within a chain, as well as beta glucose molecules in different chains.
Macrofibrils embedded with pectin (like glue).
Macrofibrils have a structure of crossing over each other for additional strength.

Question 36

What are the functions of the cell wall in plants?

Answer 36

The cell wall helps to add strength to the plant to help it stay upright- this is essential as plants do not have a rigid skeleton.
Space between the macrofibrils allows water and minerals to pass in and out of the cell- this makes the cell wall permeable.
The cell wall has high tensile strength which prevent the plant cell from bursting when it is turgid- turgidity in plant cells is important as it also supports the structure of the plant.
The plant cell wall is waterproof.

Question 37

How may we use cellulose in our everyday lives?

Answer 37

Cotton is 90% cellulose.
Cellophane and celluloid (used in photographic film) is derived from cellulose.
Component of paper is cellulose.

Question 38

What are bacteria cell walls made from?

Answer 38

Peptidoglycan.

Question 39

What is the exoskeleton of insects and crustacean made from?

Answer 39

Chitin.

Question 40

What are the three types of lipids?

Answer 40

Triglycerides, phospholipids, steroids.

Question 41

What atoms are lipids made from? How do they differ from carbohydrates?

Answer 41

Lipids are made from carbon, hydrogen and oxygen atoms. They differ from carbohydrates as they have a larger amount of carbon-hydrogen bonds.

Question 42

Describe the solubility of lipids.

Answer 42

Lipids are unable to dissolve in water, but can dissolve in alcohol.

Question 43

Describe the structure of glycerol.

Answer 43

Glycerol is a hydrocarbon chain of 3 carbons, with 3 hydroxyls group on each carbon.

Question 44

What is a triglyceride composed of?

Answer 44

One glycerol and 3 fatty acids (can be of different chain lengths between 2-20 carbons).

Question 45

What are ‘essential fatty acids’?

Answer 45

Essential fatty acids are fatty acids we do not already have made in our bodies, and must be ingested to obtain it.

Question 46

What is the difference between a saturated and unsaturated fatty acid?
What is the difference between monounsaturated and polyunsaturated fatty acids?

Answer 46

A saturated fatty acid has no double carbon bonds. An unsaturated fatty acid has double carbon bonds present.
A monounsaturated fatty acid only has one double carbon bond, a polyunsaturated has more than one double carbon bond.
Having double carbon bonds changes the shape of the fatty acid. Having more double carbon bonds makes it more fluid.

Question 47

What are the bond between the fatty acids and glycerol? How do they form?

Answer 47

The bonds between the fatty acids and glycerol is called an ester bond (oxygen bond). It forms when the hydroxyl on the -COOH of the fatty acid bonds with the -OH on the glycerol. From this, a water molecule is lost, and what is left in an oxygen bond- the ester bond. This is called a condensation reaction.

Question 48

What are the functions of triglycerides?

Answer 48

Energy source- Triglycerides can be broken down to glycerol and fatty acids via the hydrolysis of the ester bonds. The glycerol and fatty acids can go on to form different intermediates in respiration, which eventually forms ATP.
Energy store- As triglycerides are insoluble in water, they can be stored in water without affecting the water potential. It also known that 1g of a triglyceride has twice as much energy as 1g of glucose as the triglycerides have so many carbon-hydrogen bonds.
Insulation- Fat near the surface of the skin (like adipose tissue in whales) acts as heat insulation. Animals preparing for hibernation store extra fat.
Buoyancy- Fat is less dense than water, so it helps aquatic animals to float.
Protection- Fat around delicate organs, like the kidney, acts as a shock absorber.

Question 49

How are phospholipids different to triglycerides?

Answer 49

Triglycerides contains a glycerol and 3 fatty acids; phospholipid contains a glycerol and 2 fatty acids, and a phosphate group.

Question 50

Why are phospholipids described as amphipathic molecules?

Answer 50

The phosphate head of a phospholipid is hydrophilic (attracted to water) whereas the fatty acids are hydrophobic (repelled by water). As a phospholipid molecule contains both hydrophobic and hydrophilic parts, it can be described as an amphipathic molecule.

Question 51

The plasma membrane of animal cells is made of phospholipids. Describe its structure.

Answer 51

As the phosphate heads are hydrophilic, this is the part of the molecule that must interact with the cytoplasm because it is made of 70% water. The fatty acid tails are found within the actual membrane as no water is found here. So the plasma membrane is composed of a bilayer- two layers of phospholipids. The fatty acid tails of each layer faces inwards while the backbone (exterior part) is where the phosphate heads.

Question 52

Describe the movement of phospholipids in the bilayer.

Answer 52

Phospholipids are free to move around in the bilayer as long as the fatty acid tail is not exposed to any water in the outside- this gives some restriction to the movement of phospholipids and gives the membrane some stability.

Question 53

Why is the membrane described as ‘selectively permeable’?

Answer 53

The fatty acid tails are on the interior of the membrane. This means anything polar or charged is unable to pass through as it will be repelled by the fatty acid tails. For this reason, only small non-polar (and uncharged) molecules can pass through, like carbon dioxide and oxygen. For other molecules to pass through, like glucose and water, special proteins are required in the membrane, which is what makes the membrane selectively permeable.

Question 54

Describe the general structure of cholesterol and its function.

Answer 54

Cholesterol is a lipid molecule made of 4 carbon-based rings. The function of cholesterol is to regulate the fluidity of the membrane. The cholesterol sits in the hydrophobic part of the membrane.
(In animals, cholesterol is made in the liver. Steroid hormones like testosterones, oestrogen and vitamin D is made from cholesterol).

Question 55

What are the functions of amino acids?

Answer 55

Amino acids makes up proteins:
Proteins are structural components of animals- muscles are made of proteins.
Proteins have specific shapes (globular) to carry out specific function- there are enzymes, antibodies, hormones.
The membrane also has protein components, in which these protein components acts as carrier and pores.

Question 56

What are ‘essential amino acids’?

Answer 56

We can make amino acids ourselves, but amino acids ingested through food is called ‘essential amino acids’.
Plants can also make their own amino acids, but they need access to fixed nitrogen (nitrates).

Question 57

What is the general structure of an amino acid?

Answer 57

The central carbon is bonded to an amino group on one side, and a carboxylic group on the other side. The central carbon is also bonded to a hydrogen atom and an R group (as the carbon needs 4 bonds). The R group varies between different amino acids, and is what makes them different. The R group can vary in size, charge and polarity, some are hydrophilic and some are hydrophobic.

Question 58

What is a dipeptide molecule?

Answer 58

A dipeptide molecule is when 2 amino acids bond and join together.

Question 59

What bond exists in the dipeptide molecule? How does it form?

Answer 59

A peptide bond is what joins the two amino acids together. It happens through a condensation reaction, where the -OH of the carboxylic group on amino acid 1, bonds with the amine group of amino acid 2. Through this, a water molecule is lost, and what is left is a bond joining the carbon on amino acid 1, and the nitrogen amino acid 2.

Question 60

During digestion, how do protease enzymes break down proteins?

Answer 60

The protease enzymes breaks down the peptide bonds in the protein to release the different amino acids to be digested.

Question 61

What is the primary structure of a protein?

Answer 61

The primary structure of a protein is the sequence of amino acids in the polypeptide chain. The sequence of amino acids determines the shape of the protein, as it determines how the polypeptide folds. As the sequence of amino acids is determined the sequence of bases on mRNA, we can ultimately say that the shape of a protein is determined by sequences of bases in mRNA.

Question 62

Describe the secondary structure of a protein.

Answer 62

The secondary structure of a protein involves the polypeptide chains folding in on itself. The structure formed can either be an alpha-helix (coiled helix/spiral). or folds into a zigzag structure called a beta-pleated sheet. In either structure, the shape is maintained because many hydrogen bonds form (between the -NH group of one amino acids and -COOH of another amino acid).
A polypeptide bond can form both alpha helix and beta pleated structures.

Question 63

Describe the tertiary structure of a protein.

Answer 63

In the tertiary structure, the coils and pleats from the secondary structure starts to fold on itself, held together by forming many bonds. These bonds includes, hydrophobic and hydrophilic interactions, hydrogen bonds, ionic bonds and disulphide bonds.

Question 64

The arrangement of hydrophobic and hydrophilic components is dependent on the external environment that the protein will be in. Why is this?

Answer 64

In the external environment of protein will be in an aqueous solution (like the cytoplasm), the protein would arrange itself so its hydrophilic components are on the exterior of the protein. If the protein is in an environment like the plasma membrane, it needs to arrange it hydrophilic components to be in the interior, and hydrophobic on the exterior (if it was the other way around, the hydrophobic fatty acid tails of the membrane would repel and push away the protein).

Question 65

Describe the quaternary structure of a protein.

Answer 65

Proteins are made from more than 1 polypeptide chain. The quaternary structure describes how multiple polypeptide chains are arranged to make a complete protein molecule.

Question 66

What is the difference between fibrous and globular proteins?

Answer 66

Fibrous proteins have regular, repetitive sequences of amino acids. These fibrous proteins are insoluble in water, which allows them to form fibres, which gives a structural function. Examples are collagen and elastin (connective tissue) and keratin (hair).
Globular proteins tends to take a spherical shape. Hydrophilic components are tend to be found on the exterior and hydrophobic on the interior- this makes them soluble in water. They have specific shapes which gives them specific functions. Examples are enzymes, hormones, antibodies and more.

Question 67

An example of a fibrous protein is collagen. Where may it be found in the body?

Answer 67

Found in artery walls- withstands high pressure and prevents the artery from bursting.
Tendons- Fibres that connects muscle to bone.
Bone- Collagen reinforced with calcium phosphate makes the bone hard.
Cartilage and connective tissue is made of collagen.

Question 68

An example of a fibrous protein is keratin. Where may it be found in the body?

Answer 68

Keratin is a fibrous protein found in finger nails, hair (in claws, hoofs and horns of animals).
It is strong because keratin is a protein rich is a sulphur containing amino acid, cysteine. Hence, disulphide bridges along with hydrogen bonding, makes keratin very strong.

Question 69

An example of a fibrous protein is elastin. Where may it be found in the body?

Answer 69

The crosslinking and coiling makes elastin strong and extensible. It is found in parts of the body that needs to stretch or adapt shape as part of life processes:
Elastin is found in skin, which stretches around bones and muscles.
Elastin is found in the lungs which helps it to inflate and deflate.
Elastin is found in the bladder which allows it to expand to hold urine.
Helps blood vessels to pump blood by stretching.

Question 70

Describe the structure of haemoglobin.

Answer 70

Haemoglobin is a protein with the function of carrying oxygen from the lungs to tissues around the body.
The structure of haemoglobin has 4 polypeptide chains- 2 alpha globin and 2 beta globin chains. Each chain has a haem group (containing iron ion) at the end- this is an example of a prosthetic group. Proteins that are associated with prosthetic groups are called conjugated proteins.
In the lungs, oxygen binds to the iron in each of the 4 harm groups, therefore, haemoglobin cannot function without the haem groups present.

Question 71

What is the function and structure of insulin?

Answer 71

Insulin binds to glycoproteins on the outside of muscle and fat muscles to increase uptake of glucose from blood, and to increase rate of consumption of glucose.
Insulin is made of 2 polypeptide chains, the A and B chains.
The hydrophilic components of the protein is found on the exterior, making insulin soluble in water.

Question 72

What is the function of pepsin? How does the R groups of amino acids relate to where it is found?

Answer 72

Pepsin as an enzyme that breaks down proteins.
Pepsin exists in acidic environments, however, structure of pepsin is not affected by pepsin. This is because most of the amino acids have R groups that are acidic (won’t interact with H+ ions from acid), rather than basic (alkaline) R groups.

Question 73

What are the role of nucleotides?

Answer 73

Nucleotides are monomers of nucleic acids, like DNA, RNA and ATP.
Phosphorylated nucleotides, like ADP and ATP, are used in energy-requiring metabolic processes.
They can be components of many coenzymes.

Question 74

What are the three components of nucleotides (DNA or RNA)?

Answer 74

Pentose sugar, nitrogenous base and phosphate group.

Question 75

Describe what bonds are found in a nucleotide and where they are found.
How do these bonds form?

Answer 75

On carbon 1 of the pentose sugar, there is a glycosidic bond between the pentose sugar and nitrogenous base. On either carbon 3 or carbon 5 of the pentose sugar, there is a phosphodiester bond between the pentose sugar and phosphate group.
These bonds form through condensation reactions.

Question 76

How does a nucleotide of DNA and RNA differ from each other?

Answer 76

DNA contains pentose sugar, deoxyribose, whereas, RNA contains pentose sugar ribose. DNA has nitrogenous bases A, T, C, G. In RNA, instead of thymine, we have uracil.

Question 77

In nucleotides of DNA, there are 4 types. How do these 4 nucleotides differ from each other?

Answer 77

Each nucleotide has a different nitrogenous base. The 4 types of nitrogenous bases are thymine, guanine, cytosine and adenine.

Question 78

What does it mean if the bases are purine?
What does it mean if the bases are pyrimidine?

Answer 78

Purine bases refer to adenine and guanine as they have 2 carbon rings in their structure.
Pyrimidine refers to cytosine and thymine as they have 1 carbon ring in their structure.

Question 79

DNA is made from 2 polynucleotide strands. How are they joined together?
Why are the strands described as ‘antiparallel’?

Answer 79

The polynucleotide strands are joined together is joined together between hydrogen bonding between nitrogenous bases. Between adenine and thymine, 2 hydrogen bonds form. Between cytosine and guanine, 3 hydrogen bonds form.
The strands are described as ‘antiparallel’ as the strands run in opposite directions.

Question 80

Why is DNA described as having a sugar-phosphate backbone?

Answer 80

It is described as this as the deoxyribose sugar and phosphate is found on the exterior of the DNA molecule- the backbone. Whereas the nitrogenous bases are found in the middle.

Question 81

How is DNA found in eukaryotic cells?
How is DNA found in prokaryotic cells?

Answer 81

In eukaryotic cells, DNA exists in a nucleus. Each large molecule of DNA is wound around histone proteins, to form chromosomes- hence, each chromosome is one molecule of DNA. DNA also found in mitochondria and chloroplasts.
In prokaryotic cells, DNA is freely lying in the cytoplasm. It is not wound around any histone proteins, and hence described as being ‘naked’.

Question 82

How are RNA molecules different to DNA molecules?

Answer 82

RNA has a ribose sugar; DNA has a deoxyribose sugar.
RNA has nitrogenous base uracil (instead of thymine); DNA has thymine.
RNA is single stranded; DNA is double stranded.
RNA molecule is shorter; DNA is longer.

Question 83

Why is genetic code described as ‘near universal’?

Answer 83

In nearly all living organisms, the same triplet of bases (codon) codes for the same amino acid.
Also genetic code is not overlapping.

Question 84

Why is genetic code described as degenerate? Why is this beneficial?

Answer 84

For almost all amino acids (except methionine and tryptophan), different combination of bases can still code for the same amino acid. In the case of a mutation of base, there is a chance that a change in base can still code for the same amino acid, and hence, has no effect.
Also genetic code is not overlapping.

Question 85

What occurs in transcription of protein synthesis?

Answer 85

A gene and unwinds and unzips (unzips using DNA helicase which breaks hydrogen bonds between complementary nucleotide bases).
Enzyme RNA polymerase catalyses formation of temporary hydrogen bonds between RNA nucleotides and complementary DNA bases. T bonds with A, C bonds with G, G bonds with C, and A bonds with U.
The DNA strand in which the RNA nucleotides bond to is called the template strand.
The RNA produced is a copy of the other DNA strand (this DNA strand is called the coding strand), with the exception of the uracil base.
The length of DNA produced is called the mRNA and passes out from the nucleus via the nuclear pores.

Question 86

Describe the structure of tRNA molecules?

Answer 86

The tRNA molecules are made in the nucleolus and passes out of the nucleus to cytoplasm.
tRNA molecules are single stranded polynucleotides that folds into a hairpin shape.
At one end of the tRNA molecule is 3 bases (anticodon) that is complementary to a codon on the mRNA strand. At the other end of the tRNA molecule is an amino acid, specific to the same anticodon on the tRNA molecule.

Question 87

Describe the process of translation of protein synthesis.

Answer 87

Translation occurs in the ribosomes.
tRNA molecules brings amino acids and form temporary hydrogen bonds between anticodon on tRNA molecule and codon on mRNA.
A ribosome moves along the length of mRNA and reads code. When 2 amino acids are adjacent (hence when tRNA molecules are adjacent), a peptide bond forms between them.
Energy is needed for the formation of peptide bonds.
After the polypeptide is assembled, mRNA breaks down. Component molecules are recycled into new length of mRNA, with different codon sequences.

Question 88

In DNA replication, the chromosomes needs to unwind and unzip. How does this occur?

Answer 88

The unwinding of the double helix occurs at a bit at a time, catalysed by the gyrase enzyme.
The unzipping of the DNA molecule means to separate the molecule into 2 strands. This is done using the DNA helicase enzyme, which breaks the hydrogen bonds between the nitrogenous bases.

Question 89

After unzipping and unwinding of DNA, nucleotides are joined to the exposed bases.
From where did the nucleotides come from (the nucleotides that are being joined to exposed bases)?
What enzymes catalyses the joining of nucleotides to form a polynucleotide?
In what direction does this enzyme work?

Answer 89

The nucleotides that are being joined to the exposed bases are free phosphorylated nucleotides present in the nucleoplasm.
DNA polymerase catalyses the joining of the new nucleotides to form a polynucleotide.
DNA polymerase works in a 5’ to 3’ direction along the unzipped DNA strand.

Question 90

The two new strands synthesised (from each template strand in the DNA) are different- there is the leading strand and the lagging strand.
How are these two strands different?
Why are they different?

Answer 90

Firstly, the difference in strands synthesised occurs due to the nature of strands in a DNA molecule- they are anti-parallel. This means if DNA polymerase always works in a 5’ to 3’ direction, on one strand DNA polymerase works towards the unzipping of the DNA, and on the other strand, DNA polymerase works away from the unzipping of DNA (because they are anti-parallel; run in opposite directions).
The strand where DNA polymerase is working towards the unzipping of DNA is called the leading strand, where it is continuously synthesised.
The lagging strand is where DNA polymerase is working away from the unzipping of DNA- so DNA has to continuously break off and move ahead in order to link complementary bases. The continuous breaking off of DNA polymerase leaves the lagging strand in fragments, which is what makes it different to the leading strand which is unbroken and long.

Question 91

What is the role of the ligase enzyme in DNA replication?

Answer 91

The ligase enzyme glues fragments of nucleotides back together in the lagging strand.

Question 92

Energy is required to make bonds between the deoxyribose sugar on one nucleotide and phosphate group of another nucleotide.
What is this bond called?
How is this energy obtained?

Answer 92

The bond between sugar residue on one nucleotide and phosphate group on another nucleotide is called a phosphodiester bond.
The energy needed to make these bonds is obtained through the hydrolysis of activated nucleotides, which in turn releases extra phosphate groups, suppling with the energy needed.

Question 93

Why is DNA replication described as semi-conservative?

Answer 93

The product of replication is 2 DNA molecules, identical to the parent molecule. Each new molecule contains one old strand (from parent molecule) and one new strand (just synthesised), so it is termed semi-conservative replication.

Question 94

How are the risk of having a mutation (like having the wrong nucleotide inserted during DNA replication) reduced?

Answer 94

There are enzymes that proof-read and edit out incorrect nucleotides, reducing rate that mutation are produced.