Nucleic Acids

Question 1

What is DNA and what does it do?

Answer 1

DNA stands for deoxyribonucleic acid. Its main function is to carry genetic material, which works as instructions needed to grow and develop.

Question 2

What is RNA and what does it do?

Answer 2

RNA stands for ribonucleic acid. Its main function is to transfer genetic information from the DNA to the ribosomes (which are ‘protein factories’. The ribosomes read the RNA to make polypeptides and proteins.

Question 3

What is a nucleotide and what is it made out of?

Answer 3

A nucleotide is a biological molecule, used as a monomer to make up DNA/RNA, which is made out of three main components:
- Phosphate group
- A pentose sugar
- A nitrogen containing organic base.

Question 4

What are the five nitrogen containing organic bases?

Answer 4

Adenine, Thymine, Guanine, Cytosine and Uracil.

Question 5

What is a mononucleotide?

Answer 5

A monomer which forms when a phosphate group, a pentose sugar and a nitrogen containing organic base join together via condensation reactions.

Question 6

What is a dinucleotide?

Answer 6

A dinucleotide is when two mononucleotides bond together through condensation reactions. This happens through joining the pentose sugar of one nucleotide to the phosphate group of another, forming a phosphodiester bond (consisting of a phosphate group and two ester bonds).

Question 7

What is a polynucleotide?

Answer 7

Many mononucleotides join through condensation reactions, forming phosphodiester bonds between the phosphate groups and the pentose sugars.

Question 8

Describe the DNA structure.

Answer 8

DNA has a double helix structure, formed from two separate strands of polynucleotides which coil around each other. DNA is very long and coils very tightly, so they can hold a lot of information which fits in a small space in the cell nucleus.

Question 9

Describe the DNA nucleotide structure.

Answer 9

In a DNA nucleotide, there is always a phosphate group and a deoxyribose sugar (pentose sugar), creating the sugar-phosphate back bone. However, the bases change. The bases can either be Adenine, Cytosine, Guanine or Thymine (only present in DNA).

Question 10

What is complementary base pairing?

Answer 10

Two DNA polynucleotide strands are joined together through hydrogen bonds between the bases. Each base can only join between one partner, this is complementary base pairing or specific base pairing.

Question 11

What are the base pairings in DNA?

Answer 11

Adenine pairs with Thymine.
Guanine pairs with Cytosine.

Therefore, there are equal amounts of Adenine and Thymine, as well as equal amounts of Guanine and Cytosine in a DNA molecule.

Question 12

How many bonds happen in each pair of bases in a DNA molecule?

Answer 12

Between A and T, there are 2 hydrogen bonds.
Between C and G, there are 3 hydrogen bonds.

Question 13

What is meant by the term ‘antiparallel’ in context of DNA structure.

Answer 13

The two polynucleotide strands run in opposite directions (5’ to 3’ and 3’ to 5’). This is important for hydrogen bonds between paired bases, and for DNA replication.

Question 14

Name two ways DNA is a stable molecule.

Answer 14

1) The phosphodiester backbone protects the chemically reactive bases on the inside of the double helix.
2) Hydrogen bonds form between the bases, forming bridges. As there are three hydrogen bonds between C-G, the more C-G pairs, the more stable the DNA molecule.

Question 15

What is the function of the DNA?

Answer 15

The DNA’s function is to carry genetic information and to pass hereditary material from cell to cell and from generation to generation.
Its function depends on the sequence of the bases it is made out of.

Question 16

How is DNA adapted to aid its function?

Answer 16

Stable - can pass from generation to generation without significant change. Most mutations are repaired to persistent mutations are rare.
Large molecule - can hold a lot of genetic information.
Two separate strands with only hydrogen bonds - able to split.
Base pairing - able to replicate.

Question 17

Describe the RNA nucleotide structure.

Answer 17

RNA nucleotides are made out of a phosphate group, a ribose sugar and one of four bases: Adenine, Cytosine. Guanine and Uracil. These nucleotides also form polynucleotide strand with sugar-phosphate backbone.

Question 18

Describe the structural differences between DNA and RNA.

Answer 18

• DNA has a deoxyribose sugar, while RNA has a ribose sugar.
• DNA has the base thymine, while RNA has uracil.
• DNA has two polynucleotide strands, RNA has one, single polynucleotide strand.
• DNA has long strands, while RNA strands are much shorter.

Question 19

What does it mean if DNA is semi-conserved during replication?

Answer 19

Half of the strands in the new DNA molecule were from the original DNA which replicated. This strand acts as a template while the new strand forms from it.
This means there is genetic continuity between generations

Question 20

What are the two main stages of the cell cycle?

Answer 20

1. Nuclear division - the stage where the nucleus splits, this can be mitosis or meiosis.
2. Cytokinesis - when the whole cell actually splits into two.

Question 21

How does semi-conserved replication occur?

Answer 21

1. The hydrogen bonds between the bases is broken down through the enzyme DNA helicase, which causes the two strands to separate and unwind from each other.
2. Each original single strand acts as a template strand. Bases from the free floating nucleotides are attracted to their complementary bases on the template strand - A with T and C with G.
3. An enzyme called DNA polymerase catalyzes the condensation reactions that just the nucleotides on the new strand together. A new strand is formed and the molecule has one new strand and the old template strand, so is semi-conserved.

Question 22

What are the four requirements for semi-conserved replication to take place?

Answer 22

1. All four types of nucleotides with all four bases must be present.
2. A chemical energy source must be present.
3. DNA polymerase must be present.
4. Two template strands from the original DNA molecule are also needed.

Question 23

What direction does DNA polymerase work in?

Answer 23

Each end of a DNA strand is different in structure, so it is either 3’ or 5’.
DNA polymerase only works on the 3’ end of a DNA strand. This means the new strand is built in the 5’ to the 3’ direction, while the DNA polymerase moves down the template strand in the 3’ to 5’ direction.
As the DNA strands in a double helix are antiparallel, the DNA polymerase working on one template strand is working the opposite direction to the DNA polymerase on the other template strand.

Question 24

Describe Meselson and Stahl’s evidence for semi-conserved replication.

Answer 24

1. They made two bacteria samples - one in a nutrient broth containing light nitrogen, and one in a broth containing heavy nitrogen.
2. They left the bacteria to reproduce, so it took up the nitrogen for its nucleotides from the broth.
3. They took a sample from the new bacteria and spun it in a centrifuge. The heavier nitrogen containing bacteria DNA settled lower than the lighter nitrogen.
4. Then, they took a new DNA sample from the heavy nitrogen DNA and placed it in the light nitrogen bacteria.
5. They left there for one round of DNA replication, then a sample was taken and spun in a centrifuge again.
6. If replication was semi conservative, there would be original heavy DNA which would still settle at the bottom, and lighter DNA at the top.
7. However, it was semi-conserved, therefore the DNA settled in between where the light and heavy nitrogen DNA settled before.

Question 25

What does ATP stand for and how is it made?

Answer 25

Adenosine Triphosphate - cells cannot get energy directly from glucose, so the energy released is used to create ATP.

Question 26

Describe the structure of ATP.

Answer 26

Adenosine Triphosphate is a phosphorylated macromolecule made of three main parts: 1. Adenine - an inorganic nitrogen containing base. 2. Ribose - a sugar containing 5 carbons in a ring structure (pentose sugar). It acts as a backbone for the other parts to attach to. 3. Phosphate group - three phosphates joined in a chain.

Question 27

Why is ATP referred to as a nucleotide derivative?

Answer 27

It is a modified form of a nucleotide (an inorganic nitrogen containing base, a pentose sugar, a phosphate group).

Question 28

How does ATP store energy?

Answer 28

The three phosphate molecules have very unstable, weak bonds but hold a lot of energy. As these bonds are weak, they have a low activation energy and so are very easily hydrolyzed, releasing a considerable amount of energy.

Question 29

What is ATP broken down into?

Answer 29

ADP - Adenosine Diphosphate and an inorganic phosphate. This is a hydrolysis reaction and is broken down by ATP hydrolase.

Question 30

What is the word equation of the hydrolysis of ATP?

Answer 30

Adenosine Triphosphate + water --> Adenosine Diphosphate + Inorganic phosphate + energy This is catalyzed by DNA hydrolase.

Question 31

What are two extra ways how ATP can help specific things?

Answer 31

1. It can couple up with a reaction that required energy so it can supply energy directly. 2. The inorganic base can be added to another compound (phosphorylation), which makes the compound reactive.

Question 32

What is the synthesis of ATP?

Answer 32

The hydrolysis reaction of ATP --> ADP is reversible. Through a condensation reaction, energy can be used to add an inorganic phosphate to ADP and form ATP. This uses the enzyme ATP synthase.

Question 33

What are three ways an inorganic phosphate group can be added to synthesize ATP?

Answer 33

1. In chlorophyll in plants during photosynthesis. 2. In animals during respiration. 3. In plants and animals when a donor molecule transfers the phosphate group to the ADP (substrate-level phosphorylation).

Question 34

Why is ATP used as an immediate source of energy and why is this not a problem?

Answer 34

The instability in its bonds mean it is not good for long-term storage. It releases the energy in small, manageable amounts. The hydrolysis of ATP to ADP is a single reaction and happens immediately, whereas for example the breaking down of glucose is a long series of reactions, making it more suitable for storage. This is not a problem as it is easily reformed using an inorganic phosphate and ADP.

Question 35

What do high energy cells such as muscle cells and epithelium cells need a lot of?

Answer 35

Mitochondria, this is so a lot of ATP can be made.

Question 36

Name the five ways in which ATP is used.

Answer 36

1. Metabolic process - ATP is used in the process of creating molecules from their basic units. 2. Movement - provides energy for muscles to contract. 3. Active transport - allows carrier proteins to change shape in the plasma to carry molecules and ions against the concentration gradient. 4. Secretion - ATP is needed to make lysosomes which secrete cell products. 5. Activation of molecules - inorganic phosphates can be used to phosphorylate other compounds and lower their activation energy so they can be more reactive.

Question 37

What is the structure of water?

Answer 37

Water is made out of one oxygen atom and two hydrogen atoms, sharing an electron.

Question 38

How is water dipolar?

Answer 38

As the hydrogen's shared electron is pulled towards the oxygen atom, the other side of the hydrogen is left with a positive charge. In the oxygen atom, there are many negative electrons in the other side, creating a negative charged region. This makes water a polar molecule.

Question 39

How does the hydrogen bonding occur in water?

Answer 39

As opposite poles attract, the negative regions in the oxygens and the positive regions in the hydrogens attract each other, forming hydrogen bonds. Although these are individually weak, when added together they can be collectively strong. This bonding gives water useful properties.

Question 40

How is water an important metabolite?

Answer 40

Water is used in many metabolic processes, such as condensation and hydrolysis reactions. For example, in hydrolysis, water is used to break down things, such as turning proteins into amino acids. It is also a major raw material in photosynthesis.

Question 41

Why is water a good solvent and why is this useful?

Answer 41

Many important biological molecules are ionic, meaning they have a positively charged molecule and a negatively charged molecule. As water is polar, the positive region is attracted to the negative ion, and the negative region is attracted to the positive ion. Eventually, this means that the ion is completely surrounded in water molecules, so it becomes dissolved. This is useful as it means water can be used as a solvent to dissolve useful substances and transport them around an organism. It can dissolve ions, gases, wastes and enzymes.

Question 42

How does water have a high latent heat of vaporization and why is this useful?

Answer 42

Water has many hydrogen bonds, which require a lot of energy to be broken. Once they are broken, it allows some water molecules to escape as a gas from the surface. For these to break, they require a lot of energy, therefore water has a high latent heat of vaporization and needs a lot of heat to turn from liquid to a gas. This is useful as it means organisms can use water loss through vaporization to cool down, without too much water being lost. This is because a lot of energy will be released through a small amount of water.

Question 43

How does water have a high specific heat capacity and why is it useful?

Answer 43

The hydrogen bonds in water require a lot of energy to be broken. Therefore, most the heat that is applied to water is used to break the bonds, and less heat is available to actually heat the temperature of water. Therefore, a lot of energy is required to raise 1 gram of water be 1C, so it has a high specific heat capacity. This is useful as it means that water does not experience rapid temperature variations, and can keep a stable temperature. For example, it can be a good habitat as the temperature underwater is more likely to stay stable than on land. Furthermore, it can help keep the temperature stable inside living organisms so it can maintain constant internal temperature.

Question 44

How is water cohesive and why is this useful?

Answer 44

Cohesion is the attraction between molecules of the same type. Water molecules are attracted to each other due to the hydrogen bonds, meaning it can keep a flow. This is useful as it means water can flow and be used to transport substances. For example, water can flow up a xylem in columns to transport substances through a plant. Furthermore, cohesion means that water has a high surface tension when it meets air. This is the reason sweat droplets form and the body can cool down. It is also the reason certain insects can walk on surface of water on ponds.

Question 45

What are the two extra properties of water?

Answer 45

1. Water is also transparent, meaning it can allow photosynthesis to still occur for aquatic plants. Furthermore, light can enter the jelly-like substance in the eye to reach the retina. 2. Water is not easily compressed, so it can provide structural support for small animals such as earth worms and plants.

Question 46

What is an ion and what are the two types (positive/negative)?

Answer 46

An ion is an atom with a charge. An ion with a positive charge is called a cation, whereas an ion with a negative charge is called an anion.

Question 47

What is an inorganic ion?

Answer 47

An ion which does not contain any carbon (there are a few exceptions to this). They are found in the solution in the cytoplasm or in the body fluid. Ions have different functions depending on their properties, and they can come in different concentrations depending on their role.

Question 48

What are the four inorganic ions and what are their uses?

Answer 48

Iron ion - found in hemoglobin and plays a role in transporting oxygen. Hydrogen ion - can calculate the pH of a solution. The more H+ ions present, the lower the pH and the more acidic the solution. This determines the functioning of enzymes Phosphate ions - present in DNA and RNA and allows nucleotides to join up. It is also present in ATP, where the phosphate bonds store the energy. Sodium ions - used in co-transport to allow glucose and amino acids through a cell surface membrane.