Section 1 - Biological molecules: 2. Nucleic acids

Question 1

What is a nucleic acid

Answer 1

A polymer of nucleotides which store genetic information in biological systems (ie. DNA and RNA)

Question 2

What is the structure of a nucleotide (3 components)

Answer 2

• Pentose sugar (with 5 carbon atoms)
• A phosphate group
• A nitrogen-containing organic base

Components covalently bonded together in condensation reactions

Question 3

What is the name of the bond between nucleotides in a polynucleotide

Answer 3

Phosphodiester bonds

Question 4

What are the 4 bases in DNA

Answer 4

• Adenine
• Thymine
• Cytosine
• Guanine

Question 5

What are the 4 bases in RNA

Answer 5

• Adenine
• Uracil
• Cytosine
• Guanine

Question 6

What are the complementary base pairs in the double stranded helix of DNA

Answer 6

Adenine bonds with Thymine (double hydrogen bond)

Cytosine bonds with Guanine (triple hydrogen bond)

Question 7

How does the structure of DNA relate to it’s function

Answer 7

• Stable (phosphodiester backbone and hydrogen bonds): allows genetic info to be passed on without change
• Two strands joined by hydrogen bonds: allows for separation during replication
• Large molecule: hold more genetic info
• Helix structure: protects the bases from being corrupted by outside forces or chemicals
• Complementary base pairing: allows for the formation of m-RNA

Question 8

What were the stages of discovering DNA

Answer 8

• 1869: Johann Friedrich discovered nucleic acids
• 1919: Phoebus Levene described the nucleotides within them
• 1944: Oswald Avery discovered that DNA passes on genetic info down generations
• 1950: Erwin Chargaff discovered the complementary base pairing
• 1950: Maurice Wilkins and Raymond Gosling began to collect x-ray diffraction patterns of DNA
• 1952: Rosalind Franklin used improved techniques to produce ‘Photograph 51’, and began evaluating the structure
• 1953: James Watson and Frances Crick published accurate descriptions of the structure of DNA, using Franklin’s work

Question 9

What was the experiment used to determine the hereditary nature of material in organisms

Answer 9

• Mice injected with living, safe form of bacteria = healthy
• Mice injected with dead, harmful form of bacteria = healthy
• Mice injected with both = infected
  This suggests that the dead harmful bacteria can pass on the information needed to make the toxins.

This hereditary material was proven to be DNA, by removing all parts from the living harmful bacteria and adding the individually to living safe bacteria. Only the DNA caused to trait to be passed on.

Question 10

What was the experiment used to determine that DNA is responsible for passing genetic info down generations.

Answer 10

Viruses infect bacteria to produce more viruses
Viruses only contain DNA and proteins, so one of those must be responsible.
The proteins and DNA in the viruses were labelled with different radioactive elements, and the viruses produced from them only showed signs of radioactive DNA

Question 11

Where is the 3’ carbon atom located on a pentose sugar

Answer 11

Attached to the hydroxyl group (bottom left when sugar is oriented upright)

Question 12

Where is the 5’ carbon atom located on a pentose sugar

Answer 12

Attached to the phosphate group (top left when the sugar is oriented upright)

Question 13

Why can nucleic acids only be synthesised ‘in vivo’

Answer 13

They can only be synthesised in the 5’-3’ direction, as DNA polymerase can only attach nucleotides to the OH group on the 3’ carbon atom

Question 14

What is semi-conservative replication

Answer 14

The process of DNA replication where one strand of the double helix is conserved, with a new strand attached to it

Question 15

Describe the process of semi-conservative DNA replication

Answer 15

• The double helix is unzipped by DNA helicase, forming a replication fork
• On the leading strand, leaving the fork with the 3’ end first, the enzyme primase forms a small section of bases attached to it, called a primer
• DNA polymerase binds to this primer, and forms the new strand in the 5’ to 3’ direction as the original strand is passed through in the opposite direction.
• On the lagging strand, leaving the fork with the 5’ end first, the enzyme primase forms multiple primers between which DNA polymerase forms the new strand in the same direction that the original is being passed out, from the 5’ to 3’ end
• These sections of strands formed on the lagging strand are called Okazaki fragments
• The enzyme exonuclease removes all the primers and they are replaced with DNA by DNA polymerase

Question 16

What is the enzyme that unzips DNA for replication

Answer 16

DNA helicase

Question 17

What is enzyme that forms primers to begin DNA replication

Answer 17

Primase

Question 18

What is the enzyme the forms the new strand of DNA replication

Answer 18

DNA polymerase

Question 19

What is the name of the sections of DNA formed individually on the lagging strand of DNA replication

Answer 19

Okazaki fragments

Question 20

What direction does the leading strand leave the replication fork in DNA replication

Answer 20

3’ to 5’

Question 21

What direction does the lagging strand leave the replication fork in DNA replication

Answer 21

5’ to 3’

Question 22

What is the enzyme that removes the primers after DNA replication

Answer 22

Exonuclease

Question 23

What were the alternative models of DNA replication instead of semi-conservative replication

Answer 23

• Conservative: original DNA remains intact, and complete new copy is formed
• Dispersive: new and old DNA randomly alternates in both strands

Question 24

What is the experiment that supports the model of semi-conservative replication (‘the most beautiful experiment’)

Answer 24

• Bacteria grown in 14-N medium
• Bacteria moved and grown in 15-N medium for many generations and when DNA is extracted and centrifuged, all DNA is the heavy, containing 15-N
• After then being grown in 14-N medium for one generation, when extracted and centrifuged, all DNA is in the medium weight band, containing both 14-N and 15-N (disproving conservative replication)
• After 2 generations in 14-N medium, half the DNA was light containing only 14-N, half was medium containing both (disproving dispersive replication)
• After 3 generations, 75% is light and 25% is medium, proving semi-conservative replication, as there is no heavy containing only 15-N, but the proportion containing the original strands halves each generation

Question 25

What is the structure of ATP

Answer 25

Adenosine Triphosphate: - Ribose sugar - Adenine: organic nitrogenous base - Phosphates: chain of 3 phosphate groups

Question 26

How does ATP store energy

Answer 26

Bonds between the phosphates are unstable, so have a low activation energy and release lots of energy when broken by hydrolysis, forming ADP

Question 27

What enzyme catalyses the hydrolysis of ATP into ADP

Answer 27

ATP hydrolase

Question 28

How is ATP synthesised

Answer 28

A phosphate molecule is added to ADP in a condensation reaction.

Question 29

What are the 3 ways ATP can be synthesised from ADP

Answer 29

- Happens in plants during photosynthesis (photophosphorylation) - Happens in plants and animals during respiration (oxidative phosphorylation) - Happens in plants and animals when phosphate groups are transferred from donor molecules to ADP (substrate-level phosphorylation)

Question 30

What is photophosphorilation

Answer 30

The synthesis of ATP, by adding a phosphate group to ADP in chlorophyll-containing plant cells during photosynthesis.

Question 31

What is oxidative phosphorylation

Answer 31

The synthesis of ATP by adding a phosphate group to ADP in plant and animal cells during respiration

Question 32

What is substrate-level phosphorylation

Answer 32

The synthesis of ATP, when a phosphate group is donated from one ADP to another, in plant and animal cells

Question 33

What enzyme catalyses the synthesis of ATP

Answer 33

ATP synthase

Question 34

Why is ATP required as well as glucose

Answer 34

- ATP is unstable, so can't store energy like glucose - ATP can be broken down easily for immediate energy release, whereas the breakdown of glucose is a longer process

Question 35

Where is ATP made

Answer 35

Constantly made in the mitochondria of the cell that it is needed in.

Question 36

What energy requiring processes is ATP used for

Answer 36

- Metabolic processes - Movement - Active transport - Secretion: formation of lysosomes used for the secretion of cell products - Activation of molecules: the inorganic phosphate released can phosphorylate other compounds, lowering their activation energy and making them more reactive

Question 37

What are inorganic ions

Answer 37

Ions that are not directly associated with a biological molecule

Question 38

Inorganic ions: What are the functions of iron (Fe^2+)

Answer 38

- Forms prosthetic groups in haemoglobin (Haem groups), associating with O2 for transport in the red blood cells

Question 39

Inorganic ions: What are the functions of Sodium (Na^+)

Answer 39

- Sodium-potassium pumps (in co-transport) - Important in the nervous system for communication between neurons.

Question 40

Inorganic ions: What are the functions of Hydrogen (H^+)

Answer 40

- Ion pumps used to maintain pH (digestion) - Important on respiration and photosynthesis

Question 41

Inorganic ions: What are the functions of phosphate (PO4^3-)

Answer 41

- Used to form phosphodiester bonds - Forms part of the sugar-phosphate backbone of DNA and RNA - Part of ATP molecule that is removed by hydrolysis to release energy - Used to activate molecules by phosphorylation - Part of phospholipids within cell membranes

Question 42

Inorganic ions: What are the functions of Potassium (K^+)

Answer 42

- Regulation of the guard cells, opening and closing the stomata

Question 43

What is the structure of a water molecule

Answer 43

Two hydrogen atoms are covalently bonded to one oxygen atom

Question 44

Why is water dipolar

Answer 44

Although it has no overall charge, the arrangement of the electrons within the molecule mean that the oxygen side is slightly negative and the hydrogen end is slightly positive

Question 45

Why do hydrogen bonds form between water molecules

Answer 45

The negative oxygen of one molecule is attracted to the hydrogen of another, resulting in an electrostatic attraction that forms a hydrogen bond

Question 46

What are the main properties of water

Answer 46

- High specific heat capacity - High latent heat of vaporisation - Cohesive and adhesive nature of the molecules - Surface tension - Effective solvent - Common reactant in metabolism - Not compressible - Transparent

Question 47

Why does water have a high specific heat capacity

Answer 47

The hydrogen bonds between the molecules means that more energy is required to increase their kinetic energy, so increase the temperature

Question 48

What is an example of the benefits of the high specific heat capacity of water

Answer 48

Allows for sustained ecosystems in water based environments, as the temperature of the water doesn't fluctuate as much as the air around it

Question 49

Why does water have a high latent heat of vaporisation

Answer 49

Hydrogen bonds between the molecules mean that more energy is required to change form liquid to gas

Question 50

What is an example of a benefit of the high latent heat of vaporisation of water

Answer 50

Allows for thermoregulation, as the evaporation of sweat takes in loads of energy, cooling down the skin

Question 51

Why are water molecules cohesive (and adhesive)

Answer 51

The polarity of the water molecules causes them to be attracted to each other, and other molecules

Question 52

What is an example of a benefit of the cohesive nature of water molecules

Answer 52

Allows for transpiration to occur, as the cohesion of water causes it to flow up the stem (and adhesives as they are attracted to xylem cells as they pass)

Question 53

Why does water have surface tension

Answer 53

Hydrogen bonds near the surface allows water droplets to form

Question 54

What is an example of a benefit of the surface tension of water

Answer 54

Allows droplets to form, and the spherical shape has the minimum surface area, so takes longer to evaporate, supporting ecosystems that rely on the presence of water

Question 55

Why is water a good solvent

Answer 55

All small and charged particles will dissolve in water as they are attracted to the polar molecules

Question 56

What is an example of water being used as a solvent

Answer 56

- Ocean - Blood stream - Cytoplasm - Tissue fluid - Cell sap - Transpiration flow

Question 57

Why is water commonly used in metabolic reactions

Answer 57

Readily available, so is commonly present to be used

Question 58

What is an example of a metabolic reaction that uses water

Answer 58

- Hydrolysis of compounds - Photosynthesis

Question 59

Why is water not compressible

Answer 59

Sides of the molecules with the same charge will repel if too close, stopping the water from being compressed

Question 60

What is an example of a benefit of water being non-compressible

Answer 60

- Turgid pressure in plants - Hydrostatic pressure differences

Question 61

Why is water transparent

Answer 61

The constant breaking and forming of hydrogen bonds allows light to pass through the gaps between molecules

Question 62

What is an example of a benefit of water being transparent

Answer 62

- Allows for photosynthesis in underwater plants - Vital for eyesight