topic 1B: more biological molecules (nucleic acids, ATP, water + inorganic ions)

Question 1

Describe the basic functions of DNA and RNA in all living cells

Answer 1

DNA: holds genetic information which codes for polypeptides (proteins)
RNA: transfers genetic information from DNA to ribosomes

Question 2

Name the two types of molecule from which a ribosome is made

Answer 2

RNA and proteins

Question 3

definition of nucleotide

Answer 3

a nucleotide is a type of biological molecule, made from a pentose sugar (sugar with 5 carbon atoms), a nitrogen-containing organic base and a phosphate group

Question 4

draw and label a DNA nucleotide

Answer 4

components of a DNA nucleotide
-deoxyribose
-phosphate group
-nitrogen containing organic bases adenine, cytosine, guanine or thymine

Question 5

draw and label an RNA nucleotide

Answer 5

components of a RNA nucleotide
-ribose
-phosphate group
-nitrogen containing organic bases: adenine, cytosine, guanine or URACIL

Question 6

Describe the differences between a DNA nucleotide and an RNA nucleotide

Answer 6

-in a DNA nucleotide Pentose sugar is deoxyribose whereas RNA nucleotide is Pentose sugar is ribose
-in DNA nucleotide Base can be thymine whereas RNA nucleotide is Base can be uracil

Question 7

Describe how nucleotides join together to form polynucleotides

Answer 7

● Condensation reactions, removing water molecules
● Between phosphate group of one nucleotide and deoxyribose / ribose of another
● Forming phosphodiester bonds

Question 8

Why did many scientists initially doubt that DNA carried the genetic code?

Answer 8

The relative simplicity of DNA - chemically simple molecule with few components

Question 9

Describe the structure of DNA (5)

Answer 9

-double helix
-DNA is a polymer called polynucleotide made up of monomers of nucleotides joined together
-Each nucleotide is formed from a deoxyribose, a phosphate group and a nitrogen-containing organic base e.g. thymine, guamine, cytosine, adenine
-Phosphodiester bonds join adjacent nucleotides
-2 polynucleotide chains are held together by hydrogen bonds between specific complementary base pairs - adenine / thymine and cytosine / guanine

Question 10

Describe the structure of (messenger) RNA (4)

Answer 10

-a single helix
-RNA is a polymer called polynucleotide made up of monomers of nucleotides joined together
-Each nucleotide is formed from ribose, a phosphate group and a nitrogen-containing organic base e.g.- URACIL, adenine, cytosine, guanine
-phosphodiester bonds join adjacent nucleotides

Question 11

compare and contrast the structure of DNA and (messenger) RNA (5)

Answer 11

-DNA’s pentose sugar is deoxyribose whereas RNA’s Pentose sugar is ribose
-DNA Has the base thymine whereas RNA has the base uracil
-DNA is Double stranded / double helix whereas RNA is Single stranded / single helix
- DNA is Long (many nucleotides) whereas RNA is Shorter (fewer nucleotides)
-DNA Has hydrogen bonds / base pairing whereas RNA does not

Question 12

Suggest how the structure of DNA relates to its functions (7)

Answer 12

REPLICATE
-two strands → both can act as templates for semi-conservative replication
-Hydrogen bonds between bases are weak → strands can be separated for replication
-Complementary base pairing → accurate replication

STABLE MOLECULE
-Many hydrogen bonds between bases → stable / strong molecule
-Double helix with sugar phosphate backbone → protects bases / hydrogen bonds

STORAGE/COMPACT
-Long molecule → store lots of genetic information (that codes for polypeptides)
-Double helix (coiled) allows DNA to fit into nucelus → compact

Question 13

definition of Semi-conservative

Answer 13

• each new DNA molecule consists of one template (original) strand and one new strand

Question 14

Why is semi-conservative replication important?

Answer 14

Ensures genetic continuity between generations of cells

Question 15

process of semi conservative replication of DNA

Answer 15

1. the enzyme DNA helicase BREAKS hydrogen bonds between complementary bases in the two polynucleotide strands, unwinding the double helix to form two strands
2. Both strands act as TEMPLATES for new strands
3. Free-floating DNA nucleotides are attracted to their complementary exposed bases and join by specific COMPLEMENTARY BASE PAIRING
4. Hydrogen bonds form between the bases on the original and new strands (adenine-thymine and guanine-cytosine)
5. DNA polymerase joins adjacent nucleotides on new strand by CONDENSATION reactions
6. Forming phosphodiester bonds - each new DNA molecule contains one strand from the ORIGINAL DNA molecule and one new strand

Question 16

Use your knowledge of enzyme action to suggest why DNA polymerase
moves in opposite directions along DNA strands

Answer 16

● DNA has antiparallel strands
● So shapes / arrangements of nucleotides on two ends are different
● DNA polymerase is an enzyme with a specific shaped active site
● So can only bind to substrate with complementary shape (phosphate end of developing strand)

Question 17

Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication

Answer 17

Watson and Crick

Question 18

Describe the work of Meselson and Stahl in validating the Watson-Crick
model of semi-conservative DNA replication

Answer 18

1. Bacteria grown in medium containing heavy nitrogen (15N) so nitrogen is incorporated into DNA bases
   -DNA extracted & centrifuged → settles near bottom, as all DNA molecules contain 2 ‘heavy’ strands
2. Bacteria transferred to medium containing light nitrogen (14N) and allowed to divide once
   -DNA extracted & centrifuged → settles in middle, as all DNA molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand
3. Bacteria in light nitrogen (14N) allowed to divide again
   -DNA extracted & centrifuged → half settles in middle, as contains 1 original ‘heavy’ and 1 new ‘light’ strand; half settles near top, as contains 2 ‘light’ strands

Question 19

draw + describe structure of ATP

Answer 19

ATP (adenosine triphosphate) is a nucleotide derivative (modified nucleotide) and is formed from a molecule of ribose bound to a molecule of adenine (base) and 3 phosphate groups

Question 20

Describe how ATP is hydrolysed

Answer 20

RELEASES ENERGY
● ATP (+ water) → ADP (adenosine diphosphate) + Pi (inorganic phosphate)
● Hydrolysis reaction, using a water molecule
● Catalysed by ATP hydrolase (enzyme)

Question 21

Give two ways in which the hydrolysis of ATP is used in cells

Answer 21

● Coupled to energy requiring reactions within cells (releases energy)
○ eg. active transport, protein synthesis
● Inorganic phosphate released can be used to phosphorylate
(add phosphate to) other compounds, making them more reactive

Question 22

Describe how ATP is resynthesised in cells

Answer 22

ENERGY USED
● ADP + Pi → ATP (+ water)
● Condensation reaction, removing a water molecule
● Catalysed by ATP synthase (enzyme)
● During respiration and photosynthesis

Question 23

Suggest how the properties of ATP make it a suitable immediate source of
energy for cells

Answer 23

-Releases energy in (relatively) small amounts / little energy lost as heat
-Single reaction / one bond hydrolysed to release energy (so immediate release)
-Cannot pass out of cell

Question 24

Explain how hydrogen bonds occur between water molecules

Answer 24

● Water is polar molecule
● Slightly negatively charged oxygen atoms attract slightly positively
charged hydrogen atoms of other water molecules

Question 25

Explain 5 properties of water that are important in biology

Answer 25

-METABOLITE: in many metabolic reactions, including used in condensation/ hydrolysis/ photosynthesis/ respiration -SOLVENT (can dissolve solutes): -allows metabolic reactions to occur (faster in solution) -allows transport of substances e.g. nitrates in xylem, urea in blood -RELATIVELY HIGH SPECIFIC HEAT CAPACITY: BUFFERS changes in temperature, as it can gain/ lose alot of heat/ energy without changing temperature -good habitat for aquatic organisms as temperature is more stable than land -helps organisms maintain a constant internal body temperature -RELATIVELY LARGE LATENT HEAT OF VAPORISATION: -allows effective cooling with little loss of water by evaporation of a small volume -so helps organisms maintain a constant internal body temperature -STRONG COHESION BETWEEN WATER MOLECULES: -supports columns of water in tube-like transport cells of plants e.g. transpiration stream through xylem in plants -produces surface tension where water meets air, supporting small organisms (to walk on water)

Question 26

Where are inorganic ions found in the body?

Answer 26

In solution in cytoplasm and body fluid of organisms, some in high concentrations and others in very low concentrations

Question 27

Describe the role of hydrogen ions

Answer 27

-H+ -MAINTAINS PH LEVELS in the body -high concentration of hydrogen ions, the lower the pH and the more acidic -pH affects enzyme-controlled reactions as it can cause enzymes to denature

Question 28

describe the role of iron ions

Answer 28

-FE3+ -Component of haem group of haemoglobin -Allowing oxygen to bind / associate for transport as oxyhaemoglobin

Question 29

describe the role of sodium ions

Answer 29

-Na+ -Involved in co-transport of glucose / amino acids into cells -Involved in action potentials in neurons -Affects water potential of cells / osmosis

Question 30

describe the role of phosphate ions

Answer 30

-Component of nucleotides, allowing phosphodiester bonds to form in DNA / RNA -Component of ATP, allowing energy release -Phosphorylates other compounds making them more reactive -Hydrophilic part of phospholipids, allowing a bilayer to form