1.5 - Nucleic Acids, 1.6 - ATP

Question 1

What is a nucleotide?

Answer 1

A type of biological molecule, they’re the monomers that make up DNA & RNA.

Question 2

What is a nucleotide composed of?

Answer 2

1 pentose sugar
1 nitrogen-containing organic base
1 phosphate group

Question 3

What is a pentose sugar?

Answer 3

A sugar with 5 carbon atoms

Question 4

What is meant by ‘organic’?

Answer 4

It contains carbon.

Question 5

Name the pentose sugar in DNA.

Answer 5

Deoxyribose

Question 6

Name the pentose sugar in RNA

Answer 6

Ribose

Question 7

State the role of DNA in living cells.

Answer 7

Base sequence of genes for functional RNA & amino acid sequence of polypeptides.
Used to store genetic information determines inherited characteristics = influences structure & function of organisms.

Question 8

State the role of RNA in living cells.

Answer 8

Transfer genetic information from the DNA to the ribosomes.

Question 9

What does DNA stand for?

Answer 9

Deoxyribonucleic acid.

Question 10

What does RNA stand for?

Answer 10

Ribonucleic acid

Question 11

What is a polynucleotide?

Answer 11

A polymer of nucleotides.
Both DNA & RNA nucleotides form polynucleotides.

Question 12

How do polynucleotides form?

Answer 12

Via condensation reactions between the phosphate group of one nucleotide & the pentose sugar of another.
This forms a phosphodiester bond (1phosphate group + 2 ester bonds)
Strong chain of sugars and phosphate = sugar-phosphate backbone.

Question 13

Describe the structure of DNA.

Answer 13

Double helix of 2 polynucleotide strands
Hydrogen bonds between bases are complementary:
Adenine + thymine
Guanine + cytosine

Question 14

How many hydrogen bonds from between adenine and thymine?

Answer 14

2

Question 15

How many hydrogen bonds form between guanine & cytosine?

Answer 15

3

Question 16

What is base ‘A’?

Answer 16

Adenine

Question 17

What is base ‘T’?

Answer 17

Thymine

Question 18

What is base ‘G’?

Answer 18

Guanine

Question 19

What is base ‘C’?

Answer 19

Cytosine

Question 20

Name the complementary base pairs in RNA.

Answer 20

Adenine + uracil (2 hydrogen bonds)
Guanine + cytosine ( 3 hydrogen bonds)

Question 21

Which base in DNA is replaced by which base in RNA?

Answer 21

Thymine is replaced by uracil.

Question 22

Relate the structure of DNA to its functions.

Answer 22

• Sugar-phosphate backbone & many H-bonds provide stability.
• Long molecule to stores lots of information.
• Helix is compact for storage in nucleus.
• Base sequence of triplets codes for amino acids.
• Double-stranded for semi-conservative replication.
• Complementary base pairing for accurate replication.
• Weak H-bonds break so strands separate easily for replication.

Question 23

What does the m stand for in mRNA?

Answer 23

Messenger

Question 24

Describe the structure of mRNA.

Answer 24

• Long ribose polynucleotide (but shorter than DNA)
• Contains uracil instead of thymine
• Single-stranded & linear (no complementary base pairing)
• Codon sequence is complementary to exons of 1 gene from 1 DNA strand.

Question 25

Relate the structure of mRNA to its functions.

Answer 25

- Shorter polynucleotide breaks down quickly so no excess polypeptide forms. - Single-stranded and linear so ribosome can move along strand & tRNA can bind exposed bases. - Complementary codon sequence to exons of 1 gene of 1 DNA strand so can be translated into a specific polypeptide by ribosomes.

Question 26

Describe the structure of transfer RNA

Answer 26

- Single strand of about 80 nucleotides - Folded into clover shape - Anticodon on one end, amino acid binding site on the other: Anticodon binds to complementary mRNA codon, Amino acid corresponds to anticodon.

Question 27

What does the t stand for in tRNA?

Answer 27

Transfer

Question 28

Order DNA, mRNA & tRNA according to increasing length.

Answer 28

tRNA mRNA DNA

Question 29

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

Answer 29

As it’s a chemically simple molecule with few components.

Question 30

Why is DNA replication described as ‘semi-conservative’?

Answer 30

Strands from original DNA molecule act as a template. New DNA molecule contains 1 old strand + 1 new strand.

Question 31

Outline the process of semi-conservative DNA replication.

Answer 31

Enzyme DNA helicase breaks hydrogen bonds between base pairs. Helix unwinds to form 2 single strands. Each original strand acts as a template; free-floating nucleotides are attracted to their complementary exposed bases of original strand. Enzyme DNA polymerase catalyses condensation reactions that join adjacent nucleotides on new strand (phosphodiester bonds) Hydrogen bonds from between bases on new & old strand.

Question 32

Describe the Meselson-Stahl experiment.

Answer 32

2 samples of bacteria were grown - 1 in medium containing light nitrogen isotope 14N & 1 in heavy 15N. Sample from each was spun in centrifuge. Bacteria from 15N settles lower down tube that 14N as DNA nucleotides taken up during replication were heavier. Bacteria from heavy were placed into broth containing light nitrogen. After 1 round of DNA replication, sample was spun in centrifuge. Sample settles in middle, showing that it contained a mixture of light & heavy nucleotides. Bacterial DNA had replicated semi-conservatively in the light nitrogen.

Question 33

What does ATP stand for?

Answer 33

Adenosine triphosphate

Question 34

Describe the structure of ATP

Answer 34

Nucleotide base adenine Combined with a ribose sugar And 3 phosphate groups. Known as a nucleotide derivative (modified form of a nucleotide)

Question 35

Why is ATP needed to release energy?

Answer 35

Plant & animal cells release energy from glucose (respiration). But a cell can’t get energy directly from glucose. In respiration, energy released by glucose is used to make ATP.

Question 36

How does ATP release energy?

Answer 36

ATP diffuses to part of cell that needs energy. Energy in ATP is stored in bonds between phosphate groups. It’s released via hydrolysis reactions.

Question 37

Describe how ATP is broken down?

Answer 37

Through a hydrolysis reaction (Which requires a water molecule) Catalysed by ATP hydrolase Where a phosphate bond is broken Products: ADP + Pi

Question 38

What is ADP?

Answer 38

Adenosine diphosphate

Question 39

What is a Pi?

Answer 39

Inorganic phosphate

Question 40

Describe how ATP hydrolysis can be ‘coupled’ to energy-requiring reactions?

Answer 40

The energy released by ATP hydrolysis can be used directly to make coupled reaction happen, rather than being lost as heat.

Question 41

What is ‘phosphorylation’?

Answer 41

This is where the inorganic phosphate released in ATP hydrolysis can be added to another compound, making the compound more reactive.

Question 42

How is ATP resynthesised in cells?

Answer 42

Enzyme ATP synthase catalysed condensation reaction between ADP & Pi Happens during photosynthesis & respiration

Question 43

Explain why ATP is suitable as the ‘energy currency’ of cells.

Answer 43

High energy bonds between phosphate groups. Small amounts of energy released at a time = less energy wasted as heat. Single-step hydrolysis means energy’s available quickly. Readily re-synthesised.