Nucleic acids, ATP - Biological Molecules

Question 1

Draw the structure of a nucleotide.

Answer 1

check google

phosphate group, pentose sugar, organic base

Question 2

Name the pentose sugars in DNA & RNA.

Answer 2

DNA: deoxyribose RNA: ribose

Question 3

State the role of DNA in living cells.

Answer 3

Base sequence of genes codes for functional RNA & amino acid sequence of polypeptides.

Genetic information determines inherited characteristics = influences structure & function of organisms.

Question 4

State the role of RNA in living cells.

Answer 4

mRNA: Complementary sequence to 1 gene from DNA with introns (non-coding regions) spliced out. Codons can be translated into a polypeptide by ribosomes.

to carry protein information from the DNA in a cell’s nucleus to the cell’s cytoplasm

rRNA: component of ribosomes (along with proteins)

tRNA: supplies complementary amino acid to mRNA codons during translation

Question 5

How do polynucleotides form?

Answer 5

Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone).

Question 6

Describe the structure of DNA.

Answer 6

double helix of 2 polynucleotide strands (deoxyribose)

H-bonds between complementary purine & pyrimidine base pairs on opposite strands:
adenine (A) + thymine (T)
guanine (G) + cytosine (C)

Question 7

Which bases are purine and which are pyrimidine?

Answer 7

A & G = 2-ring purine bases
T & C & U = 1-ring pyrimidine bases

Question 8

Name the complementary base pairs in DNA.

Answer 8

2 H-bonds between adenine (A) + thymine (T)
3 H-bonds between guanine (G) + cytosine (C)

Question 9

Name the complementary base pairs in RNA.

Answer 9

2 H-bonds between adenine (A) + uracil (U)
3 H-bonds between guanine (G) + cytosine (C)

Question 10

Relate the structure of DNA to its functions.

Answer 10

● sugar-phosphate backbone & many H-bonds provide stability
● long molecule 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 for replication

Question 11

Describe the structure of messenger RNA (mRNA).

Answer 11

● 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 12

Relate the structure of messenger RNA (mRNA) to its functions.

Answer 12

NB: functions given in same order as related structural feature on previous slide
● Breaks down quickly so no excess polypeptide forms.
● Ribosome can move along strand & tRNA can bind to
exposed bases.
● Can be translated into a specific polypeptide by
ribosomes.

Question 13

Describe the structure of transfer RNA (tRNA).

Answer 13

● Single strand of about 80 nucleotides.
● Folded into clover shape (some paired bases).
● Anticodon on one end, amino acid binding site
on the other:
a) anticodon binds to complementary mRNA codon
b) amino acid corresponds to anticodon

Question 14

Order DNA, mRNA and tRNA according to increasing length.

Answer 14

tRNA mRNA DNA

Question 15

Why is DNA replication described as ‘semiconservative’?

Answer 15

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

Question 16

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

Answer 16

Chemically simple molecule with few components.

Question 17

Outline the process of semiconservative DNA replication.

Answer 17

1. DNA helicase breaks H-bonds between base pairs.
2. Each strand acts as a template.
3. Free nucleotides from nuclear sap attach to exposed
   bases by complementary base pairing.
4. DNA polymerase catalyses condensation reactions that
   join adjacent nucleotides on new strand. 5. H-bonds reform.

Question 18

Describe the Meselson-Stahl experiment.

Answer 18

1. Bacteria were grown in a medium containing heavy isotope 15N for many generations.
2. Some bacteria were moved to a medium containing light isotope 14N. Samples were extracted after 1 & 2 cycles of DNA replication.
3. Centrifugation formed a pellet. Heavier DNA (bases made from 15N) settled closer to bottom of tube.

Question 19

Explain how the Meselson-Stahl experiment validated semiconservative replication.

Answer 19

check google

Question 20

Describe the structure of adenosine triphosphate (ATP).

Answer 20

nucleotide derivative of adenine with 3 phosphate groups

check google for photo

Question 21

Explain the role of ATP in cells.

Answer 21

ATP hydrolase catalyses ATP → ADP + Pi
● Energy released is coupled to metabolic reactions.
● Phosphate group phosphorylates compounds to make them more reactive.

Question 22

How is ATP resynthesised in cells?

Answer 22

● ATP synthase catalyses condensation reaction between ADP & Pi
● during photosynthesis & respiration

Question 23

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

Answer 23

● High energy bonds between phosphate groups.
● Small amounts of energy released at a time =
less energy wasted as heat.
● Single-step hydrolysis = energy available
quickly.
● Readily resynthesised.