Biological Molecules- DNA ( Year 12 content )

Question 1

Draw the structure of a nucleotide

Answer 1

1) phosphate group
2) pentose sugar
3) nitrogenous base

Question 2

name the pentose sugars in DNA and RNA

Answer 2

1) DNA: deoxyribose
2) RNA: ribose

Question 3

what is the role of DNA in living cells

Answer 3

1)base sequence of gene codes for functional RNA and amino acid sequences of polypeptides.
2) genetic information determines inherited characteristics which influence function and structure 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
tRNA: Supplies complementary amino acids to mRNA codons during translation

Question 5

How do polynucleotides form?

Answer 5

Condemnation reaction 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 and pyrimidine base pairs on opposite strands:
Adenine(A) + Thymine(T)
Guanine(G) + Cytosine(C)

Question 7

What bases are purine and which bases are pyrimidine?

Answer 7

Adenine and Guanine=2-ring purine bases
Thymine, Cytosine and Uracil= 1-ring pyrimidine bases

Question 8

Name the complementary base pairs in DNA

Answer 8

2 Hydrogen bonds between
Adenine(A) + Thymine(T)
3 Hydrogen bonds between
Guanine(G) + Cytosine(C)

Question 9

Name the complementary base pairs in RNA

Answer 9

2 Hydrogen bonds between
Adenine(A) + Uracil(U)
3 hydrogen bonds between
Guanine(G) + Cytosine(C)

Question 10

Relate the structure of DNA to its function

Answer 10

● sugar-phosphate backbone and many hydrogen bonds provide stability
●double-stranded for semi-conservative replication
●weak hydrogen breaks so strands seperate for replication

Question 11

Describe the structure of messenger RNA(mRNA)

Answer 11

●long ribose polynucleotide(but shorter than DNA)
●contains the base Uracil instead of thymine
●single-stranded and linear

Question 12

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

Answer 12

●breaks down quickly so no excess polypeptide forms
●Ribosome can move along the strand and tRNA can bind to exposed bases

Question 13

Describe the structure of transfer RNA(tRNA)

Answer 13

●single strand of about 80 nucleotides
●folded into a 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 acids correspond to anticodon

Question 14

Order DNA, mRNA and tRNA according to increasing length

Answer 14

tRNA
mRNA
DNA

Question 15

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

Answer 15

Chemically simple molecule with few components

Question 16

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

Answer 16

●strands from original DNA molecule act as a template
●New DNA molecule contains 1 old strand and 1 new strand

Question 17

Outline the process of semi-conservative DNA replication.

Answer 17

1) DNA helicase breaks the weak hydrogen bonds between bases which unzips the DNA forming 2 template strands
2) free nucleotides from the cytoplasm attach to exposed bases by complementary base pairing
3) DNA polymerase catalyses condensation reacts that join adjacent nucleotides on new strand
4) hydrogen bond reform

Question 18

Describe the meselson-stahl experiment

Answer 18

1) bacteria were grown in a medium containing heavy isotope N15 for many generations
2) some bacteria were moved to a medium containing light isotope N14. Samples were extracted after 1&2 cycles of DNA replication
3)centrifugation formed a pellet. Heavier DNA( bases made from N15) settled closer to bottom of tube
4)

Question 19

Explain how the meselson-stahl experiment validated semi-conservative replication

Answer 19

the DNA from the second round of centrifugation the band formed in the middle of the tube showing thar each DNA molecule contained a mixture of the heavier and lighter isotope of nitrogen

Question 20

Describe the structure of Adenosine triphosphate(ATP)

Answer 20

Nucleotides derivative of adenine with 3 phosphate groups

Question 21

Explain the role of 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 and Pi
●during photosynthesis and 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
●readily resynthesised