nucleic acids + ATP

Question 1

explain what is meant by nucleic acids ?

Answer 1

• long chains (polymers) made up of many nucleotide monomers joined together by phosphodiester bonds .
• DNA + RNA

Question 2

what are the three components that each nucleotide is made up of ?

Answer 2

• pentose sugar (either deoxyribose/ribose)
• nitrogenous base (guanine, cytosine, adenine, thymine or uracil)
• phosphate .

Question 3

name the pentose sugars in DNA + RNA.

Answer 3

• DNA = deoxyribose
• RNA = ribose

Question 4

state the role of DNA in living cells .

Answer 4

• base sequence of genes codes for functional RNA + amino acid sequence of polypeptides
• genetic info determines inherited characteristics / influence structure + organisms functions
• huge, so store vast amounts info in small volumes
• small variations in structure that acts as simple code
• stable / so info not easily corrupted
• reproduce themselves so copy info

Question 5

describe the structure of DNA .

Answer 5

• consists of two nucleic acid strands bonded together by complementary base pairing , strands twisted around each other to form double helix
• 2 strands same sequence running in opposite directions = anti-parallel.

Question 6

which bases are purine and which are pyrimidine ?

Answer 6

• H bonds between complementary purine + pyrimidine base pairs on opposite strands :
• Adenine + Thymine
• Guanine and Cytosine
• A + G = 2-ring purine bases = large double ring structure
• T + C + U = 1-ring pyrimidine bases = smaller as they are composed of a single ring

Question 7

explain why a purine will always base pairs with a pyrimidine .

Answer 7

• it’s the only way DNA can keep regular shape and not become too bulky / narrow .

Question 8

how do polypeptides form?

Answer 8

• condensation reactions between nucleotides between strong phosphodiester bonds (sugar-phosphate backbone.)

Question 9

explain the differences between a molecule of DNA and RNA .

Answer 9

• different sugars - pentose sugar in DNA is deoxyribose and in RNA is ribose. (deoxyribose has one less oxygen atom.)
• RNA contains uracil instead of thymine .
• DNA is double stranded (antiparallel strands) but RNA is single stranded .
• RNA is shorter than DNA molecules = contain fewer nucleotides joined together
• DNA has hydrogen bonds between 2 complementary strands .

Question 10

state for role of RNA.

Answer 10

• mRNA: complementary sequence to 1 gene from DNA with introns (non-coding regions) spliced out . = codons can be translated into a polypeptide by ribosomes .
• rRNA: component of ribosomes (along with proteins .)
• tRNA: supplies complementary amino acid to mRNA codons during translation .

Question 11

name the complementary base pairs in DNA .

Answer 11

• 2 H-bonds between Adenine (A) + Thymine (T)
• 3 H-bonds between guanine (G) + cytosine (C)

Question 12

name the complementary base pairs in RNA.

Answer 12

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

Question 13

relate the structure of DNA to its functions .

Answer 13

• sugar-phosphate backbone + many H-bonds provides stability.
• long molecule stores lots of info.
• 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 14

describe the structure of messenger RNA. (mRNA)

Answer 14

• long ribose polynucleotide (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 15

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

Answer 15

• breaks down quickly so no excess polypeptide forms.
• ribosomes can move along strand + tRNA can bind to exposed bases.
• can be translated into a specific polypeptide by ribosomes.

Question 16

describe the structure of transfer RNA (tRNA)

Answer 16

• single strand of 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 17

order DNA, mRNA and tRNA according to increasing length.

Answer 17

• tRNA, mRNA, DNA

Question 18

explain what happens during RNA/DNA synthesis.

Answer 18

• nucleotides connected through formation of phosphodiester bonds between phosphate group on one nucleotide + pentose sugar on next nucleotide
• condensation reaction - water formed during reaction + catalysed by enzyme : DNA/RNA polymerase
• breaking phosphodiester bonds needs addition of water molecule - hydrolysis reaction .

Question 19

why is DNA replication described as “semiconservstive?”

Answer 19

• strands from original DNA molecule acts as a template.
• new DNA molecule contains 1 old strand + 1 new strand.

Question 20

Outline the process of semiconservative DNA replication.

Answer 20

(DNA copied perfectly by semiconservative replication.)

1) DNA helicase unwinds the double helix, breaking the hydrogen bonds between base pairs.

2) each strand acts as template (for synthesis of other strand)
3) complementary nucleotides attach to template strand by hydrogen bonding

4) DNA polyermase catalyses condensation reaction, forming phosphodiester bonds between nucleotides - form complementary strand alongside template parent strand

5) 2 daughter DNA molecules formed , each contain half of original DNA molecule .

Question 21

Describe the Meselson-Stahl experiment.

Answer 21

• used a heavy isotope of nitrogen (N-15) with the extra neutron compared to normal N-14

1) bacteria were grown in a medium containing heavy isotope N-15 for many generations

2) some bacteria were moved to a medium containing light isotope N-14 . samples 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 22

Explain how the Meselson-Stahl experiment validated semiconservative replication.

Answer 22

• after second DNA replication - 2 bands of DNA.
• one band has intermediate weight but further newly synthesised DNA now only made using lighter isotope

—> proved that DNA is replicated semi-conservatively

Question 23

Describe the structure of adenosine troposphere (ATP.)

Answer 23

(a phosphorylated nucleotide)

• nucleotide derivative made of an adenine molecule, a ribose molecule + 3 phosphate groups attached .
• hydrolysis of ATP to ADP removes 1 of phosphate groups = release small quantities energy in single reaction so less wasted

Question 24

Explain the role of ATP in cells.

Answer 24

• when ATP hydrolysed - converted into adenine diphosphate + inorganic phosphate.
  —> reaction catalysed by enzyme: ATP hydrolase.
• phosphate can be attached to other molecules making them more reactive - phosphate group phosphorylates compounds (inactive —> active form)

Question 25

How is ATP resynthesised in cells?

Answer 25

• ATP synthase catalysed condensation reaction between ADP + Pi to reform ATP = catalysed by enzyme ATP synthase.
• during photosynthesis + respiration

Question 26

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

Answer 26

• 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 re synthesised .