Flashcards - Topic 1.5 Nucleic acids 1.6 ATP - AQA Biology A-level
(23 cards)
Draw the structure of a nucleotide.
nitrogen-containing base, phosphate group, pentose sugar
Name the pentose sugars in DNA & RNA.
DNA: deoxyribose, RNA: ribose
State the role of DNA in living cells.
Base sequence of genes codes for functional RNA & amino acid sequence of polypeptides. Genetic information determines inherited characteristics = influences structure & function of organisms.
State the role of RNA in living cells.
mRNA: Complementary sequence to 1 gene from DNA with introns spliced out. Codons can be translated into a polypeptide by ribosomes. rRNA: component of ribosomes. tRNA: supplies complementary amino acid to mRNA codons during translation.
How do polynucleotides form?
Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone).
Describe the structure of DNA.
double helix of 2 polynucleotide strands (deoxyribose) with H-bonds between complementary purine & pyrimidine base pairs: adenine (A) + thymine (T), guanine (G) + cytosine (C).
Which bases are purine and which are pyrimidine?
A & G = 2-ring purine bases; T & C & U = 1-ring pyrimidine bases.
Name the complementary base pairs in DNA.
2 H-bonds between adenine (A) + thymine (T); 3 H-bonds between guanine (G) + cytosine (C).
Name the complementary base pairs in RNA.
2 H-bonds between adenine (A) + uracil (U); 3 H-bonds between guanine (G) + cytosine (C).
Relate the structure of DNA to its functions.
? 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.
Describe the structure of messenger RNA (mRNA).
? 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.
Relate the structure of messenger RNA (mRNA) to its functions.
? 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.
Describe the structure of transfer RNA (tRNA).
? Single strand of about 80 nucleotides; ? Folded into clover shape (some paired bases); ? Anticodon on one end, amino acid binding site on the other.
Order DNA, mRNA and tRNA according to increasing length.
tRNA, mRNA, DNA
Why did scientists initially doubt that DNA carried the genetic code?
Chemically simple molecule with few components.
Why is DNA replication described as ‘semiconservative’?
? Strands from original DNA molecule act as a template; ? New DNA molecule contains 1 old strand & 1 new strand.
Outline the process of semiconservative DNA replication.
- 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.
Describe the Meselson-Stahl experiment.
- 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.
Explain how the Meselson-Stahl experiment validated semiconservative replication.
Describe the structure of adenosine triphosphate (ATP).
nucleotide derivative of adenine with 3 phosphate groups.
Explain the role of ATP in cells.
ATP hydrolase catalyses ATP ? ADP + Pi; ? Energy released is coupled to metabolic reactions; ? Phosphate group phosphorylates compounds to make them more reactive.
How is ATP resynthesised in cells?
? ATP synthase catalyses condensation reaction between ADP & Pi during photosynthesis & respiration.
Explain why ATP is suitable as the ‘energy currency’ of cells.
? 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.
