Nucleic acids 1

What are DNA and RNA classified as?

Nucleic acids.

What role does DNA have in cells?

Stores genetic information.

What role does RNA have in cells?

Transfers genetic information from DNA to ribosomes.

What are ribosomes made from?

RNA and proteins.

What are nucleotides?

Monomers of nucleic acids.

What three components make up a nucleotide?

Pentose sugar, phosphate group, nitrogen-containing base.

What pentose sugar is in DNA nucleotides?

Deoxyribose.

What pentose sugar is in RNA nucleotides?

Ribose.

What are the four bases found in DNA?

Adenine, thymine, cytosine, guanine.

What are the four bases found in RNA?

Adenine, uracil, cytosine, guanine.

Which DNA base is replaced by uracil in RNA?

Thymine.

What reaction forms the phosphodiester bond between nucleotides?

Condensation reaction.

What bond links two nucleotides?

Phosphodiester bond.

What is formed when many nucleotides join?

A polynucleotide chain.

What structure does DNA take?

A double helix.

What holds the two polynucleotide strands together in DNA?

Hydrogen bonds between complementary bases.

What is complementary base pairing?

Bases pair in specific pairs only.

Which bases pair together in DNA?

A with T; C with G.

Why is the DNA molecule described as having antiparallel strands?

The strands run in opposite 3’→5’ directions.

Why is RNA usually single-stranded?

It does not form a stable double helix.

Why is RNA relatively short compared with DNA?

It carries short-term instructions, not long-term information.

Why did scientists once doubt that DNA carried genetic information?

DNA appeared too chemically simple.

What key evidence overturned this doubt?

Its consistent ratios of complementary bases, structure and replication mechanism.

Why does complementary base pairing allow accurate replication?

Each base only pairs with its complement.

What is meant by semi-conservative replication?

Each daughter molecule contains one original strand and one new strand.

What enzyme unwinds the DNA double helix?

DNA helicase.

How does DNA helicase separate the two strands of DNA?

It breaks hydrogen bonds.

What happens to the hydrogen bonds during unwinding?

They break, separating the strands.

What exposes the bases on each strand?

The two strands are pulled apart.

What attracts new DNA nucleotides to the template strands?

Complementary base pairing.

What enzyme joins the new nucleotides together?

DNA polymerase.

What bond is formed between the nucleotides in replication?

Phosphodiester bond.

Why is DNA polymerase direction-specific?

It only adds nucleotides in the 5’→3’ direction.

Why does DNA replication produce two genetically identical molecules?

Each original strand templates an identical new strand.

What experimental evidence supported the semi-conservative model?

Meselson and Stahl’s isotopic labelling experiment.

Why does conservative replication not fit the evidence?

Conservative replication would produce a fully heavy and a fully light molecule, not hybrids.

Why does dispersive replication not fit the evidence?

Dispersive replication would produce molecules with mixed segments, not distinct hybrid + light patterns.

What ensures genetic continuity between cells?

Accurate copying of DNA before cell division.

Why is fidelity in DNA replication essential for organisms?

Prevents harmful mutations accumulating.

What feature of DNA structure enables stability?

Sugar-phosphate backbone and strong covalent bonding.

Why is the double-helix structure advantageous for repair?

The complementary template allows damaged sequences to be rebuilt.

How does hydrogen bonding contribute to replication accuracy?

They pair only correctly matched bases, ensuring precision.

Why is breaking hydrogen bonds easier than covalent bonds during replication?

They are weak individually, allowing easy strand separation.

Why is DNA replication described as enzyme-controlled?

Enzymes catalyse unwinding and bonding steps.

Why must the DNA template remain intact during replication?

It ensures correct genetic information is preserved.

What direction does DNA polymerase work in?

5’→3’.

Why do the antiparallel strands result in a leading and lagging strand?

DNA polymerase can only build continuously on one strand; the other requires discontinuous synthesis.

How does nucleotide concentration affect replication rate?

Low nucleotide supply slows polymerisation.

How can base frequencies be used to infer complementary sequences?

Complementary bases occur in equal ratios (A=T, C=G).

Why is correct base pairing essential for maintaining coding sequences?

Incorrect pairing changes triplet codes and protein sequences.