nucleic acids and their functions

What are nucleic acids?

Large biological molecules (polymers) made of nucleotide monomers that store and transfer genetic information.

What is a nucleotide?

The basic unit of nucleic acids — made of a phosphate group, a pentose sugar, and a nitrogenous base.

What are the two types of nitrogenous bases?

• Purines: adenine (A) and guanine (G) — double-ring structures.

• Pyrimidines: cytosine (C), thymine (T), and uracil (U) — single-ring structures.

What are the two types of nucleic acid?

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

What type of sugar does DNA contain?

Deoxyribose (a five-carbon sugar lacking one oxygen atom compared to ribose).

What type of sugar does RNA contain?

Ribose (a five-carbon sugar with one more oxygen than deoxyribose).

How are nucleotides joined together?

By condensation reactions forming phosphodiester bonds between the phosphate of one nucleotide and the sugar of the next.

Describe the structure of DNA

A double helix made of two antiparallel polynucleotide strands held together by hydrogen bonds between complementary bases.

Which bases pair together in DNA?

Adenine with thymine (A–T) and cytosine with guanine (C–G).

How many hydrogen bonds form between base pairs?

A–T pairs form 2 hydrogen bonds; C–G pairs form 3 hydrogen bonds

What is meant by “antiparallel” strands?

The two DNA strands run in opposite directions — one 5′→3′ and the other 3′→5′.

Describe the structure of RNA.

A single polynucleotide chain with the base uracil (U) replacing thymine.

What are the three types of RNA and their functions?

• mRNA (messenger): carries genetic code from DNA to ribosomes.

• tRNA (transfer): brings amino acids to the ribosome.

• rRNA (ribosomal): forms part of ribosome structure and catalyses peptide bond formation.

When does DNA replication occur?

During interphase, before cell division

What is the purpose of DNA replication?

To produce two identical DNA molecules so that each daughter cell receives a complete copy of genetic information.

What is meant by “semi-conservative replication”?

Each new DNA molecule contains one original (template) strand and one newly synthesised strand.

Outline the steps of DNA replication

1. DNA helicase breaks hydrogen bonds between strands, separating them.

2. Free nucleotides align with complementary bases.

3. DNA polymerase joins nucleotides by forming phosphodiester bonds.

4. Two identical DNA molecules are produced.

What experiment provided evidence for semi-conservative replication?

The Meselson–Stahl experiment using heavy (¹⁵N) and light (¹⁴N) nitrogen isotopes

What is a gene?

A sequence of DNA bases that codes for a specific polypeptide.

What is the genetic code?

The set of rules by which sequences of three DNA bases (triplets) code for specific amino acids.

What is a codon?

A sequence of three bases on mRNA that codes for one amino acid.

Why is the genetic code described as “degenerate”?

Because most amino acids are coded for by more than one codon.

Why is the genetic code “non-overlapping”?

Each base is part of only one triplet and is read once during translation.

Why is the genetic code “universal”?

The same codons code for the same amino acids in almost all organisms.

What are the two stages of protein synthesis?

Transcription and translation

Where does transcription occur?

In the nucleus.

Outline the process of transcription.

1. DNA helicase unwinds the gene and breaks hydrogen bonds.

2. Free RNA nucleotides pair with complementary DNA bases on the template strand.

3. RNA polymerase joins nucleotides to form pre-mRNA.

4. In eukaryotes, introns are removed (splicing) to form mature mRNA, which leaves the nucleus.

Where does translation occur?

On ribosomes in the cytoplasm

Outline the process of translation.

1. mRNA attaches to a ribosome.

2. tRNA molecules with complementary anticodons bring specific amino acids.

3. Peptide bonds form between amino acids, catalysed by rRNA.

4. The polypeptide chain grows until a stop codon is reached.

What determines the sequence of amino acids in a protein?

The order of codons on the mRNA, which reflects the base sequence of the DNA.

What is ATP?

A nucleotide derivative that acts as the universal energy currency of the cell.

What is the structure of ATP?

Adenine (base), ribose (sugar), and three phosphate groups.

How does ATP release energy?

The terminal phosphate bond is hydrolysed by ATP hydrolase, producing ADP, Pi (inorganic phosphate), and energy.

How is ATP regenerated?

By ATP synthase during respiration or photosynthesis when ADP and Pi are recombined.

Why is ATP a good energy source?

• Releases energy in small, manageable amounts.

• Hydrolysis is quick and reversible.

• Can be used directly for cell processes

How is DNA adapted for its function?

Stable double helix for storage, complementary base pairing for accurate replication, and long structure for storing lots of information.

How is RNA adapted for its function?

Short-lived, single-stranded, allowing rapid synthesis and breakdown to regulate protein production.

How are ATP and nucleic acids related?

ATP is a modified nucleotide; both contain adenine and ribose and use phosphodiester bonds.