2.1.3 NUCLEIC ACIDS SUPERSTACK

What is translation?

Converting genetic information from the mRNA to a sequence of amino acids

Where does translation occur in eukaryotic cells?

At ribosomes in the cytoplasm.

What are ribosomes made of?

Two subunits (one large, one small) composed of roughly equal amounts of protein and ribosomal RNA (rRNA).

What is the role of rRNA in ribosomes?

It maintains structural stability and catalyses peptide bond formation between amino acids.

What happens when mRNA leaves the nucleus?

It binds to a specific site on the small subunit of a ribosome.

What is the function of the ribosome during translation?

It holds the mRNA in place and facilitates base pairing between mRNA codons and tRNA anticodons.

What is tRNA?

A type of RNA that carries specific amino acids to the ribosome during translation.

What is the structure of a tRNA molecule?

A single strand of RNA folded into a cloverleaf shape with an anticodon at one end and an amino acid attachment site at the other.

What is an anticodon?

A triplet of three bases on tRNA that is complementary to a codon on mRNA.

What is the function of the anticodon?

It base pairs with the complementary codon on mRNA to ensure the correct amino acid is added to the growing polypeptide.

What determines which amino acid each tRNA carries?

The specific anticodon sequence on the tRNA.

What happens when tRNA anticodons bind to mRNA codons?

The amino acids carried by each tRNA are positioned in the correct order to form a polypeptide.

How are amino acids joined together during translation?

By peptide bonds formed between adjacent amino acids.

Which molecule catalyses peptide bond formation?

rRNA within the large ribosomal subunit.

How does the polypeptide chain grow during translation?

Amino acids are added one at a time as the ribosome moves along the mRNA.

In what direction is the mRNA read?

From the 5′ to the 3′ end.

What happens to tRNA after it delivers its amino acid?

It is released from the ribosome and can pick up another amino acid.

What signals the end of translation?

A stop codon on the mRNA is reached, causing the ribosome to release the completed polypeptide chain.

What is the final product of translation?

A primary polypeptide chain with a specific sequence of amino acids.

How are transcription and translation linked?

Transcription produces mRNA in the nucleus; translation uses that mRNA in the cytoplasm to assemble proteins.

Why does translation require RNA rather than DNA?

RNA can leave the nucleus and carry genetic information to ribosomes, while DNA remains protected inside the nucleus.

Where does transcription occur in eukaryotic cells?

In the nucleus, where DNA is contained and protected by the nuclear envelope.

Why does transcription occur instead of DNA leaving the nucleus?

DNA is too large to leave the nucleus

so mRNA is made to carry the genetic code to the ribosomes

What is transcription?

Process of copying a section of DNA into a complementary strand of RNA.

What enzyme controls the unwinding and unzipping of DNA during transcription?

DNA helicase.

What happens when DNA helicase acts during transcription?

It breaks hydrogen bonds between base pairs, separating the two strands.

Which DNA strand codes for the protein?

The sense strand (5′ to 3′).

Which strand acts as the template during transcription?

The antisense strand (3′ to 5′), which is complementary to the sense strand.

Why does the antisense strand act as the template strand?

So that the complementary mRNA formed has the same base sequence as the sense strand

What base replaces thymine in RNA?

Uracil (U)

What enzyme catalyses the formation of phosphodiester bonds in the RNA strand?

RNA polymerase.

What happens when RNA polymerase reaches the end of the gene?

Transcription stops, and the mRNA strand detaches from the DNA template.

What is the product of transcription called?

Messenger RNA (mRNA).

How does the base sequence of mRNA compare to the gene sequence?

It is complementary to the antisense strand and identical to the sense strand

Where does the mRNA go after it is formed?

It leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm for translation.

Why is mRNA shorter than DNA?

It only copies the base sequence of a single gene rather than the whole chromosome.

Why is transcription essential for protein synthesis?

It produces mRNA carrying the genetic code needed for translation at the ribosome.

What are the three components of a nucleotide?

A phosphate group, a pentose sugar, and a nitrogenous base.

What is the difference between DNA and RNA nucleotides?

DNA contains deoxyribose and thymine

RNA contains ribose and uracil.

Which bases are purines and which are pyrimidines?

Purines: A + G

Pyrimidines: C + T (or uracil in RNA).

What are the base pairing rules in DNA and RNA?

In DNA:

A pairs with T

G pairs with C

In RNA:

A pairs with U.

How many hydrogen bonds form between complementary bases?

A–T (or A–U): 2 hydrogen bonds; G–C: 3 hydrogen bonds.

Why are mRNA molecules shorter than DNA?

Because only one gene is transcribed into a short mRNA strand

(not the entire chromosome)

Why can RNA leave the nucleus but DNA cannot?

RNA is small and single-stranded; DNA is large and double-stranded, remaining in the nucleus.

How are polynucleotides formed from nucleotides?

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

What happens to RNA after protein synthesis?

Broken down by hydrolysis of phosphodiester bonds

nucleotides are recycled

What is uracil and how does it pair?

Uracil is a pyrimidine base in RNA that pairs with adenine via two hydrogen bonds.

Why is transcription described as a short copy of a gene?

Only the section of DNA coding for 1 protein is copied into mRNA.

Why is DNA kept cold during extraction?

Low temperature slows down DNases , preventing DNA degradation.

Why is detergent used in DNA extraction?

It disrupts the phospholipid bilayer and releases cellular contents.

What role does salt play in DNA extraction?

It neutralises the DNA charge and helps it clump together for precipitation.

Why is protease used in DNA extraction?

It digests proteins (like histones) bound to DNA, freeing it

Why does alcohol cause DNA to precipitate?

Alcohol is less polar than water, so DNA becomes insoluble and forms visible strands.

How is DNA’s structure suited for storing genetic information?

• Double helix + strong sugar-phosphate backbone provide stability

• Base sequences store information and allow accurate replication.

How does complementary base pairing ensure accurate replication?

Each strand acts as a template for correct nucleotide pairing + copying

Why is DNA more stable than RNA?

• Deoxyribose lacks the 2′-OH group of ribose, making it less reactive

• The double helix protects the bases

Why is it useful that DNA is a long polymer?

It can store a large quantity of information — many genes across long base sequences.

What are the three main cellular uses of energy?

• Synthesis (building macromolecules)

• Transport (active transport)

• Movement (muscle contraction)

What is ATP made of?

Adenine, ribose, and three phosphate groups.

Why is ATP known as the universal energy currency?

It is used by all living cells to transfer energy for biological processes.

How does ATP release energy?

By hydrolysis: breaking the bond of the final phosphate group releases energy.

What enzyme catalyses ATP hydrolysis? (state the reaction)

ATPase

(ATP to ADP + Pi)

Why does ATP hydrolysis release more energy than it requires to start?

The weak bond between the final phosphate is easily broken so new bonds formed during reactions release much more energy.

What is the reaction for ATP hydrolysis?

ATP + H₂O → ADP + Pi + energy.

Why is ATP hydrolysis described as a coupled reaction?

It happens alongside energy-requiring reactions, transferring released energy directly.

Why isn’t ATP used for long-term energy storage?

It is unstable; carbohydrates and fats are used instead

How is ATP regenerated from ADP?

By phosphorylation: ADP + Pi → ATP using energy from respiration or photosynthesis.

What type of reaction forms ATP from ADP and Pi?

A condensation reaction, as water is removed when the phosphate reattaches.

Why do cells not store much ATP?

It’s an unstable, short-term energy carrier, and its bonds are easily broken to release energy for immediate use

What properties make ATP well suited for energy transfer?

• Small

• Water-soluble

• Releases moderate amounts of energy in small quantities

• Easily regenerated

Why is ATP described as a phosphorylated nucleotide?

It has the same base and sugar structure as nucleotides, but with three phosphate groups instead of one.

What is the purpose of DNA replication?

To produce genetically identical DNA molecules before cell division so that daughter cells receive identical genetic information.

What does ‘semi-conservative replication’ mean?

Each new DNA molecule consists of one (template) strand and one newly-synthesised strand.

What is the role of DNA helicase in DNA replication?

It unwinds and separates the DNA strands by breaking hydrogen bonds between complementary bases.

What is the role of DNA polymerase in DNA replication?

It catalyses the formation of phosphodiester bonds between adjacent nucleotides on the new strand.

Why can DNA polymerase only add nucleotides in one direction?

It can only bind to and extend the 3′ end of a DNA strand, so synthesis occurs in a 5′ → 3′ direction.

How are the leading and lagging strands synthesised?

The leading strand is synthesised continuously, while the lagging strand is synthesised discontinuously in Okazaki fragments.

What are Okazaki fragments?

Short sections of DNA formed on the lagging strand during discontinuous replication.

What is a mutation?

A random, spontaneous change in the DNA base sequence due to errors in replication.

How is the accuracy of DNA replication ensured?

Complementary base pairing ensures that new nucleotides are added correctly to the template strand.

What is the genetic code?

The sequence of DNA bases that determines the sequence of amino acids in a polypeptide.

What is meant by a triplet code?

Each amino acid is coded for by a sequence of three DNA bases called a codon.

What is a gene?

A section of DNA that contains the complete sequence of bases (codons) to code for an entire protein.

Why is the genetic code described as universal?

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

What does it mean that the genetic code is degenerate?

Most amino acids are coded for by more than one codon.

What is the role of the start codon?

It signals the beginning of a sequence coding for a protein and codes for methionine if in the middle of a gene.

What is the role of stop codons?

They do not code for any amino acid and signal the end of the polypeptide chain.

What does it mean that the genetic code is non-overlapping?

Each base is read only once and forms part of only one codon.

Why are enzymes essential in DNA replication?

They catalyse the reactions required for unwinding, base pairing, and bonding, ensuring replication is fast and accurate.

What are the products of DNA replication?

Two identical double-stranded DNA molecules, each with one original and one new strand.

How can mutations affect proteins?

A change in the DNA base sequence can alter the codons, leading to different amino acids and potentially a non-functional protein.