d1.2 new

What is transcription?

Copying a gene's DNA sequence into complementary mRNA.

Where does transcription occur in eukaryotic cells?

In the nucleus.

What acts as the template during transcription?

One strand of DNA; RNA nucleotides pair with its exposed bases.

How does complementary base pairing work in transcription?

DNA A pairs with RNA U; DNA T with A; DNA C with G; DNA G with C.

What is the role of hydrogen bonds in transcription?

They form between complementary bases and help guide correct RNA nucleotide pairing.

Why must DNA bases be exposed during transcription?

The DNA strands separate locally so RNA nucleotides can pair with the template strand.

What makes the DNA template stable?

Complementary base pairing and hydrogen bonds between DNA strands, plus strong covalent bonds in each backbone.

What is produced by transcription?

An mRNA molecule complementary to the DNA template strand.

What is translation?

Using the codon sequence of mRNA to assemble amino acids into a polypeptide.

Where does translation occur?

At ribosomes in the cytoplasm or on the rough endoplasmic reticulum.

What is the role of mRNA in translation?

It carries codons copied from DNA and provides the template for amino acid order.

What is the role of the ribosome?

It reads mRNA codons, positions tRNAs, and catalyses peptide-bond formation.

What is the role of tRNA?

It carries a specific amino acid and has an anticodon complementary to an mRNA codon.

What is an mRNA codon?

A sequence of three mRNA nucleotides specifying an amino acid or a stop signal.

What is a tRNA anticodon?

A three-nucleotide sequence that pairs with a complementary mRNA codon.

Why is complementary pairing important in translation?

It matches each mRNA codon with the tRNA carrying the correct amino acid.

What does degeneration of the genetic code mean?

Most amino acids are specified by more than one codon.

What does universality of the genetic code mean?

Codons generally specify the same amino acids in nearly all organisms.

How do you read the genetic code?

Read mRNA in triplets and use a codon table to identify each amino acid.

What is the start codon on mRNA?

AUG; it usually codes for methionine and initiates translation.

What are the three mRNA stop codons?

UAA, UAG, and UGA.

How does a ribosome move along mRNA?

It advances one codon at a time, allowing successive tRNAs to deliver amino acids.

How are amino acids joined during translation?

The ribosome catalyses peptide bonds between adjacent amino acids.

What happens to the growing polypeptide during translation?

It lengthens as amino acids are added in the order specified by mRNA codons.

How can a mutation affect protein structure?

It can change a codon, altering an amino acid and potentially the polypeptide's folding and function.

What is a point mutation?

A change in a single nucleotide in DNA.

How can a point mutation change an amino acid?

A base substitution can change an mRNA codon so it specifies a different amino acid.

Give an example of a point mutation affecting protein structure.

In sickle-cell disease, a beta-globin gene substitution changes glutamic acid to valine, altering haemoglobin.