Protein Synthesis

What is gene expression?

Gene expression is the process by which information stored in a gene is used to produce a functional product. For protein-coding genes, this usually involves producing a protein through transcription and translation.

What are the two major processes involved in producing a protein from a gene?

The two major processes are transcription and translation.

What is Transcription?

Transcription is the process of using a DNA sequence as a template to synthesise a complementary RNA molecule.

What is translation?

Translation is the process in which a ribosome reads the codons of mRNA and uses this information to assemble amino acids into a polypeptide.

What is the central dogma of genetics?

DNA → RNA → protein. DNA is transcribed into RNA, and mRNA is translated into a polypeptide.

What is the main difference between transcription and translation?

Transcription converts DNA → RNA, while translation converts mRNA → polypeptide.

Where does transcription occur in prokaryotes?

In the cytoplasm, because prokaryotes do not have a nucleus.

Where does transcription occur in eukaryotes?

In the nucleus.

Where does translation occur?

Translation occurs at ribosomes in the cytoplasm.

What are three major differences between DNA and RNA?

DNA is usually double-stranded and contains deoxyribose and thymine. RNA is usually single-stranded and contains ribose and uracil.

Which base replaces thymine in RNA?

Uracil (U).

If the DNA template is TAC, what mRNA sequence is produced?

AUG.

What are the three types of RNA involved in protein synthesis?

mRNA, tRNA and rRNA.

What does mRNA do?

mRNA carries the genetic information from DNA to the ribosome in the form of codons

What does tRNA do?

tRNA carries amino acids to the ribosome and uses its anticodon to recognise mRNA codons.

What does rRNA do?

rRNA combines with proteins to form ribosomes and helps with protein synthesis.

What are the three ribosome binding sites?

A, P and E sites. A receives incoming tRNA, P holds the growing polypeptide, and E is where empty tRNA exits

What enzyme carries out transcription?

RNA polymerase.

What does RNA polymerase do?

It binds to DNA, separates the strands and uses the template strand to build a complementary RNA molecule.

In which direction is RNA synthesised?

5′ → 3′.

Does RNA polymerase require a primer?

No

What is a promoter?

A promoter is a DNA sequence where the transcription machinery binds and where transcription begins.

What is the TATA box?

The TATA box is a common sequence within many eukaryotic promoters that helps transcription factors recognise and position the transcription machinery.

What helps RNA polymerase recognise promoters in prokaryotes?

A sigma factor helps RNA polymerase recognise and bind to the promoter.

What helps initiate transcription in eukaryotes?

Transcription factors recognise promoter sequences and help recruit and position RNA polymerase.

What are introns?

Introns are sequences in pre-mRNA that are removed during RNA processing.

What are exons?

Exons are sequences that remain in the mature mRNA after splicing.

What is RNA splicing?

RNA splicing removes introns and joins exons together. Spliceosomes carry out

What are the three major modifications made to eukaryotic pre-mRNA?

5′ cap, intron removal/splicing, and 3′ poly-A tail.

Why is the mature mRNA important?

Mature mRNA can leave the nucleus and carry the genetic information to a ribosome for translation.

What is a codon?

A codon is a group of three nucleotides on mRNA that specifies an amino acid or a start/stop signal.

What is the start codon?

AUG.

What amino acid does AUG code for?

Methionine.

What are the three stop codons?

UAA, UAG and UGA.

What does it mean that the genetic code is redundant?

More than one codon can code for the same amino acid.

What does it mean that the genetic code is unambiguous?

Each individual codon has only one specific meaning.

What is wobble?

Wobble is flexibility in base pairing at the third position of a codon, allowing one tRNA to recognise multiple codons.

What are the three stages of translation?

Initiation → elongation → termination.

What happens during initiation?

The ribosome binds to mRNA, recognises the AUG start codon, and the initiator tRNA carrying methionine binds.

What happens during elongation?

tRNAs bring amino acids to the ribosome, peptide bonds form between amino acids, and the ribosome moves along the mRNA.

What happens during termination?

When a stop codon is reached, a release factor binds and the completed polypeptide is released.

What enzyme attaches amino acids to tRNA?

Aminoacyl-tRNA synthetase.

What enzyme catalyses peptide bond formation?

Peptidyl transferase.

Why does translation begin with methionine?

Translation begins at the AUG start codon, which codes for methionine.

What is a substitution mutation?

One nucleotide is replaced by another nucleotide.

What is a frameshift mutation?

An insertion or deletion that is not a multiple of three nucleotides shifts the reading frame and changes the downstream codons.

What is a silent mutation?

A mutation that changes a codon but does not change the amino acid.

What is a missense mutation?

A mutation that changes a codon so that a different amino acid is produced.

What is a nonsense mutation?

A mutation that changes a codon into a premature stop codon, producing a shortened polypeptide.

What mutation causes sickle-cell anaemia?

A missense substitution in the β-globin gene.

What amino acid change occurs in sickle-cell anaemia?

Glutamic acid → valine in the β-globin protein.