Protein Synthesis Flashcards

Comprehensive flashcards covering the Central Dogma, the concept of the gene, the genetic code, transcription, post-transcriptional modifications, and translation as detailed in the lecture notes.

Define the central dogma of molecular biology as coined by Francis Crick.

The unidirectional flow of genetic information from DNA (gene) through mRNA to protein.

What discovery challenged the unidirectional flow of the central dogma?

The discovery of reverse transcription.

What is a gene?

A unit of inheritance which contains the nucleotide sequence for synthesis of a functional gene product (e.g., a polypeptide chain, tRNA, rRNA), including both coding sequences (exons) and non-coding sequences (introns).

What is the difference between exons and introns?

Exons are coding sequences of DNA that code for the sequence of amino acids in a polypeptide chain, while introns are non-coding sequences interspersed between exons.

How many nucleotides are required to code for one amino acid if there are 20 amino acids found in nature?

3 nucleotides (triplet code), because $4^{3} = 64$ possible combinations, which is sufficient to cover 20 amino acids.

What does it mean for the genetic code to be 'degenerate'?

It means that more than one codon can code for the same amino acid.

Identify the start codon and the three stop codons in the genetic code.

The start codon is AUG; the stop codons are UGA, UAG, and UAA.

What are the three main requirements for transcription?

1. RNA polymerase, 2. Free ribonucleotides, 3. DNA template.

In which direction does RNA polymerase synthesize a polyribonucleotide chain?

In the $5' \rightarrow 3'$ direction.

What is the role of the TATA-Binding Protein (TBP) during transcription initiation?

It is a general transcription factor that recognizes and binds to the TATA box, distorting the DNA and causing the helix to partially unwind.

What sequence on the pre-mRNA signals the end of transcription in eukaryotes?

The termination or polyadenylation signal sequence AAUAAA.

List the three steps of post-transcriptional modification in eukaryotes.

1. Addition of 5' cap, 2. Polyadenylation at 3' end (addition of poly(A) tail), 3. RNA splicing.

What is the function of a spliceosome?

It consists of snRNPs and cuts pre-mRNA at specific splice sites to release introns and splice exons together.

What is the significance of alternative splicing?

It allows one gene to give rise to different proteins by joining different combinations of exons.

What molecule carries a specific amino acid to the ribosome and contains an anticodon?

tRNA (transfer RNA)

How is an amino acid attached to tRNA during amino acid activation?

Specific aminoacyl-tRNA synthetases catalyze the attachment of a specific amino acid to the $3'$ CCA end of a tRNA molecule using ATP.

Which component of the large ribosomal subunit acts as the enzyme peptidyl transferase?

rRNA (ribosomal RNA)

What are the S values for the subunits of a eukaryotic (80S) ribosome?

A large subunit of 60S and a small subunit of 40S.

Describe the roles of the A, P, and E sites in a ribosome.

The P site (Peptidyl-tRNA site) binds the tRNA holding the growing polypeptide; the A site (Aminoacyl-tRNA site) binds the next aminoacyl-tRNA; the E site (Exit site) is where tRNA is released.

What are polyribosomes and what is their significance?

A cluster of ribosomes translating a single mRNA molecule simultaneously; they increase the rate of polypeptide synthesis.

What happens during the translocation step of translation elongation?

The ribosome moves along mRNA to the next codon in the $5' \rightarrow 3'$ direction; the tRNA at the E site is released, and the tRNA at the A site moves to the P site.

How does translation terminate?

Termination occurs when a stop codon (UAA, UAG, or UGA) occupies the A site, allowing release factors to bind and hydrolyze the bond between the polypeptide and the tRNA.

Identify three differences between prokaryotic and eukaryotic protein synthesis.

1. Location: Prokaryotes (cytosol), Eukaryotes (transcription in nucleus, translation in cytoplasm). 2. Timing: Prokaryotes (simultaneous), Eukaryotes (sequential). 3. Introns/Splicing: Prokaryotes (none), Eukaryotes (present).

Compare the chemical stability of DNA and RNA.

DNA is chemically stable due to deoxyribose and base stacking; RNA is less stable due to the $2'-OH$ group in the ribose sugar which is susceptible to hydrolysis.