Protein Synthesis: Transcription and RNA Processing in Eukaryotes

Flashcards covering the stages of transcription (initiation, elongation, and termination), the comparison between transcription and DNA replication, and post-transcriptional RNA processing (end modifications and RNA splicing) in eukaryotes.

What is transcription in eukaryotic cells?

The synthesis (production) of RNA using a DNA template, in which genetic information contained in DNA is transcribed into mRNA using RNA polymerase (II).

Where does RNA polymerase (II) bind on DNA during the initiation stage of transcription?

It binds to the promoter on DNA.

Where does the promoter typically extend relative to the start point of transcription?

The promoter typically extends upstream from the start point.

How does RNA polymerase (II) unwind the DNA double helix during initiation?

By breaking the hydrogen bonds and separating the two DNA strands.

During transcription, how many DNA strands act as a template, and in what direction does the template strand run?

Only one DNA strand acts as a template, oriented in the $3'$ to $5'$ direction.

Where on the template strand does RNA synthesis begin?

At the start point on the template strand.

How does the number of template strands used differ between DNA replication and transcription?

DNA replication uses both strands as a template, whereas transcription uses only one DNA strand as a template.

How do the resulting products of DNA replication and transcription differ?

DNA replication produces a double-stranded DNA molecule, whereas transcription produces one single-stranded mRNA (or single-stranded RNA).

Which major polymerases are involved in DNA replication versus transcription?

DNA replication involves DNA polymerase III, whereas transcription involves RNA polymerase II.

When does DNA replication occur compared to transcription in a cell's lifecycle?

DNA replication happens specifically during S phase, whereas transcription occurs throughout the life of the cell.

What base-pairing rules does RNA polymerase (II) follow when adding complementary RNA nucleotides during elongation?

Cytosine (C) pairs with Guanine (G), and Uracil (U) pairs with Adenine (A).

To which end of the growing pre-mRNA transcript are RNA nucleotides added during elongation?

At the $3'$ end of the growing RNA transcript/pre-mRNA.

In which direction does the synthesis of pre-mRNA occur?

Synthesis occurs from the $5'$ to $3'$ direction ($5' \rightarrow 3'$).

What occurs to the pre-mRNA transcript and the DNA template behind RNA polymerase (II) as elongation proceeds downstream?

The newly synthesised pre-mRNA molecule peels away/detaches from the DNA template, and the DNA double helix re-forms.

What sequence on DNA does RNA polymerase (II) transcribe during the termination stage?

The polyadenylation signal sequence.

What is the specific nucleotide sequence of the polyadenylation signal found in pre-mRNA?

AAUAAA.

How is pre-mRNA freed from RNA polymerase (II) during termination?

Proteins cut the pre-mRNA free from RNA polymerase (II), releasing it from the DNA template.

What happens to RNA polymerase (II) after the pre-mRNA is released in termination?

RNA polymerase (II) detaches from the DNA template.

What is the purpose of post-transcriptional RNA processing in eukaryotic cells?

Enzymatically modifying pre-mRNA after transcription to produce a mature mRNA molecule ready for translation.

What two modifications are made to alter the ends of eukaryotic pre-mRNA during RNA processing?

The $5'$ end receives a $5'$ cap, and the $3'$ end forms a poly-A tail.

What are the three functions of the $5'$ cap and poly-A tail added to mRNA?

1. Promote the export of the mature mRNA from the nucleus.
2. Help protect the mRNA from degradation by hydrolytic enzymes.
3. Help ribosomes attach to the $5'$ end of the mRNA in the cytoplasm.

What is the difference between introns and exons in split genes?

Introns are noncoding segments that do not code for amino acids, whereas exons are coding segments that code for amino acids.

What occurs during the process of RNA splicing?

RNA splicing removes introns and joins exons together to create an mRNA molecule with a continuous coding sequence only.

What molecular complex catalyses RNA splicing in eukaryotes?

The spliceosome.