6.3 Transcription & RNA Processing

DNA to RNA

Transcription copies a gene's DNA into messenger RNA (mRNA). The enzyme RNA polymerase reads the DNA template strand and builds mRNA in the 5′→3′ direction. In eukaryotes, this happens in the nucleus.

Processing the transcript

Before leaving the nucleus, eukaryotic mRNA is processed: a protective 5′ cap and a poly-A tail are added, and introns (non-coding sequences) are spliced out while exons (coding sequences) are joined together. Only the finished mRNA is translated.

Key terms

Transcription

Copying a gene's DNA into messenger RNA (mRNA).

RNA polymerase

The enzyme that builds mRNA from a DNA template, 5′→3′.

mRNA

Messenger RNA — carries the gene's code from DNA to the ribosome.

Introns vs. exons

Introns are non-coding (spliced out); exons are coding (kept).

RNA processing

Adding a 5′ cap and poly-A tail and splicing out introns before translation.

Try an AP-style question

A DNA template strand reads 3'-TACGGATTC-5'. What mRNA is transcribed?

1. 5'-AUGCCUAAG-3'
2. 3'-AUGCCUAAG-5'
3. 5'-UACGGAUUC-3'
4. 5'-ATGCCTAAG-3'