Transcription is the process of copying genetic information from DNA into mRNA (messenger RNA). The DNA base sequence is converted into complementary mRNA by RNA polymerase.
Basic Knowledge
Mechanism of transcription (prokaryote and Escherichia coli model): ① RNA polymerase binds to a promoter (a specific base sequence). ② The DNA double strand locally unwinds, forming a transcription bubble. ③ The enzyme reads the template strand (antisense strand) in the 3'→5' direction and synthesizes complementary RNA in the 5'→3' direction (U corresponds to T). ④ When it reaches the termination sequence, RNA polymerase dissociates and single-stranded mRNA is completed.
Post-transcriptional processing in eukaryotes: ① A 5' cap (7-methylguanosine) is added; this is required for translation initiation and transport out of the nucleus. ② A 3' poly-A tail (Poly-A tail) is added; this increases mRNA stability. ③ Splicing: The spliceosome removes introns (noncoding sequences) and joins exons (coding sequences). Alternative splicing allows multiple proteins to be produced from one gene.
Types of RNA: mRNA (protein coding), tRNA (transports amino acids during translation), rRNA (component of ribosomes), snRNA (splicing), miRNA/siRNA (suppression of gene expression), and lncRNA (various regulatory functions).
Central dogma (Crick, 1958): DNA→(transcription)→RNA→(translation)→protein. Reverse transcription (RNA→DNA) occurs in retroviruses and is performed by reverse transcriptase.
Explain how alternative splicing in eukaryotes contributes to biological diversity.
Answer: In alternative splicing, different combinations of exons are selected from the same precursor mRNA (pre-mRNA), allowing one gene to produce multiple different mRNAs and proteins. For example, the human Dscam gene can theoretically produce tens of thousands of types of proteins, creating protein diversity far greater than the approximately 20,000 genes in humans.
- RNA polymerase = binds to the promoter and synthesizes mRNA
- Template strand = antisense strand (read in the 3'→5' direction)
- 5' cap and poly-A tail = stabilize eukaryotic mRNA
- Splicing = intron removal and exon joining
- Alternative splicing = one gene→multiple proteins
- Central dogma = DNA→RNA→protein
- Reverse transcription = RNA→DNA (reverse transcriptase in retroviruses)
Points to Note
① During transcription, uracil (U) pairs with adenosine (A) instead of thymine (T). ② Prokaryotes lack a nuclear membrane, so transcription and translation occur simultaneously. In eukaryotes, transcription occurs in the nucleus, mRNA is transported outside the nucleus, and translation occurs in the cytoplasm. ③ Exon is short for “expressed,” while intron is short for “intervening.”
Practice
- If the template DNA base sequence is 5'-ATGCTA-3', state the sequence of the transcribed mRNA in the 5'→3' direction.
- Name the three types of post-transcriptional processing performed on precursor mRNA in eukaryotes.
- Give an example of a virus that has reverse transcriptase, an exception to the central dogma.