The information written in DNA is first copied into RNA. This process is called transcription. It is the first half of protein synthesis.
Basic Knowledge
RNA (ribonucleic acid) resembles DNA but differs in several ways:
① Its sugar is ribose (DNA has deoxyribose; ribose has an OH group on the 2' carbon).
② The base U (uracil) is used instead of T.
③ It is usually single-stranded (DNA is double-stranded).
Process of transcription:
① DNA partially unwinds in a gene region.
② RNA polymerase uses the antisense strand (the strand opposite the sense strand) as a template and joins complementary RNA nucleotides.
③ One RNA strand (mRNA, messenger RNA) is produced.
In eukaryotes, newly formed mRNA contains introns (regions that do not contain protein information) and exons (regions that contain information). Through splicing, introns are removed and mature mRNA is produced. Prokaryotes generally do not have introns.
RNA (ribonucleic acid; sugar = ribose, bases = A・U・G・C, single-stranded)
Transcription (the process of synthesizing RNA using the DNA base sequence as a template)
mRNA (messenger RNA) (RNA that carries information for a protein's amino acid sequence)
RNA polymerase (an enzyme that synthesizes RNA)
Introns and exons (nontranslated and translated regions within eukaryotic genes)
Splicing (the process of removing introns and joining exons)
Central dogma (the flow of information: DNA→RNA→protein)
Further Study
Replication and transcription differ as follows: replication is DNA→DNA (making the same double strand), whereas transcription is DNA→RNA (making single-stranded RNA). Replication uses DNA polymerase, while transcription uses RNA polymerase. They belong to similar enzyme families, but their functions and substrates (the nucleotides they incorporate) differ.
Direction of transcription: RNA polymerase synthesizes RNA in the 5'→3' direction, reading the template DNA in the 3'→5' direction. The RNA sequence is complementary to the bases on the template strand (A↔U, G↔C).
The discovery of splicing (1977, Roberts & Sharp) was an important achievement that led to the Nobel Prize (1993). Alternative splicing can also occur, producing different proteins from the same gene through different splicing patterns. Humans have about 20,000 genes, but more than 100,000 types of proteins are estimated to be produced through alternative splicing and other processes, creating biological complexity.
The central dogma was proposed by Crick in 1958 as the flow of genetic information “DNA→RNA→protein.” Although there are exceptions, such as reverse transcription from RNA→DNA in retroviruses, the basic flow of biological information follows this direction.
- RNA = ribose sugar, A・U・G・C bases, single-stranded
- Transcription = DNA→RNA, catalyzed by RNA polymerase
- mRNA = messenger RNA that carries protein information
- Introns = nontranslated; exons = translated
- Splicing = removal of introns and joining of exons
- Alternative splicing = many proteins from one gene
- Central dogma = DNA→RNA→protein
Points to Note
① Clearly distinguish DNA (double-stranded, T) from RNA (single-stranded, U). ② Do not reverse transcription (DNA→RNA) and translation (RNA→protein). Remember that transcription copies information, whereas translation converts it into another language. ③ Do not confuse introns (nontranslated and removed) with exons (translated and retained). ④ Sense strand (coding strand) and antisense strand (template strand): the antisense strand is used as the template. The mRNA sequence is the same as the sense-strand sequence except that T is replaced by U.
Practice
- Give one base found in RNA that differs from a base in DNA.
- What is the name of the region in a eukaryotic gene that does not contain protein information and is removed after transcription?
- What is the general name for the flow of genetic information “DNA→RNA→protein”?