is the process of copying a gene's DNA into RNA.
It involves initiation, elongation, and termination.
It is carried out by RNA Polymerase II and factors.
The template strand is read to synthesize complementary mRNA.
The pre-mRNA undergoes processing: 5' capping, splicing, and poly-A tail addition.
Mature mRNA is then exported to the cytoplasm for translation into protein.
Transcription machinery and molecular logic
Introduction: Setting the Stage for Gene Expression
At the heart of biology is a simple yet powerful idea, the central dogma, which explains the flow of genetic information from DNA to RNA to protein. This directional flow underlies the function of every living cell and defines how genotype leads to phenotype. In this chapter, we begin with the first crucial step in this process: , the conversion of a DNA sequence into a complementary RNA strand.
To understand fully, we must first grasp the structure and chemical composition of nucleotides, and the key differences between DNA and RNA. These distinctions are far more than academic; they influence how genes are regulated, how cells respond to stress, and how errors in expression lead to disease.
Section I: The Building Blocks - Nucleotides, Nucleosides, and the DNA/RNA Backbone
Every molecule of DNA or RNA is composed of repeating monomers called nucleotides. A nucleotide consists of three main components:
A five-carbon sugar (either deoxyribose or ribose),
A phosphate group,
A nitrogenous base.
If the phosphate group is missing, the molecule is no longer a nucleotide but a nucleoside, a compound made only of a sugar and a nitrogenous base. This distinction is critical in biochemistry and pharmacology, where synthetic nucleoside analogs are used as drugs to interfere with viral replication and cancer cell proliferation (e.g., AZT for HIV, 5-fluorouracil for cancer).
The five-carbon sugar is central to identifying whether the nucleotide belongs to DNA or RNA. The second carbon (2') of the sugar is especially important:
In deoxyribose, found in DNA, the 2' carbon has only a hydrogen atom (H).
In ribose, found in RNA, the 2' carbon contains a hydroxyl group (OH).
This single oxygen atom has major consequences. The hydroxyl group in RNA makes the molecule more chemically reactive and less stable, which is why RNA is usually single-stranded and more transient than DNA.
Each sugar's carbons are numbered 1' to 5':
Carbon 1': Site of nitrogenous base attachment.
Carbon 2': The sugar's identity, ribose (RNA) has an OH; deoxyribose (DNA) has an H.