Central Dogma of Molecular Biology

Overview

The Central Dogma of biology describes the flow of genetic information from DNA to RNA to protein. This process is fundamental to gene expression and cellular function.

• Transcription: The process by which RNA is synthesized using DNA as a template.

• Translation: The process by which proteins are synthesized using the encoded message of mRNA.

• Transcription and translation together are often referred to as gene expression.

Example: DNA → RNA → Protein

• DNA is replicated; RNA can be transcribed into DNA, but transfer of nucleic acid to protein is irreversible.

Key Terms

• mRNA: Messenger RNA, the intermediate molecule in the flow of information from DNA to protein.

• Gene Expression: The process by which genotype becomes expressed as phenotype.

Transcription: Mechanism and Steps

Definition and Components

Transcription is the process that builds RNA using DNA within a gene as the coding template. Genes are small units of DNA that encode a product (e.g., protein).

• Promoter: DNA sequence where transcription begins; site of RNA polymerase attachment.

• RNA Polymerase: Enzyme that polymerizes/binds RNA from scratch (no primer needed).

• Terminator: DNA sequence where transcription ends.

"Upstream" refers to DNA sequences in the 5' direction of transcription; "downstream" refers to DNA sequences in the 3' direction.

Gene Definition

• A gene is a sequence of DNA that encodes a product, such as RNA or protein.

Transcription Steps

• Transcription consists of three main steps:

  1. Initiation: RNA polymerase binds to the promoter on DNA and unwinds the two DNA strands.

  2. Elongation: RNA polymerase synthesizes an RNA molecule by pairing RNA nucleotides with the DNA template, moving in the 5' to 3' direction.

  3. Termination: Transcription ends when RNA polymerase reaches the terminator sequence, releasing the RNA molecule.

Prokaryotes vs. Eukaryotes

• In prokaryotes, RNA polymerase binds directly to the promoter.

• In eukaryotes, transcription factor proteins are required for RNA polymerase to bind to the promoter.

Transcription: DNA Strands and Base Pairing

Strand Terminology

• The two strands of DNA in a gene are referred to as the coding strand and the template strand.

• RNA molecules have the same sequence as the coding DNA strand (except replacing thymine (T) with uracil (U)).

Base Pairing

• Nucleotide pairing occurs via Watson & Crick base-pairing rules:

  • Adenine (A) pairs with Uracil (U) in RNA.

  • Cytosine (C) pairs with Guanine (G).

RNA Processing in Eukaryotes

Overview

Unlike prokaryotic mRNA, eukaryotic mRNA requires further modification after transcription termination. This is known as RNA processing and splicing.

• Pre-mRNA: The initial RNA transcript before modification.

• Modified mRNA: The mature RNA ready for translation.

RNA Processing Steps

• Addition of a 5' cap (modified guanine nucleotide) to the 5' end of the pre-mRNA.

• Addition of a Poly-A tail (sequence of adenine nucleotides) to the 3' end of the pre-mRNA.

• Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions).

Functions of 5' Cap and Poly-A Tail

• Facilitate export of mRNA from the nucleus to the cytoplasm.

• Help ribosomes attach to mRNA for translation.

• Protect mRNA from degradation by enzymes.

Practice Questions and Applications

• Identify the intermediate molecule in the central dogma: mRNA.

• The process by which genotype becomes expressed as phenotype: Gene expression.

• Best definition of a gene: A sequence of DNA that encodes a product like RNA or protein.

• Transcription steps: Initiation, Elongation, Termination.

• Transcription in eukaryotes requires transcription factors in addition to RNA polymerase.

• Correct mRNA transcript from DNA template: Replace T with U and use complementary base pairing.

• During transcription, RNA polymerase moves along DNA in the 5' to 3' direction.

• RNA processing in eukaryotes includes addition of 5' cap and poly-A tail.

Comparison Table: Transcription vs. Translation

Key Equations

• Base pairing during transcription:

Additional info: RNA processing is essential for eukaryotic gene expression, ensuring that only mature, properly modified mRNA is translated into protein.