2.28 Central Dogma of Life

The Central Dogma describes the fundamental process of how genetic information flows from a DNA sequence to a functional protein product. This process, also known as gene expression, involves two main stages: transcription and translation. It was first proposed by Francis Crick in 1957.

The flow of biological information follows the path: DNA → RNA → Protein. Genes within the DNA contain the instructions for synthesizing proteins.

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Transcription

Transcription is the first step of gene expression, where the genetic information from a DNA segment (a gene) is copied into a messenger RNA (mRNA) molecule. This process occurs within the nucleus of eukaryotic cells.

Ultrastructure of Animal and Plant Cells→

Process:

1. Initiation: The two DNA strands of a specific gene unwind and separate.
2. Elongation: One of the DNA strands, the non-coding (template) strand, acts as a template.
3. Synthesis of mRNA: RNA polymerase synthesizes a single strand of mRNA by pairing complementary RNA nucleotides to the exposed DNA template.
4. Termination: Once the gene is transcribed, the mRNA molecule detaches, and the two DNA strands re-join.

Base Pairing Rule in Transcription:

• In RNA, Uracil (U) replaces Thymine (T).
• Uracil in mRNA pairs with Adenine (A) on the DNA template strand.

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Translation

Translation is the second step of gene expression, where the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids, forming a polypeptide chain (protein). It occurs in the cytoplasm on ribosomes.

Key Terms:

• Codon: A sequence of three consecutive nucleotides on an mRNA molecule that specifies a particular amino acid.
• Anticodon: A complementary triplet sequence on a tRNA molecule that base-pairs with an mRNA codon.
• tRNA (Transfer RNA): Carries specific amino acids to the ribosome, matching each codon via its anticodon.

Process:

1. Location: The mRNA molecule travels from the nucleus to the cytoplasm.
2. Ribosome Binding: The mRNA binds to a ribosome, the site of protein synthesis.
3. Codon Reading: The ribosome reads the mRNA sequence in sets of three nucleotides (codons). Each codon specifies a particular amino acid.
4. tRNA Role: tRNA molecules, each carrying a specific amino acid, have an anticodon complementary to an mRNA codon. The tRNA delivers the correct amino acid to the ribosome.
5. Polypeptide Formation: As the ribosome moves along the mRNA, amino acids are linked by peptide bonds, forming a growing polypeptide chain.

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DNA vs. RNA Stability

The chemical structure of DNA and RNA accounts for their different roles and stability within the cell.

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Reverse Transcription

While the Central Dogma generally describes a unidirectional flow (DNA → RNA → Protein), certain viruses called Retroviruses (e.g., HIV) possess an enzyme called Reverse Transcriptase. This enzyme allows them to synthesize DNA from an RNA template — a process known as reverse transcription — which is an exception to the standard Central Dogma.

$\text{RNA}\stackrel{\text{Reverse Transcriptase}}{\to }\text{DNA}$