The Genetic Code and Transcription: Structure, Function, and Mechanisms

Chapter: The Genetic Code and Transcription

Transcription and Translation in Cells

Overview of Information Flow

Cells use transcription and translation to convert genetic information stored in DNA into functional proteins. This process is fundamental to all living organisms.

• Transcription: Synthesis of RNA using DNA as a template.

• Translation: Synthesis of protein using the information encoded in RNA.

Diagram Explanation

The diagram illustrates the central dogma: DNA is transcribed into mRNA, which is then translated by ribosomes with the help of tRNA to produce polypeptides that fold into functional proteins.

DNA Template and Coding Strands

Strand Selection in Transcription

During transcription, only one strand of DNA is used as a template for RNA synthesis. The other strand is known as the coding strand.

• Template strand: The DNA strand that is copied by RNA polymerase.

• Coding strand: The DNA strand with the same sequence as the RNA (except T is replaced by U).

Example

Given the following DNA:

• Coding strand: 5'-ATGGGCTCC-3'

• Template strand: 3'-TACCCGAGG-5'

• mRNA: 5'-AUGGGCUCC-3'

This mRNA sequence encodes the amino acids: Met-Gly-Ser.

Types and Functions of RNA

Main Cellular RNAs

Cells produce several types of RNA, each with distinct roles in gene expression and protein synthesis.

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis.

• Ribosomal RNA (rRNA): Forms the core structural and catalytic components of ribosomes.

• Transfer RNA (tRNA): Serves as an adaptor, bringing amino acids to the ribosome during translation.

Example

During translation, tRNA molecules match their anticodon sequences to mRNA codons, ensuring the correct amino acid is added to the growing polypeptide chain.

The Genetic Code

Properties and Interpretation

The genetic code defines how nucleotide sequences in mRNA are translated into amino acid sequences in proteins.

• Triplet code: Each amino acid is specified by a sequence of three nucleotides (codon).

• Four bases: A, U, G, C (in RNA).

• 64 possible codons: $4^3 = 64$ combinations.

• 20 amino acids: Multiple codons can specify the same amino acid (degeneracy).

• Unambiguous: Each codon specifies only one amino acid.

• Start codon: AUG (methionine).

• Stop codons: UAA, UAG, UGA (signal termination of translation).

Table: Codon Assignments (Summary)

Codon	Amino Acid
AUG	Methionine (Start)
UUU, UUC	Phenylalanine
UAA, UAG, UGA	Stop
GCU, GCC, GCA, GCG	Alanine
... (others)	...

Additional info: The full codon table includes all 64 codons and their corresponding amino acids.

Mechanisms of Transcription

Stages of Transcription

Transcription is the process by which RNA is synthesized from a DNA template. It involves several key stages:

1. Binding: RNA polymerase binds to the promoter region of DNA.

2. Initiation: RNA synthesis begins at the transcription start site.

3. Elongation: RNA polymerase moves along the DNA, synthesizing RNA.

4. Termination: RNA synthesis ends, and the RNA molecule is released.

RNA Polymerases in Eukaryotes

Eukaryotic cells have three main RNA polymerases, each responsible for synthesizing different types of RNA:

• RNA polymerase I: Synthesizes rRNA (except 5S rRNA).

• RNA polymerase II: Synthesizes mRNA and some small nuclear RNAs.

• RNA polymerase III: Synthesizes tRNA, 5S rRNA, and other small RNAs.

Promoters and Transcription Factors

Promoters are DNA sequences that define where transcription begins. General transcription factors (TFs) are proteins required for RNA polymerase binding and initiation.

• TFIID: Recognizes the TATA box in the promoter via its TATA-binding protein (TBP) subunit.

• TFIIH: Has helicase and kinase activity; unwinds DNA and phosphorylates RNA polymerase II to initiate transcription.

• Initiation complex: Multiple TFs and RNA polymerase assemble at the promoter.

Elongation and Termination

During elongation, RNA polymerase synthesizes RNA complementary to the DNA template. Termination mechanisms differ among polymerases:

• RNA polymerase I: Recognizes an 18-nucleotide signal in the RNA.

• RNA polymerase II: Transcripts are cleaved at a site 10-35 nucleotides downstream of the AAUAAA sequence.

• RNA polymerase III: Termination involves a run of U's; no protein factors required.

RNA Processing and Turnover

Primary Transcript and Processing

Newly synthesized RNA (primary transcript) must undergo processing before functioning in the cell.

• Splicing: Removal of introns and joining of exons.

• 5' capping: Addition of a 7-methylguanosine cap.

• 3' polyadenylation: Addition of a poly(A) tail (50-250 adenines).

Table: mRNA Processing Steps

Step	Function
5' Cap Addition	Stabilizes mRNA, aids ribosome binding
Splicing	Removes introns, joins exons
Poly(A) Tail Addition	Protects mRNA, aids export and translation

rRNA and tRNA Processing

Ribosomal and transfer RNAs are processed from larger precursors:

• rRNA: Cleavage and methylation of a common precursor transcript; forms 18S, 28S, and 5.8S rRNAs.

• tRNA: Removal of leader and trailer sequences, addition of CCA at 3' end, and chemical modification of bases.

mRNA Stability and Turnover

mRNA molecules have a limited lifespan, measured by half-life (time for 50% degradation).

• Eukaryotic mRNAs: Half-lives range from several hours to days.

• Bacterial mRNAs: Half-lives are typically a few minutes.

Exons and Introns

Splicing Mechanism

Exons are coding sequences retained in mature mRNA, while introns are non-coding sequences removed during splicing.

• Splice sites: Conserved sequences at exon-intron boundaries guide splicing.

• Spliceosome: A complex of proteins and RNAs that mediates splicing.

Example

Primary transcript: Exon-Intron-Exon structure. After splicing, only exons remain in mature mRNA.

Additional info: Alternative splicing allows a single gene to produce multiple protein variants.