Genetic Code and Transcription: Study Notes

Genetic Code and Transcription

Genetic Code Overview

The genetic code is the set of rules by which the information encoded in messenger RNA (mRNA) is translated into proteins by living cells. It is fundamental to the process of gene expression and protein synthesis.

• Triplet code: Each codon consists of three nucleotides, which together specify a single amino acid.

• Number of codons: There are 64 possible codons (43 combinations) that encode 20 amino acids.

• Unambiguous: Each codon specifies only one amino acid.

• Degenerate: Multiple codons can code for the same amino acid.

• Nearly universal: The genetic code is almost the same in all organisms, with few exceptions.

Important Features of the Genetic Code

• Start codon: AUG (codes for methionine) signals the start of translation.

• Stop codons: UAA, UAG, and UGA signal the termination of translation and do not code for amino acids.

• Nonoverlapping: Codons are read one after another, without overlapping nucleotides.

• Commaless: There are no breaks or punctuation between codons in the mRNA sequence.

• Colinear: The sequence of codons in the gene corresponds directly to the sequence of amino acids in the protein.

Frameshift Mutations

Frameshift mutations occur when nucleotides are inserted into or deleted from the coding sequence, altering the reading frame of the gene. This usually results in a completely different and nonfunctional protein product.

• Example: Insertion of a single nucleotide shifts all downstream codons, changing the amino acid sequence.

Deciphering the Genetic Code

Key experiments in the 1960s helped determine which codons correspond to which amino acids.

• Nirenberg and Matthaei: Used RNA homopolymers (e.g., poly-U) to identify codon assignments.

• Triplet binding assay: Helped match specific codons to their corresponding amino acids.

• Repeating copolymers: Used to further refine codon assignments.

Wobble Hypothesis

The wobble hypothesis explains how a single tRNA can recognize multiple codons due to flexible base pairing at the third position of the codon.

• Significance: Reduces the number of tRNAs needed and contributes to the degeneracy of the code.

Exceptions to the Universal Code

While the genetic code is nearly universal, some exceptions exist, particularly in mitochondria and certain microorganisms.

• Example: In human mitochondria, UGA codes for tryptophan instead of serving as a stop codon.

Overlapping Genes

In some cases, a single stretch of DNA or mRNA can be read in multiple reading frames, producing different proteins from the same nucleotide sequence.

• Example: Certain viruses use overlapping genes to maximize coding capacity.

Transcription

Transcription Overview

Transcription is the process by which RNA is synthesized from a DNA template. This is the first step in gene expression.

• Occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes.

• Carried out by the enzyme RNA polymerase.

• Does not require a primer to initiate synthesis.

RNA Polymerase

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template.

• Prokaryotic RNA polymerase: Consists of a core enzyme and a sigma (σ) factor.

• Sigma factor: Recognizes and binds to promoter regions to initiate transcription.

• Direction of synthesis: RNA is synthesized in the 5' to 3' direction.

Promoters and Initiation

Promoters are specific DNA sequences where RNA polymerase binds to initiate transcription.

• Pribnow box: A conserved sequence (TATAAT) located about 10 bases upstream of the transcription start site in prokaryotes.

• RNA polymerase binds to the promoter with the help of the sigma factor.

Elongation

During elongation, RNA polymerase moves along the DNA template, adding ribonucleotides to the growing RNA strand.

• RNA strand grows as nucleotides are added complementary to the DNA template.

Termination

Transcription ends when RNA polymerase encounters a termination signal.

• Rho-dependent termination: Requires the rho (ρ) protein to release the RNA transcript.

• Rho-independent termination: Involves the formation of a hairpin structure in the RNA followed by a string of uracils (U), causing the polymerase to dissociate.

Eukaryotic Transcription

Transcription in eukaryotes is more complex and involves multiple RNA polymerases and regulatory factors.

• Occurs in the nucleus.

• Requires transcription factors for initiation.

• Uses three main RNA polymerases: I (rRNA), II (mRNA), III (tRNA and other small RNAs).

• Regulation is more intricate due to chromatin structure and additional control elements.

RNA Processing

In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.

• 5’ cap addition: A modified guanine nucleotide is added to the 5’ end for stability and ribosome recognition.

• Poly-A tail addition: A stretch of adenine nucleotides is added to the 3’ end to protect mRNA from degradation.

• Splicing: Non-coding sequences (introns) are removed, and coding sequences (exons) are joined together.

Introns and Exons

• Introns: Non-coding sequences that are removed during RNA splicing.

• Exons: Coding sequences that remain in the mature mRNA and are translated into protein.

RNA Editing

RNA editing refers to post-transcriptional modifications that alter the nucleotide sequence of the RNA.

• Substitution editing: Individual nucleotides are changed.

• Insertion/deletion editing: Nucleotides are inserted or deleted from the RNA sequence.

Review Questions and Answers

Multiple Choice Questions

Question	Correct Answer
How many nucleotides make up a codon?	3
What is the start codon?	AUG
What type of mutation shifts the reading frame?	Frameshift
Which enzyme synthesizes RNA?	RNA polymerase
Which structure signals transcription termination without proteins?	Hairpin
What is removed during RNA splicing?	Introns
What is the function of the sigma factor?	Promoter recognition
Which RNA carries genetic information?	mRNA
What is added to the 5’ end of mRNA?	Cap
Which codon does NOT code for an amino acid?	UAA

Short Answer Questions (with Sample Answers)

1. Explain degeneracy of the genetic code. The genetic code is degenerate because most amino acids are encoded by more than one codon. This redundancy helps protect against some mutations, as changes in the third base of a codon often do not alter the amino acid specified.

2. Describe the role of RNA polymerase. RNA polymerase is the enzyme that synthesizes RNA from a DNA template during transcription. It binds to the promoter region, unwinds the DNA, and catalyzes the formation of phosphodiester bonds between ribonucleotides.

3. Compare prokaryotic vs eukaryotic transcription. In prokaryotes, transcription occurs in the cytoplasm, involves a single RNA polymerase, and requires a sigma factor for initiation. In eukaryotes, transcription occurs in the nucleus, involves three different RNA polymerases, and requires multiple transcription factors for initiation and regulation. Eukaryotic transcripts also undergo extensive processing.

4. Explain how frameshift mutations affect proteins. Frameshift mutations, caused by insertions or deletions of nucleotides, shift the reading frame of the genetic code. This alters the downstream amino acid sequence and usually results in a nonfunctional protein.

5. Describe RNA processing steps. Eukaryotic pre-mRNA undergoes 5’ capping, addition of a poly-A tail at the 3’ end, and splicing to remove introns and join exons, resulting in a mature mRNA ready for translation.

Key Terms and Definitions

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid or stop signal.

• Anticodon: A sequence of three nucleotides in tRNA complementary to a codon in mRNA.

• Promoter: A DNA sequence that signals the start site for transcription.

• Exon: A coding region of a gene that remains in the mature mRNA.

• Intron: A non-coding region of a gene that is removed during RNA processing.

• RNA polymerase: The enzyme that synthesizes RNA from a DNA template.

• Frameshift mutation: A genetic mutation caused by insertions or deletions that change the reading frame.

Formulas and Equations

• Number of possible codons: $4^3 = 64$

Summary Table: Comparison of Prokaryotic and Eukaryotic Transcription

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus
RNA Polymerases	One	Three (I, II, III)
Initiation Factors	Sigma factor	Multiple transcription factors
RNA Processing	Minimal	Extensive (capping, polyadenylation, splicing)