Genetics Exam 3 Study Guide: Chapters 8–11 (Transcription, Translation, Chromosome Structure, and Mutation)

Transcription and RNA Processing

Transcribed DNA and Chromatin Structure

Transcription is the process by which RNA is synthesized from a DNA template. The accessibility of DNA for transcription is influenced by its chromatin state.

• Transcribed DNA is generally found in euchromatin, which is less condensed and more accessible to transcription machinery.

• Heterochromatin is tightly packed and typically transcriptionally inactive.

• Constitutive heterochromatin (e.g., centromeric regions) remains condensed and is usually not transcribed.

The Central Dogma and Information Flow

The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein.

• Permitted transfers: DNA to RNA (transcription), RNA to protein (translation).

• Forbidden transfers: Protein to RNA or DNA (information cannot flow from protein back to nucleic acids).

• Exceptions: RNA to DNA (reverse transcription in retroviruses).

G+C Content in Nucleic Acids

The guanine-cytosine (G+C) content of nucleic acids affects their stability and is characteristic of species.

• If the G+C content of E. coli RNA is 30%, the DNA is likely to have a similar G+C content, as both are transcribed from the same genome.

Transcribing Enzymes

RNA synthesis is catalyzed by specific enzymes.

• RNA polymerase is the primary enzyme responsible for transcription in both prokaryotes and eukaryotes.

• RNA primase synthesizes short RNA primers during DNA replication, not transcription.

• Reverse transcriptase synthesizes DNA from an RNA template (in retroviruses).

Initiation of Transcription in Prokaryotes

Transcription initiation requires precise recognition of promoter sequences.

• Sigma factor is a protein that enables RNA polymerase to recognize promoter regions and initiate transcription with high fidelity.

• The AUG codon is the start codon for translation, not transcription.

Promoter Function

Promoters are DNA sequences that define where transcription of a gene by RNA polymerase begins.

• Primary function: Recognition by RNA polymerase and associated factors (e.g., sigma factor in prokaryotes).

• Promoters are not involved in ribosomal recognition or stem-loop formation.

Promoter Elements: Pribnow Box and TATA Box

Promoter regions contain conserved sequences that facilitate transcription initiation.

• Pribnow box (TATAAT) is found in prokaryotes.

• TATA box (Goldstein-Hogness box) is the eukaryotic equivalent, located about 25–35 bases upstream of the transcription start site.

DNA Strands and Transcription

During transcription, the coding and template strands of DNA have distinct roles.

• Coding strand: Has the same sequence as the mRNA (except T for U).

• Template strand: Is complementary to both the coding strand and the mRNA.

Ribosomal RNA Segments

Ribosomes are composed of rRNA and proteins, with the number of rRNA segments differing between prokaryotes and eukaryotes.

• Prokaryotes: 3 rRNA segments (16S, 23S, 5S).

• Eukaryotes: 4 rRNA segments (18S, 28S, 5.8S, 5S).

Post-Transcriptional Modification of mRNA

Eukaryotic mRNAs undergo several modifications after transcription.

• 5' cap: Addition of a methylated guanine nucleotide to the 5' end.

• Poly-A tail: Addition of a stretch of adenine nucleotides to the 3' end.

• Splicing: Removal of introns and joining of exons.

Self-Splicing and Ribozymes

Some RNA molecules can catalyze their own splicing, acting as ribozymes.

• Ribozyme: An RNA molecule with enzymatic activity.

• Self-splicing is a property of certain introns (Group I and II).

Translation and Protein Synthesis

Translation: mRNA to Protein

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Primary structure: The linear sequence of amino acids in a protein.

• Secondary, tertiary, quaternary structures: Higher levels of protein folding and assembly.

Aminoacyl-tRNA Synthetases

These enzymes attach amino acids to their corresponding tRNAs.

• There are typically 20 aminoacyl-tRNA synthetases, one for each amino acid.

Directionality of Protein Synthesis

Proteins are synthesized from the amino (N) terminus to the carboxyl (C) terminus.

• The first amino acid is at the amino (N) terminus.

Initiation of Translation in Prokaryotes

In E. coli and other prokaryotes, the first amino acid incorporated is usually N-formylmethionine (fMet).

• N-formylmethionine is recognized by the initiator tRNA.

Peptide Bond Formation

Peptide bonds are formed by the ribosome during translation.

• Peptidyl transferase is the ribozyme activity of the large ribosomal subunit that catalyzes peptide bond formation.

Termination of Translation

Translation ends when a stop codon is encountered.

• Release factors recognize stop codons and promote the release of the newly synthesized polypeptide.

Chromosome Structure and Abnormalities

Chromosome Types

Chromosomes are classified based on the position of the centromere.

Type	Description
Metacentric	Centromere in the middle; arms of equal length
Submetacentric	Centromere slightly off center; arms unequal
Acrocentric	Centromere near one end; very short p arm
Telocentric	Centromere at the end; no p arm

Chromosome Inversions

Inversions are chromosomal rearrangements that reverse the orientation of a segment.

• Paracentric inversion: Does not include the centromere.

• Pericentric inversion: Includes the centromere.

Effects of Inversions on Meiosis

Inversion heterozygotes can form inversion loops during meiosis, affecting crossover and recombination.

• Crossover suppression: Reduced recombination within the inverted region.

• Consequences: Paracentric inversions can produce dicentric and acentric chromatids; pericentric inversions can lead to duplications and deletions.

Translocations and Segregation Patterns

Reciprocal translocations involve exchange of segments between nonhomologous chromosomes.

• Alternate segregation: Produces viable gametes.

• Adjacent-1 and adjacent-2 segregation: Often produce unbalanced gametes.

Nondisjunction

Nondisjunction is the failure of chromosomes to separate properly during meiosis or mitosis, leading to aneuploidy.

• Aneuploidy: Abnormal number of chromosomes in a cell.

Gene Mutation and DNA Repair

Types of Mutations

Mutations are changes in the DNA sequence that can affect gene function.

• Frameshift mutation: Insertion or deletion of bases that alters the reading frame.

• Inversion: Reversal of a DNA segment.

• Suppression: Restoration of function by a second mutation (intragenic or intergenic).

Mutagenic Agents

Certain chemicals can induce mutations in DNA.

• Acridine orange: Intercalates into DNA, causing frameshift mutations.

• Ethyl methane sulfonate (EMS): Alkylates DNA bases, causing mispairing.

• 5-bromouracil: Base analog that induces transitions via tautomeric shifts.

Consensus Sequence

A consensus sequence is the most common sequence found at a particular location in DNA or RNA, important for binding proteins such as transcription factors.

Null Mutation

A null mutation results in complete loss of function of the gene product.

Constitutive Expression

Constitutive genes are expressed continuously, regardless of environmental conditions.

Inversion Heterozygote

An individual with one normal chromosome and one chromosome with an inversion.

Additional info:

• Sample test problems cover key concepts from chapters 8–11, including transcription, translation, chromosome structure, and mutation.

• Office hours are provided for exam preparation.