Gene Expression and Regulation

Gene Expression and Regulation

  • Importance in Exam

    • Approximately 12-16% of exam questions will focus on this topic.

The Roles and Functions of DNA and RNA

  • Nucleic Acids' Function

    • Basis for heredity in all life forms; DNA is primary form in most, with RNA in some species.

  • Structure of DNA

    • Eukaryotes: Multiple linear chromosomes within a nucleus.

    • Prokaryotes: Single circular chromosome.

    • Extra-chromosomal DNA found in both, notably plasmids and mitochondrial/chloroplast DNA in eukaryotes.

  • DNA Replication

    • Low error rates allow heritable information transfer.

    • Semiconservative Replication: Each new double strand consists of one original and one new strand.

    • Key enzymes:

    • Helicase: Unwinds double helix, forming a replication fork.

    • Topoisomerase: Prevents supercoiling; breaks and rejoins DNA strands.

    • DNA Polymerase III: Synthesizes new DNA from 5' to 3', requires RNA primers to initiate.

    • Lagging vs. Leading Strands:

    • Leading Strand: Continuous replication.

    • Lagging Strand: Discontinuous replication using Okazaki fragments; RNA primers needed, ligated by DNA ligase.

  • Circular vs. Linear Replication

    • Circular: Has a single origin of replication.

    • Linear (Eukaryotic): Multiple origins, telomere shortening with each replication round.

The Mechanisms of Gene Expression

  • Process Overview

    • DNA contains genes that encode proteins through a multi-step process.

  • Transcription

    • mRNA is synthesized from DNA by RNA polymerase, forming pre-mRNA.

    • Post-transcriptional Modifications:

    • Addition of 5' GTP cap and poly(A) tail to stabilize mRNA.

    • Splicing to remove introns; exons are retained.

    • Alternative Splicing: Generates different protein variants from a single gene.

  • Translation

    • mRNA moves to ribosomes where tRNA translates mRNA into amino acids.

    • Translation occurs in three phases: initiation, elongation, termination.

    • Start codon signals the beginning of translation and stop codons signal termination.

  • Key Differences in Gene Expression:

    • In Prokaryotes: No nucleus, transcription and translation are simultaneous; no introns, simpler post-translational modifications.

    • In Retroviruses: RNA reverse transcribed into DNA, which integrates into host genome for further expression.

How Genotype Affects Phenotype

  • Gene Expression Variability

    • Different cell types express unique genes, affecting the phenotype despite identical DNA.

    • Transcription Factors: Proteins regulating gene expression during development.

  • Gene Regulation

    • Regulatory Sequences: Control transcription:

    • Promoters: Binding sites for RNA polymerase.

    • Enhancers: Increase transcription likelihood.

    • Silencers: Repress transcription.

    • Operons in Prokaryotes: Control gene regulation via a single promoter (e.g., lac operon).

  • Epigenetics: Changes in gene expression not involving DNA sequence alterations (e.g., DNA methylation, histone modifications).

Mutations, Genetic Diversity, and Natural Selection

  • Types of Mutations:

    • Missense: Change an amino acid.

    • Nonsense: Create a premature stop codon.

    • Insertion/Deletion: Add/delete base pairs, affecting reading frame and possibly leading to frameshift mutations.

    • Duplication: Segment copied multiple times.

  • Impact of Mutations:

    • Mutations can be negative, neutral, or positive, influencing evolutionary advantageous traits (e.g., sickle cell favorable against malaria).

  • Chromosomal Changes: Errors during cell division can alter chromosome numbers leading to developmental issues or new phenotypes (e.g., Down syndrome).

Genetic Engineering and Biotechnology

  • Biotechnology Techniques:

    • Electrophoresis: Separates molecules by size and charge for analysis.

    • Polymerase Chain Reaction (PCR): Amplifies DNA to produce many copies, aiding analysis and manipulation.

    • Bacterial Transformation: Introduces new DNA to bacteria to produce proteins (e.g., insulin).

    • DNA Sequencing: Determines nucleotide sequence for various applications, including disease identification and ancestry tracing.