This cheat sheet covers the key concepts of AP Biology Unit 4: Gene Expression and Regulation. Remember to consult your textbook and class notes for a complete understanding. This is designed as a concise review, not a replacement for thorough study.
I. Gene Expression: From DNA to Protein
A. Central Dogma: The flow of genetic information: DNA → RNA → Protein.
B. Transcription: The process of creating an RNA molecule from a DNA template. * Initiation: RNA polymerase binds to the promoter region of the DNA. * Elongation: RNA polymerase synthesizes RNA, adding nucleotides complementary to the DNA template strand (5' to 3'). * Termination: RNA polymerase reaches a termination sequence and detaches from the DNA. * Key Players: RNA polymerase, transcription factors, promoter region, terminator sequence.
C. RNA Processing (Eukaryotes Only): Modifications to the pre-mRNA molecule before translation. * 5' capping: Addition of a modified guanine nucleotide to the 5' end. Protects mRNA from degradation and aids in ribosome binding. * 3' polyadenylation: Addition of a poly(A) tail (a string of adenine nucleotides) to the 3' end. Protects mRNA from degradation and aids in export from the nucleus. * Splicing: Removal of introns (non-coding sequences) and joining of exons (coding sequences). Performed by spliceosomes.
D. Translation: The process of synthesizing a polypeptide (protein) from an mRNA molecule. * Initiation: Ribosome binds to the mRNA at the start codon (AUG). * Elongation: tRNA molecules carrying amino acids bind to the mRNA codons, and peptide bonds form between amino acids. * Termination: Ribosome reaches a stop codon (UAA, UAG, UGA), and the polypeptide chain is released. * Key Players: Ribosomes (rRNA and proteins), tRNA, mRNA, amino acids, codons, anticodons.
II. Gene Regulation: Controlling Gene Expression
A. Prokaryotic Gene Regulation (Operons): A coordinated unit of genes regulated together. * Lac Operon (E. coli): Inducible operon; genes for lactose metabolism are turned on only in the presence of lactose. Includes a promoter, operator, and structural genes. Repressor protein binds to the operator, preventing transcription in the absence of lactose. Lactose acts as an inducer, binding to the repressor and preventing it from binding to the operator. * Trp Operon (E. coli): Repressible operon; genes for tryptophan synthesis are turned off in the presence of tryptophan. Tryptophan acts as a corepressor, binding to the repressor protein and allowing it to bind to the operator.
B. Eukaryotic Gene Regulation: More complex than prokaryotic regulation, involving multiple levels of control. * Transcriptional Regulation: Control of the rate of transcription initiation. Includes: * Transcription factors: Proteins that bind to DNA and regulate transcription. Activators increase transcription, repressors decrease transcription. * Enhancers and silencers: DNA sequences that bind transcription factors, influencing transcription from a distance. * Chromatin remodeling: Changes in chromatin structure that affect accessibility of DNA to RNA polymerase. Histone modification (acetylation, methylation) plays a crucial role. * Post-transcriptional Regulation: Control after transcription, including: * RNA processing: Alternative splicing can produce different mRNA molecules from a single pre-mRNA. * mRNA stability: The lifespan of mRNA molecules can be regulated. * Translation regulation: Control of the rate of translation initiation. * Post-translational modification: Modifications to the protein after translation (e.g., phosphorylation, glycosylation). These modifications can affect protein activity and stability.
III. Mutations and their Effects
A. Types of Mutations: * Point mutations: Changes in a single nucleotide. Include substitutions (missense, nonsense, silent), insertions, and deletions. * Frameshift mutations: Insertions or deletions that shift the reading frame of the mRNA, leading to altered amino acid sequences. * Chromosomal mutations: Larger-scale changes in chromosome structure or number.
B. Mutagens: Agents that cause mutations (e.g., radiation, certain chemicals).
C. Effects of Mutations: Mutations can be beneficial, harmful, or neutral. Harmful mutations can lead to genetic disorders.
This cheat sheet provides a condensed overview. Thorough review of your textbook and class materials is essential for success on the AP Biology exam. Remember to practice applying these concepts through problem-solving and essay writing. Good luck!
