AP+Biology+Unit+6_+Gene+Expression+and+Regulation+Review+2

Unit 6: Gene Expression and Regulation

Topics Overview

6.1 DNA and RNA Structure

Heritable information provides continuity of life, which is crucial for the reproduction and functioning of all living organisms.Key Objectives: Understanding the structure and transmission of genetic information through DNA and RNA.Important Concepts:

• DNA and RNA as primary sources of heritable information: DNA (deoxyribonucleic acid) is the molecule that carries the genetic blueprint of organisms, composed of nucleotide monomers that include a deoxyribose sugar, a phosphate group, and nitrogenous bases. RNA (ribonucleic acid), which includes ribose sugar, plays various roles including mediating the synthesis of proteins, structuring ribosomes, and acting as a genetic carrier in some viruses.

• Prokaryotic vs. Eukaryotic Chromosomes: Prokaryotic organisms possess circular chromosomes that float freely within the cytoplasm, lacking a defined nucleus, which allows for quick replication and accessibility. Conversely, eukaryotic organisms have linear chromosomes organized within a defined nucleus, causing a more complex process of cell division.

• Plasmids: These are small circular DNA molecules found in prokaryotes (in the cytoplasm) and eukaryotes (in mitochondria and chloroplasts). They often carry genes that confer advantageous traits, such as antibiotic resistance or the ability to metabolize unusual substances, providing survival advantages under specific environmental conditions.

• Purines vs. Pyrimidines: DNA and RNA consist of purines (adenine (A) and guanine (G), which have a double-ring structure) and pyrimidines (cytosine (C), thymine (T) in DNA, and uracil (U) in RNA). The pairing between these bases (A with T and G with C in DNA; A with U and G with C in RNA) is crucial for the stability and replication of genetic material.

6.2 Replication

Continuity of hereditary information through DNA replication is essential for proper cell division.Mechanisms:

• Synthesis Direction: DNA replication proceeds in the 5’ to 3’ direction, utilizing the antiparallel nature of DNA strands to ensure proper alignment and binding of nucleotides.

• Semiconservative Process: Each newly synthesized DNA molecule consists of one original strand and one newly formed strand, ensuring genetic fidelity and maintaining the original sequence in subsequent generations.

Key Enzymes and Proteins:

• Helicase: Unwinds the DNA double helix, creating two single strands and enabling access for other enzymes.

• Topoisomerase: Relieves torsional strain generated by unwinding the DNA, preventing supercoiling that could impede replication.

• DNA Polymerase: Responsible for adding nucleotides to the growing DNA strand, ensures accurate base-pairing, and has proofreading abilities to correct errors during synthesis.

• Leading vs. Lagging Strand: The leading strand is synthesized continuously in the direction of the fork, while the lagging strand is synthesized in short fragments known as Okazaki fragments, which require additional processing and rejoining by DNA ligase after synthesis.

6.3 Transcription and RNA Processing

Information flow: from DNA to RNA and then to Protein.Key Functions:

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes for protein synthesis, and is often subject to processing before it can be translated.

• tRNA (transfer RNA): Transfers specific amino acids to ribosomes during translation, matching amino acids to their corresponding codons on the mRNA strand.

• rRNA (ribosomal RNA): A structural and functional component of ribosomes, facilitating protein synthesis by providing a site for translation and catalyzing peptide bond formation.

Transcription Process: Involves the synthesis of mRNA from the antisense (template) DNA strand, facilitated by RNA polymerase, which binds to a promoter region at the gene start site.

Post-Processing of mRNA: After transcription, mRNA undergoes several modifications such as the addition of a poly-A tail, a 5’ cap (GTP cap), and removal of non-coding sequences (introns) through splicing. Alternative splicing allows a single gene to produce multiple mRNA variants, contributing to protein diversity.

6.4 Translation

The process of synthesizing a polypeptide from mRNA involves complex steps that ensure correct protein synthesis.Steps of Translation:

1. Initiation: The ribosome assembles around the start codon (AUG) of the mRNA, with the first tRNA molecule bringing the amino acid methionine, establishing the reading frame for the sequence.

2. Elongation: tRNAs successively bring amino acids to the growing polypeptide chain based on codon-anticodon pairing, with peptide bonds formed between amino acids by the ribosomal enzyme peptidyl transferase.

3. Termination: A stop codon on the mRNA (UAA, UAG, or UGA) triggers the release of the completed polypeptide from the ribosome, ending translation. Release factors help disassemble the ribosomal complex, allowing the new protein to fold and undergo post-translational modifications.

6.5 Regulation of Gene Expression

Gene expression plays a crucial role in determining an organism's phenotype and responding to environmental changes.Mechanisms:

• Regulatory Sequences: Specific DNA sequences (enhancers, silencers, and promoters) interact with transcription factors and other proteins to enhance or inhibit transcription initiation, allowing for precise control over gene activity.

• Epigenetic Changes: Modifications such as DNA methylation and histone acetylation can alter gene expression levels without changing the DNA sequence itself. These changes can affect the accessibility of genes for transcription and can be influenced by environmental factors.

6.6 Gene Expression and Cell Specialization

Transcription factors bind to promoter regions to regulate gene expression, contributing to differential gene expression required for cell specialization.Differentiation: Specialized cells arise from stem cells through controlled gene expression changes, leading to distinct cell types with unique functions that are essential for organism development, homeostasis, and adaptation to various environments.

6.7 Mutations

Genetic variations can result in different phenotypes, influencing survival and reproduction.Types of Mutations:

• Positive Mutations: Provide benefits that enhance survival and adaptability to changing environments.

• Negative Mutations: Have detrimental effects, often leading to decreased fitness and increased vulnerability to diseases.

• Neutral Mutations: Have no significant effect on the organism, serving as a reservoir for potential future adaptations.Examples:

• The CFTR gene mutation can lead to cystic fibrosis, a serious genetic disorder affecting respiratory function, indicating how mutations can lead to significant health challenges.

• Variants of the MC1R gene can lead to melanism, affecting pigmentation in certain animal species and impacting their survival rates in various habitats.

6.8 Biotechnology

Techniques for DNA analysis and manipulation are widely applied in research and industry to harness biological mechanisms for various applications.Key Processes:

• Electrophoresis: A method used to separate DNA fragments based on size and charge, allowing for analysis and comparison of genetic material.

• Polymerase Chain Reaction (PCR): A powerful technique that amplifies specific DNA sequences, making millions of copies for analysis, diagnosis, or research purposes.

• Bacterial Transformation: A technique for introducing foreign DNA into bacteria, enabling cloning or protein production for therapeutic or research uses.

• DNA Sequencing: Determines the exact nucleotide sequence of a DNA molecule, allowing for a detailed understanding of genetic information.

Applications: Applications extend from forensic analysis for crime scene investigations to genetic engineering in agriculture for crop improvement, and the creation of transgenic organisms engineered to express desired traits beneficial in various commercial and medical fields.

Summary of Concepts

• Central Dogma: The framework which states that DNA transcribes into RNA, which then translates into protein, outlining the flow of genetic information within a biological system.

• Mutations and Natural Selection: Variants resulting from mutations can enhance survival and reproduction, driving the process of evolution by providing the raw material for natural selection.