Molecular Basis of Inheritance and Regulation of Gene Expression: Study Guide

Chapter 16: The Molecular Basis of Inheritance

DNA Structure and Function

The structure and function of DNA are central to understanding inheritance and gene expression. DNA is a double-stranded molecule that encodes genetic information.

• Function of DNA: DNA stores genetic information used for the development, functioning, and reproduction of all living organisms.

• DNA Nucleotide Structure: Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The 3’ end has a free hydroxyl group, while the 5’ end has a free phosphate group.

• Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• Condensation Synthesis: Nucleotides are joined by phosphodiester bonds, forming the sugar-phosphate backbone.

• Complementary Base Pairing: A pairs with T, C pairs with G, via hydrogen bonds.

• Double Helix Structure: DNA forms a right-handed double helix, with antiparallel strands.

• Antiparallel Configuration: One strand runs 5’ to 3’, the other 3’ to 5’.

DNA Replication

DNA replication is the process by which DNA is copied before cell division. It is semiconservative, meaning each new DNA molecule contains one old and one new strand.

• Semiconservative Replication: Each daughter DNA molecule consists of one parental and one newly synthesized strand.

• Replication Fork: The Y-shaped region where DNA is actively being unwound and replicated.

• Helicase: Unwinds the DNA double helix, creating replication bubbles.

• Leading vs. Lagging Strand: The leading strand is synthesized continuously; the lagging strand is synthesized in short segments called Okazaki fragments.

• RNA Primers: Short RNA sequences initiate DNA synthesis; replaced by DNA nucleotides later.

• DNA Ligase: Joins Okazaki fragments to complete the lagging strand.

• Telomeres: Repetitive DNA sequences at chromosome ends; protect against loss of genetic information.

• 5’ End Limitation: The RNA primer at the 5’ end cannot be replaced, leading to telomere shortening.

Enzymes and Proteins in DNA Replication

• Primase: Synthesizes RNA primers.

• Single-Stranded Binding Proteins: Stabilize unwound DNA.

• Ligase: Joins DNA fragments.

• Helicase: Unwinds DNA.

• Topoisomerase: Relieves strain ahead of the replication fork.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA.

• DNA Polymerase III: Synthesizes new DNA strands.

Chromatin Structure

• Histones: Proteins around which DNA winds.

• Nucleosome: DNA wrapped around histone octamer.

• Chromatin: Complex of DNA and proteins; can be tightly (heterochromatin) or loosely (euchromatin) packed.

• Heterochromatin: Densely packed, transcriptionally inactive.

• Euchromatin: Loosely packed, transcriptionally active.

Chapter 17 & 18: Gene Expression and Regulation

Relationship Between DNA and Proteins

The sequence of nucleotides in DNA determines the sequence of amino acids in proteins, via transcription and translation.

• Protein Synthesis: The process of making proteins from DNA instructions.

• Transcription: DNA is copied into mRNA in the nucleus.

• Translation: mRNA is decoded at the ribosome to build a protein.

• tRNA: Transfers amino acids to the ribosome; matches mRNA codons with anticodons.

• Genetic Code: DNA triplets correspond to mRNA codons, which specify amino acids.

Transcription Process

• Three Steps: Initiation, Elongation, Termination.

• Promoter: DNA sequence before a gene; binds transcription factors and RNA polymerase.

• Transcription Factors: Proteins that regulate transcription initiation.

• Elongation: RNA polymerase synthesizes complementary RNA from DNA template.

• Terminator: DNA sequence signaling the end of transcription.

RNA Processing

• Introns: Non-coding regions; removed from primary transcript.

• Exons: Coding regions; spliced together to form mature mRNA.

• Four Steps: Addition of 5’ cap, addition of poly(A) tail, removal of introns, splicing of exons.

• Spliceosome: Complex that removes introns and joins exons.

• Mature mRNA: Processed transcript ready for translation.

mRNA Movement and Modification

• mRNA Export: mRNA moves from nucleus to ribosome.

• 5’ Cap: Protects mRNA and aids ribosome binding.

• Poly(A) Tail: Stabilizes mRNA and facilitates export.

Translation Process

• Ribosome Structure: Composed of rRNA and proteins; has A, P, and E sites.

• Three Steps: Initiation, Elongation, Termination.

• Initiation Complex: mRNA, ribosome, and initiator tRNA assemble.

• Elongation: Amino acids are joined to form a polypeptide.

• Codon Recognition: tRNA anticodons pair with mRNA codons.

• Translocation: tRNA moves through A, P, and E sites.

• Termination: Stop codon and release factor end translation.

Mutations

• Silent Mutation: No change in protein function.

• Nonsense Mutation: Introduces premature stop codon; usually nonfunctional protein.

• Missense Mutation: Changes one amino acid; may affect function.

• Frameshift Mutation: Alters reading frame; often severely affects protein.

Operons and Gene Regulation

• Operon: Cluster of genes under control of a single promoter.

• Promoter: Initiates transcription.

• Operator: Regulatory sequence; binds repressor.

• Repressor: Protein that blocks transcription.

• Structural Genes: Encode proteins.

• Repressible vs. Inducible Operons: Repressible operons are usually on, can be turned off; inducible operons are usually off, can be turned on.

Chromatin and Epigenetic Regulation

• Differential Gene Expression: Different cells express different genes.

• Heterochromatin vs. Euchromatin: Heterochromatin is tightly packed and inactive; euchromatin is loosely packed and active.

• DNA Methylation: Addition of methyl groups; silences genes.

• Histone Acetylation: Addition of acetyl groups; activates transcription.

• Epigenetic Inheritance: Heritable changes not involving DNA sequence.

Transcriptional Control Elements

• Control Elements: DNA sequences that regulate transcription.

• General Transcription Factors: Required for all genes.

• Specific Transcription Factors: Regulate particular genes.

• Promoters, Enhancers, Activators, Repressors: Regulate transcription initiation and rate.

Alternative RNA Splicing

• Process: Different exons are joined to produce multiple mRNAs from one gene.

• Significance: Increases protein diversity.

Translational and Post-Translational Control

• Translational Control: Regulates mRNA translation.

• Post-Translational Control: Modifies proteins after synthesis.

MicroRNAs and Proteasomes

• miRNAs: Small RNAs that regulate gene expression by binding mRNA.

• Proteasome: Protein complex that degrades unneeded or damaged proteins.

Example Table: Types of Mutations and Their Effects

Mutation Type	Definition	Effect on Protein
Silent	Change in nucleotide, no change in amino acid	No effect
Nonsense	Change to stop codon	Truncated, usually nonfunctional
Missense	Change in amino acid	May affect function
Frameshift	Insertion/deletion shifts reading frame	Often severely affects function

Key Equations and Concepts

• Central Dogma:

• $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Base Pairing:

• $\text{A} \text{ pairs with } \text{T}, \quad \text{C} \text{ pairs with } \text{G}$

• Genetic Code:

• $\text{Codon} = 3 \text{ nucleotides} \rightarrow 1 \text{ amino acid}$

Additional info: Some explanations and examples were expanded for clarity and completeness, including the table and key equations.

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