Study Guide: Molecular Basis of Inheritance & Gene Expression (Chapters 13 & 14)

Chapter 13: The Molecular Basis of Inheritance

Section 13.1: Structure of DNA

This section explores the chemical structure of DNA, the foundational molecule of genetic information, and the historical discoveries that led to our understanding of its double helix structure.

• Nucleotides: The building blocks of nucleic acids, each consisting of a phosphate group, a five-carbon sugar (deoxyribose in DNA), and a nitrogenous base (adenine, thymine, cytosine, or guanine).

• Sugar-phosphate backbone: The repeating chain of sugar and phosphate groups that forms the structural framework of DNA strands.

• Nitrogenous base: Organic molecules (A, T, C, G) that pair specifically (A with T, C with G) via hydrogen bonds, forming the rungs of the DNA ladder.

• Chargaff’s rule: In any DNA sample, the amount of adenine equals thymine, and the amount of cytosine equals guanine: $A = T$, $C = G$.

• Watson, Crick, Wilkins, and Franklin: Scientists who elucidated the double helix structure of DNA using X-ray diffraction data (Franklin) and model building (Watson & Crick).

• Antiparallel: The two strands of DNA run in opposite directions: one strand runs 5’ to 3’, the other 3’ to 5’.

• 5’ and 3’: Refer to the carbon numbers in the DNA’s sugar backbone; the 5’ end has a phosphate group, and the 3’ end has a hydroxyl group.

Example: The DNA double helix is stabilized by hydrogen bonds between complementary bases and by hydrophobic interactions among stacked bases.

Section 13.3: Chromatin Structure

DNA in eukaryotic cells is packaged with proteins to form chromatin, which can exist in more or less condensed forms, affecting gene expression.

• Chromatin: The complex of DNA and proteins (mainly histones) that makes up chromosomes in the nucleus.

• Histone: Basic proteins around which DNA winds, forming nucleosomes, the fundamental units of chromatin structure.

• Heterochromatin: Densely packed chromatin, generally transcriptionally inactive.

• Euchromatin: Loosely packed chromatin, generally accessible for transcription and gene expression.

Example: During interphase, most chromatin is euchromatin, allowing active gene transcription, while heterochromatin is found at centromeres and telomeres.

Chapter 14: Gene Expression: From Gene to Protein

Section 14.1: The Flow of Genetic Information

This section introduces the central dogma of molecular biology: DNA is transcribed into RNA, which is then translated into protein.

• Gene expression: The process by which information from a gene is used to synthesize a functional gene product (usually a protein).

• Transcription: The synthesis of messenger RNA (mRNA) from a DNA template.

• Messenger RNA (mRNA): The RNA copy of a gene that carries genetic information from the nucleus to the cytoplasm for translation.

• Triplet code: The genetic code is read in sets of three nucleotides (codons), each specifying an amino acid.

• Codon: A sequence of three nucleotides in mRNA that codes for a specific amino acid or stop signal.

• Template strand: The DNA strand that is read by RNA polymerase to synthesize mRNA.

• Reading frame: The way nucleotides are grouped into codons; shifting the reading frame changes the resulting protein sequence.

Example: The codon AUG codes for methionine and also serves as the start signal for translation.

Section 14.2: Transcription in Detail

Transcription involves several steps and regulatory elements that ensure accurate and efficient synthesis of RNA.

• Promoter: A DNA sequence upstream of a gene where RNA polymerase binds to initiate transcription.

• Terminator: A DNA sequence signaling the end of transcription.

• Transcription factor: Proteins that help RNA polymerase recognize and bind to the promoter.

• TATA box: A common promoter sequence rich in thymine and adenine, crucial for forming the transcription initiation complex.

• Transcription elongation: The process by which RNA polymerase moves along the DNA, synthesizing RNA in the 5’ to 3’ direction.

• RNA polymerase: The enzyme that synthesizes RNA from a DNA template.

Example: In eukaryotes, RNA polymerase II transcribes mRNA, while RNA polymerase I and III transcribe other types of RNA.

Section 14.3: RNA Processing in Eukaryotes

Before mRNA can be translated, it undergoes several modifications to become mature mRNA.

• RNA processing: The modification of pre-mRNA, including capping, polyadenylation, and splicing.

• 5’ cap: A modified guanine nucleotide added to the 5’ end of mRNA, protecting it from degradation and aiding in ribosome binding.

• Poly-A tail: A stretch of adenine nucleotides added to the 3’ end of mRNA, enhancing stability and export from the nucleus.

• RNA splicing: The removal of non-coding sequences (introns) from pre-mRNA and joining of coding sequences (exons).

• Exon: Coding regions of a gene that remain in mature mRNA.

• Intron: Non-coding regions removed during RNA splicing.

Example: Alternative splicing allows a single gene to code for multiple proteins by including or excluding different exons.

Section 14.4: Translation and Protein Synthesis

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

• Transfer RNA (tRNA): Adaptor molecules that bring amino acids to the ribosome, matching their anticodon to the mRNA codon.

• Codon: Three-nucleotide sequence in mRNA specifying an amino acid.

• Anticodon: Three-nucleotide sequence in tRNA complementary to an mRNA codon.

• Aminoacyl-tRNA synthetases: Enzymes that attach the correct amino acid to its corresponding tRNA.

• Wobble: Flexibility in base pairing at the third position of the codon, allowing some tRNAs to pair with more than one codon.

• Ribosomes: Complexes of rRNA and proteins that facilitate the coupling of tRNA anticodons with mRNA codons during protein synthesis.

• rRNA: Ribosomal RNA, a structural and catalytic component of ribosomes.

• A-site: The ribosomal site where incoming aminoacyl-tRNA binds.

• P-site: The site holding the tRNA with the growing polypeptide chain.

• E-site: The exit site where discharged tRNAs leave the ribosome.

• Polypeptide: A chain of amino acids linked by peptide bonds, which folds into a functional protein.

• Ribosome initiation: Assembly of the translation machinery at the start codon of mRNA.

• Polypeptide elongation: Sequential addition of amino acids to the growing chain.

• Polypeptide termination: Release of the completed polypeptide when a stop codon is reached.

• Protein folding (primary to tertiary structure): The process by which a polypeptide folds into its functional three-dimensional shape.

Example: The ribosome moves along the mRNA, catalyzing peptide bond formation and ensuring the correct sequence of amino acids.

Section 14.5: Mutations and Their Effects

Mutations are changes in the genetic material that can affect protein structure and function in various ways.

• Mutations: Heritable changes in the DNA sequence.

• Point mutations: Changes in a single nucleotide pair.

• Nucleotide-pair substitution: Replacement of one nucleotide and its partner with another pair.

• Silent mutation: A mutation that does not change the amino acid sequence due to redundancy in the genetic code.

• Nonsense mutation: A mutation that changes a codon to a stop codon, resulting in a truncated protein.

• Missense mutation: A mutation that changes one amino acid to another.

• Insertion: Addition of one or more nucleotide pairs into a gene.

• Deletion: Loss of one or more nucleotide pairs from a gene.

• Frameshift mutation: Insertions or deletions that alter the reading frame, usually resulting in a nonfunctional protein.

• Mutagen: Physical or chemical agents that increase the rate of mutation.

Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.

Summary Table: Types of Mutations and Their Effects

Mutation Type	Description	Effect on Protein
Silent	Change in nucleotide that does not alter amino acid	No effect
Missense	Change in nucleotide that changes one amino acid	May alter protein function
Nonsense	Change in nucleotide that creates a stop codon	Premature termination; usually nonfunctional protein
Frameshift	Insertion or deletion not in multiples of three	Alters reading frame; usually nonfunctional protein

Additional info: The central dogma of molecular biology is often summarized as: $DNA \rightarrow RNA \rightarrow Protein$