Chapter 14: DNA Structure and Function Flashcards
Describe the double-helix structure of DNA.
Composed of two antiparallel strands forming a right-handed helix.
Nucleotides (sugar-phosphate backbone + nitrogenous bases) linked by phosphodiester bonds.
Base pairing: Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds; Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds.
Discovered by Watson, Crick, and Franklin (1953).
What are the three parts of a nucleotide?
Deoxyribose sugar (5-carbon).
Phosphate group.
Nitrogenous base (A, T, C, G).
What did Chargaff’s experiments reveal about DNA?
In DNA, the amount of A = T and C = G.
Base composition varies between species, supporting DNA as genetic material.
What is the semiconservative model of DNA replication?
Each new DNA molecule consists of one original strand and one new strand.
Demonstrated by the Meselson-Stahl experiment (1958) using isotopic nitrogen (15N and 14N).
List key enzymes in DNA replication and their roles.
Helicase: Unwinds DNA by breaking hydrogen bonds.
DNA Polymerase III: Synthesizes new strands (5’→3’ direction).
Primase: Adds RNA primers to initiate synthesis.
Ligase: Seals nicks between Okazaki fragments.
Topoisomerase: Relieves supercoiling ahead of the replication fork.
How do leading and lagging strands differ?
Leading strand: Synthesized continuously toward the replication fork.
Lagging strand: Synthesized discontinuously as Okazaki fragments (later joined by ligase).
How does DNA polymerase ensure accuracy?
Proofreading: DNA Polymerase III checks and excises mismatched bases (3’→5’ exonuclease activity).
Mismatch repair: Post-replication enzymes (e.g., MutS/MutL) correct errors missed by polymerase.
What are telomeres, and why are they important?
Telomeres: Repetitive sequences (e.g., TTAGGG) at chromosome ends, preventing DNA loss during replication.
Telomerase: Enzyme extending telomeres in germ/stem cells; inactive in most somatic cells (linked to aging).
How is DNA packaged in eukaryotic cells?
Histones (proteins) organize DNA into nucleosomes (DNA wrapped around histone octamers).
Nucleosomes coil into chromatin fibers → looped domains → condensed chromosomes.
How did Hershey and Chase confirm DNA is genetic material?
Labeled viral DNA (32P) and proteins (35S).
Only 32P entered bacteria during infection, proving DNA (not protein) transmits genetic information.
Define point mutations and chromosomal mutations.
Point mutations:
Silent: No amino acid change (e.g., CGA → CGC).
Missense: Altered amino acid (e.g., sickle cell anemia: GAG → GTG).
Nonsense: Premature stop codon (e.g., CGA → TGA).
Chromosomal mutations: Deletions, duplications, inversions, translocations.
Name two DNA repair mechanisms.
Base excision repair (BER): Fixes small, non-helix-distorting lesions (e.g., uracil mismatches).
Nucleotide excision repair (NER): Removes bulky lesions (e.g., UV-induced thymine dimers).
How does PCR amplify DNA?
Denaturation: Heat separates DNA strands.
Annealing: Primers bind to target sequences.
Extension: Taq polymerase synthesizes new strands.
Used in forensics, cloning, and COVID-19 testing.
What is the Sanger sequencing method?
Uses dideoxynucleotides (ddNTPs) to terminate DNA synthesis at specific bases.
Electrophoresis separates fragments by size, revealing the sequence (e.g., Human Genome Project).
How does CRISPR-Cas9 edit genes?
Cas9 enzyme cuts DNA at sequences guided by sgRNA.
Applications: Correct genetic disorders (e.g., cystic fibrosis), engineer crops, study gene function.
Summarize the central dogma of molecular biology.
DNA → RNA → Protein.
Replication (DNA copied), Transcription (DNA → RNA), Translation (RNA → Protein).
Why are DNA strands antiparallel?
One strand runs 5’→3’, the other 3’→5’.
Ensures complementary base pairing and accurate replication/transcription.
What causes DNA supercoiling, and how is it resolved?
Caused by helicase unwinding DNA during replication.
Topoisomerases (e.g., DNA gyrase) cut and rejoin strands to relieve tension.
Label the replication fork and describe its components.
Components:
Helicase (unwinds DNA).
Single-strand binding proteins (stabilize strands).
Leading/lagging strands.
RNA primer.
What are ethical concerns with DNA technologies?
Privacy: Genetic data misuse (e.g., discrimination).
Gene Editing: Unintended consequences (e.g., CRISPR “off-target” effects).
Patenting: Ownership of genetic sequences.
