Genetics and Meiosis Flashcards

Overview

• Flashcards covering key concepts from biology chapters on genetics, meiosis, Mendelian inheritance, DNA structure, and DNA replication.
• Focus on definitions, processes, examples, and problem-style questions with concise answers.

Genetics: Genome and Chromosomes

• Genome: complete set of an organism's genes and other DNA sequences.
• Homologous chromosomes: carry information for the same traits.
• Diploid (2n) and haploid (n) numbers:
  • Example: quaking aspen gametes n = 19 → asexual stem cells have 2n = 38.
  • If an organism has 46 chromosomes, gametes have 23.
• Chromosome composition after meiosis and mitosis:
  • After telophase I: cells are haploid; chromosomes composed of two chromatids.
  • Completion of meiosis vs. prophase I cells: gametes have half the chromosome number and one-fourth the DNA amount compared to prophase I.
  • For meiosis I duration each chromosome consists of two sister chromatids joined at centromere.

Meiosis: Key Events and Consequences

• Purpose: reduce chromosome number to haploid and create genetic diversity.
• Events unique to meiosis:
  • Synapsis of homologous chromosomes (prophase I).
  • Crossing over (exchange between corresponding segments of non-sister chromatids).
  • Separation of homologous chromosomes (conclusion of meiosis I).
• Meiosis II vs. mitosis:
  • Meiosis II separates sister chromatids in haploid cells.
  • Mitosis separates sister chromatids in diploid or haploid cells, but typically in diploid somatic context.
• Centromere behavior: centromeres split and sister chromatids migrate in anaphase II.
• Chiasmata visible indicates prophase I.
• If dnaB (helicase) mutated in E. coli → no replication fork formed.

Genetic Variation Mechanisms

• Independent assortment: random orientation of homologous pairs at metaphase I.
  • Result: for n = 23 haploid number → ~8 million combinations from independent assortment.
• Crossing over: recombines alleles by exchanging non-sister chromatid segments.
• Combined sources of diversity: mutations, crossing over, independent assortment.
• Ecological implication: sexually reproducing species better adapt to changing environments due to more genetic variation.

Mendelian Inheritance

• Mendel's major conclusion: traits are inherited as discrete units (genes), not by blending.
• Segregation of alleles corresponds to anaphase I of meiosis.
• Independent assortment basis: alignment of tetrads at equator in meiosis I.
• Dominance relationships:
  • Dominant allele masks recessive in heterozygote (e.g., Mendel's F1 phenotype).
  • Example crosses:
    • Black (dominant) × albino (recessive) producing all black indicates albino is recessive.
    • Heterozygote (Tt) × homozygous tall (TT) → probability short = 0.
    • Two true-breeding parents (red axial × white terminal) with independent assortment: probability white axial in F2 = 3/16.
• Mendel continued F2/F3 to reveal recessive trait reappearance.

Extensions Of Mendelian Genetics

• Pleiotropy: one allele influences multiple phenotypic traits (example: Marfan syndrome).
• Polygenic inheritance: multiple genes contribute to a single trait (example: human skin pigmentation).
• Environmental effects interact with genotype (example: hydrangea flower color affected by soil pH).
• Epistasis: one gene masks effect of another (example: ee genotype prevents fur color in rabbits).
• Incomplete dominance: heterozygote shows intermediate phenotype (example: red × white → purple in radish).
• Codominance and multiple alleles: ABO blood group
  • Alleles: IA, IB (codominant), i (recessive).
  • AB × O parents → children can be type A or B.

Key Terms and Definitions

• Genome: all genes and DNA sequences of an organism.
• Homologous chromosomes: chromosome pairs carrying same trait loci.
• Haploid (n): one set of chromosomes (gametes).
• Diploid (2n): two sets of chromosomes (somatic cells).
• Chiasma: cytological evidence of crossing over.
• Synapsis: pairing of homologous chromosomes in prophase I.
• Pleiotropy: single gene influences multiple traits.
• Polygenic inheritance: multiple genes affect one trait.
• Epistasis: interaction where one gene affects expression of another.
• Incomplete dominance: heterozygote phenotype is intermediate.
• Codominance: both alleles expressed in heterozygote.

DNA Structure and Evidence for Genetic Material

• Chargaff's rules: A = T and G = C in double-stranded DNA.
  • Example calculation: if C = 42% → G = 42%, remaining 16% split A = T = 8% each.
• Watson and Crick: DNA stores information in base sequence.
• Hershey-Chase experiment: DNA (not protein) enters E. coli during T2 bacteriophage infection.
  • Reason experimental labels matter: DNA contains phosphorus; proteins do not.
  • Labeling nitrogen would fail because proteins also contain nitrogen.

DNA Replication: Mechanism and Enzymes

• Antiparallel strands: one strand runs 5'→3', the other 3'→5'.
• Semiconservative replication: each daughter DNA has one parental strand as template.
• DNA polymerase synthesizes new strand in 5'→3' direction; requires free 3'-OH.
• Leading vs. lagging strand:
  • Leading synthesized continuously in replication fork movement direction.
  • Lagging synthesized discontinuously as Okazaki fragments opposite fork direction.
  • DNA ligase joins Okazaki fragments.
• Primase: synthesizes RNA primer (5–10 nucleotides) to start DNA synthesis.
  • If primase mutated → replication cannot initiate on leading or lagging strand.
• Topoisomerase: relieves torsional strain ahead of replication fork.
• Single-strand binding proteins: hold separated DNA strands during replication.
• Prokaryote vs eukaryote replication origins:
  • Prokaryotes: single origin of replication.
  • Eukaryotes: multiple origins on linear chromosomes.

Formulas and Short Answers

• Diploid chromosome pairs: 2n = 16 → 8 homologous pairs.
• Gamete chromosome count: if karyotype 46 → gametes n = 23.
• Number of gamete combinations from independent assortment: 2^n (e.g., n = 23 → ~8 million).
• Base-pair percentage example: given C = 42% → T = 8%.

Action Items / Study Steps

• Review meiosis phases and identify where segregation and independent assortment occur.
• Practice Punnett squares for dominance, codominance, incomplete dominance, and epistasis.
• Memorize DNA replication enzymes and their roles (helicase, primase, DNA pol, ligase, topoisomerase).
• Solve chromosome-number problems converting between diploid and haploid contexts.
• Work through Chargaff percentage calculations and understand base-pairing rules.