Chapter 13: The Molecular Basis of Inheritance

DNA Structure and Function

Deoxyribonucleic acid (DNA) is the hereditary material in almost all living organisms. Its structure and replication are central to genetics.

• Double Helix: DNA consists of two antiparallel strands twisted into a double helix.

• Nucleotide: The building block of DNA, composed of a phosphate group, deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, guanine).

• Backbone: Alternating sugar and phosphate groups form the sides of the helix.

• Rungs: Nitrogenous bases pair via hydrogen bonds (A with T, C with G) to form the rungs.

• Antiparallel Strands: The two DNA strands run in opposite 5' to 3' directions.

Chargaff's Rules: In any DNA sample, the amount of adenine equals thymine, and cytosine equals guanine. This underlies complementary base pairing.

• Formula:

Key Scientists and Discoveries:

• Griffith: Discovered transformation in bacteria.

• Hershey and Chase: Demonstrated DNA is the genetic material using bacteriophages.

• Morgan: Linked genes to chromosomes using fruit flies.

• Franklin and Wilkins: Used X-ray crystallography to reveal DNA's helical structure.

• Watson and Crick: Proposed the double helix model of DNA.

DNA Replication

DNA replication is the process by which DNA makes a copy of itself during cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Semi-conservative Model: Each daughter DNA has one parental and one new strand.

• Origin of Replication: Specific sequence where replication begins.

• Replication Fork: Y-shaped region where new DNA strands are synthesized.

Major Steps in Replication:

1. Initiation: Helicase unwinds DNA at the origin, forming replication forks.

2. Elongation: Primase synthesizes RNA primers. DNA polymerase adds nucleotides to the 3' end.

3. Leading Strand: Synthesized continuously toward the replication fork.

4. Lagging Strand: Synthesized discontinuously away from the fork in Okazaki fragments.

5. DNA Ligase: Joins Okazaki fragments on the lagging strand.

6. Proofreading: DNA polymerase checks and corrects errors.

Key Enzymes and Components:

• Helicase: Unwinds the DNA double helix.

• Primase: Synthesizes RNA primers.

• DNA Polymerase: Adds nucleotides to growing DNA strand; also proofreads.

• Exonuclease: Removes RNA primers and incorrect nucleotides.

• DNA Ligase: Seals nicks between DNA fragments.

Telomeres and Telomerase:

• Telomere: Repetitive DNA at chromosome ends, protecting genes from erosion.

• Telomerase: Enzyme that extends telomeres in germ cells and some cancer cells.

Example: In eukaryotes, replication occurs in the nucleus during the S phase of the cell cycle.

Chapter 9: The Cell Cycle and Mitosis

Cell Division and Its Purpose

Cell division is essential for growth, repair, and reproduction. In eukaryotes, it involves mitosis and cytokinesis.

• Genome: The complete set of genetic material in a cell.

• Chromosome: DNA molecule with associated proteins; humans have 46 chromosomes in somatic cells.

• Somatic Cells vs. Sex Cells: Somatic cells are diploid (2n); sex cells (gametes) are haploid (n).

The Cell Cycle

The cell cycle consists of interphase (G1, S, G2) and the mitotic (M) phase.

• G1 Phase: Cell growth.

• S Phase: DNA replication.

• G2 Phase: Preparation for mitosis.

• M Phase: Mitosis and cytokinesis.

Mitosis

Mitosis is the division of the nucleus, producing two genetically identical daughter cells.

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

Cytokinesis: Division of the cytoplasm. In animals, a cleavage furrow forms; in plants, a cell plate forms.

Key Structures and Terms

• Sister Chromatids: Identical copies of a chromosome, joined at the centromere.

• Centromere: Region where sister chromatids are attached.

• Mitotic Spindle: Microtubule structure that separates chromosomes.

• Kinetochore: Protein complex at the centromere for spindle attachment.

Regulation and Cancer

• Checkpoints: Control points in the cell cycle (e.g., G1 checkpoint).

• Density-dependent Growth: Cells stop dividing when crowded.

• Anchorage-dependent Growth: Cells must be attached to a surface to divide.

• Growth Factors: Proteins that stimulate cell division.

• Cancer Cells: Divide uncontrollably, ignore growth signals.

• Benign Tumor: Non-invasive growth.

• Malignant Tumor: Invasive, can metastasize (spread).

Binary Fission: Asexual reproduction in prokaryotes; simpler than mitosis.

Chapter 10: Meiosis and Sexual Life Cycles

Reproduction and Chromosomes

Organisms reproduce asexually or sexually. Sexual reproduction increases genetic diversity.

• Asexual Reproduction: Offspring are clones of the parent.

• Sexual Reproduction: Involves fusion of gametes (egg and sperm).

• Homologous Chromosomes: Chromosome pairs, one from each parent.

• Autosomes: Non-sex chromosomes (22 pairs in humans).

• Sex Chromosomes: X and Y chromosomes; determine sex.

• Gametes: Haploid sex cells (n); produced by meiosis.

• Haploid (n): One set of chromosomes.

• Diploid (2n): Two sets of chromosomes.

• Zygote: Fertilized egg; diploid.

Meiosis

Meiosis reduces chromosome number by half, producing four genetically unique gametes.

• Stages: Meiosis I (homologs separate), Meiosis II (sister chromatids separate).

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I; creates recombinant chromosomes.

• Karyotype: Display of an individual's chromosomes; used to detect chromosomal abnormalities.

Genetic Variation

• Mechanisms: Independent assortment, crossing over, random fertilization.

• Role in Evolution: Genetic variation is the raw material for natural selection.

Comparison: Mitosis vs. Meiosis

Chapter 11: Mendelian Genetics

Basic Principles of Inheritance

Gregor Mendel's experiments established the laws of inheritance using pea plants.

• Allele: Alternative form of a gene.

• Dominant: Expressed allele in heterozygote.

• Recessive: Masked allele in heterozygote.

• Homozygous: Two identical alleles (AA or aa).

• Heterozygous: Two different alleles (Aa).

• Phenotype: Observable traits.

• Genotype: Genetic makeup.

Patterns of Inheritance

• Monohybrid Cross: Involves one gene.

• Dihybrid Cross: Involves two genes.

• Punnett Square: Diagram to predict genetic crosses.

• Test Cross: Cross with homozygous recessive to determine genotype.

• Law of Independent Assortment: Genes on different chromosomes assort independently during gamete formation.

• Rules of Probability: Used to predict outcomes of genetic crosses.

Non-Mendelian Inheritance

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles expressed (e.g., AB blood type).

• Polygenic Inheritance: Multiple genes affect a trait (e.g., skin color).

• Epigenetics: Heritable changes not involving DNA sequence (e.g., DNA methylation).

Human Genetics

• Pedigree Analysis: Chart showing inheritance patterns in families; used to identify carriers.

• Carrier: Heterozygote for a recessive disorder.

• Huntington's Disease: Dominant genetic disorder.

Chapter 12: Chromosomal Basis of Inheritance

Sex Determination and Sex-linked Genes

• Sex-linked Genes: Genes located on sex chromosomes (X or Y).

• Barr Body: Inactivated X chromosome in female cells.

• Human Sex Determination: XX = female, XY = male; probability is 50% for each sex.

Chromosomal Abnormalities

• Nondisjunction: Failure of chromosomes to separate during meiosis; leads to aneuploidy.

• Aneuploidy: Abnormal number of chromosomes.

• Down Syndrome: Trisomy 21 (extra chromosome 21).

• Klinefelter's Syndrome: XXY males.

• Turner's Syndrome: XO females.

• Cri du Chat: Deletion on chromosome 5; causes intellectual disability and cat-like cry.

Extranuclear Inheritance

• Extranuclear DNA: Found in mitochondria and chloroplasts; inherited maternally.

• Affected Genes: Genes involved in energy production (mitochondria) or photosynthesis (chloroplasts).

Example: Color blindness and hemophilia are X-linked recessive disorders more common in males.

*Additional info: Some explanations and examples were expanded for clarity and completeness based on standard biology curricula.*