Chapter 12: The Cell Cycle

Overview of the Cell Cycle

The cell cycle is the series of events that cells undergo to grow and divide. It ensures the transmission of genetic material to daughter cells and is essential for growth, development, and tissue repair.

• Interphase: The period of cell growth and DNA replication, consisting of G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).

• Mitosis: Division of the nucleus into two genetically identical daughter nuclei. Stages include prophase, prometaphase, metaphase, anaphase, and telophase.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells. In animal cells, this occurs via a cleavage furrow; in plant cells, a cell plate forms.

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

• Chromosomes: Structures that carry genetic information; composed of chromatin.

• Somatic cells vs. Gametes: Somatic cells are body cells (diploid), gametes are reproductive cells (haploid).

• Sister chromatids: Identical copies of a chromosome connected at the centromere.

• Mitotic spindle: Structure made of microtubules that separates chromosomes during mitosis.

• Centrosomes: Organize the mitotic spindle.

• Kinetochore: Protein structure on chromatids where spindle fibers attach.

• Binary fission: A form of asexual reproduction in bacteria.

• Cancer cells: Cells with uncontrolled division due to defects in cell cycle control.

• Cell cycle control system: Regulates progression through the cell cycle; includes checkpoints (G1, G2, M).

• G0 phase: A non-dividing state.

• Benign vs. Malignant tumor: Benign tumors do not invade other tissues; malignant tumors can metastasize.

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

Comparison: Mitosis vs. Meiosis

Chapter 13: Meiosis

Meiosis and Genetic Variation

Meiosis is the process by which gametes are produced, reducing chromosome number by half and increasing genetic diversity.

• Genes: Units of heredity located on chromosomes.

• Gametes: Reproductive cells (sperm and egg).

• Somatic cells: All other body cells.

• Locus: Location of a gene on a chromosome.

• Asexual vs. Sexual reproduction: Asexual produces identical offspring; sexual combines genetic material from two parents.

• Karyotype: Display of chromosome pairs.

• Homologous chromosomes: Chromosome pairs with the same genes.

• Autosomes vs. Sex chromosomes: Autosomes are non-sex chromosomes; sex chromosomes determine gender.

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

• Haploid (n): One set of chromosomes.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate; results in two haploid cells.

• Meiosis II: Sister chromatids separate; results in four haploid cells.

• Prophase I: Crossing over occurs at chiasmata, increasing genetic variation.

• Metaphase I: Homologous pairs align randomly at the metaphase plate.

• Anaphase I: Homologous chromosomes separate.

Mechanisms of Genetic Variation

• Independent assortment: Random distribution of chromosomes.

• Crossing over: Exchange of genetic material between homologous chromosomes.

• Random fertilization: Any sperm can fertilize any egg.

Chapter 14: Mendel and the Gene Idea

Mendelian Genetics

Gregor Mendel's experiments with pea plants established the basic principles of heredity.

• Characters and traits: Characters are heritable features; traits are variants.

• True-breeding: Organisms that produce offspring of the same trait.

• P, F1, F2 generations: Parental, first filial, and second filial generations.

• Dominant vs. Recessive traits: Dominant traits mask recessive traits.

• Mendel’s 4 Concepts: 1) Alternative versions of genes (alleles), 2) Two alleles per gene, 3) Dominant and recessive alleles, 4) Law of segregation.

• Alleles: Different versions of a gene.

• Law of segregation: Alleles separate during gamete formation.

• Homozygous vs. Heterozygous: Homozygous has identical alleles; heterozygous has different alleles.

• Phenotype vs. Genotype: Phenotype is physical appearance; genotype is genetic makeup.

• Testcross: Used to determine genotype.

• Dihybrid cross: Cross involving two traits.

Non-Mendelian Genetics

• Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers).

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

• Multiple alleles: More than two alleles for a gene (e.g., blood types).

• Pleiotropy: One gene affects multiple traits (e.g., sickle cell disease).

• Epistasis: One gene affects expression of another.

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

• Environmental impact: Environment can influence phenotype.

• Inherited disorders: Recessive (e.g., sickle cell), dominant, multifactorial disorders.

• Carriers: Heterozygotes for recessive disorders.

Chapter 15: Chromosomal Basis of Inheritance

Chromosomes and Inheritance

Chromosomes carry genes and are responsible for inheritance patterns. Studies with fruit flies (Morgan) provided evidence for the chromosomal theory of inheritance.

• Wild type vs. Mutants: Wild type is the normal phenotype; mutants have altered genes.

• Chromosomal basis of sex: XY system in humans.

• Sex-linked genes: Genes located on sex chromosomes; X-linked disorders are more common in males (e.g., color blindness).

• Barr body: Inactivated X chromosome in females.

• Linked genes: Genes located close together on a chromosome.

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

• Aneuploidy: Abnormal chromosome number (monosomic, trisomic).

• Polyploidy: More than two sets of chromosomes (common in plants).

• Chromosomal alterations: Deletion, duplication, inversion, translocation.

• Genetic disorders: Down Syndrome (Trisomy 21), Klinefelter syndrome, Triple X syndrome, Turner Syndrome, cri du chat.

• Genomic imprinting: Expression of genes depends on parent of origin.

• Extranuclear genes: Genes in mitochondria and chloroplasts are inherited maternally.

Chapter 16: Molecular Basis of Inheritance

Discovery of DNA as Genetic Material

Experiments by Morgan, Griffith, Oswald, McCarty, MacLeod, Hershey, and Chase established DNA as the molecule of inheritance.

• Chargaff’s rules: Amount of A = T, G = C.

• Rosalind Franklin: Provided X-ray images of DNA.

• Watson and Crick: Proposed the double helix structure.

• Antiparallel: DNA strands run in opposite directions.

DNA Replication

• Semiconservative model: Each new DNA molecule has one old and one new strand.

• Origins of replication: Sites where replication begins.

• Replication fork: Y-shaped region where DNA is unwound.

• Key enzymes:

  • Helicase: Unwinds DNA.

  • Single-Strand Binding Proteins: Stabilize unwound DNA.

  • Topoisomerase: Relieves strain ahead of fork.

  • Primase: Synthesizes RNA primer.

  • DNA Polymerases: Synthesize new DNA.

  • DNA Ligase: Joins Okazaki fragments.

• Leading vs. Lagging strand: Leading strand synthesized continuously; lagging strand in fragments (Okazaki fragments).

• Mismatch repair: Corrects errors in DNA.

• Nucleotide excision repair: Removes damaged DNA.

• Telomeres: Protect chromosome ends.

• Chromatin, histones, nucleosome: DNA packaging in the nucleus.

DNA Replication Equation

DNA polymerase adds nucleotides to the 3' end:

Chapter 17: Gene Expression

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is transcribed to RNA, which is translated to protein.

• Transcription: Synthesis of RNA from DNA template; occurs in nucleus (eukaryotes) or cytoplasm (prokaryotes).

• Translation: Synthesis of protein from mRNA; occurs in cytoplasm.

• Genetic code: 20 amino acids, triplet code (codons), 64 codons (61 for amino acids, 3 stop codons).

• Start codon: AUG (methionine).

• Universal code: Same codons specify same amino acids in all organisms.

• RNA polymerase: Enzyme for transcription.

• mRNA modification: 5’ cap, poly-A tail, RNA splicing (removal of introns, joining of exons).

• Alternative splicing: Produces different proteins from same gene.

• tRNA: Transfers amino acids to ribosome; has anticodon.

• Ribosome sites: A (aminoacyl), P (peptidyl), E (exit).

• Stages of translation: Initiation, elongation, termination.

• Post-translational modification: Protein folding and chemical modifications.

• Polyribosomes: Multiple ribosomes translating same mRNA.

• Point mutations: Single nucleotide changes; can be silent, missense, or nonsense.

• Mutagens: Agents causing mutations.

• CRISPR: Genome editing tool.

• Gene: Sequence of DNA encoding a functional product.

Types of Point Mutations

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