Comprehensive Study Notes on DNA Replication, Cell Division, Sexual Reproduction, and Patterns of Inheritance

Study Notes: DNA Replication, Cell Division, Sexual Reproduction, and Patterns of Inheritance

Chapter 8: DNA Replication, Binary Fission, and Mitosis

1. Cell Division Overview

  • All cells divide:

    • Unicellular organisms divide to reproduce.

    • Multicellular organisms divide to replace cells and support growth.

  • Eukaryotic cells:

    • Divide by mitosis, serving purposes of growth, development, repair, and asexual reproduction.

  • Prokaryotic cells:

    • Divide by binary fission, an asexual process resulting in identical daughter cells.

2. DNA Replication

  • Precedes cell division:

    • Ensures daughter cells receive identical copies of DNA.

  • Process of DNA replication:

    • DNA unwound by helicase; strands held apart by binding proteins.

    • Primase adds RNA primer to initiate replication.

    • DNA polymerase synthesizes new DNA strands, working from the 3' ends.

    • The leading strand synthesized continuously; the lagging strand synthesized in fragments known as Okazaki fragments.

    • Ligase seals gaps left by RNA primers after replacement with DNA.

    • Replication occurs at multiple origins on chromosomes simultaneously.

3. Binary Fission in Prokaryotes

  • Chromosome Structure:

    • Prokaryotes contain a single circular chromosome.

  • Process of Binary Fission:

    • DNA replication occurs, and DNA attaches to the cell membrane.

    • The cell membrane grows inward, dividing the cell into two genetically identical daughter cells.

4. Mitosis in Eukaryotes

  • Pre-division DNA Condensation:

    • DNA condenses into visible chromosomes prior to division.

  • Cell Cycle Overview:

    • Comprises interphase (G1, S, G2) and mitosis.

    • Interphase details:

    • G1 phase: Cell growth and normal cellular functions.

    • S phase: DNA replication occurs, resulting in two sister chromatids per chromosome.

    • G2 phase: Preparation for mitosis with additional protein synthesis.

  • Phases of Mitosis:

    1. Prophase: Chromosomes condense; spindle fibers form.

    2. Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores of the chromosomes.

    3. Metaphase: Chromosomes align along cell equator.

    4. Anaphase: Sister chromatids separate and migrate to opposite poles.

    5. Telophase: Nuclear envelopes reform, and chromosomes de-condense.

  • Cytokinesis: Division of the cytoplasm resulting in two daughter cells.

5. Cell Cycle Control and Cancer

  • Checkpoints in the Cell Cycle:

    • Monitor for DNA damage, replication accuracy, and proper chromosome alignment.

  • Origin of Cancer:

    • Cancers arise from mutations that disrupt control genes:

    • Proto-oncogenes: Become overactive.

    • Tumor suppressor genes: Become underactive.

  • Tumor Types:

    • Benign tumors: Encapsulated and contained; less harmful.

    • Malignant tumors: Can spread throughout the body.

  • Prevention Strategies:

    • Healthy diet, exercise, avoiding tobacco, limiting UV exposure, and regular screenings.

  • Apoptosis: Programmed cell death, crucial for removing defective or excess cells.

Chapter 9: Sexual Reproduction and Meiosis

1. Modes of Reproduction

  • Asexual Reproduction:

    • Single parent produces genetically identical offspring; common in unicellular organisms.

  • Sexual Reproduction:

    • Two parents contribute genetic material, leading to genetically diverse offspring.

2. Chromosome Basics

  • Somatic Cells: Most are diploid (2n), with homologous chromosome pairs (one from each parent).

    • In humans, there are 23 pairs of chromosomes: 22 autosomes and 1 pair of sex chromosomes (XX for females and XY for males).

  • Homologous Chromosomes: Carry the same genes but can possess different alleles (alternate gene versions).

3. Gametes and Fertilization

  • Gametes:

    • Sperm and egg cells that are haploid (n), carrying one set of chromosomes.

  • Fertilization:

    • Fusion of haploid gametes results in a diploid zygote, the first cell of a new organism.

    • The zygote grows via mitosis into multicellular organisms.

4. Meiosis Overview

  • Meiosis Process:

    • Specialized diploid germ cells undergo meiosis, producing haploid gametes.

    • Involves one round of DNA replication followed by two nuclear divisions (Meiosis I and II).

    • Meiosis I: Homologous chromosomes pair, undergo crossing over, align, and then separate.

    • Meiosis II: Sister chromatids separate, yielding four genetically distinct haploid cells.

5. Sources of Genetic Variation in Meiosis

  • Crossing Over:

    • Exchange of chromosome segments during prophase I creates new allele combinations.

  • Independent Assortment:

    • Random orientation of chromosome pairs during metaphase I leads to varied allele combinations in gametes.

6. Genetic Disorders Related to Meiosis

  • Nondisjunction:

    • Failure of chromosomes to separate properly, causing abnormal chromosome numbers.

    • E.g., trisomy 21 (Down syndrome).

  • Sex Chromosome Anomalies:

    • XXX (Triplo-X), XXY (Klinefelter syndrome), XYY (Jacobs syndrome), XO (Turner syndrome).

    • Effects vary from mild to severe developmental issues.

  • Structural Mutations:

    • Chromosome defects like deletions, duplications, inversions, and translocations can cause issues.

7. Gamete Production

  • Spermatogenesis:

    • Produces four sperm cells from each germ cell.

  • Oogenesis:

    • Produces one egg cell from each germ cell.

Chapter 10: Patterns of Inheritance

1. Genetics Fundamentals

  • Gene: Segment of DNA encoding instructions for protein synthesis.

  • Chromosomes: DNA packets containing multiple genes; genes have specific loci on chromosomes.

  • Diploid Cells: Have homologous pairs of chromosomes and therefore two alleles per gene.

  • Alleles: Can be identical (homozygous) or different (heterozygous).

2. Mendel’s Laws of Inheritance

  • Law of Segregation: Each gamete receives only one allele from each gene during meiosis.

  • Law of Independent Assortment: Alleles of different genes assort independently, provided the genes are located on different chromosomes.

3. Dominant and Recessive Alleles

  • Dominant Alleles: Produce visible trait when present; e.g., yellow seed color (dominant) vs. green seed color (recessive) in pea plants.

  • Genotypes and Phenotypes:

    • Homozygous dominant (YY): Trait expressed.

    • Heterozygous (Yy): Dominant trait expressed.

    • Homozygous recessive (yy): Recessive trait expressed.

4. Punnett Squares and Probability

  • Use of Punnett Squares:

    • Predict genotypes and phenotypes of offspring from parental alleles.

    • Monohybrid Crosses: Analyze inheritance of one gene.

    • Dihybrid Crosses: Analyze inheritance of two genes.

  • Product Rule: Probability of independent events is the product of individual probabilities.

5. Linked Genes and Crossing Over

  • Linked Genes: Genes that are close to each other on the same chromosome are inherited together.

  • Crossing Over: Can separate linked genes, and the frequency is proportionate to distance between genes.

6. Alternative Inheritance Patterns

  • Incomplete Dominance: Heterozygote shows an intermediate phenotype (e.g., red + white = pink flowers).

  • Codominance: Both alleles are fully expressed (e.g., ABO blood groups, where IA and IB are codominant).

  • Pleiotropy: One gene affects multiple traits (e.g., Marfan syndrome).

  • Epistasis: Interaction where one gene's product affects the expression of another gene.

7. Sex-Linked Inheritance

  • X-Linked Traits: Genes on the X chromosome have unique inheritance patterns, particularly in males (XY) who express any inherited allele on the X.

  • X-Linked Recessive Disorders: More common in males (e.g., hemophilia, Duchenne muscular dystrophy, red-green color blindness).

  • X-inactivation: Randomly silences one X chromosome in females to equalize gene dosage across sexes; leads to phenomena like calico coloring in cats.

8. Genetic Disorders and Pedigree Analysis

  • Pedigrees: Track inheritance of traits through families and deduce genotype/inheritance mode (autosomal dominant/recessive, X-linked).

  • Mendelian Genetics: Applicable to human genetic diseases (e.g., cystic fibrosis - recessive; achondroplasia - dominant).

9. Environmental Effects and Polygenic Traits

  • Environmental Influence: Expression of genes can be affected by environmental factors (e.g., temperature impacting phenotype in Siamese cats).

  • Polygenic Traits: Traits influenced by multiple genes (e.g., skin color), exhibiting continuous variation rather than discrete categories.

Summary

These notes encompass significant biological processes and genetic principles including DNA replication, mechanisms of cell division (binary fission, mitosis, meiosis), sexual reproduction, and patterns of inheritance. Understanding these concepts forms a foundation for further studies in biology and medicine.