Genetics and Chromosomes

Key Terms and Concepts

This section introduces fundamental genetic terminology and chromosome structure, essential for understanding inheritance and cell division.

• Allele: Alternative forms of a gene found at the same locus on homologous chromosomes.

• Haploid: A cell or organism with one set of chromosomes (n).

• Diploid: A cell or organism with two sets of chromosomes (2n).

• Homologous chromosomes: Chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location.

• DNA polymerase: Enzyme responsible for DNA synthesis during replication.

• Telomere: Protective end region of a chromosome.

• Centromere: Region of a chromosome where sister chromatids are joined and spindle fibers attach during cell division.

• Histones, nucleosome: Proteins and structural units that package and organize DNA in eukaryotic cells.

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

Cell Cycle and Mitosis

The cell cycle describes the sequence of events in cell growth and division, including mitosis and cytokinesis.

• Phases: Interphase (G1, S, G2), Mitosis, Cytokinesis

• Mitosis stages:

  • Prophase: Chromosomes condense, spindle fibers form.

  • Metaphase: Chromosomes align at the cell equator.

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

  • Telophase: Nuclear membranes reform around separated chromatids.

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

• Centrosome, centrioles, spindle fibers: Structures involved in organizing and separating chromosomes during mitosis.

• Cancer, mitosis: Uncontrolled cell division can lead to cancer.

• Proto-oncogenes, oncogenes, tumor suppressor genes: Genes that regulate cell growth and division; mutations can lead to cancer.

Meiosis

Overview and Stages

Meiosis is a specialized form of cell division that produces gametes (sperm and egg cells) with half the chromosome number of the parent cell.

• Purpose: To produce haploid gametes for sexual reproduction.

• Stages:

  • Meiosis I: Homologous chromosomes separate.

  • Meiosis II: Sister chromatids separate.

• Key events: Synapsis, crossing-over (exchange of genetic material between homologous chromosomes), independent assortment.

• Phases: Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II.

• Result: Four genetically unique haploid cells.

• Nondisjunction: Failure of chromosomes to separate properly, leading to disorders such as Down syndrome.

Mendelian Genetics

Principles and Patterns of Inheritance

Mendelian genetics explains how traits are inherited through dominant and recessive alleles.

• Homozygous: Two identical alleles for a trait.

• Heterozygous: Two different alleles for a trait.

• Parental (P), F1, F2 generations: Generational terms used in genetic crosses.

• Monohybrid and dihybrid crosses: Crosses involving one or two traits, respectively.

• Dominance, recessiveness, co-dominance: Patterns of allele expression.

• Punnett square: Diagram used to predict the outcome of genetic crosses.

• Law of segregation: Each gamete receives one allele from each gene pair.

• Law of independent assortment: Genes for different traits are inherited independently.

Example: In a monohybrid cross between two heterozygous individuals (Aa x Aa), the expected genotypic ratio is 1:2:1 (AA:Aa:aa), and the phenotypic ratio is 3:1 (dominant:recessive).

Human Blood Groups

Blood groups are determined by multiple alleles and show co-dominance.

• ABO blood group: Three alleles (IA, IB, i) determine four blood types (A, B, AB, O).

• Rh factor: Another antigen determining positive or negative blood type.

Chromosomal Inheritance and Sex Determination

Some traits are linked to sex chromosomes, and sex is determined by specific chromosomal combinations.

• Sex-linked traits: Traits associated with genes on sex chromosomes (e.g., color blindness).

• Determination of sex: XX (female), XY (male) in mammals.

Gene Expression and Mutations

Transcription and Translation

Gene expression involves transcription (DNA to RNA) and translation (RNA to protein).

• Transcription: Synthesis of RNA from DNA template by RNA polymerase.

• Promoter, terminator, operon: Regulatory DNA sequences controlling gene expression.

• Translation: Process by which ribosomes synthesize proteins using mRNA as a template.

• Codon, anticodon: Triplet nucleotide sequences in mRNA and tRNA, respectively.

• Mutations: Changes in DNA sequence, including silent, missense, nonsense, and frameshift mutations.

Example: The Beadle-Tatum experiment demonstrated that genes encode enzymes.

Tissues and Cells

Types of Animal Tissues

Animal tissues are specialized for different functions and are classified into four main types.

• Epithelial tissue: Covers surfaces, lines cavities, forms glands. Example: alveoli of lungs, glomeruli of kidney.

• Connective tissue: Supports and binds other tissues. Includes bone, cartilage, blood, adipose tissue.

• Muscle tissue: Responsible for movement. Types: skeletal (voluntary), cardiac (involuntary), smooth (involuntary).

• Nervous tissue: Conducts electrical impulses. Includes neurons and glial cells.

Functions and Examples

• Blood: Connective tissue transporting oxygen, nutrients, and immune cells.

• Bone: Connective tissue providing structure and support.

• Muscle: Enables movement and force generation.

• Nervous system: Controls and coordinates body activities.

Skeletal Muscles

Structure and Function

Skeletal muscles are composed of fibers organized into sarcomeres, enabling contraction and movement.

• Myofibrils: Bundles of actin and myosin filaments within muscle fibers.

• Sarcomere: Functional unit of muscle contraction.

• Sliding filament theory: Muscle contraction occurs as actin and myosin filaments slide past each other.

• ATP: Provides energy for muscle contraction.

• Calcium ions (Ca2+): Trigger contraction by binding to troponin and exposing binding sites on actin.

Equation:

Example: Fast and slow muscle fibers differ in contraction speed and fatigue resistance.

Skeletons: Types and Functions

Classification of Skeletons

Animals possess different types of skeletons, each with unique advantages and disadvantages.

• Hydrostatic skeleton: Fluid-filled cavity provides support (e.g., earthworms).

• Endoskeleton: Internal skeleton made of bone or cartilage (e.g., vertebrates).

• Exoskeleton: External skeleton made of chitin or calcium carbonate (e.g., insects, crustaceans).

Advantages and disadvantages: Hydrostatic skeletons allow flexibility but lack protection; endoskeletons provide support and growth but are vulnerable to injury; exoskeletons offer protection but limit growth and movement.

Bone Formation and Maintenance

Bones are dynamic tissues formed and maintained by specialized cells.

• Osteoblasts: Cells that form new bone.

• Osteoclasts: Cells that break down bone tissue.

• Osteoporosis: Condition characterized by decreased bone density and increased fracture risk.

Locomotion and Energy Use

Locomotion in animals involves muscle and skeletal interactions, with energy use varying by size and gait.

• Type of gait: Different walking or running styles affect energy expenditure.

• Body mass vs. bone diameter: Larger animals require proportionally thicker bones for support.

• Relative energy use: Smaller mammals use more energy per unit mass than larger mammals.

Comparison Table: Skeleton Types

The following table summarizes the main features of hydrostatic, endoskeleton, and exoskeleton types.

Additional info: Some explanations and examples have been expanded for clarity and completeness.