Chapter Two: Mitosis and Meiosis

Basic Structure of the Cell

The cell is the fundamental unit of life, consisting of various organelles that perform essential functions.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Cytoplasm: Gel-like substance where organelles are suspended.

• Cell membrane: Regulates entry and exit of substances.

• Other organelles: Mitochondria (energy production), ribosomes (protein synthesis), endoplasmic reticulum, Golgi apparatus.

Centromere Locations and Chromosome Shapes

Chromosomes are classified based on the position of their centromere:

• Metacentric: Centromere is in the middle; arms are equal length.

• Submetacentric: Centromere is slightly off-center; arms are unequal.

• Acrocentric: Centromere is near one end; one arm is much shorter.

• Telocentric: Centromere is at the end; only one arm is visible.

Diploid and Haploid Numbers

Organisms can have two sets of chromosomes (diploid) or one set (haploid).

• Diploid (2n): Two sets of chromosomes, one from each parent.

• Haploid (n): One set of chromosomes, typical of gametes.

• Example: Humans have 2n = 46 chromosomes; n = 23 in gametes.

Phases of the Cell Cycle

The cell cycle consists of interphase and mitotic phase:

• G1 phase: Cell growth and preparation for DNA replication.

• S phase: DNA synthesis; chromosomes are replicated.

• G2 phase: Further growth and preparation for mitosis.

• M phase: Mitosis and cytokinesis.

Stages of Mitosis

Mitosis is the process by which somatic cells divide, producing two identical daughter cells.

• Prophase: Chromosomes condense, spindle forms, nuclear envelope dissolves.

• Metaphase: Chromosomes align at the cell's equator.

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

• Telophase: Chromosomes decondense, nuclear envelope reforms.

• Cytokinesis: Division of cytoplasm, forming two cells.

Genetic Checkpoints in the Cell Cycle

Checkpoints ensure proper cell cycle progression and prevent errors.

• G1/S checkpoint: Checks for DNA damage before replication.

• G2/M checkpoint: Ensures DNA is fully replicated and undamaged.

• Spindle checkpoint: Ensures chromosomes are properly attached to spindle before anaphase.

Meiosis and Its Differences from Mitosis

Meiosis produces gametes with half the chromosome number, introducing genetic variation.

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate (similar to mitosis).

• Key differences: Meiosis involves two divisions, produces four haploid cells, and includes crossing over.

Sperm and Egg Formation

Gamete formation differs between males and females.

• Spermatogenesis: Produces four functional sperm from each precursor cell.

• Oogenesis: Produces one functional egg and three polar bodies.

• Differences: Timing, number of gametes, and cytoplasmic distribution.

Role of Meiosis in Sexual Reproduction

Meiosis ensures genetic diversity and maintains chromosome number across generations.

• Reduces chromosome number: Prevents doubling in each generation.

• Genetic variation: Crossing over and independent assortment.

Chapter Three: Mendelian Genetics

Monohybrid Crosses

Monohybrid crosses involve one gene with two alleles.

• Example: Crossing pea plants with tall (T) and dwarf (t) alleles.

• Expected ratio: 3:1 in F2 generation for dominant:recessive phenotype.

Dominant and Recessive Concepts

Alleles can be dominant (expressed) or recessive (masked).

• Dominant allele: Expressed in heterozygotes.

• Recessive allele: Expressed only in homozygotes.

Mendel’s Four Postulates

Mendel established principles of inheritance based on pea plant experiments.

• Postulate 1: Unit factors exist in pairs.

• Postulate 2: Dominance and recessiveness.

• Postulate 3: Segregation of alleles during gamete formation.

• Postulate 4: Independent assortment of different genes.

Test-Cross

A test-cross determines the genotype of an individual with a dominant phenotype.

• Performed by: Crossing with a homozygous recessive individual.

• Results: Ratio of offspring reveals genotype.

Dihybrid Cross and Independent Assortment

Dihybrid crosses involve two genes; independent assortment leads to new combinations.

• Example: Crossing plants for seed color and shape.

• Expected ratio: 9:3:3:1 in F2 generation.

Trihybrid Crosses and Forked-Line/Branch Diagram

Trihybrid crosses involve three genes; forked-line diagrams help predict outcomes.

• Branch diagram: Visualizes possible combinations and their probabilities.

Calculating Probabilities: Product Law

The product law states that the probability of independent events occurring together is the product of their individual probabilities.

• Formula:

Binomial Theorem in Genetics

The binomial theorem calculates the probability of a specific combination of outcomes.

• Formula:

• Application: Used for predicting ratios in offspring.

Chi-Square Analysis

Chi-square analysis tests whether observed data fit expected ratios.

• Formula:

• Interpretation: Compares calculated value to critical value to accept or reject hypothesis.

Chapter Four: Extensions of Mendelian Genetics

Mutant Allele and Wild Type Allele

Alleles can be classified as wild type (common) or mutant (variant).

• Wild type: Most common allele in a population.

• Mutant: Allele that differs from wild type, often causing altered phenotype.

Trait Description Systems

Traits can be described by different systems, such as qualitative or quantitative.

• Qualitative traits: Discrete categories (e.g., flower color).

• Quantitative traits: Continuous variation (e.g., height).

Incomplete or Partial Dominance

Incomplete dominance occurs when heterozygotes show an intermediate phenotype.

• Example: Red and white snapdragons produce pink offspring.

• Ratio: 1:2:1 in F2 generation (distinct from Mendelian 3:1).

Co-Dominance

Co-dominance occurs when both alleles are fully expressed in heterozygotes.

• Example: ABO blood group system; both A and B antigens are expressed.

Multiple Alleles for One Trait

Some traits are controlled by more than two alleles.

• Example: ABO blood group has three alleles: IA, IB, and i.

ABO Blood Groups, Secretor Locus, and Bombay Phenotype

The ABO blood group system is influenced by multiple alleles and loci.

• ABO system: IA and IB are co-dominant; i is recessive.

• Secretor locus: Determines if antigens are present in body fluids.

• Bombay phenotype: Rare genotype where H antigen is absent, masking ABO expression.

Lethal Alleles

Lethal alleles cause death when present in certain genotypes.

• Dominant lethal: Only one copy needed for lethality (e.g., Huntington's disease).

• Recessive lethal: Two copies needed for lethality (e.g., cystic fibrosis).