Meiosis, Mendelian Genetics, Chromosomal Inheritance, DNA Structure & Replication, Gene Expression, and Mutations

Meiosis & Sexual Reproduction

What is Meiosis?

Meiosis is a specialized type of cell division that produces gametes—sperm and egg cells—in sexually reproducing organisms. Its main purposes are to reduce the chromosome number by half and to generate genetic variation among offspring.

• Produces: Sperm and egg cells (gametes)

• Purpose: Reduces chromosome number from diploid (2n) to haploid (n); creates genetic diversity

• Human Chromosome Numbers: Diploid (2n) = 46; Haploid (n) = 23

Key Vocabulary

• Homologous Chromosomes: Chromosome pairs, one from each parent, carrying the same genes

• Sister Chromatids: Identical copies of a chromosome formed during DNA replication

Stages of Meiosis

• Prophase I: Homologous chromosomes pair up; crossing over occurs, exchanging DNA and creating genetic variation

• Metaphase I: Homologous pairs align at the cell's equator

• Anaphase I: Homologous chromosomes separate to opposite poles

• Telophase I: Two haploid cells form

• Meiosis II: Similar to mitosis; separates sister chromatids

  • Prophase II: Spindle forms

  • Metaphase II: Sister chromatids align

  • Anaphase II: Sister chromatids separate

  • Telophase II: Four haploid cells result

Sources of Genetic Variation

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I

• Independent Assortment: Random arrangement of homologous pairs during Metaphase I

Mitosis vs Meiosis

Mitosis	Meiosis
Produces 2 cells	Produces 4 cells
Cells are genetically identical	Cells are genetically different
Occurs in body cells	Occurs in sex cells
No crossing over	Crossing over occurs

Mendelian Genetics

Mendel's Pea Plants & Vocabulary

Gregor Mendel's experiments with pea plants established foundational principles of heredity. He distinguished between characters (general features) and traits (specific forms).

• Character: Observable feature (e.g., flower color)

• Trait: Specific form of a character (e.g., purple or white)

• Gene: Unit of heredity

• Allele: Different versions of a gene

Dominant vs Recessive

• Dominant: Expressed with a capital letter (e.g., P)

• Recessive: Expressed with a lowercase letter (e.g., p)

Genotype and Phenotype

• Genotype: Genetic makeup (e.g., Pp)

• Phenotype: Physical appearance (e.g., purple flower)

Homozygous vs Heterozygous

• Homozygous: Two identical alleles (PP or pp)

• Heterozygous: Two different alleles (Pp)

Law of Segregation

Alleles for a gene separate during gamete formation, ensuring offspring inherit one allele from each parent.

Law of Independent Assortment

Genes for different traits segregate independently during gamete formation.

Punnett Squares

Punnett squares are used to predict the genotypes and phenotypes of offspring from genetic crosses.

Parent Genotypes	Offspring Genotypes	Offspring Phenotypes
Pp × Pp	1 PP, 2 Pp, 1 pp	3 Purple, 1 White

Monohybrid vs Dihybrid Cross

• Monohybrid Cross: Involves one trait (e.g., flower color)

• Dihybrid Cross: Involves two traits (e.g., color and shape)

Beyond Mendel

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

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

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

Chromosomal Theory of Inheritance

Chromosomal Theory

Genes are located on chromosomes, and chromosomes undergo segregation and independent assortment during meiosis.

Thomas Hunt Morgan & Fruit Flies

• Model Organism: Drosophila melanogaster (fruit fly)

• Advantages: Fast reproduction, many offspring

Wild Type vs Mutant

• Wild Type: Most common trait in nature (e.g., red eyes)

• Mutant: Alternative trait (e.g., white eyes)

Sex-Linked Traits

• Location: X chromosome

• Examples: Color blindness, hemophilia

• Males (XY): Only one recessive allele needed to express trait

• Females (XX): Two recessive alleles needed

Barr Body & Mosaicism

• Barr Body: Inactive X chromosome in female cells

• Mosaic: Females with different cells expressing different X chromosomes

DNA Structure & Replication

Discovery of DNA as Genetic Material

• Griffith Experiment: Demonstrated transformation—dead bacteria can change live bacteria

• Avery-MacLeod-McCarty: Identified DNA as the genetic material

• Hershey-Chase: Used viruses to show DNA enters cells, not protein

Chargaff's Rules

• Base Pairing: A = T, G = C

DNA Structure

• Double Helix: Two strands twisted together

• Key Scientists: Watson, Crick, Franklin

Semiconservative Replication

Each new DNA molecule contains one old strand and one new strand.

DNA Replication Process

• Origin of Replication: Where replication begins

• Enzymes:

  • Helicase: Unzips DNA

  • SSB Proteins: Stabilize separated strands

  • Topoisomerase: Prevents tangling

  • Primase: Synthesizes RNA primer

  • DNA Polymerase: Builds new DNA strand

  • Ligase: Joins DNA fragments

• Leading Strand: Synthesized continuously

• Lagging Strand: Synthesized in fragments (Okazaki fragments)

Telomeres

• Function: Protect chromosome ends; shorten with age

Mutagens

• Definition: Agents that cause mutations

• Examples: UV light, radiation, chemicals

Gene Expression

Central Dogma

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

• DNA → RNA → Protein

Transcription

• Process: DNA is copied into mRNA

• Location: Nucleus

• Enzyme: RNA Polymerase

• Steps: Initiation, Elongation, Termination

• Promoter: DNA sequence where transcription starts; contains TATA box

RNA Processing (Eukaryotes)

• 5' Cap: Added to protect mRNA

• Poly-A Tail: Added to protect mRNA

• Splicing: Removes introns, keeps exons

• Alternative Splicing: Allows one gene to produce multiple proteins

Translation

• Process: mRNA is decoded to synthesize protein

• Location: Ribosome

• Codons: Three-base sequences; specify amino acids

• Start Codon: AUG (methionine)

• Stop Codons: UAA, UAG, UGA

• Genetic Code: 64 codons; 61 code for amino acids, 3 are stop codons

• Polyribosomes: Multiple ribosomes translating a single mRNA

• Termination: Release factor binds stop codon; protein released

Mutations

Types of Mutations

• Silent Mutation: No change in amino acid sequence

• Missense Mutation: Changes one amino acid

• Nonsense Mutation: Creates a stop codon; protein synthesis stops early

• Frameshift Mutation: Alters reading frame; usually most severe

Summary Table: Mutation Types

Mutation Type	Effect
Silent	No amino acid change
Missense	One amino acid changed
Nonsense	Early stop codon
Frameshift	Reading frame altered; severe effect

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

Additional info: The notes have been expanded to include definitions, examples, and context for each topic, as well as tables for comparison and classification. All key terms and processes are explained for clarity and completeness.