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 variation

• 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 distribution of homologous chromosomes during Metaphase I

Mitosis vs Meiosis

Mitosis and meiosis are two distinct processes of cell division. The table below summarizes their differences:

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

Mendelian Genetics

Mendel's Pea Plants & Vocabulary

Gregor Mendel's experiments with pea plants established the foundational principles of genetics. 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 flowers)

• 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)

Law of Segregation

Alleles for a gene separate during gamete formation, ensuring each gamete receives only one allele.

Law of Independent Assortment

Genes for different traits segregate independently during gamete formation.

Punnett Squares

Punnett squares are used to predict the outcome of genetic crosses. Example: Pp × Pp cross.

	P	p
P	PP	Pp
p	Pp	pp

• Genotypes: 1 PP, 2 Pp, 1 pp

• Phenotypes: 3 Purple, 1 White

Monohybrid vs Dihybrid Cross

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

• Dihybrid Cross: 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

The chromosomal theory of inheritance states that genes are located on chromosomes, which are the vehicles of genetic transmission.

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): Need only one recessive allele to express trait

• Females (XX): Need two recessive alleles

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 and carries genetic information

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 consists of 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 into RNA, which is then translated into protein.

• Process: DNA → RNA → Protein

Transcription

• Location: Nucleus

• Enzyme: RNA Polymerase

• Steps: Initiation, Elongation, Termination

• Promoter: Starting point; contains TATA box

RNA Processing (Eukaryotes)

• 5' Cap: Protects mRNA

• Poly-A Tail: Protects mRNA

• Splicing: Removes introns, keeps exons

• Alternative Splicing: One gene can produce multiple proteins

Translation

• 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 one mRNA

• Termination: Release factor binds stop codon; protein released

Mutations

Types of Mutations

• Silent Mutation: No change in amino acid

• Missense Mutation: Changes one amino acid

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

• Frameshift Mutation: Alters reading frame; usually most severe

Summary Table: Mutation Types

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