Asexual vs. Sexual Reproduction

Asexual vs. Sexual Reproduction — Essential Notes (AP Bio)

Reproduction (General)

  • Purpose: ensure continuation of the species

Asexual Reproduction

  • One parent

  • Produces genetically identical offspring (clones)

  • No gametes or fertilization

  • Advantages:

    • Fast

    • Efficient in stable environments

  • Low genetic variation

  • Examples:

    • Amoebas

    • Sponges

Sexual Reproduction

  • Involves gametes (sperm + egg)

  • Requires fertilization

  • Produces genetically diverse offspring

  • Advantages:

    • Increases genetic variation

    • Better adaptation to changing environments

  • Common in:

    • Plants

    • Animals (insects, fish, mammals)

Alternation of Reproductive Modes

  • Some organisms switch between asexual and sexual reproduction

  • Triggered by environmental conditions

Heterogamy (AP-relevant example)

  • Organisms can alternate reproductive strategies

  • Daphnia (plankton):

    • Spring: asexual reproduction → female clones

    • Harsh conditions (winter): sexual reproduction

      • Produces males

      • Fertilized eggs protected inside female

  • Adaptive advantage: survival through unfavorable conditions

Asexual Reproduction — AP Bio Essentials

General

  • One parent

  • Offspring are genetically identical (clones)

  • No gametes, no fertilization

  • Common in unicellular organisms, fungi, plants, simple animals

Types of Asexual Reproduction

Fission

  • Occurs in unicellular organisms

  • Steps:

    1. DNA is copied

    2. Nucleus divides

    3. Cytoplasm divides

  • Produces genetically identical daughter cells

  • Types:

    • Binary fission: 2 cells

    • Multiple fission: more than 2 cells

  • Examples:

    • Amoeba

    • Paramecium

Fragmentation

  • Parent body breaks into pieces

  • Each fragment can grow into a new organism

  • Examples:

    • Fungi (molds, mushrooms)

    • Yeast

Budding

  • New organism grows out of parent

  • Offspring may:

    • Stay attached (hydra)

    • Break off and live independently (sponges)

  • Examples:

    • Hydra

    • Sponges

    • Coral

    • Jellyfish

Spore Formation

  • Parent produces spores

  • Spores released when conditions are favorable

  • Each spore → new organism

  • Examples:

    • Bread mold

    • Mosses

Vegetative Propagation

  • New plants grow from roots, stems, or leaves

  • No seeds involved

  • Example:

    • Potato tuber

Key AP Bio Takeaway

  • All asexual methods = rapid reproduction + low genetic variation

Sexual Reproduction — AP Bio Essentials

Definition

  • Requires two parents (male + female of same species)

  • Involves fusion of gametes (sperm + egg)

  • Fusion forms a zygote

  • Fusion process = fertilization

Fertilization Types

External Fertilization

  • Eggs fertilized outside female’s body

  • Common in:

    • Fish

    • Frogs

  • Disadvantages:

    • Many gametes wasted

    • Low offspring survival

Internal Fertilization

  • Eggs fertilized inside female’s body

  • Higher success rate

  • Occurs in:

    • Reptiles

    • Birds

    • Mammals

    • Some fish (sharks)

    • Some invertebrates

Cell Types in Sexually Reproducing Organisms

Somatic Cells

  • Body (non-sex) cells

  • Diploid (2n)

  • Two sets of chromosomes:

    • One from mother

    • One from father

Homologous Chromosomes

  • Chromosome pairs in somatic cells

  • Same:

    • Size

    • Shape

    • Gene sequence

    • Gene location (loci)

Chromosome Number (Species-Specific)

  • Fruit fly: 4 pairs (2n = 8)

  • Humans: 23 pairs (2n = 46)

Gametes

  • Sex cells (sperm, egg)

  • Haploid (n)

  • One set of chromosomes

  • Half the number of somatic cells

Fertilization Outcome

  • n (sperm) + n (egg) → 2n zygote

  • Restores diploid chromosome number

Key AP Bio Takeaway

Sexual reproduction increases genetic variation and maintains chromosome number across generations.

Asexual vs. Sexual Reproduction — AP Bio Comparison

Evolutionary Context

  • Asexual reproduction appeared first (earliest organisms)

  • Sexual reproduction evolved later

  • Today:

    • Simple organisms → often asexual

    • Complex organisms → usually sexual

    • Some organisms → both

Asexual Reproduction

  • One parent

  • Offspring are genetically identical (clones)

  • Fast

  • No gametes or mating required

  • No genetic diversity

  • Advantage: rapid population growth

  • Disadvantage: poor adaptation to environmental change

Sexual Reproduction

  • Two parents

  • Offspring have genetic combination of both parents

  • Slower

  • Requires gamete formation + fertilization

  • High genetic diversity

  • Advantage: variation improves survival and evolution

Key AP Bio Takeaway (memorize):

Asexual reproduction is faster but produces no variation; sexual reproduction is slower but increases genetic diversity through recombination.

Mitosis and Meiosis — AP Bio Essentials

Mitosis

  • Produces somatic (body) cells

  • Function:

    • Growth and repair (multicellular organisms)

    • Reproduction in unicellular organisms

  • One parent cell → 2 daughter cells

  • Daughter cells are:

    • Genetically identical

    • Same chromosome number as parent (diploid, 2n)

  • DNA is replicated once, then divided equally

  • Does NOT produce gametes

Why Mitosis Can’t Make Gametes

  • Gametes with 2n chromosomes would cause:

    • Fertilization → chromosome number doubles each generation

  • This is biologically unstable ❌

Meiosis

  • Produces gametes (sex cells)

  • Occurs in germ cells (immature reproductive cells)

  • Purpose:

    • Reduce chromosome number by half

  • Result:

    • Gametes are haploid (n)

    • Fertilization restores diploid (2n)

Meiosis Structure

  • Two divisions:

    1. Meiosis I: reduction division (2n → n)

    2. Meiosis II: separates chromatids

  • One germ cell → 4 haploid cells

Meiosis in Males

  • 1 germ cell → 4 functional sperm

  • All equal size

  • Plant equivalent:

    • Pollen grains (haploid)

Meiosis in Females

  • 1 germ cell → 4 haploid cells

  • Only 1 functional egg

  • Other 3 = polar bodies (nonfunctional)

  • Plant equivalent:

    • Ovule (haploid)

Key AP Bio Takeaway (memorize):

Mitosis maintains chromosome number; meiosis reduces it to ensure stability across generations.

Genetic Variation — AP Bio Essentials

Why Sexual Reproduction Creates Variation

  • Uses meiosis

  • Combines genetic material from two parents

  • Produces genetically unique offspring

Sources of Genetic Variation in Meiosis

Crossing Over

  • Occurs during meiosis I

  • Homologous chromosomes exchange segments

  • Creates new gene combinations

  • Increases variation within chromosomes

Independent Assortment

  • Homologous chromosomes separate randomly

  • Each gamete receives a random mix of maternal and paternal chromosomes

  • Produces genetically unique gametes

Random Fertilization

  • Any sperm can fertilize any egg

  • One sperm out of millions fertilizes one ovum

  • Produces a genetically unique zygote

Key AP Bio Takeaway (memorize):

Genetic variation in sexual reproduction arises from crossing over, independent assortment, and random fertilization.