Cell division: Mitosis and Meiosis

• Cells are the basic unit of life

Learning Objectives

• Describe the stages of the cell cycle, including mitosis and meiosis
• Explain the life cycles of selected organisms
• Differentiate between reproductive processes in flowering plants and humans
• Appreciate patterns governing living systems

Cell Cycle

• Represents the life cycle of a cell
• Involves continuous cell growth and protein production
• Involves DNA and centrosome duplication
• Cytoplasmic materials are doubled during this cycle

Cell Division

• Involves Karyokinesis and Cytokinesis
• Functions include Reproduction, Growth, and Repair

Karyokinesis

• Refers to division of the nucleus
• "Karyo" means nucleus
• "Kinesis" means division

Cytokinesis

• Refers to division of the cytoplasm
• "Cyto" means cytoplasm
• "Kinesis" means division
• In animal cells this involves cleavage furrow
• In plant cells this involves cell plate formation

Mitosis

• Involves one division
• Results in two identical cells
• Cells are diploid (2n)
• Occurs in somatic or body cells

Meiosis

• Involves two divisions
• Includes reductional and equational division
• Results in four varied cells
• Cells are haploid (n)
• Occurs in gametes or germ cells and is directly linked to genetic variability

Diploid and Haploid Cells

• Diploid (2n): two sets of chromosomes
• Haploid (n): one set of chromosomes

Phases of Mitosis

• G2, Late Interphase: Cell checks for complete DNA replication
• Early Prophase: Chromosomes condense and become visible, spindle forms as centrosomes move to opposite poles
• Prometaphase: Nuclear envelope breaks up, spindle fibers attach to kinetochores on chromosomes
• Metaphase: Chromosomes line up along equator of cell.
• Anaphase: Centromeres split as sister chromatids separate and move to opposite poles of cell
• Telophase: Nuclear envelope and nucleolus form at each pole, chromosomes decondense and spindle disappears
• Cytokinesis: Division of the cytoplasm occurs into two cells
• G1, Early Interphase: Cells resume normal functions or enter another division cycle

Meiosis I: Reductional Division

• Interphase: DNA replicates, cell produces proteins needed for cell division
• Prophase I (Early): Chromosomes condense and become visible (2n = 4)
• Prophase I (Late): Crossing over occurs, spindle forms, nuclear envelope breaks up
• Metaphase I: Paired homologous chromosomes line up along equator of cell
• Anaphase I: Homologous chromosomes separate to opposite poles of cell, sister chromatids remain joined
• Telophase I & Cytokinesis: Nuclear envelopes form around chromosomes, which may temporarily decondense, spindle disappears, cytokinesis may divide cell into two

Meiosis II: Equational Division

• Prophase II: Spindles form, nuclear envelopes break up
• Metaphase II: Chromosomes line up along equator of cell
• Anaphase II: Centromeres split as sister chromatids separate to opposite poles of cell
• Telophase II & Cytokinesis: Nuclear envelopes assemble around daughter nuclei, chromosomes decondense, spindles disappear, cytokinesis divides cells
• Results in four nonidentical haploid daughter cells

Spermatogenesis

• Production of sperm
• Spermatogonium develop into primary spermatocytes through growth/maturation
• Primary spermatocytes undergo Meiosis I to form secondary spermatocytes
• Secondary spermatocytes undergo Meiosis II to form spermatids
• Spermatids undergo differentiation to form spermatozoa

Oogenesis

• Production of oocytes
• Oogonium develop into primary oocytes through growth/maturation
• Primary oocytes undergo Meiosis I to form secondary oocyte and first polar body
• Secondary oocyte undergoes Meiosis II to form ootid and second polar body
• Ootid undergoes differentiation to form ovum

Nondisjunction

• Results in extra or missing chromosomes
• Can occur in Meiosis I or Meiosis II

Comparison of Mitosis and Meiosis

• Mitosis: One nuclear division, no synapsis or crossing over, centromeres separate in anaphase, somatic cells, two diploid daughter cells, genetically identical, involved in asexual reproduction/repair/growth
• Meiosis: Two nuclear divisions, synapsis and crossing over occurs, centromeres separate in anaphase I, sex cells, four haploid daughter cells, genetically different, involved only in sexual reproduction

Plant Life Cycles

• 3 Major types are: Haplontic, Diplontic, and Haplodiplontic

Haplontic Life Cycle

• Also called zygotic meiosis
• Haploid gametophytic generation is dominant over diploid sporophytic generation
• Zygote is the only diploid cell that undergoes meiosis
• Spores are haploid cells that can grow and be considered fully functional organisms.

Haplodiplontic Life Cycle

• Also called diplohaplontic
• Involves alternation of generations
• Both sporophyte and gametophyte are multicellular and fully functioning individuals
• Sporophyte is in diploid condition (dominant in pteridophytes or ferns)
• Gametophyte is in haploid condition (dominant in bryophytes or mosses)

Diplontic Life Cycle

• Also called gametic meiosis
• Body or somatic cells of the organism are diploid
• Sex cells (gametes) are haploid à fertilization à zygote
• Zygote is diploid and grows by mitosis
• Dominant sporophytic (diploid generation)

Angiosperm Life Cycle

• Both gametophytes are reduced to a few cells
• Antheridia & archegonia are lacking
• Pollination occurs followed by the development of pollen tube of mature male gametophyte
• The megaspore develops into the embryo sac which consists of a polar nuclei and an egg nuclei
• Double fertilization occurs.
• Angiosperms are flowering plants with perfect flower structures, which contains both male and female parts

Flower Structure

• Non-fertile parts: Sepals and Receptacle
• Fertile parts: Male stamen and Female carpel
• Pollination occurs when a pollen grain lands on the stigma of a carpel
• A pollen tube grows down through the style of the carpel to the ovary.
• One sperm fuses with the central cell and One sperm fuses with the egg cell

From Flower to Fruit and Ovule to Seed

• The ovary develops into a fruit and the ovule into a seed
• The integuments form the seed coat.
• The central cell (triploid) forms the endosperm
• The zygote (diploid) forms the embryo

Human Life Cycle

• Involves meiosis and fertilization, which lead to sexual reproduction
• Meiosis results in haploid gametes
• Fertilization leads to a diploid zygote
• Mitosis and development leads to multicellular diploid adults
• The new organism is genetically distinct from both parents.

Reproductive Patterns

• Asexual reproduction leads to genetically identical offspring
• Sexual reproduction leads to genetically variable offspring with mixed traits

Asexual Reproduction

• Efficient way to reproduce
• Organisms can thrive without sex if they live in a stable environment
• Harmful mutations may arise and remain in the population

Sexual Reproduction

• Widespread of occurrence from simplest to complex
• Can provide advantage to adaptability, evolutionary flexibility, genetic diversity
• Take out harmful mutation

Types of Asexual Reproduction

• Budding: A new organism begins to form at the side of the parent and enlarges until an individual is created
• Regeneration (Fragmentation): The ability to restore lost or damaged tissues, organs or limbs
• Fission: A type of cell division where one organism becomes two complete organisms
• Parthenogenesis: An unfertilized egg develops into an adult animal
• Vegetative Propagation: Asexual plant propagation methods that produce new plants from vegetative parts of the original plant like the leaves, stems and roots

Types of Sexual Reproduction

• Monoecious reproduction: Hermaphroditic which means both male and female gonads can be found in only one organism
• Dioecious reproduction: Evolved from hermaphroditism and the sexes are separate
• Gender bending (Sequential Hermaphrodite): Protandrous species first develop as makes but can become female
• Protogyny: An individual is first a female then then could become a male
• Courtship for successful mating: Visual and ritual displays occur because of 'choosy' females