Lecture 2: Cell cycle and mitosis; Embryogenesis and Animal Development (Campbell 12.1-12.2)

The Eukaryotic Cell Cycle: Phases and Key Concepts

• Interphase consists of G1, S, and G2 phases

• M phase includes mitosis and cytokinesis

• G1: cell growth; proteins for DNA replication are synthesized

• S: DNA replication occurs, resulting in identical sister chromatids for each chromosome

• G2: cell growth continues; preparation for cell division; centrosomes replicate

• M phase: mitosis (division of the nucleus) followed by cytokinesis (cytoplasm division)

• The order of phases in the cell cycle: G1 → S → G2 → M

• The spindle apparatus moves and separates chromosomes during mitosis

• Centrosomes (and their centriole pairs in some organisms) organize the spindle

• Chromosomes condense during mitosis to facilitate separation

• The nucleolus and nuclear envelope dynamics change during interphase and mitosis

• Duplication of chromosomes results in sister chromatids held together at the centromere until separation

• Key terms to know for mitosis and cell cycle:

  • Sister chromatids

  • Centromere

  • Centrosomes

  • Kinetochores

  • Kinetochore microtubules

  • Non-kinetochore microtubules

  • Spindle

  • Cleavage furrow in cytokinesis (animal cells)

• Important numerical references:

  • Diploid chromosome number in humans for somatic cells: $2n=46$

  • Haploid number in humans for gametes: $n=23$

Embryogenesis and Animal Development: Core Concepts

• Embryogenesis is the development of a functional embryo from a single cell zygote

• An animal life cycle includes alternating haploid and diploid stages produced by meiosis and fertilization

• Major stages in the animal life cycle and embryogenesis:

  • Fertilization: fusion of sperm and egg to form a diploid zygote

  • Zygote: the single fertilized cell (diploid, $2n=46$ in humans)

  • Cleavage: rapid mitotic divisions without growth, increasing cell number

  • Blastula: a hollow ball of cells formed after cleavage

  • Gastrulation: processes that form germ layers and establish body plan

  • Organogenesis: development of organs and organ systems

  • Larval stages and metamorphosis: later life stages that may transform form and function

• Connections to cellular processes in embryogenesis:

  • Cell division (mitosis) increases cell number during development

  • Cell differentiation: cells become specialized into various tissue types

  • Morphogenesis: organization of cells into a body plan with defined axes and structures

• Foundational sequence linking to broader life cycles:

  • MEIOSIS occurs to produce haploid gametes (n)

  • FERTILIZATION creates a diploid zygote (2n) that enters MITOSIS for growth and development

  • MITOSIS repeatedly divides to form multicellular embryo and beyond

• Key terms and transitions illustrated in the figures:

  • Haploid vs diploid states: n and 2n definitions, gametes are haploid

  • MEIOSIS and FERTILIZATION pathways that establish zygote formation

  • MITOSIS pathways that drive embryo growth

The Cell Cycle in Animal Development: Detailed Stages

• The cell cycle must coordinate DNA replication and division while allowing for developmental milestones

• Fig. 12.5 summary: The Eukaryotic Cell Cycle

  • G1: cell growth; proteins for DNA replication synthesized

  • S: DNA replication; DNA is copied

  • G2: cell growth; preparation for cell division; centrosomes replicate

  • M: mitosis (division of the nucleus) and cytokinesis (cytoplasm division)

• Interphase vs M phase distinctions:

  • Interphase includes G1, S, G2 where the cell grows and DNA is replicated

  • M phase includes mitosis and cytokinesis where nuclei and cytoplasm divide

Chromosome Content and Figures: Spider Mite Example

• Spider mites: somatic cells have 4 total chromosomes, i.e., 2n = 4

• Q5 asks to draw a diploid spider mite cell before cell cycle begins

  • Baseline: 2n = 4; four chromosomes present in G1, each chromosome consists of a single chromatid

• After the cell cycle begins (Q6):

  • G1: 4 chromosomes, each with a single chromatid (4 DNA molecules)

  • S phase: DNA replication occurs; each chromosome now consists of two sister chromatids; still 4 chromosomes but 8 DNA molecules

  • G2: chromosomes duplicated, centrosomes present; (2n = 4 with duplicated chromosomes)

  • M phase: mitosis begins; at completion, two daughter cells are formed, each with 2n = 4

• Important note: the zygote and subsequent embryo stages maintain the same number of chromosomes per somatic cell in mitosis across cycles

• Q6 prompt to show each stage of the cell cycle for the spider mite cell ends with two daughter cells after cytokinesis

Metaphase Labeling and Mitosis Architecture

• Q7 asks for a metaphase diagram of a spider mite cell with labels:

  • Sister chromatids

  • Centrosomes

  • Kinetochore

  • Kinetochore microtubules

  • Non-kinetochore microtubules

• The mitotic spindle comprises two main microtubule populations:

  • Kinetochore microtubules attach to kinetochores on chromosomes and move them toward the metaphase plate

  • Nonkinetochore microtubules interact with each other to elongate the cell during anaphase

Stages of Mitosis: Key Features and Structures

• Prophase: chromosomes condense; centrosomes organize the mitotic spindle

• Prometaphase: the spindle is fully formed; microtubules attach to kinetochores on chromosomes

• Metaphase: chromosomes align at the metaphase plate at the equator; centromeres divide late in metaphase

• Anaphase: sister chromatids separate and are pulled toward opposite poles; microtubules shorten and spindle lengthens to elongate the cell

• Telophase: nuclear membranes reform around the separated chromosomes; chromosomes de-condense

• Cytokinesis: cytoplasm divides; in animal cells, a contractile ring of microfilaments forms a cleavage furrow that pinches the cell membrane to create two daughter cells

• The mitotic spindle components:

  • Centrosomes (spindle organizers)

  • Kinetochore microtubules (attach to kinetochores on chromosomes)

  • Non-kinetochore microtubules (overlap at the spindle midzone to push poles apart)

Cytokinesis in Animal Cells

• Mechanism: contractile ring of actin filaments (microfilaments) forms a contractile ring

• Cleavage furrow forms as the ring tightens, splitting the cytoplasm into two daughter cells

• Visual: cleavage furrow deepens to separate the two daughter cells

Practice and Visualization: Stage Diagrams and Examples

• Interphase, mitotic phases, and cytokinesis organized as a standard sequence

• The following visualization prompts are given in the materials:

  • Draw a spider mite diploid cell before cell cycle: 2n = 4

  • Draw the same cell after each stage of the cell cycle, excluding detailed mitotic sub-stages (Q6)

  • Draw and label a cell during metaphase with the required labels (Q7)

  • Draw a muntjac cell with 2n = 6 through all stages of the cell cycle including mitosis (Q8)

Practical Applications and Real-World Relevance

• Developmental biology links cell division to organismal form and function through morphogenesis and differentiation

• Proper regulation of the cell cycle is essential for development and tissue maintenance

• Disruptions in cell cycle control are associated with diseases such as cancer; understanding mitosis helps in thinking about targeted therapies and diagnostics

Learning and Study Strategies: Forgetting Curves and Best Practices

• Hermann Ebbinghaus forgetting curve overview:

  • Without review, retention declines over time after initial learning

  • Early forgetting is steep; reviews at short intervals help stabilize memory

• General forgetting curve patterns:

  • Initial learning retention decays over days if unchecked

• The effect of study methods on retention (typical data from the slides):

  • Verbal processing alone yields relatively low retention after 24 hours

  • Verbal plus visual processing improves retention modestly

  • Reading plus audiovisual demonstration yields higher retention

  • Discussion groups raise retention further

  • Practice by doing leads to the highest retention among the listed methods

  • Teaching others or immediate application produces the strongest retention

• Key takeaway: Reviewing questions and objectives soon after a lecture dramatically improves retention; attending recitation and doing active practice further enhances memory

• Specific course guidance for success:

  • Prepare for class by readings in advance

  • Attend every class and participate

  • Take organized notes by hand

  • Review learning objectives, notes, and slides at least twice a week

  • Use office hours and recitation for reinforcement

  • Stay connected with the instructional team via email and Canvas

  • LAs and TAs assist with review in class and recitation

Neuroscience-Informed Study Question and Answers (Q1–Q4, Q5–Q8)

• Q1: Based on neuroscience research, the best way to study and retain the information is to: answer the learning objectives on your own and then discuss the answers with other students or the instructional team

• Q2: Animal life cycle diagram terms to include: Adult, Embryo, Zygote, Meiosis, Mitosis, Fertilization, Egg, Sperm, Haploid, Diploid, Gametes

• Q3: In animals, which cell types are produced by mitosis? Correct: Embryos and Adults (i.e., B and C)

• Q4: From a single celled zygote to a functional embryo and adult, at least two cellular processes to occur are: 1) cell division (mitosis), 2) cell differentiation, 3) morphogenesis, and 4) gastrulation as applicable in developmental contexts

• Q5: Spider mite diploid cell before cell cycle begins: 2n = 4; four chromosomes; single DNA molecules per chromosome in G1

• Q6: Spider mite cell through the cell cycle (excluding detailed mitotic sub-stages): G1 has 4 chromosomes; S has 4 chromosomes with duplicated DNA (8 DNA molecules total); G2 has duplicated chromosomes with centrosomes present; Mitosis yields two daughter cells each with 2n = 4

• Q7: Metaphase diagram labeling requirements: Sister chromatids, Centrosomes, Kinetochores, Kinetochore microtubules, Non-kinetochore microtubules

• Q8: Muntjac cell with 2n = 6 drawn through all stages of the cell cycle including mitosis

Connections to Foundational Principles and Real-World Relevance

• The cell cycle coordinates growth, DNA replication, and division with developmental events

• Embryogenesis demonstrates how a single cell transitions to a complex multicellular organism via regulated mitosis, differentiation, and morphogenesis

• Meiosis and fertilization establish genetic diversity and species-specific chromosome sets

• The study of mitosis reveals targets for cancer therapies and informs regenerative medicine and developmental biology

Quick References and Key Terms

• Embryogenesis: development of a functional embryo from a single celled zygote

• Cleavage: rapid mitotic divisions following fertilization

• Blastula: hollow ball of cells after cleavage

• Gastrulation: formation of germ layers and body axes

• Organogenesis: formation of organs and organ systems

• Morphogenesis: organization of cells into a body plan

• Cell differentiation: specialization of cells into distinct types

• Interphase: G1, S, G2 phases; growth and DNA replication

• Mitosis: prophase, prometaphase, metaphase, anaphase, telophase

• Cytokinesis: division of cytoplasm; cleavage furrow in animals

• Chromosome terms: chromosome, chromatid, centromere, sister chromatids, kinetochores, centrosomes, spindle, kinetochore microtubules, non-kinetochore microtubules

• Chromosome numbers: haploid $n = 23$, diploid $2n = 46$ in humans; spider mite 2n = 4; muntjac 2n = 6

• Major stages of the cell cycle in order: G1 → S → G2 → M

Notes on Figures and Practice Diagrams

• Fig. 12.5 The Eukaryotic Cell Cycle illustrates G1, S, G2, M with accompanying preparatory steps like DNA replication and centrosome duplication

• Imagery for metaphase, anaphase, and cytokinesis demonstrates the alignment of chromosomes at the metaphase plate and subsequent separation and cell division

• The provided Q prompts serve as practical exercises to reinforce the structural components of mitosis and the chromosomal changes through the cycle

Summary of Key Equations and Numbers

• Human somatic diploid: $2n = 46$

• Human gamete haploid: $n = 23$

• Spider mite somatic diploid: $2n = 4$

• Muntjac somatic diploid: $2n = 6$

• Zygote chromosome count in humans: $2n = 46$

• Mitosis stages: prophase, prometaphase, metaphase, anaphase, telophase; cytokinesis follows