Mitosis and Cell Cycle Quiz

Unit C: Cells, Chromosomes and DNA

• Chapters 17 and 20 cover cell division and molecular genetics (DNA)
• Cell division is crucial for growth, maintenance, and repair in all life forms.
• There are approximately 150 billion cell divisions daily in the human body, originating from a single fertilized egg.
• Mitosis is the process of asexual division for growth and repair, maintaining chromosome number.
• Meiosis creates gametes (germ cells) for sexual reproduction and reduces the chromosome number by half.

Cell Division

• Chapter 17 discusses the cell cycle
• Cell cycle: the cell cycle, mitosis, and meiosis
• Chapter 17, page 556-595 in Metro Biology 30
• 17.1, pages 558-563 (The Cell Cycle)

Why Divide?

• All life depends on the ability to grow, maintain, and repair cells.
• Cells arise from pre-existing cells.
• In a human, 100 trillion cells originate from a single fertilized egg.
• Cell division ensures accurate reproduction of genetic and cellular matter.

Mitosis

• Responsible for body (somatic) cell growth, repair, and maintenance;
• Maintains the normal chromosome number (2n)
• Considered a form of asexual cellular division.

Meiosis

• Creates sex cells (gametes) for sexual reproduction - germ cells
• Reduces the chromosome number (2n to 1n) in gametes
• Enables genetic variety through crossing over in prophase I

Genetic Material in Eukaryotes

• Genetic information is found within the nucleus as DNA in chromatin.
• Chromatin is a complex of DNA and proteins (histones).
• DNA is tightly wrapped around histone proteins, forming nucleosomes.
• DNA is compact in the cell, fitting inside a nucleus much smaller than the DNA itself: ~150 km string in 5 micrometer diameter nucleus.
• Chromatin (uncoiled, thin strands of DNA) vs. Chromosomes (condensed structures of DNA)
• Chromatin condenses into visible chromosomes during cell division.

Chromatin vs. Chromosomes

• DNA is super-coiled to form chromatin or chromosomes, depending on the cell cycle stage.
• Chromatin (interphase): uncondensed DNA
• Chromosomes (mitosis): condensed into visible, distinct structures.
• Nucleosomes are the basic structural units of chromatin, consisting of DNA coiled around histone proteins.

Chromosome Structure

• Replicated chromosomes are called sister chromatids, attached at a centromere.
• Each chromatid identical to its sister chromatid, though may have different alleles.
• The centromere is the region of the chromosome where sister chromatids attach.

Centromeres and Kinetochores

• The centromere is a specialized region of a chromosome that holds the sister chromatids together.
• Kinetochores are protein structures located on the centromeres.
• Kinetochores serve as "docking sites" on the centromeres for microtubules of the mitotic spindle.
• Microtubules are involved in chromosome movement during cell division.

Chromosome Number

• Chromosome numbers vary considerably among different species.
• Even closely related organisms can have markedly different chromosome numbers.
• Humans have 46 chromosomes.

Ploidy

• Ploidy refers to the number of chromosome sets in a cell.
• Haploid (n): single set of chromosomes (gametes)
• Diploid (2n): two sets of chromosomes (somatic cells)
• Some organisms are polyploid (more than two sets of chromosomes)

Genes on Chromosomes

• Genes are sections of DNA that contain instructions to make proteins.
• Genes are located at specific locations on chromosomes called loci.
• Alleles are different forms of a gene.

The Cell Cycle - Interphase

• Interphase is the longest phase of the cell cycle (approx. 90%).
• It consists of three stages: G1 (growth), S (DNA synthesis/replication), and G2 (growth).
• Chromosomes are duplicated during the S phase, though they are still called chromosomes (because they are attached at the centromere).

Gap 1 (G₁), Synthesis (S), and Gap 2 (G₂) Phases of Interphase

• G1: growth and metabolic activity
• S: DNA replication and chromosome duplication
• G2: preparation for mitosis, growth

The Cell Cycle - Mitosis & Cytokinesis

• The M phase includes mitosis and cytokinesis.
• Mitosis: the process of nuclear division
• Four phases of mitosis: prophase, metaphase, anaphase, and telophase;
• Cytokinesis: The division of the cytoplasm to form two separate daughter cells.

Microtubules, and Mitotic Spindle

• Microtubules form the mitotic spindle, a structure of protein fibers responsible for organizing and moving chromosomes during mitosis and meiosis.
• Microtubules are assembled during prophase.
• Microtubules are disassembled during anaphase.

Plant Toxins Interrupt Microtubules

• Colchicine: derived from autumn crocus (Colchicinium autumnale), binds to tubulin subunits and disrupts microtubule assembly, preventing cell division. Used medically in gout treatment.
• Taxol: derived from western yew (Taxus brevifolia), an anti-cancer medicine, which prevents microtubule disassembly, which halts cell division.

Phases of Mitosis

• Based on chromosome arrangement and cell structures.

Prophase (Early & Late)

• Chromatin condenses into chromosomes.
• Nucleoli disappear.
• Nuclear envelope fragments.
• Centrosomes move to opposite poles and the mitotic spindle forms.
• In late prophase, microtubules attach to kinetochores of the sister chromatids.

Metaphase

• Chromosomes align midway on mitotic plate (equator).

Anaphase

• Centromeres divide, separating sister chromatids.
• Chromatids move to opposite poles.

Telophase

• Chromosomes reach opposite poles.
• Nuclear envelopes reform—2 new nuclei form.
• Chromosomes begin to decondense.
• Cell furrow begins to develop. Cellular division is completed through cytokinesis.

Cytokinesis

• Cytoplasm divides, forming two identical daughter cells.
• In animals, a cleavage furrow forms.
• In plants, a cell plate forms to construct new cell walls.

Mitosis in Onion Cells

• Most onion root cells are in interphase during cell observation.

Microscopic Observations of Mitosis

• Pictures/micrographs of the various stages are included.

Plant Mitosis

• Plant cells lack centrioles.
• Plant cytokinesis involves cell plate formation.

You Try: Onion Root Slides

• Slides of different phases of mitosis in onion root cells

Chromosome Number: Ploidy (Again)

• For humans, haploid (n) number = 23, diploid (2n) = 46

Meiosis I

• Separation of homologous chromosome pairs.
• Prophase I: Homologous chromosomes pair and may exchange segments (crossing over). Tetrads form. Homologues are separated.
• Metaphase I: Tetrads line up at the metaphase plate.
• Anaphase I: Homologous chromosomes separate and move to opposite poles.
• Telophase I: Chromosomes arrive at poles, the nuclear membrane re-forms, and the cell divides through cytokinesis to form two daughter cells—each haploid, each with a (n) chromosome number.

Interkinesis

• Short period between meiosis I and meiosis II (No DNA Replication again).

Meiosis II

• Similar to mitosis, but separates sister chromatids
• Prophase II: Duplicated chromosomes condense and nuclear envelopes disintegrate.
• Metaphase II: Duplicated chromosomes line up along each cell's metaphase plate.
• Anaphase II: The sister chromatids separate and migrate to opposite poles.
• Telophase II: Nuclear Envelopes reform. The cell divides again via cytokinesis, resulting in 4 haploid daughter cells.

Gametogenesis: Sperm & Egg Production in Humans

• Meiosis produces gametes (sex cells–ova and sperm).

Timing of Meiosis for Female Gametogenesis

• First meiotic division initiated in the embryo but stops at prophase I;
• Secondary meiotic division (Meiosis II) starts after puberty (and is arrested again until fertilization).

Male Gametogenesis

• Meiosis I creates two secondary spermatocytes (haploid);
• Meiosis II creates 4 spermatids (haploid), which develop into sperm.

Summary of Gametogenesis

• Details about timing and processes of oogenesis and spermatogenesis – diagrams provided
• Summary of gametogenesis in plants (including alternation of generations)

Mitosis vs. Meiosis

• Mitosis: produces genetically identical daughter cells for growth, repair, and asexual reproduction.
• Meiosis: produces genetically diverse gametes, reducing the chromosome number.

Origins of Genetic Variation

• Differences in meiosis, including independent assortment (random orientation at Metaphase I) and crossing-over (exchange of genetic material at Prophase I).
  • Numbers for possible chromosome combinations resulting from multiple independent assortment events (for humans).

Abnormal Meiosis - Nondisjunction

• The failure of chromosomes to segregate properly during meiosis (either during meiosis I or meiosis II). Possible outcomes include trisomy or monosomy conditions.

• The effects of nondisjunctions

• Common nondisjunction-related syndromes, including Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY).

• Karyotyping: used to diagnosing nondisjunction-related disorders

Karyotyping

• Karyotyping is a technique used to visualize and analyze the chromosomes of an organism—used in diagnosing disorders.
• Steps involved.
• Analyzing banding patterns, satellite/centromere positions.

Human Karyotypes

• Diagrams depicting normal human karyotypes—autosomes, sex chromosomes.

Additional Information:

• Note that the information about the symptoms and frequency of genetic disorders are not to be memorized.
• Included are learning activities and resources to encourage active learning.

Test your knowledge on the stages of mitosis and the cell cycle with this quiz. From anaphase to cytokinesis, explore key concepts such as chromatin arrangement, telomere function, and cancer cell characteristics. Perfect for students studying biology and cell division.