Cell Division, Cell Cycle, Mitosis, Meiosis, and Chromosomal Disorders: Study Guide

Cell Division & Reproduction

Asexual vs. Sexual Reproduction

Organisms reproduce either asexually or sexually, each with distinct mechanisms and outcomes for genetic diversity.

• Asexual Reproduction: Involves a single parent producing genetically identical offspring (clones). No gametes are involved. Examples: binary fission in bacteria, budding in yeast.

• Sexual Reproduction: Involves the fusion of gametes (egg and sperm), resulting in genetically unique offspring. Increases genetic variation through mechanisms such as crossing over and independent assortment during meiosis.

Key Terms: Gametes (sex cells), Zygote (fertilized egg, first cell of a new organism).

Binary Fission (Prokaryotic Cell Division)

Binary fission is the primary method of reproduction in prokaryotes (bacteria and archaea).

• Chromosome replicates (usually a single circular chromosome).

• Cell elongates and divides into two identical daughter cells.

• No mitosis or meiosis involved.

Cell Cycle & Mitosis

Chromatin vs. Chromosomes

DNA exists in two forms within the cell, depending on the stage of the cell cycle.

• Chromatin: Uncoiled, loose DNA wrapped around histone proteins; present during interphase.

• Chromosomes: Tightly packed, condensed DNA visible during cell division (mitosis/meiosis).

• Sister Chromatids: Two identical copies of a chromosome joined at the centromere after DNA replication.

Cell Cycle Stages

The cell cycle is the sequence of growth and division that a cell undergoes.

• Interphase: Includes G1 (Gap 1), S (Synthesis), and G2 (Gap 2) phases.

• G1 Phase: Cell grows, performs normal functions.

• S Phase: DNA is replicated.

• G2 Phase: More growth, preparation for mitosis, and error checking.

• M Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis (M Phase) & Cytokinesis

Mitosis produces two genetically identical daughter cells. It is divided into several stages:

1. Prophase: Chromatin condenses into chromosomes; spindle apparatus begins to form.

2. Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.

3. Metaphase: Chromosomes align at the metaphase plate (center of the cell).

4. Anaphase: Sister chromatids separate and move to opposite poles.

5. Telophase: Chromosomes arrive at poles; nuclear envelope reforms; chromosomes decondense.

Cytokinesis: Division of the cytoplasm. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.

Cell Cycle Regulation

Cell division is tightly regulated to ensure proper growth and development.

• Anchorage Dependence: Cells must be attached to a solid surface to divide.

• Density-Dependent Inhibition: Cells stop dividing when crowded.

• Growth Factors: External signals that stimulate cell division.

• Checkpoints: Control points in the cell cycle (G1, G2, M) where the cell assesses whether to proceed with division.

G0 Phase: A non-dividing state; some cells (e.g., neurons, muscle cells) remain here permanently.

Cancer

Cancer results from the loss of cell cycle control, leading to uncontrolled cell division.

• Benign Tumor: Abnormal mass of cells that does not invade other tissues.

• Malignant Tumor: Can invade other tissues and metastasize (spread to other parts of the body).

• Cancer Treatments: Surgery, radiation, chemotherapy, immunotherapy.

Meiosis & Sexual Life Cycles

Homologous Chromosomes

Pairs of chromosomes with the same genes but possibly different alleles (versions of a gene). Humans have 23 pairs (46 chromosomes).

Purpose of Meiosis

Meiosis produces gametes (sperm and egg) with half the number of chromosomes (haploid, n) to maintain chromosome number across generations.

• Somatic cells: Diploid (2n = 46 in humans)

• Gametes: Haploid (n = 23 in humans)

• Zygote: Diploid (2n) formed by fertilization

Steps of Meiosis

Meiosis consists of two divisions: Meiosis I and Meiosis II.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate (similar to mitosis).

Key Stages:

1. Prophase I: Homologous chromosomes pair up (synapsis); crossing over occurs, increasing genetic variation.

2. Metaphase I: Tetrads align at the metaphase plate; orientation is random (independent assortment).

3. Anaphase I: Homologous chromosomes separate.

4. Telophase I: Two haploid cells form, each with duplicated chromosomes.

5. Meiosis II: Sister chromatids separate, resulting in four unique haploid gametes.

Sources of Genetic Variation

• Crossing Over: Exchange of DNA between homologous chromosomes during Prophase I.

• Independent Assortment: Random orientation of chromosome pairs during Metaphase I.

• Random Fertilization: Any sperm can fertilize any egg, resulting in a huge number of possible genetic combinations.

Formula for possible gamete combinations:

$2^n$ (where n = haploid number of chromosomes)

Chromosomal Disorders

Nondisjunction

Occurs when chromosomes fail to separate properly during meiosis, leading to abnormal chromosome numbers in gametes.

• Trisomy (2n + 1): Extra chromosome (e.g., Down syndrome, Trisomy 21)

• Monosomy (2n - 1): Missing a chromosome (e.g., Turner syndrome, XO)

Common Disorders

• Down Syndrome: Trisomy 21 (three copies of chromosome 21)

• Sex Chromosome Abnormalities:

  • Turner Syndrome: XO

  • Klinefelter Syndrome: XXY

  • Triple-X: XXX

  • Jacob's Syndrome: XYY

People with extra X chromosomes survive because of X-inactivation (one X is inactivated in each cell).

Chromosomal Alterations

• Deletion: Loss of a chromosome fragment.

• Duplication: Extra copy of a chromosome segment.

• Inversion: Segment reattaches in reverse orientation.

• Translocation: Segment attaches to a non-homologous chromosome.

Drug/Therapy Connection: Methotrexate

Methotrexate is a chemotherapy drug that blocks the synthesis of thymine, a DNA base, preventing DNA replication and cell division. Without thymine, cells cannot replicate DNA, cannot divide, and rapidly dividing cells (like cancer cells) are most affected.

Key Vocabulary and Definitions

Term	Definition
Asexual reproduction	Reproduction involving one parent that produces genetically identical offspring.
Sexual reproduction	Reproduction involving gametes (egg and sperm) that fuse to form genetically unique offspring.
Gametes	Sex cells (sperm or egg) containing half the number of chromosomes (haploid).
Zygote	The first cell of a new organism, formed when sperm and egg fuse.
Chromatin	Loose, uncondensed DNA wrapped around proteins; present when the cell is not dividing.
Chromosome	Tightly packed DNA, visible during cell division.
Sister chromatids	Identical copies of a chromosome joined together at the centromere.
Homologous chromosomes	Chromosomes with the same genes and structure, but possibly different alleles.
Allele	Different versions of a gene (e.g., blue vs. brown eye color).
Cell cycle	The repeating sequence of growth and division a cell undergoes.
Interphase	Stage of the cell cycle when the cell grows and prepares for division (G1, S, G2).
Mitosis	Division of the nucleus that produces two genetically identical daughter cells.
Cytokinesis	Division of the cytoplasm following mitosis.
Growth factors	External signal molecules that stimulate cell division.
Anchorage dependence	Requirement that cells must be attached to a surface to divide.
Density-dependent inhibition	Cells stop dividing when crowded.
Checkpoint	A control point in the cell cycle where "go" or "stop" signals are given.
Cancer	Disease caused by uncontrolled cell division.
Benign tumor	A mass of abnormal cells that does not invade other tissues.
Malignant tumor	A cancerous mass that can invade and spread to other tissues.
Metastasis	The spread of cancer cells to other parts of the body.
Meiosis	Type of cell division that reduces chromosome number by half to form gametes.
Diploid (2n)	A cell with two sets of chromosomes (somatic cells).
Haploid (n)	A cell with one set of chromosomes (gametes).
Synapsis	Pairing of homologous chromosomes during prophase I of meiosis.
Tetrad	A group of four chromatids formed when homologous chromosomes pair up.
Crossing over	Exchange of DNA between homologous chromosomes, increasing genetic variation.
Chiasma (plural: chiasmata)	The physical location where crossing over occurs.
Independent assortment	Random orientation of chromosome pairs during metaphase I.
Nondisjunction	Failure of chromosomes to separate properly during meiosis.
Trisomy	A cell with an extra chromosome (2n + 1).
Monosomy	A cell missing a chromosome (2n - 1).
Karyotype	An organized display of chromosomes, used to identify abnormalities.
Deletion	Loss of a chromosome segment.
Duplication	Extra copy of a chromosome segment.
Inversion	A segment reattaches in reverse orientation.
Translocation	A chromosome piece attaches to a nonhomologous chromosome.
Methotrexate	A drug that blocks thymine production, preventing DNA replication; used in cancer treatment.

Example: Down Syndrome

Down syndrome is caused by trisomy 21, where an individual has three copies of chromosome 21. This results from nondisjunction during meiosis.

Example: Cancer Cell Cycle Defects

Cancer cells often ignore density-dependent inhibition and anchorage dependence, dividing uncontrollably and forming tumors.

Additional info:

• Meiosis increases genetic diversity through crossing over, independent assortment, and random fertilization.

• Checkpoints in the cell cycle (G1, G2, M) are critical for preventing the division of damaged or incomplete cells.

• Chromosomal alterations can be detected using karyotyping and prenatal screening techniques such as amniocentesis and chorionic villus sampling (CVS).