Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2 phases) and the mitotic (M) phase, which includes mitosis and cytokinesis.

• G1 Phase: Cell grows and carries out normal functions.

• S Phase: DNA replication occurs, doubling the genetic material.

• G2 Phase: Cell prepares for division by producing necessary proteins and organelles.

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

Example: In human cells, the cell cycle ensures that each daughter cell receives a complete set of chromosomes.

Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two genetically identical daughter cells.

• Prophase: Chromosomes condense, spindle fibers form, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Nuclear envelopes reform around each set of chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Formula:

Example: If a cell starts with 46 chromosomes, each daughter cell will also have 46 chromosomes after mitosis.

Homologous Chromosomes vs. Sister Chromatids

Homologous chromosomes are pairs of chromosomes (one from each parent) that have the same genes but may have different alleles. Sister chromatids are identical copies of a single chromosome, connected by a centromere, formed during DNA replication.

• During metaphase of mitosis, sister chromatids align at the equator.

• During metaphase I of meiosis, homologous chromosomes align together.

Cytokinesis

Cytokinesis is the division of the cytoplasm following mitosis or meiosis, resulting in two separate daughter cells.

• In animal cells, a cleavage furrow forms to split the cell.

• In plant cells, a cell plate forms to divide the cell.

Major differences: Plant cells build a new cell wall, while animal cells pinch inwards.

Meiosis and Sexual Reproduction

Overview of Meiosis

Meiosis is a type of cell division that reduces the chromosome number by half, producing four genetically unique gametes (sperm or egg cells). It consists of two consecutive divisions: meiosis I and meiosis II.

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

• Meiosis II: Sister chromatids separate, similar to mitosis.

Formula:

Example: Human gametes have 23 chromosomes, half the number of somatic cells.

Genetic Variation in Meiosis

Meiosis introduces genetic variation through independent assortment and crossing-over.

• Independent Assortment: Random distribution of maternal and paternal chromosomes to gametes.

• Crossing-over: Exchange of genetic material between homologous chromosomes during prophase I.

Significance: These processes ensure each gamete is genetically unique.

Key Terms in Sexual Reproduction

• Allele: Different forms of a gene.

• Fertilization: Fusion of gametes to form a zygote.

• Diploid (2n): Cells with two sets of chromosomes.

• Haploid (n): Cells with one set of chromosomes (gametes).

• Zygote: Fertilized egg cell.

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to disorders like Down's syndrome.

• Monosomy: Condition of having only one copy of a chromosome.

Special Topics

Oncogenes and Growth Factors

Oncogenes are mutated genes that can cause uncontrolled cell division, leading to cancer. Growth factors are proteins that stimulate cell division.

• Normal cells respond to growth factors to divide only when needed.

• Oncogenes can override normal controls, causing tumors.

Down's Syndrome and Chromosomal Disorders

Down's syndrome is caused by trisomy 21, where an individual has three copies of chromosome 21 due to nondisjunction during meiosis.

• Extra chromosome is passed during gamete formation.

• Fertilization results in a zygote with 47 chromosomes.

Comparison Table: Mitosis vs. Meiosis

Stages of Meiosis (with Key Events)

Additional info:

• Cell cycle checkpoints (G1, G2, M) ensure proper division and prevent errors.

• Errors in meiosis (nondisjunction) can lead to genetic disorders.

• Chromosome number in humans: 46 (diploid), 23 (haploid).