Cell Division and Reproduction: Mitosis, Meiosis, and Chromosomal Alterations

Cell Division and Reproduction

Asexual and Sexual Reproduction

Cell division is fundamental for the continuity of life, enabling organisms to reproduce, grow, repair tissues, and maintain their genetic integrity. There are two main types of reproduction:

• Asexual reproduction: Produces offspring genetically identical to the parent, involving only one parent. Common in single-celled organisms and for growth and repair in multicellular organisms.

• Sexual reproduction: Produces genetically varied offspring by combining genes from two parents, involving the formation of gametes (sperm and egg).

Prokaryotic Cell Division: Binary Fission

Mechanism of Binary Fission

Prokaryotes (bacteria and archaea) reproduce by binary fission, a process that results in two genetically identical daughter cells. The prokaryotic chromosome is a single, circular DNA molecule.

• Step 1: Chromosome duplication and separation of copies

• Step 2: Cell elongation and movement of chromosome copies

• Step 3: Division into two daughter cells

Example: Escherichia coli dividing by binary fission.

Eukaryotic Cell Cycle and Mitosis

Structure of Eukaryotic Chromosomes

Eukaryotic cells are more complex, containing multiple linear chromosomes within a nucleus. Chromosomes are composed of chromatin (DNA and proteins).

• Before division, chromosomes duplicate, forming sister chromatids joined at a centromere.

• During cell division, sister chromatids separate, ensuring each daughter cell receives an identical set of chromosomes.

The Cell Cycle

The cell cycle is an ordered sequence of events from the formation of a cell to its own division. It consists of:

• Interphase: Cell growth and chromosome duplication (G1, S, G2 phases)

• Mitotic (M) phase: Division of the nucleus (mitosis) and cytoplasm (cytokinesis)

Phases of Mitosis

Mitosis is divided into several stages:

• Prophase: Chromosomes condense, spindle forms

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores

• Metaphase: Chromosomes align at the cell equator

• Anaphase: Sister chromatids separate and move to opposite poles

• Telophase: Nuclear envelopes reform, chromosomes decondense

Cytokinesis

Cytokinesis divides the cytoplasm, resulting in two daughter cells. The process differs between animal and plant cells:

• Animal cells: Formation of a cleavage furrow by a contractile ring of microfilaments.

• Plant cells: Formation of a cell plate from vesicles, which develops into a new cell wall.

Functions of Mitosis

• Growth of multicellular organisms

• Repair and replacement of damaged cells

• Asexual reproduction in some organisms

Meiosis and Sexual Life Cycles

Homologous Chromosomes and Chromosome Number

Somatic cells are diploid (2n), containing pairs of homologous chromosomes. Gametes are haploid (n), containing one set of chromosomes.

Overview of Meiosis

Meiosis reduces the chromosome number by half, producing four genetically unique haploid gametes from a diploid cell. It consists of two divisions: meiosis I and meiosis II.

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate

  • Prophase I: Synapsis and crossing over form tetrads

  • Metaphase I: Tetrads align at the equator

  • Anaphase I: Homologous chromosomes separate

  • Telophase I and Cytokinesis: Two haploid cells form

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

  • Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis

Comparison of Mitosis and Meiosis

• Mitosis: Produces two genetically identical diploid cells for growth and repair.

• Meiosis: Produces four genetically unique haploid gametes for sexual reproduction.

Genetic Variation in Sexual Reproduction

Sources of Genetic Variation

• Independent assortment: Random orientation of homologous pairs during metaphase I creates many possible combinations (2n where n = haploid number).

• Random fertilization: Any sperm can fertilize any egg, increasing variability.

• Crossing over: Exchange of genetic material between nonsister chromatids during prophase I produces recombinant chromosomes.

Alterations of Chromosome Number and Structure

Nondisjunction

Nondisjunction is the failure of chromosomes or chromatids to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers. Fertilization involving such gametes leads to aneuploidy (e.g., trisomy or monosomy).

Karyotyping

A karyotype is an ordered display of an individual's chromosomes, used to detect chromosomal abnormalities.

Down Syndrome (Trisomy 21)

Down syndrome is caused by an extra copy of chromosome 21. Symptoms include intellectual disability, characteristic facial features, heart defects, and increased risk of certain diseases. The risk increases with maternal age.

Sex Chromosome Abnormalities

Abnormal numbers of sex chromosomes (e.g., XXY, XYY, XXX, XO) usually have less severe effects due to X-inactivation and the small size of the Y chromosome.

Sex Chromosomes	Syndrome	Origin of Nondisjunction	Frequency in Population
XXY	Klinefelter syndrome (male)	Meiosis in egg or sperm formation	1/2,000
XYY	None (normal male)	Meiosis in sperm formation	1/2,000
XXX	None (normal female)	Meiosis in egg or sperm formation	1/1,000
XO	Turner syndrome (female)	Meiosis in egg or sperm formation	1/5,000

Polyploidy and Chromosomal Rearrangements

• Polyploidy: Organisms with more than two sets of chromosomes, common in plants.

• Chromosomal rearrangements: Deletions, duplications, inversions, and translocations can cause genetic disorders or cancer.

Example: Chronic myelogenous leukemia (CML) results from a reciprocal translocation between chromosomes 9 and 22, creating the "Philadelphia chromosome." Cancer is usually not inherited because it often arises from somatic cell mutations.