The Cellular Basis of Reproduction and Inheritance: Mitosis, Meiosis, and Genetic Variation

Ch. 8 The Cellular Basis of Reproduction and Inheritance

Introduction to Cell Division

Cell division is a fundamental process by which a single parent cell divides to produce two or more daughter cells. This process is essential for reproduction, growth, and tissue repair in all living organisms.

• Types of Cell Division:

  • Binary Fission: A form of asexual cell division in prokaryotes, producing two genetically identical cells.

  • Mitosis: Eukaryotic cell division that produces two genetically identical somatic (body) cells. Human somatic cells are diploid (2n), containing two copies of each chromosome.

  • Meiosis: Eukaryotic cell division that produces haploid (n) gametes (sex cells), each with one copy of each chromosome.

• Importance: Cell division is crucial for reproduction, development, and tissue renewal.

Asexual vs. Sexual Reproduction

Organisms reproduce either asexually or sexually, each with distinct genetic outcomes.

• Asexual Reproduction: Involves one parent and produces genetically identical offspring (clones). Common in prokaryotes and some eukaryotes.

• Sexual Reproduction: Involves two parents, resulting in genetically diverse offspring due to the combination of genetic material from both parents.

Functions of Cell Division

Cell division serves several key roles in multicellular organisms:

• Reproduction: Producing new organisms or cells.

• Growth and Development: Increasing cell number during embryonic and fetal development.

• Tissue Repair and Renewal: Replacing damaged or dead cells to maintain tissue health.

Organization of DNA in the Cell

Genome Structure

The genome is the complete set of an organism's DNA, organized into chromosomes. DNA is packaged with proteins called histones to form nucleosomes, which further coil to create chromatin or condense into chromosomes during cell division.

• Chromatin: Loosely packed DNA-protein complex in non-dividing cells.

• Chromosomes: Highly condensed DNA-protein structures visible during cell division.

DNA Replication and Chromosome Structure

Before cell division, DNA is replicated to ensure each daughter cell receives an identical set of genetic material. Each replicated chromosome consists of two identical sister chromatids joined at a centromere.

• Chromatid: One of two identical halves of a replicated chromosome.

• Centromere: Region where sister chromatids are attached.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle describes the sequence of events in the life of a cell, from its formation to its division into daughter cells. It consists of interphase (cell growth and DNA replication) and the mitotic (M) phase (nuclear and cytoplasmic division).

• Interphase: Includes G1 (cell growth), S (DNA synthesis/replication), and G2 (preparation for mitosis).

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

• G0 Phase: A non-dividing state where cells exit the cycle and perform specialized functions.

Centrosomes and Mitotic Spindle

During the S phase, centrosomes are duplicated. Centrosomes organize the mitotic spindle, a structure made of microtubules that separates chromosomes during mitosis.

• Centrosome: Microtubule-organizing center in animal cells.

• Mitotic Spindle: Array of microtubules that segregates chromosomes during cell division.

Mitosis: Phases and Mechanisms

Overview of Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and genetic material, resulting in two genetically identical daughter cells. It consists of five phases: prophase, prometaphase, metaphase, anaphase, and telophase.

Detailed Phases of Mitosis

• Prophase: Chromatin condenses into visible chromosomes; nucleolus disappears; centrosomes move to opposite poles and begin forming the spindle.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores on chromosomes.

• Metaphase: Chromosomes align at the metaphase plate (cell equator).

• Anaphase: Sister chromatids are separated and pulled toward opposite poles by the spindle fibers.

• Telophase: Chromosomes decondense; nuclear envelope and nucleolus reform; spindle disassembles, resulting in two nuclei.

Cytokinesis

Cytokinesis is the division of the cytoplasm, following mitosis, to produce two separate daughter cells. The mechanism differs between animal and plant cells.

• Animal Cells: Formation of a cleavage furrow by actin and myosin filaments pinches the cell in two.

• Plant Cells: Vesicles from the Golgi apparatus form a cell plate that develops into a new cell wall, separating the daughter cells.

Regulation of the Cell Cycle

Cell Cycle Checkpoints

Cell division is tightly regulated by checkpoints that ensure the cell is ready to proceed to the next phase. Checkpoints are controlled by proteins such as growth factors and tumor suppressors (e.g., p53).

• Major Checkpoints:

  • G1 Checkpoint: Checks for DNA damage before replication.

  • G2 Checkpoint: Ensures DNA is fully replicated and undamaged before mitosis.

  • M (Metaphase) Checkpoint: Confirms all chromosomes are properly attached to the spindle before separation.

• Apoptosis: Programmed cell death triggered if errors cannot be repaired.

Genes, Alleles, and Chromosome Number

Genes and Alleles

Genes are segments of DNA that encode proteins and determine traits. Alleles are alternative forms of a gene found at the same locus on homologous chromosomes.

• Alleles: Represented by different letters (e.g., B for blue eyes, b for brown eyes).

Haploid vs. Diploid Cells

Cell ploidy refers to the number of chromosome sets in a cell.

• Haploid (n): One set of chromosomes (e.g., gametes).

• Diploid (2n): Two sets of chromosomes (e.g., somatic cells).

Homologous Chromosomes and Karyotypes

Homologous chromosomes are pairs that are similar in size, shape, and gene content but may have different alleles. A karyotype is an ordered display of all chromosomes in a cell.

• Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY).

Meiosis and Sexual Life Cycles

Overview of Meiosis

Meiosis is a two-part cell division process that produces four genetically diverse haploid gametes from a diploid germ cell. It is essential for sexual reproduction and genetic diversity.

• Meiosis I (Reductional Division): Homologous chromosomes separate, reducing chromosome number by half.

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

Genetic Variation in Meiosis

Meiosis generates genetic diversity through two main mechanisms:

• Crossing Over: Exchange of genetic material between non-sister chromatids during prophase I, resulting in recombinant chromosomes.

• Independent Assortment: Random alignment and separation of homologous chromosome pairs during metaphase I, leading to numerous possible genetic combinations. The number of possible combinations is $2^n$, where n is the haploid number of chromosomes.

Nondisjunction

Nondisjunction is an error in meiosis when chromosomes fail to separate properly, resulting in aneuploid cells (cells with abnormal chromosome numbers). This can lead to genetic disorders such as Down syndrome (trisomy 21).

Comparison of Mitosis and Meiosis

• Mitosis: Occurs in somatic cells, produces two genetically identical diploid cells, used for growth and repair.

• Meiosis: Occurs in germ cells, produces four genetically diverse haploid gametes, used for sexual reproduction.

• Key Differences: Homologous chromosomes pair and undergo crossing over in meiosis I, but not in mitosis. Cytokinesis occurs twice in meiosis but only once in mitosis.

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