Module Two: Cellular Basis of Heredity and Reproduction

Fundamental Classification of Cells

All living organisms are composed of one or more cells, which serve as the basic unit of life. These cells are broadly categorized into two types: prokaryotic and eukaryotic. Prokaryotic cells are found primarily in bacteria and cyanobacteria. In these cells, chromosomes are not enclosed within a membrane-bound nucleus and are instead concentrated in a specialized region of the cytoplasm known as the nucleoid. Eukaryotic cells, found in plants, animals, fungi, and protists, possess a distinct nucleus surrounded by a nuclear membrane. This membrane separates the nuclear material from the cytoplasmic material. Eukaryotic cells exhibit high complexity through various membrane-bound organelles such as the endoplasmic reticulum, mitochondria, Golgi apparatus, chloroplasts, and lysosomes. Despite these differences, all cells share four universal components: a plasma membrane, cytoplasm, DNA, and ribosomes.

Anatomy of the Eukaryotic Cell

The plasma membrane is a semi-permeable phospholipid bilayer that regulates the passage of molecules into and out of the cell. The nucleus is the largest and most conspicuous organelle, surrounded by a double membrane known as the nuclear membrane or envelope, which separates the cytoplasm from the nucleoplasm. Within the nuclear membrane are nuclear pores, which are tiny openings that allow the exchange of materials between the nucleus and the cytoplasm. Inside the nucleus, the nucleolus consists of spherical bodies responsible for manufacturing ribosomes. The nucleus also contains chromatin, a complex of DNA and histone proteins that form the chromosomes, which are threadlike structures containing the genetic information of the organism.

Cytoplasmic Organelles and the Cytoskeleton

The cytoplasm is the part of the cell surrounding the nucleus where various organelles perform active functions. Mitochondria are sausage-shaped or rod-shaped organelles enclosed by a double membrane; the inner membrane forms internal foldings called cristae. Mitochondria are termed the powerhouse of the cell because they provide energy through oxidative metabolism or aerobic respiration. The Golgi apparatus consists of stacks of flat membranous sacs involved in the packaging, modification, and transport of molecules. The endoplasmic reticulum (ER) is a network of tubules and flattened sacs, divided into two types: the smooth endoplasmic reticulum (SER) and the rough endoplasmic reticulum (RER). The SER lacks ribosomes and is involved in lipid and carbohydrate metabolism and detoxification. The RER has attached ribosomes and is the site of protein synthesis. Lysosomes are membrane-bound organelles containing digestive enzymes for various macromolecules. Ribosomes are small particles acting as the main sites of protein synthesis. They consist of two subunits with specific sedimentation coefficients. In bacteria, the subunits are $30S$ and $50S$. In eukaryotic plants and animals, the subunits are $40S$ and $60S$ respectively.

Additional specialized structures include the centriole, a cytoplasmic organelle involved in forming spindle fibers during cell division, and the chloroplast, which is enclosed by a two-layered membrane containing chlorophyll for photosynthesis. The cytoskeleton is a network of microtubules, intermediate filaments, and microfilaments that provides shape, strength, and anchors for organelles, while also allowing the cell to move and divide.

Chromosome Structure and Numerical Variations

Chromosomes contain the genetic material of organisms, and their number varies significantly between species. For instance, in humans (Homo sapiens), the diploid chromosome number is $2n = 46$. Other species examples include the Dog (Canis familiaris) at $2n = 48$, the Housefly (Musca domestica) at $2n = 12$, the Mouse (Mus musculus) at $2n = 40$, the Fruit fly (Drosophila melanogaster) at $2n = 8$, Rice (Oryza sativa) at $2n = 24$, Corn (Zea mays) at $2n = 20$, and the Cat (Felis catus) at $2n = 38$.

Structurally, a mitotic chromosome consists of several distinct parts. The centromere, or primary constriction, is a permanent, well-defined region. Associated with the centromere is the kinetochore, a protein that serves as the attachment point for spindle fibers during division. Secondary constrictions are narrow regions found at points other than the centromere. A satellite is a bulge at the telomeric end containing repetitive, heterochromatic DNA sequences. Finally, telomeres are regions of repetitive nucleotide sequences at each end of a chromatid that protect the chromosome from deterioration or fusion.

Classification of Chromosomes

Chromosomes are classified by size (small, medium, or large) and by the location of the centromere. Telocentric chromosomes have the centromere at the terminal end. Acrocentric chromosomes have the centromere near the terminal end. Submetacentric chromosomes have it near the center, and metacentric chromosomes have the centromere exactly at the center. Furthermore, chromosomes are divided into autosomes (somatic chromosomes) and sex chromosomes. Autosomes control somatic characteristics. In humans, there are $22$ pairs of autosomes and $2$ sex chromosomes. Individuals with $XX$ chromosomes are female, while those with $XY$ are male.

The Cell Cycle and Mitosis

The cell cycle is an ordered sequence of events from a cell's formation to its own division, consisting of interphase and the M phase. Interphase is the stage between divisions where the cell grows and functions. It includes the $G1$ phase (first gap), where the nucleus and cytoplasm enlarge and organelles form; the $S$ phase (synthesis), where DNA and histones are synthesized and chromosomes are duplicated, doubling the DNA amount; and the $G2$ phase (second gap), where biochemical events and protein/RNA synthesis occur for division.

The M phase involves mitosis (nuclear division or karyokinesis) and cytokinesis (cytoplasmic division). Mitosis is divided into four stages. During Prophase, chromosomes condense, centrosomes move apart, nucleoli disappear, and the nuclear envelope disintegrates. In Metaphase, chromosomes align at the equatorial plane. During Anaphase, the cohesin protein breaks down, centromeres divide, and sister chromatids separate toward opposite poles. Finally, in Telophase, nuclear envelopes form around each set of chromosomes at the poles, nucleoli reappear, and spindle microtubules disintegrate. Mitosis produces two genetically identical daughter cells with the full parental chromosome complement ($2n$).

Cytokinesis and Meiotic Division

Cytokinesis varies by cell type: in animal cells, a cleavage furrow forms from the periphery toward the center, whereas in plant cells, a cell plate forms from the center toward the periphery. Meiosis is the process required for sexual reproduction, occurring in gonads (testes/ovaries in animals; stamens/pistils in plants). Meiosis reduces the chromosome number from diploid ($2n$) to haploid ($n$) to maintain a constant number across generations upon fertilization. It consists of two successive division cycles: Meiosis I and Meiosis II.

Meiosis I (Reductional Division) includes Prophase I, the longest and most complex stage. Prophase I is subdivided into five sub-stages: Leptotene (chromosomes condense as thin threads with beadlike chromomeres); Zygotene (homologous chromosomes undergo synapsis to form bivalents and the synaptonemal complex); Pachytene (crossing over occurs at points called chiasmata between non-sister chromatids, providing genetic variation); Diplotene (synaptonemal complex disintegrates, and homologs separate except at chiasmata); and Diakinesis (further condensation, disappearance of the nuclear membrane and nucleolus, and spindle formation). Metaphase I involves bivalents aligning at the equator. In Anaphase I, homologs (univalents) separate. Telophase I results in two haploid nuclei.

Meiosis II (Equational Division) follows a process similar to mitosis but starting with haploid cells. It results in four haploid ($n$) nuclei. The genetic consequences of meiosis include the reduction of chromosome count and the generation of genetic differences through crossing over and random separation of chromosomes.

Reproduction in Flowering Plants and Animals

The flowering plant life cycle alternates between the diploid sporophyte and the haploid gametophyte. Microsporogenesis is the formation of male gametophytes (pollen grains) in microsporangia. A diploid microsporocyte undergoes meiosis to produce four haploid microspores, which then undergo mitosis to form a generative cell (which produces two sperm nuclei) and a tube cell. Megasporogenesis is the formation of the female gametophyte (embryo sac) in the ovule. A megasporocyte undergoes meiosis to produce four megaspores, though only one typically survives. The surviving megaspore undergoes three mitotic divisions to produce a large cell with $8$ nuclei: $3$ antipodal nuclei, $2$ polar nuclei, $1$ egg nucleus, and $2$ synergids. Double fertilization occurring after pollination results in a zygote ($2n$) and a triploid endosperm ($3n$).

In animals, gametogenesis produces mature reproductive cells. Spermatogenesis occurs in the seminiferous tubules, where a diploid spermatogonium develops into a primary spermatocyte, then into two haploid secondary spermatocytes via Meiosis I, then into four haploid spermatids via Meiosis II, which differentiate into functional spermatozoa. Oogenesis occurs in the ovary. A diploid oogonium develops into a primary oocyte. Through unequal division in Meiosis I, it produces a large secondary oocyte and a small first polar body. Meiosis II produces an ootid and a second polar body. The ootid differentiates into a single functional ovum, while polar bodies disintegrate. Fertilization of the ovum by a sperm carrying an $X$ or $Y$ chromosome determines the sex of the offspring.