Cell Division: Reproduction, Chromosomes, Mitosis, Meiosis, and Viral Replication

Cell Division

Asexual and Sexual Reproduction

Organisms reproduce to ensure the continuation of their species and to pass genetic information to the next generation. Reproduction can occur through asexual or sexual means, each with distinct biological implications.

• Asexual Reproduction: Involves a single parent and produces genetically identical offspring. Common in prokaryotes and some eukaryotes.

• Sexual Reproduction: Involves two parents and the fusion of gametes, resulting in genetically diverse offspring.

Types of Asexual Reproduction:

Binary Fission	Budding
Parent cell divides into two equal parts; common in bacteria.	New organism grows from the body of the parent; seen in yeast and hydra.

• Other Types: Fragmentation, spore formation, vegetative propagation.

Sexual Reproduction: Involves specialized cells called gametes (sperm and egg). Fertilization restores the diploid chromosome number.

Chromosomes: Structures made of DNA and proteins that carry genetic information. Humans have 46 chromosomes in somatic cells.

Prekaryotic vs Eukaryotic DNA

Prokaryotic	Eukaryotic
Circular DNA, usually one chromosome, found in the nucleoid region.	Linear DNA, multiple chromosomes, contained within a nucleus.

Chromosome Structure

• Chromatid: Each half of a duplicated chromosome.

• Centromere: Region where sister chromatids are joined.

• Telomere: Protective ends of chromosomes.

Karyotypes

• Diploid Cells (2n): Contain two sets of chromosomes (somatic cells).

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

• Karyotype: A visual profile of all the chromosomes in a cell, used to detect chromosomal abnormalities.

• Limitations: Cannot detect small mutations or gene-level changes.

Cell Replication and the Cell Cycle

Purpose of Cell Replication

Cell replication allows organisms to grow, repair tissues, and reproduce. In developing organisms, it enables growth; in adults, it maintains and repairs tissues.

The Cell Cycle

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

• S Phase: DNA is replicated.

• G2 Phase: Cell prepares for division.

• M Phase (Mitosis): Division of the nucleus and cytoplasm.

Mitosis

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells.

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform around chromosomes.

• Cytokinesis: Cytoplasm divides, forming two cells.

Cell Cycle Checkpoints

• Ensure proper division and prevent errors.

• Major checkpoints: G1, G2, and M phase checkpoints.

Cancer

• Results from uncontrolled cell division due to checkpoint failures or mutations in regulatory genes.

Meiosis

Purpose

Meiosis reduces chromosome number by half, producing haploid gametes and increasing genetic diversity through recombination.

Crossing Over

• Exchange of genetic material between homologous chromosomes during Prophase I, increasing genetic variation.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate.

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

Spermatogenesis and Oogenesis

• Spermatogenesis: Formation of sperm cells.

• Oogenesis: Formation of egg cells.

Mitosis vs Meiosis

Mitosis	Meiosis
One division, two identical cells, diploid.	Two divisions, four unique cells, haploid.

Chromosomal Mutation

• Dangers: Can cause genetic disorders, cancer, or cell death.

• Types of Mutations: Deletion, duplication, inversion, translocation, nondisjunction.

Nondisjunction

• Failure of chromosomes to separate properly during meiosis, leading to abnormal chromosome numbers in gametes.

Genetic Disorders Caused by Nondisjunction

• Examples: Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY).

Viral Replication

Bacteria Review

• Prokaryotic Cells: Lack a nucleus, have circular DNA.

• Eukaryotic Cells: Have a nucleus, linear DNA.

Viral Structure

• Composed of genetic material (DNA or RNA) and a protein coat (capsid).

• Some have an envelope derived from host cell membranes.

Bacteriophage

• Definition: Viruses that infect bacteria.

• Structure: Head (capsid), tail, tail fibers.

• Role: Can be used in genetic engineering and as antibacterial agents.

Viral Reproduction

• Lytic Cycle: Virus takes over host machinery, produces new viruses, and lyses the cell.

• Lysogenic Cycle: Viral DNA integrates into host genome and replicates with it; can later enter lytic cycle.

Lytic	Lysogenic
Immediate replication and destruction of host cell.	Viral DNA remains dormant in host genome.

• Comparison: Lytic cycle is rapid and destructive; lysogenic cycle is latent and can persist for generations.

Example: Bacteriophage lambda can undergo both cycles in Escherichia coli.