Cell Cycle and Mitosis

Purpose of Cell Division

Cell division is a fundamental process in all living organisms, enabling growth, tissue repair, development, and reproduction. In eukaryotes, this process is tightly regulated to ensure genetic stability.

• Growth and Development: Multicellular organisms grow and develop from a single cell through repeated cell divisions.

• Tissue Repair: Damaged or dead cells are replaced by new cells produced via division.

• Asexual Reproduction: Some organisms reproduce by mitotic division, producing genetically identical offspring.

• Basic Steps of Cellular Replication:

  1. Copying the DNA

  2. Separating the DNA copies

  3. Dividing the cytoplasm (cytokinesis)

The Eukaryotic Cell Cycle

The cell cycle describes the ordered sequence of events that a cell undergoes from its formation to its division into two daughter cells.

• Interphase: The phase where the cell grows, performs its normal functions, and prepares for division. It consists of three subphases:

  • G1 (First Gap): Cell growth, organelle replication, and normal metabolic roles. Cells may enter a non-dividing state called G0.

  • S (Synthesis) Phase: DNA is replicated, resulting in duplicated chromosomes.

  • G2 (Second Gap): Further growth and protein synthesis to prepare for mitosis.

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

Chromosome Structure and Mitosis

Chromosomes are the carriers of genetic information, composed of DNA and associated proteins.

• Chromosome: A single, long double helix of DNA wrapped around histone proteins.

• Replication: Before mitosis, each chromosome is duplicated, forming two sister chromatids joined at a centromere.

• Mitosis Subphases:

  1. Prophase: Chromosomes condense, spindle apparatus forms.

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

  3. Metaphase: Chromosomes align at the metaphase plate.

  4. Anaphase: Sister chromatids separate and move to opposite poles.

  5. Telophase: Nuclear envelopes reform around the two sets of chromosomes.

• Cytokinesis:

  • Animal Cells: Actin-myosin ring contracts to form a cleavage furrow, splitting the cell.

  • Plant Cells: Vesicles guided by microtubules form a cell plate that develops into a new cell wall.

• Bacterial Division: Bacteria divide by binary fission, a simpler process without mitosis.

Cell Cycle Control and Cancer

Cell cycle progression is regulated by checkpoints to ensure proper division and prevent errors.

• Checkpoints: Occur at G1, G2, and M phases, monitoring cell size, nutrient availability, DNA integrity, and spindle attachment.

• Cancer: Results from checkpoint failures, leading to uncontrolled cell division and tumor formation.

  • Benign Tumors: Non-invasive and localized.

  • Malignant Tumors: Invasive, can spread (metastasize) to distant tissues.

Meiosis

DNA Structure Basics

Nucleic acids, such as DNA, are polymers made of nucleotide monomers. Each nucleotide consists of:

• A five-carbon sugar (deoxyribose in DNA)

• A phosphate group attached to the 5' carbon

• A nitrogenous base attached to the 1' carbon

Principles of Meiosis and Ploidy

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes for sexual reproduction.

• Ploidy:

  • Haploid (n): One set of chromosomes.

  • Diploid (2n): Two sets of chromosomes (one from each parent).

  • Autosomes: Non-sex chromosomes.

  • Sex Chromosomes: Determine the sex of an individual (e.g., X and Y in humans).

• Meiosis I: Homologous chromosomes separate, reducing chromosome number (reduction division).

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

• Genetic Variation:

  • Synapsis and Crossing Over: Homologous chromosomes exchange genetic material during Prophase I (chiasmata formation).

  • Independent Assortment: Random orientation of homologous pairs during Metaphase I.

  • Random Fertilization: Any sperm can fertilize any egg, increasing genetic diversity.

• Nondisjunction: Failure of homologs or sister chromatids to separate properly, resulting in aneuploidy (e.g., Down syndrome).

DNA Synthesis (Replication)

Structure and Replication of DNA

DNA is the hereditary material in all living organisms. Its structure and replication were elucidated through key experiments and observations.

• Hershey-Chase Experiment (1952): Demonstrated that DNA, not protein, is the genetic material.

• Chargaff's Rules: In DNA, the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C).

• Complementary Base Pairing: A pairs with T, G pairs with C via hydrogen bonds.

• Antiparallel Strands: DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Semi-Conservative Replication: Each new DNA molecule consists of one old and one new strand.

Proteins and Mechanism of Replication

DNA replication is a complex process involving multiple enzymes and proteins to ensure accurate copying of genetic information.

• Helicase: Unwinds and separates the DNA double helix.

• Single-Strand DNA-Binding Proteins (SSBPs): Stabilize unwound DNA strands.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

• Primase: Synthesizes short RNA primers to initiate DNA synthesis.

• DNA Polymerase III: Extends DNA from the primer in the 5' to 3' direction; synthesizes both leading and lagging strands.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA.

• DNA Ligase: Joins Okazaki fragments on the lagging strand.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork in short Okazaki fragments.

• Proofreading and Repair: DNA polymerases have proofreading activity; mismatch repair enzymes correct errors on the newly synthesized (unmethylated) strand.

Mendelian Genetics

Core Vocabulary

• Allele: A variant form of a gene.

• Genotype: The combination of alleles an individual possesses (homozygous or heterozygous).

• Phenotype: Observable physical traits determined by genotype.

• Locus (plural: Loci): The specific location of a gene on a chromosome.

Mendel’s Principles

Gregor Mendel's experiments with pea plants established the foundational principles of inheritance.

• Principle of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete receives only one allele.

  • Monohybrid Cross: Cross between individuals differing in one trait; F2 generation shows a 3:1 phenotypic ratio.

• Principle of Independent Assortment: Alleles of different genes assort independently during gamete formation.

  • Dihybrid Cross: Cross involving two traits; F2 generation shows a 9:3:3:1 phenotypic ratio.

• Chromosome Theory of Inheritance: Genes are located on chromosomes, which segregate and assort independently during meiosis.

• Exceptions to Mendel's Rules:

  • Gene Linkage: Genes located close together on the same chromosome tend to be inherited together.

  • Sex-Linked Traits: Traits determined by genes on sex chromosomes.

  • Incomplete Dominance: Heterozygotes show an intermediate phenotype.

  • Codominance: Both alleles are expressed equally in the phenotype.

  • Multiple Allelism: More than two alleles exist for a gene in a population.

  • Pleiotropy: One gene affects multiple traits.

  • Polygenic Traits: Traits controlled by multiple genes, often showing continuous variation.

Key Ratios and Equations

• Monohybrid Cross Phenotypic Ratio:

• Dihybrid Cross Phenotypic Ratio:

• Complementary Base Pairing:

• Reduction Division in Meiosis:

Table: Comparison of Mitosis and Meiosis

Example Applications

• Down Syndrome: Caused by nondisjunction leading to trisomy 21 (an extra chromosome 21).

• Blood Types: Exhibit multiple allelism (A, B, O alleles) and codominance (A and B both expressed in AB blood type).

• Cancer: Results from mutations that disrupt cell cycle control, leading to unchecked cell division.

Additional info: The above notes expand on the provided study guide by including definitions, examples, and a comparison table for mitosis and meiosis to enhance understanding and exam preparation.