Cell Divison

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION

• Asexual Reproduction:

  • Mechanism: Chromosomes are duplicated followed by cell division.

  • Result: Each daughter cell is genetically identical to the parent and each other.

• Sexual Reproduction:

  • Mechanism: Offspring inherit a unique gene combination from both parents.

  • Result: Greater genetic variation among offspring.

CELL DIVISION PRINCIPLES

• Preexisting Cells Concept:

  • Quote: "Every cell from a cell" supports life perpetuation.

  • Functions: Enables unicellular organism reproduction, allows multicellular growth, and renewal.

PROKARYOTIC REPRODUCTION

• Binary Fission:

  • Process:

  • Genes are located on a single circular DNA molecule.

  • Cell replicates DNA, elongates, and begins division as the plasma membrane grows inward.

  • Result: Two genetically identical daughter cells.

EUKARYOTIC CELL CYCLE AND MITOSIS

• Chromosome Duplication: Eukaryotic chromosomes are complex, composed of DNA and proteins, mostly in the form of chromatin.

• Cell Cycle Stages:

  • Interphase:

  • G1 phase: Cellular growth.

  • S phase: DNA synthesis (replication).

  • G2 phase: Further growth and preparation for division.

  • Mitotic Phase:

  • Mitosis: Division of the nucleus.

  • Cytokinesis: Division of cytoplasm, distributing chromosomes evenly into daughter cells.

MITOTIC STAGES

• Mitosis Stages:

  • Prophase: Mitotic spindle starts forming; centrosomes move apart.

  • Prometaphase: Chromatins condense to chromosomes; nuclear envelope breaks down.

  • Metaphase: Chromosomes align on metaphase plate, attached to spindle fibers.

  • Anaphase: Sister chromatids separate to opposite poles.

  • Telophase: Nuclear envelopes reform around separated chromosomes; chromosomes uncoil.

• Cytokinesis:

  • Animal cells: Cleavage furrow forms due to microfilament contraction.

  • Plant cells: Formation of a new cell wall via vesicle fusion.

CONTROL OF CELL DIVISION

• Factors Affecting Division:

  • Anchorage dependence: Cells need to contact a surface to divide.

  • Density-dependent inhibition: Cell division stops when a single layer is formed or cells touch each other.

  • Growth factors signal cell cycle control system to regulate division.

CANCER CELL BEHAVIOR

• Defective Checkpoints:

  • Cancer cells divide uncontrollably, potentially leading to tumors.

• Types of Tumors:

  • Benign: remain localized.

  • Malignant: invade other tissues; can metastasize.

MEIOSIS AND GENETIC VARIATION

• Gametes Formation:

  • Somatic cells: Diploid (2n), paired chromosomes.

  • Gametes: Haploid (n), single chromosome set.

  • Meiosis involves two rounds of division to reduce chromosome number and creates gametes with genetic variation through independent orientation and crossing over.

COMPARISON OF MITOSIS AND MEIOSIS

• Mitosis:

  • Functions in growth, repair, and asexual reproduction.

  • Produces genetically identical daughter cells.

• Meiosis:

  • Functions in sexual reproduction.

  • Produces genetically varied daughter cells, essential for genetic diversity in offspring.

GENETIC VARIABILITY THROUGH MEIOSIS

• Crossing Over:

  • Occurs during prophase I when homologous chromosomes exchange segments.

  • Increases genetic diversity in gametes.

ALTERATIONS IN CHROMOSOME NUMBER AND STRUCTURE

• Karyotype Analysis:

  • Involves chromosomal arrangement to identify abnormalities.

  • Normal human configuration includes 22 pairs of autosomes and 2 sex chromosomes.

• Down Syndrome:

  • Caused by trisomy of chromosome 21; associated with various anomalies.

• Nondisjunction:

  • Failure of chromosomes to separate leads to abnormal chromosome numbers, potentially causing syndromes including Down syndrome.

• Structural Chromosome Alterations:

  • Deletions, duplications, inversions, and translocations can lead to genetic disorders or cancer development.

MEIOSIS AND GENETIC VARIATION

Gametes Formation:
Somatic cells: Diploid (2n), paired chromosomes.
Gametes: Haploid (n), single chromosome set.
Meiosis involves two rounds of division to reduce chromosome number and creates gametes with genetic variation through independent orientation and crossing over.

Details on Meiosis:
Meiosis I: In meiosis I, homologous chromosomes pair up and undergo crossing over, a process where genetic material is exchanged between them. This creates recombinant chromosomes with new combinations of genes. The homologous pairs then separate, with each chromosome moving to opposite poles of the cell. This reduces the chromosome number from diploid (2n) to haploid (n).
Meiosis II: In meiosis II, the sister chromatids separate, similar to what happens in mitosis. This results in four haploid daughter cells, each with a single set of chromosomes. These cells are genetically unique due to the crossing over and independent assortment of chromosomes during meiosis I.

Significance of Meiosis:
Genetic Diversity: Meiosis is essential for sexual reproduction because it generates genetic diversity in offspring. The combination of genes from two parents during fertilization,