Cell Division

Overview of Cell Division

• Cell Division Necessity:

  • All cells must divide; no living organism can reproduce without this process.

  • Unicellular organisms reproduce through cell division.

  • In multicellular organisms, it continuously replaces cells.

Types of Cell Division

• Mitosis:

  • Occurs in eukaryotes for purposes such as skin repair and zygote division.

  • Results in two identical daughter cells.

• Meiosis:

  • Produces sex cells (gametes) with half the genetic material of the parent cells, crucial for sexual reproduction.

DNA Replication and Cell Division

• Link Between DNA Replication and Cell Division:

  • Cells must replicate their DNA before division to ensure that each new cell receives one complete copy of the genome.

Semiconservative Nature of DNA Replication

• Definition of Semiconservative Replication:

  • The term "semiconservative" indicates that during DNA replication, each newly formed molecule consists of one parental strand (original) and one daughter strand (new).

Replication Enzymes and Process

• Origins of Replication:

  • DNA replication begins at multiple points along chromosomes called origins of replication, proceeding bidirectionally from each origin.

Mitosis Overview

• Function of Mitosis:

  • Allows organisms to grow, develop, repair tissues, and regenerate lost body parts.

  • In some organisms, mitosis leads to asexual reproduction.

The Cell Cycle

• Definition of the Cell Cycle:

  • A series of events that starts with one cell division and ends with the next. It consists of dividing phases (mitosis) and non-dividing phases (interphase).

Mitosis Stages

• M Phase (Mitosis):

  • Cells exiting the G₂ phase enter the M phase leading to nuclear division and followed by cytokinesis (division of the cytoplasm).

  • The process of mitosis includes:

    • Prophase: Chromosomes condense and become visible; the nuclear envelope begins to break down.

    • Metaphase: Chromosomes align at the cell's equator ensuring proper separation.

    • Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.

    • Telophase: Chromosome decondensation occurs, and nuclear envelopes reform around each set of chromosomes.

Cytokinesis

• Definition:

  • Cytokinesis is the division of the cytoplasm, resulting in two distinct daughter cells.

  • It technically is not a part of mitosis but a separate process.

  • Involves the formation of a contractile ring in animal cells.

Regulation of the Cell Cycle

• Cell Cycle Checkpoints:

  • The cell cycle is tightly regulated through several checkpoints that ensure:

    • All DNA has replicated successfully.

    • DNA is free of damage.

    • Chromosomes are properly aligned and separated during mitosis.

Impact of Cancer on Cell Division

• Cancer and Cell Division:

  • Cancerous cells typically divide uncontrollably, bypassing normal regulatory checkpoints, leading to tumors.

Role of P53 Protein

• P53 Function:

  • Known as a master tumor suppressor gene, each individual receives a copy from both parents.

  • If activated, P53 can initiate apoptosis (programmed cell death) as a protective measure against malignant cells.

  • Many solid tumors demonstrate inactivation of P53.

Cancer Treatment Options

• Methods:

  • Surgical removal of tumors.

  • Chemotherapy with anticancer drugs to hinder or stop cell division.

  • Radiation therapy aimed at destroying targeted tumor cells.

Meiosis and Sexual Reproduction

• Purpose of Meiosis:

  • To generate sperm and egg cells (gametes), ensuring each is genetically unique, essential for sexual reproduction.

Chromosome Composition in Organisms

• Diploid Cell Definition:

  • Organisms consist primarily of diploid cells, which contain two complete sets of chromosomes, one from each parent.

Homologous Chromosomes

• Definition of Homologous Chromosomes:

  • Pairs of chromosomes that are the same size and shape and contain the same genes, with one inherited from each parent.

Genetic Variability in Homologous Chromosomes

• Characteristics:

  • Although homologous chromosomes have the same genes, they can carry different alleles (versions of genes).

Haploid Gametes

• Function in Reproduction:

  • Human sperm (23 chromosomes) and egg cells (23 chromosomes) fuse to form a zygote with 46 chromosomes, completing the genetic contribution from both parents.

Meiosis Similarities to Mitosis

• Common Phases:

  • Both processes share phases such as interphase, prophase, metaphase, anaphase, telophase, and cytokinesis.

Meiosis Mechanics

• DNA Replication:

  • DNA replicates once before meiosis I (during S phase) but divides twice in total (meiosis I and II), with no additional DNA replication during meiosis II.

Crossing Over in Meiosis

• Phase Specifics:

  • During prophase I, homologous chromosomes undergo crossing over, whereby portions of homologous chromosomes exchange segments, increasing genetic diversity.

Independent Assortment in Meiosis

• Mechanism:

  • In metaphase I, the alignment of chromosome pairs happens randomly, creating diverse combinations of chromosomes in gametes. For example, if there are 3 pairs of chromosomes, the possible number of gametic combinations is $2^3 = 8$.

Meiosis Effects on Chromosome Number

• Reduction in Chromosome Number:

  • Meiosis reduces the chromosomal count by half, transforming diploid germ cells (4 chromosomes) into haploid gametes (2 chromosomes).

Distinction Between Mitosis and Meiosis

• Different Functions:

  • Mitosis generates identical cells for growth and repair, whereas meiosis produces haploid gametes with unique genetic material.

Nondisjunction Events in Meiosis

• Anaphase I:

  • Leads to abnormalities in all resulting gametes due to improper separation of homologous chromosomes.

• Anaphase II:

  • Results in abnormalities in some gametes, with some being normal and others having one chromosome extra or missing.

Down Syndrome and Chromosomal Abnormalities

• Cause:

  • Down syndrome is caused by a trisomy, specifically having three copies of chromosome 21, resulting from nondisjunction.

Chromosomal Types

• Autosomes vs. Sex Chromosomes:

  • Humans possess 22 pairs of autosomes (chromosomes 1-22) and one pair of sex chromosomes, which determine biological sex (XX or XY).