Biology Chapter on Cell Division and Genetics

Cell Cycle and Division

• Cells originate from pre-existing cells
• New cells are produced for growth and replacement of damaged or old cells
• Cell division differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, and animals)

DNA Replication

• DNA must be copied before cell division
• Each new cell receives a complete copy of the DNA

Identical Daughter Cells

• Daughter cells contain the same number of chromosomes as the parent cell.
• They are identical to each other but smaller.
• They grow in size in G1 of Interphase to become mature cells.

Chromosomes

• DNA is packaged tightly into structures called chromosomes
• Long thread-like shapes made of DNA and protein, which coil up tightly into X shapes during cell division, becoming visible under a microscope.
• Chromosomes in eukaryotic cells are diploid (2n) and contain 23 pairs of homologous chromosomes, for a total of 46.

Chromosome Definitions

• Homologous chromosomes are the same size and shape and carry the same types of genes in the same order.
• Diploid (2n) refers to a cell containing two sets of homologous chromosomes.
• Haploid (n) refers to a cell containing one set of chromosomes.

Sets of Human Chromosomes

• The first 22 pairs of chromosomes in a human cell are called autosomes.
• The 23rd pair (XX or XY) are the sex chromosomes.

The Cell Cycle

• The series of events from the time a cell forms until it divides is called the cell cycle.
• It is divided into two major phases: Interphase and Mitosis (M phase).

Interphase

• Interphase is the first major phase of the cell cycle

• Includes G1 (Gap 1), S (Synthesis), and G2 (Gap 2) phases.

• G1 (Gap 1)

• 1st growth stage after cell division

• Cells mature by making more cytoplasm and organelles

• Cell carries out normal metabolic activities

• S (Synthesis)

• DNA is copied or replicated

• Each chromosome replicates to form sister chromatids

• Sister chromatids are attached at the centromere

• G2 (Gap 2)

• 2nd growth stage after DNA replication

• All cell structures needed for cell division are made

• e.g., centrioles

• Organelles and proteins are synthesized

DNA Replication in Chromosomes

• Cells replicate their DNA during Interphase's S phase.
• Each duplicated chromosome has two sister chromatids joined at the centromere.

Mitosis

• Mitosis is a process that divides the nucleus of a cell.
• It has four stages: Prophase, Metaphase, Anaphase, and Telophase
• The cell divides into two identical copies

Prophase

• Chromatin in nucleus condenses to form visible chromosomes; Each chromosome is composed of two sister chromatids joined at the centromere.
• Centrioles move to opposite sides of nucleus; Spindle microtubules appear in the cytoplasm; Nuclear envelope and nucleolus disassemble.

Metaphase

• Chromosomes, attached to kinetochore fibers, move to the center of the cell; Chromosomes line up at the equator of the cell.

Anaphase

• Sister chromatids are pulled apart to opposite poles of the cell by kinetochore fibers; Centromeres split separating sister chromatids.

Telophase

• Sister Chromatids are at opposite poles; Spindle disassembles; Nuclear envelope forms around each set of sister chromatids; Nucleolus reappears; Chromosomes reappear as chromatin; Cytokinesis occurs.

Cytokinesis

• Cytoplasm divides, creating two identical daughter cells.

Meiosis

• Meiosis is a type of cell division that produces gametes (eggs and sperm).
• The process of meiosis involves two meiotic divisions.
• The original cell is diploid (2n), but the resulting gametes are haploid (n).
• This halving of chromosomes is essential for sexual reproduction.

Meiosis I: Reduction Division

• Early Prophase I: Homologs pair and crossing over occurs
• Late Prophase I: Chromosomes condense, spindle forms, and nuclear envelope breaks down.
• Metaphase I: Homologous pairs align along the cell's equator.
• Anaphase I: Homologous chromosomes separate and move to opposite poles, while sister chromatids remain attached.
• Telophase I: Nuclear envelopes reform, and cytokinesis occurs, producing two haploid cells.

Meiosis II

• Prophase II: Chromosomes condense, and spindle apparatus forms again.
• Metaphase II: Chromosomes align along the cell's equator.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II: Nuclear envelopes reform, spindle disappears, and cytokinesis occurs, resulting in four haploid daughter cells.

Crossing-Over

• During Prophase I, homologous chromosomes exchange pieces of chromosome or genes
• Results in genetic recombination in offspring.

Results of Meiosis

• Produces gametes (eggs and sperm)
• Four haploid cells with one copy of each chromosome, with different combinations of alleles for different genes.

Spermatogenesis

• Production of sperm in males, involving mitosis and meiosis to generate haploid sperm cells.

Oogenesis

• Production of eggs in females, involving mitosis and meiosis to create a single mature egg and polar body cells.

Mitosis vs. Meiosis

• Key Differences:
• Mitosis: One division, identical daughter cells, used for growth and repair.
• Meiosis: Two divisions, unique daughter cells, used for sexual reproduction.

Fertilization

• The fusion of a haploid egg and a haploid sperm to form a diploid zygote.