Understanding Chromosome Structure

• Chromosomes are structures within cells containing an organism's genetic information in the form of DNA.
• When a cell is not dividing chromosomes are not visible in the cell's nucleus, even under a microscope.
• During cell division, DNA that makes up chromosomes becomes more tightly packed making them visible under a microscope.

Chromosome Structure

• Each chromosome has a constriction point, the centromere, which divides the chromosome into two sections, or arms.
• The short arm of the chromosome is labeled the "p arm."
• The long arm of the chromosome is labeled the "q arm."
• Chromosomes are typically described using their length and the position of the centromere.
• A chromosome is made of DNA tightly coiled around proteins called histones, supporting its structure.
• Chromosomes exist as single-stranded structures during most of the cell cycle.
• During cell division, each chromosome duplicates; the resulting identical sister chromatids are joined at the centromere.
• The number of chromosomes varies depending on the organism; humans have 46 chromosomes arranged in 23 pairs.

Cell Cycle

• The cell cycle involves a repeating series of growth, DNA replication, and division, resulting in two new "daughter" cells.
• The cell cycle consists of two major phases: interphase and the mitotic (M) phase.
• Interphase includes G1 phase, S phase, and G2 phase.
• The M phase includes mitosis and cytokinesis.

Interphase

• Interphase is the longest part of the cell cycle.
• During interphase, the cell grows and copies its DNA to prepare for the M phase.
• G1 phase: The cell grows in size and synthesizes mRNA and proteins for DNA replication.
• S phase: The cell duplicates its DNA, and each chromosome now consists of two identical sister chromatids.
• G2 phase: The cell continues to grow and produce new proteins.

Mitotic (M) Phase

• During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells.
• The M phase involves two distinct division-related processes: mitosis and cytokinesis.

Mitosis

• Mitosis refers to the process of nuclear division.
• Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase.
• Prophase: The chromosomes condense and become visible, the nuclear envelope breaks down, and the centrosomes move to opposite poles of the cell.
• Metaphase: The chromosomes line up along the metaphase plate (the middle of the cell).
• Anaphase: The sister chromatids separate and move to opposite poles of the cell.
• Telophase: The chromosomes arrive at opposite poles and begin to decondense; the nuclear envelope reforms.

Cytokinesis

• Cytokinesis refers to the division of the cytoplasm to form two new cells.
• In animal cells, cytokinesis occurs through the formation of a cleavage furrow.
• In plant cells, cytokinesis occurs through the formation of a cell plate.

Cell Division: Mitosis

• Mitosis is a type of cell division resulting in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, which is typical of ordinary tissue growth.
• Mitosis facilitates growth, repair, and asexual reproduction.
• The stages of mitosis are prophase, metaphase, anaphase, and telophase.

Cell Division: Meiosis

• Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.
• Meiosis is used for sexual reproduction.
• Meiosis involves two rounds of cell division: meiosis I and meiosis II.
• Meiosis I: Homologous chromosomes separate.
• Meiosis II: Sister chromatids separate.
• Meiosis introduces genetic variation through crossing over and independent assortment.

Chromosomal Abnormalities

• Chromosomal abnormalities can occur during cell division.
• These abnormalities can lead to genetic disorders.
• Numerical abnormalities involve the loss or gain of entire chromosomes.
• Structural abnormalities involve changes in the structure of individual chromosomes.