B i o l o g y : C h r o m o s o m e s a n d I n h e r i t a n c e

Chapter 5: Chromosomes and Inheritance

Introduction to Cells and Cell Division

  • Cell Life and Death:

    • Between 5050 and 7070 billion cells die in the human body each day.

    • The body continuously produces new cells for various purposes.

    • Rapidly replaced cells: Examples include skin cells and blood cells.

    • Slowly or never replaced cells: Examples include neurons (nerve cells) and certain muscle cells.

  • Cell Theory - A Fundamental Concept:

    1. All life is cellular.

    2. All cells arise from preexisting cells.

    • Organisms can range from single-celled entities to those composed of trillions of cells.

  • Functions of Cell Division:

    • Cell division is the process by which new cells are formed from existing cells.

    • It serves three primary purposes:

      1. Growth: Essential for an organism to increase in size and complexity from a single cell (zygote) to an adult.

      2. Repair: Replaces lost or damaged cells, crucial for healing wounds and maintaining tissues.

      3. Reproduction: Allows organisms to create new individuals.

    • Reproduction can occur in two main ways: sexually or asexually.

Modes of Reproduction

  • Sexual Reproduction:

    • Involves two parents.

    • Produces genetically unique offspring.

    • Requires gametes (specialized sex cells: egg and sperm) formed via cell division in adult gonads (testes in males, ovaries in females).

    • Fertilization occurs when sperm and egg fuse.

    • Life Cycle:

      1. Gametes (egg and sperm) are produced in the gonads.

      2. Fertilization occurs, forming a zygote (fertilized egg).

      3. The zygote receives half of its chromosomes from the egg and half from the sperm, creating a unique diploid set.

      4. Development proceeds through repeated rounds of cell division (mitosis), transforming the single-celled zygote into an embryo, fetus, baby, and eventually an adult.

      5. Growth and Repair continue throughout life, using cell division to replace cells and expand tissues.

  • Asexual Reproduction:

    • Involves only one parent.

    • Produces genetically identical offspring (clones).

    • Does not involve sperm or egg.

  • Comparison of Sexual vs. Asexual Reproduction:

Feature

Sexual Reproduction

Asexual Reproduction

Number of parents needed

22

11

Gametes involved? (yes/no)

Yes

No

Fertilization? (yes/no)

Yes

No

Number of chromosome sets

22

11

Offspring genetically unique?

Yes

No

Deoxyribonucleic Acid (DNA) and Chromosomes

  • DNA as Genetic Material:

    • All life forms on Earth utilize DNA as their genetic material.

    • In eukaryotes, the nucleus of every cell contains long strands of DNA called chromosomes.

  • Genes: Units of Inheritance:

    • A gene is a specific length of DNA, representing a small segment of a chromosome.

    • Each gene carries the essential information required to synthesize one or more protein strands or one or more RNA molecules.

  • Chromosome Structure:

    • Inside the nucleus, the chromosomal DNA is tightly wound around proteins, forming a complex called chromatin.

    • Most of the time, chromosomes exist in an unraveled, loose chromatin state.

  • Human Chromosome Number:

    • Every human body cell (somatic cell) typically contains 4646 chromosomes.

  • Chromosomes During Cell Division:

    • Prior to cell division, chromosomes undergo significant changes:

      1. They become tightly packed and visible.

      2. They duplicate, forming identical copies known as sister chromatids.

    • Sister chromatids are a pair of duplicated chromosomes, physically joined at a constricted region called the centromere.

    • The formation of sister chromatids signals that the cell is preparing to divide.

The Cell Cycle

  • Regulated Cell Division:

    • Healthy cells only initiate division when replication is necessary (e.g., for growth or repair).

    • Unregulated cell division is characteristic of unhealthy cells, such as those in cancer.

    • Cancer originates when a cell divides inappropriately, disregarding normal regulatory signals.

    • Treating cancer is challenging because cancer cells are fundamentally the body's own cells, making it difficult to target them without harming healthy tissues.

  • Phases of the Cell Cycle:

  • cell cycle:

    • 1. DNA check point #1

    • Interphase: Performs its normal functions, grows, and prepares for division by duplicating its chromosomes

    • Miotic phase: Splits two offspring (cytokinesis)

      • Early interphase: Cell is carrying out its normal activities

      • Late Interphase: Chromesomes duplicate

        • Cell is preparing to divide; generates sister chromatids

      • Prophase: chromosomes condense

        • Nuclear membrane dissolves. Cell lays down mitotic spindle

      • Metaphase: chromosomes align

        • Sister chromatics line up and attach to mitotic spindle

      • Anaphase: chromosomes split

        • Sister chromatids are pulled apart as mitoic spindle retracts

      • Telophase: Nucleus reforms

        • Two duplicated nuclei are formed

      • Cytokinesis: division of the cytoplasm

        • The process of cytokinesis is different for plant and animals cells

  • Cytokinesis in plant cells

    • Plant cells divide by forming a cell plate along the center of the cell

Cloning