Understanding Cell Division and Chromosomes

What is Life?

  • All life uses free energy from the environment to create order (reduce its entropy, create gradients to do work, etc)

  • Many non-living systems also utilize energy to organize themselves.

  • A unique feature of life is the ability to reproduce, which is accomplished via cell division.

Cell Division

  • Plays vital roles in life:

    • Single-celled organisms: Cell division creates new organisms.

    • Multicellular organisms:

    • Causes embryonic development and growth.

    • Renews and repairs tissue.

Key Functions of Cell Division

  • Cell division involves:

    1. Copying DNA: Each time a cell divides, it must replicate its genetic material.

    2. Cytokinesis: The process of splitting into two cells.

  • Heredity: The resemblance between parents and offspring, facilitated by the transmission of copied genetic material.

Cellular Organization of the Genetic Material

  • In eukaryotic cells, DNA is located in the nucleus.

  • A genome consists of:

    • A single DNA molecule (most prokaryotes)

    • Many DNA molecules (most eukaryotes)

Chromosomes

  • DNA is packaged into chromosomes.

  • Each chromosome carries hundreds to thousands of genes.

  • Following DNA replication, there are two identical sister chromatids.

  • Sister chromatids are held together at the centromere by proteins known as cohesins.

  • Eukaryotic chromosomes are made of chromatin (a complex of DNA and protein) that condenses during cell division.

Human Chromosomes

  • Humans possess 23 different chromosomes in the cell nucleus, named in order of size.

  • The 23rd chromosome is the sex chromosome, with two versions in mammals, X and Y.

  • Each chromosome carries a unique set of genes.

Identifying Chromosomes

  • Counting Chromosomes: Deductions on how many chromosomes are present based on visual identification.

Homologous Chromosomes

  • Many cells, including those in most plants and animals, contain two versions of each chromosome, known as homologous chromosomes.

  • Homologous chromosomes have:

    • Same genes in the same location along the chromosome.

    • Same size and position of the centromere.

    • Different versions of the same genes (example: one for A blood type, the other for B blood type).

Why Diploid Cells Have Two Versions of Each Chromosome

  • Originates from having two parents: one homologue comes from the mother, the other from the father.

  • Diploid cells contain two versions of each chromosome (2n), while haploid cells contain one (n).

Fertilization

  • In a diploid cell, one homologue comes from the mother's egg, and the other from the father's sperm.

  • Resulting in a diploid zygote.

Eye Color Genetics

  • Genetic determination of eye color involves the HERC2 gene at chromosome 15.

  • The T allele corresponds to brown eyes; the C allele corresponds to blue eyes (only when homozygous).

  • Since individuals are diploid, they possess two copies of the HERC2 gene, which influence eye color.

Counting Chromosomes Analogy

  • Comparing chromosomes to shoes:

    • A pair represents homologous chromosomes (same type but not identical).

    • Identical shoes (like sister chromatids) are not a pair.

Eukaryotic Species Chromosome Count

  • Every eukaryotic species has a characteristic number of chromosomes.

  • Example:

    • Humans have 23 pairs (46 total chromosomes).

    • Chimpanzees have 24 pairs.

    • Drosophila melanogaster (fruit fly) has 8 chromosomes.

Somatic Cells vs Germ Cells

  • Somatic Cells:

    • Non-reproductive cells that are diploid (2n), with 46 chromosomes in humans.

    • Examples include skeletal, muscle, blood, stem, fat, and neuron cells.

  • Germ Cells:

    • Reproductive cells (sperm and eggs) that are haploid (n), possessing only one copy of each chromosome.

The Stages of Cell Division

  • Cell Cycle Overview:

    • A parent cell gives rise to two genetically identical daughter cells through the cell cycle.

    • Consists of two main phases:

    • Interphase: Cell grows and prepares for division; DNA is precisely duplicated.

    • Mitosis: Chromosome copies are separated and moved to different cell ends.

    • Cytokinesis: The cell divides into two daughter cells.

Phases of the Cell Cycle

  • Interphase Components:

    • G1 phase: Metabolic activity and growth.

    • S phase: DNA synthesis occurs, with 8 hours in human cells.

    • G2 phase: Preparation for cell division, lasting 4 hours.

    • G0 Phase: Non-dividing, where cells such as muscle cells and neurons reside.

Stages of Mitosis

  • Mitosis has five stages:

    1. Prophase

    2. Prometaphase

    3. Metaphase

    4. Anaphase

    5. Telophase

  • During mitosis, structures such as chromosomes, nuclear envelope, and the mitotic spindle (centrosomes and microtubules) play key roles.

Chromosome Behavior

  • Chromosomes condense to become easier to manage during mitosis.

  • The nuclear envelope is absorbed by the endoplasmic reticulum as it breaks down.

  • Chromosomes are aligned at the metaphase plate with spindle fibers facilitating their movement.

Prokaryotic Cell Division

  • Prokaryotes reproduce via binary fission, which differs from mitosis.

  • Involves:

    1. Chromosome replication initiation.

    2. Movement of origins to opposite ends of the cell.

    3. Completion of replication and formation of two daughter cells.

Cell Cycle Control

  • Regulatory proteins: cyclins and cyclin-dependent kinases (Cdks) play essential roles in cell cycle regulation.

  • Cyclins bind to Cdks to form MPF (Maturation Promoting Factor), which then activates mitosis.

  • After mitosis, cyclins degrade, effectively shutting down the process.

  • Errors in this regulation can lead to cancer.

Checkpoints in Cell Cycle Control

  • The cell cycle includes checkpoints to ensure everything is correct before proceeding.

  • Checkpoints monitor:

    • DNA damage

    • Proper chromosome alignment during metaphase

  • If errors are found, signals halt the cell cycle until issues are resolved.