Chromosomes, Mitosis and Cyclins

Mitosis and Chromosomes

The image provided illustrates key aspects of mitosis, particularly focusing on chromosomes and their behavior during cell division. Here are some important concepts:

1. Chromosomes and Genes:

   • Chromosomes contain genetic material in the form of DNA. In the diagram, the chromosomes inherited from a person's father (labeled "Dad") and mother ("Mom") are shown.
   • These chromosomes have specific regions known as genes, which code for various traits (e.g., eye color). In the image, a gene for eye color is highlighted as G on one chromosome and g on the homologous chromosome.

2. Human Chromosome Count:

   • Humans have 46 chromosomes in total, arranged in 23 pairs. Each pair consists of one chromosome from the father and one from the mother.
   • The image shows two chromosomes that are homologous pairs. This means they are similar in structure and contain the same genes but might have different alleles (versions) of those genes (e.g., G and g).

3. Homologous Pairs:

   • Homologous chromosomes pair up during cell division. Each chromosome from the father pairs with the corresponding chromosome from the mother.
   • These chromosomes are crucial in maintaining genetic consistency when cells divide, ensuring that each new cell gets a complete set of genetic instructions.

4. Sister Chromatids:

• During the process of cell division, each chromosome replicates, forming two identical copies known as sister chromatids. These are connected at a region called the centromere, creating the familiar X-shaped structure.
• The image shows homologous chromosomes that have duplicated into two sister chromatids. These sister chromatids are identical copies of the original chromosome and are separated during mitosis to ensure each new cell has the same genetic material.
5. Centromere: The point where two sister chromatids are attached.
6. Kinetochore: A protein structure on the centromere where spindle fibers attach during mitosis.

Mitosis Overview:

• Mitosis is the process of nuclear division that ensures each daughter cell gets an identical set of chromosomes. This process is important for growth, repair, and asexual reproduction in organisms.
• The stages of mitosis include:
• Interphase: DNA is uncondensed as chromatin, and the cell prepares for division by replicating its DNA and organelles.
• Prophase: Chromatin condenses into visible chromosomes, and each chromosome now has two sister chromatids. The nuclear membrane dissolves, and spindle fibers start to form.
• Metaphase: Chromosomes align at the cell's equator, attached to spindle fibers at their centromeres. In this stage, there are 4 chromosomes (with each chromosome consisting of 2 chromatids, totaling 8 chromatids).
• Anaphase: Sister chromatids separate and are pulled to opposite poles by the spindle fibers, making each chromatid an independent chromosome. The cell now has 8 chromosomes.
• Telophase: Chromosomes reach the poles, decondense back into chromatin, and nuclear membranes reform around each set of chromosomes.
• Cytokinesis: The cell membrane pinches in, dividing the cytoplasm, resulting in two identical daughter cells, each with the same chromosome number as the original cell.

Chromosome and DNA Content During the Cell Cycle

• G₁ Phase: Cell growth phase with 46 chromosomes.
• S Phase (Synthesis): DNA replication occurs, resulting in each chromosome having two sister chromatids. The total DNA doubles, but the chromosome count remains 46.
• G₂ Phase: The cell prepares for mitosis, with 46 chromosomes (each with two chromatids).
• M Phase (Mitosis): The number of chromosomes temporarily doubles during anaphase when sister chromatids separate. By cytokinesis, each daughter cell ends up with 46 chromosomes.
• Graph Summary: The chromosome count remains constant at 46, but the DNA content (chromatids) doubles in S phase and returns to normal after cell division.

Cyclin Regulation in the Cell Cycle

• Cyclins are proteins that regulate progression through the cell cycle, with different cyclins active at different stages:
  • Cyclin D: Constantly present, supports overall cell cycle progression.
  • Cyclin E: Peaks to initiate the S phase.
  • Cyclin A: Responsible for ensuring completion of S phase and progression through G₂.
  • Cyclin B: Prepares the cell for mitosis (M phase).
• Importance of Timing: Cyclins ensure that cell cycle events happen in the correct sequence. For example, if Cyclin B spikes before S phase, the cell might prematurely enter mitosis, leading to errors in chromosome distribution.

In summary, mitosis ensures that each new cell receives a full set of chromosomes, maintaining genetic consistency across cells. Homologous chromosomes and sister chromatids are key players in this process, ensuring accurate distribution of genetic material.