Lecture 3 - Cell Cycle

The Cell Cycle

• Cell Division:

  • For Reproduction: Binary fission in bacteria.

  • For Growth: Embryo development in sea urchins.

  • For Renewal and Repair: Dividing bone marrow cells give rise to new blood cells.

  • Importance of Cell Division: One cell is impressive, but two (or more) cells are essential for life.

Key Concepts – Cell Cycle

• New cells arise from pre-existing cells through cell division, a fundamental part of the cell cycle.

• Definition of Cell Cycle:

  • Comprises cell growth and the processes of nucleus division (mitosis) and cytoplasm/organelles division (cytokinesis).

• DNA Replication:

  • Before cell division, DNA is replicated.

  • Some cells divide through Mitosis: resulting in daughter cells that are genetically identical.

  • Other cells undergo Meiosis: resulting in genetically different daughter cells (eggs and sperm) with half the genetic material.

• Regulation: The cell cycle is tightly regulated by molecular checkpoints. Breakdown of this control can lead to cancer.

Phases of the Cell Cycle (Fig. 12.3)

• M Phase:

  • The dividing phase, which includes mitosis and cytokinesis.

• I Phase: Non-dividing phase.

  • G1 Phase: First gap phase.

  • S Phase: Synthesis phase where DNA replication occurs.

  • G2 Phase: Second gap phase.

• Growth: Occurs in all three interphase phases; replication of all organelles and membranes occurs during this time.

Understanding Genetic Processes

• DNA Replication:

  • Involves making a copy of DNA using it as a template with nucleotides.

• Transcription:

  • Making an mRNA copy of DNA using RNA nucleotides.

• Translation:

  • Making proteins using the mRNA template and amino acids.

• Note: Utilize the glossary for definitions of scientific terms.

Components of Cell Division

• Three Main Steps of Cell Division:

  • Copying DNA.

  • Separating DNA copies.

  • Dividing the cytoplasm and organelles.

• Key Structures:

  • Plasma membrane.

  • Chromosomes.

  • Microtubules.

Chromosome Structure

• Definition of Chromosome:

  • Consists of a single, long DNA molecule wrapped around histone proteins.

• Chromatin:

  • Combination of DNA and proteins to form a compact structure.

• Genome:

  • Total amount of DNA in the cell composed of multiple chromosomes.

• Gene Carrying: Each chromosome contains several hundred to a few thousand genes.

Mitosis Overview

• Before Mitosis: Each chromosome is replicated during the S phase, resulting in sister chromatids.

• Movement: Sister chromatids are held together at the centromere by cohesin proteins.

• Distribution: During mitosis, sister chromatids are distributed to daughter cells.

The Mitotic Spindle

• Structure: A microtubule structure with motor proteins that assembles in prophase.

• Function:

  • Kinetochore microtubules bind and move sister chromatids.

  • Polar microtubules help maintain spindle structure.

Phases of Mitosis (PPMAT)

• Interphase:

  • Cellular processes occur, nuclear envelope intact, chromosomes not yet condensed.

• Prophase:

  • Chromosomes condense and centrosomes move apart as the spindle begins to form.

• Prometaphase:

  • Nuclear envelope disintegrates, and kinetochore microtubules engage with chromosomes.

• Metaphase:

  • Chromosomes align at the center of the cell.

• Anaphase:

  • Sister chromatids separate and are pulled to opposite poles via shortening of kinetochore microtubules.

• Telophase:

  • Nuclear envelope reforms, and spindle disintegrates.

• Cytokinesis:

  • Division of cytoplasm occurs, resulting in two daughter cells.

Checkpoints in the Cell Cycle

• Function: Ensure processes required for division are completed correctly (cell size, DNA integrity, replication checks).

• Key Checkpoints:

  • G1, G2, and M-phase checkpoints capable of halting the cell cycle.

Cancer and Cell Cycle Regulation

• Impact: Cell division frequency varies among cell types (skin and gut cells divide frequently; mature muscle and nerve cells do not).

• Molecular Control:

  • Loss of checkpoints due to mutations can lead to uncontrolled cell division (cancer).

  • Cancer cells can evade apoptosis, proliferate indefinitely, and develop metastasis.

Conclusion on Mitosis

• A diploid (2n) parent cell duplicates its chromosomes before mitosis, leading to genetically identical diploid daughter cells.

• Meiosis: Results in daughter cells with half the number of chromosomes, making them haploid (n).

  • Explore Table 13.2 for differences between mitosis and meiosis.

SIMPLIFIED

The Cell Cycle

What is Cell Division?

Cell division is when one cell splits into two or more cells. This process is important for several reasons:

• Reproduction: In simple organisms like bacteria, a type of division called binary fission helps them reproduce.

• Growth: When an organism is developing, like an embryo in a sea urchin, it grows by dividing its cells.

• Repair: When our body needs to repair itself, such as creating new blood cells from dividing bone marrow cells.

Why is Cell Division Important?

Cell division is crucial because one cell by itself is not enough for complex life. Cells need to multiply to maintain health and perform various functions in an organism.

The Cell Cycle Explained

The cell cycle includes all the stages a cell goes through to grow and divide:

• Definition: The cell cycle is the series of events that take place in a cell leading to its division and duplication. It includes cell growth and division into new cells.

Key Steps in the Cell Cycle:

1. DNA Replication: Before dividing, a cell makes a copy of its DNA.

2. Types of Division:

   • Mitosis: This kind of division creates two new cells that are exactly like the original cell.

   • Meiosis: This leads to the creation of cells (like eggs and sperm) that are genetically different from the original cell and have half the genetic material.

3. Checkpoints: There are controls in place that check whether everything is going smoothly during the cell cycle. If problems arise, such as damaged DNA, the cycle can stop. This helps prevent issues like cancer.

Phases of the Cell Cycle:

• M Phase: The phase where the cell actually divides (includes both mitosis and cytokinesis).

• Interphase: The phase where the cell is growing and preparing for division. It has three parts:

  • G1 Phase: Cell grows and makes new proteins.

  • S Phase: DNA is copied.

  • G2 Phase: The cell continues to grow and prepares for division.

Cell Division Process:

When a cell divides, it does so in three main steps:

1. Copying DNA: The DNA is duplicated.

2. Separating DNA Copies: The two sets of DNA are pulled apart.

3. Dividing the Cytoplasm: The rest of the cell's contents are split into two new cells.

Chromosome Basics

• Chromosomes: These are long strands of DNA that contain the instructions for how an organism develops and functions.

• Genome: The entire set of genetic information (all the DNA) in a cell.

• Genes: Sections of DNA within chromosomes that contain instructions for making proteins.

Mitosis Explained

Mitosis is the process through which one cell divides to become two identical cells:

1. Before Mitosis: The cell has copied its DNA into sister chromatids.

2. Prophase: The DNA condenses and prepares to split.

3. Metaphase: Chromosomes line up in the middle of the cell.

4. Anaphase: The sister chromatids are pulled apart to opposite sides of the cell.

5. Telophase: New nuclear membranes form around each set of chromosomes.

6. Cytokinesis: The cytoplasm divides, resulting in two separate cells.

Cell Cycle Checkpoints

These checkpoints act like quality control. They ensure that the cell is ready to proceed to the next stage of division, checking if:

• The cell is the right size.

• The DNA is undamaged and has been correctly copied.

If problems are found, the cell cycle can halt, preventing potential issues such as cancer.

Conclusion

After mitosis, the result is two diploid (2n) daughter cells, meaning they have the same number of chromosomes as the original cell. On the other hand, meiosis produces haploid (n) cells with half the number of chromosomes for reproduction.

Note

For specific definitions and terms, consider utilizing a glossary to explore scientific vocabulary.