10 Chapter 10

Learning Objectives

  1. Recognize the role of cellular division and associated vocabulary
  2. Identify four phases of mitotic division
  3. Examine the characteristics of cancer cells

Cell Reproduction

A cell’s DNA, packaged as a double-helix DNA molecule, is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in a loop or circle. Prokaryotes, including bacteria and archaea, have a single, circular chromosome located in a central region called the nucleoid.

Eukaryotes have multiple, linear chromosomes surrounded by a nuclear membrane. The 46 chromosomes of human somatic (body) cells are composed of 22 autosome pairs and a pair of sex chromosomes, which may or may not be matched. This is the 2n or diploid state. Human gametes (egg or sperm) have 23 chromosomes or one complete set of chromosomes. This is the n or haploid state. Humans have 23 pairs of chromosomes, for a total of 46 chromosomes, in each somatic cell.

Matched pairs of chromosomes in a diploid organism are called homologous chromosomes, which are the same length and have specific nucleotide segments called genes. Genes determine specific characteristics by coding for specific proteins. Traits are the variations of those characteristics. Chromosomes are compacted DNA wrapped around histone proteins. Several classes of protein are involved in the organization and packing of the chromosomal DNA into this highly condensed structure. The condensing complex compacts chromosomes, and the resulting condensed structure is necessary for chromosomal segregation during mitosis.

Each copy of a homologous pair of chromosomes originates from a different parent, so the genes are not identical. The variation of individuals is due to the specific combination of genes inherited from both parents. Even a slightly altered sequence of nucleotides within a gene can result in an alternative trait.

The cell cycle is an ordered series of events producing two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division, producing two identical (clone) cells. During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated, and the cell divides.

Like a clock, the cell cycles from interphase to the mitotic phase and back to interphase. Most of the cell cycle is spent in interphase, which is subdivided into G subscript 1 baseline, S, and G subscript 2 baseline phases. Cell growth occurs during G subscript 1 baseline, D N A synthesis occurs during S, and more growth occurs during G subscript 2 baseline. The mitotic phase consists of mitosis, in which the nuclear chromatin is divided, and cytokinesis, in which the cytoplasm is divided, resulting in two daughter cells.

The cell cycle in multicellular organisms consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is replicated. Interphase is followed by the mitotic phase. During mitosis, the duplicated chromosomes are segregated and distributed into daughter nuclei. Following mitosis, cytoplasm is usually divided by cytokinesis, resulting in two genetically identical daughter cells.

Interphase has 3 parts:

  • G1 Phase (First Gap) cell grows, producing organelles as needed
  • S Phase (Synthesis of DNA) DNA replication occurs, centrioles organize
  • G2 Phase (Second Gap) cell grows, producing organelles as needed

 

This diagram shows the five phases of mitosis and cytokinesis. During prophase, the chromosomes condense and become visible, spindle fibers emerge from the centrosomes, the nuclear envelope breaks down, and the nucleolus disappears. During prometaphase, the chromosomes continue to condense and kinetochores appear at the centromeres. Mitotic spindle microtubules attach to the kinetochores, and centrosomes move toward opposite poles. During metaphase, the mitotic spindle is fully developed, and centrosomes are at opposite poles of the cell. Chromosomes line up at the metaphase plate and each sister chromatid is attached to a spindle fiber originating from the opposite pole. During anaphase, the cohesin proteins that were binding the sister chromatids together break down. The sister chromatids, which are now called chromosomes, move toward opposite poles of the cell. Non-kinetochore spindle fibers lengthen, elongating the cell. During telophase, chromosomes arrive at the opposite poles and begin to decondense. The nuclear envelope reforms. During cytokinesis in animals, a cleavage furrow separates the two daughter cells. In plants, a cell plate separates the two cells.

Prophase is the first stage in mitosis. The nuclear envelope begins to break down and chromosomes condense and are now visible. Spindle fibers start to appear and centrosomes begin to move towards opposite poles. During prometaphase, chromosomes continue to condense and are more visible. Kinetochores appear at the centromere and kinetochore microtubules attach. Centrosomes continue to move towards opposite poles. During metaphase, the mitotic spindle is fully developed and centrosomes are at opposite poles. Chromosomes are aligned at the “equatorial plate”, and each sister chromatid rests on one side of the plate, with spindle fibers attached to them. During anaphase, sister chromatids are pulled apart by spindle fibers and are separated from each other. Each chromatid is now a chromosome. During telophase, chromosomes arrive at opposite poles and start to decondense and become less visible. The nuclear envelope reassembles and begins to surround each new set of chromosomes. The mitotic spindle assembly breaks down. During cytokinesis, the division of the cytoplasm begins and the two cells separate. Credit: Rao, A., Hawkins, A.and Fletcher, S. Department of Biology, Texas A&M University.

Mitosis Summary

Prophase chromosomes condense, spindle fibers emerge, nuclear membranes break down

Metaphase spindle poles formed, chromosomes lined up at metaphase plate

Anaphase chromosomes pulled toward opposite poles, spindles elongate cell

Telophase chromosomes at poles, nuclear formation begins, spindles break down

Cell cycle is regulated to maintain homeostasis. Cell signals initiate the phases of the cell cycle.  Many signals are hormones, like growth hormone. Three checkpoint signals prevent a compromised cell from continuing to divide.

This illustration shows the three major checkpoints of the cell cycle: G subscript 1 baseline checkpoint restriction; G subscript 2 baseline checkpoint , and M checkpoint, mitotic phase.

The cell cycle is controlled at three checkpoints. The integrity of the DNA is assessed at the G1 checkpoint. Proper chromosome duplication is assessed at the G2 checkpoint. Attachment of each kinetochore to a spindle fiber is assessed at the M checkpoint.

The G1 Checkpoint determines if conditions are favorable for cell division to proceed. The G1 checkpoint is a point at which the cell irreversibly commits to cell division. External influences play a large role in the cell passing G1 checkpoint. In addition, there is a genomic DNA damage check at G1 checkpoint. A cell not meeting all the requirements will not be allowed to progress to S phase. The cell can halt the cycle or await further signals when conditions improve. If the DNA cannot be repaired, apoptosis (cell death) signals prevent the duplication of damaged chromosomes.

G2 checkpoint halts mitosis if certain conditions are not met. The most important role of the G2 checkpoint is to ensure all chromosomes have been replicated and replicated DNA is not damaged. If the checkpoint mechanisms detect DNA problems, cell cycle is halted. The cell either completes DNA replication or repairs damaged DNA.

M checkpoint occurs near the end of metaphase. The M checkpoint is also known as the spindle checkpoint, because it determines whether all sister chromatids are correctly attached to spindle microtubules.

Cancer includes many different diseases caused by the common mechanism of uncontrolled cell growth. Cancer cells display unchecked cell division caused by a breakdown of the checkpoint mechanisms that regulate the cell cycle. This could be due to an inherited mutation or an accumulation of mutations. The loss of control begins with a change in the DNA sequence of a gene coding for a checkpoint regulatory molecule. Any disruption could allow other mistakes to be passed on to daughter cells. Each successive cell division has potential for accumulated damage.

Genes and the environment can both increase cancer risk. The risk factors include radiation, chemicals, UV light, and errors in replication. When cancer metastasizes its cells can travel in the bloodstream and colonize other areas of the body.

Exercises

Reading Quiz

Key Takeaways

  1. In humans, cellular division begins with the zygote and generates identical somatic cells. Most of a cell’s life is spent in interphase.
  2. Prophase: chromosomes condense and nucleus dissolves; metaphase: chromosomes align; anaphase: chromosomes migrate to poles; telophase: two nuclear membranes form
  3. Cancer cells are the result of an accumulation of mutations that impact cell division.

 

Biology-2e. (2018). Houston, RX: website: OpenStax Book title: Biology 2e .
Access for free at https://openstax.org/books/biology-2e/pages/1-introduction

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Introductory Biology Copyright © 2023 by Mona Easterling is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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