Chromatin is an uncoiled mass of DNA and associated proteins called histones.
When cell division begins, DNA coils around the proteins forming visible structures called chromosomes.
Below: Human chromosomes (female)
Diploid cells (2N) have two complete sets of chromosomes. The body cells of animals are diploid.
Haploid cells have one complete set of chromosomes. In animals, gametes (sperm and eggs) are haploid.
Homologous chromosomes are two chromosomes that are the same. This happens because humans and other diploid organisms have two of each chromosome. Each of the pairs is a homologous pair. One of the homologous chromosomes was inherited from the individual's mother and the other one was inherited from the individual's father. For example, the two chromosomes #1 are homologous. However, a chromosome #1 and a chromosome #2 are not homologous because they are different chromosomes.
A small segment of DNA that contains the information necessary to construct a protein or part of a protein (polypeptide) is called a gene. Genes are the unit of inheritance.
Single-celled organisms divide to reproduce.
Cell division in multicellular organisms enables the organism to grow larger while the cells remain small. A large surface:volume ratio is due to small cell size.
Organisms with many cells can have cells which are specialized for different functions and tasks. For example, red blood cells are specialized for carrying oxygen but neurons (nervous tissue) are specialized for conducting signals from one cell to another.
Some cells of multicellular organisms must divide to produce sex cells (gametes).
Mitosis produces two daughter cells that are identical to the parent cell. If the parent cell is haploid (N), then the daughter cells will be haploid. If the parent cell is diploid, the daughter cells will also be diploid.
This type of cell division allows multicellular organisms to grow and repair damaged tissue.
Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell.
Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid.
Meiosis involves two divisions producing a total of four daughter cells.
A chromatid is a single DNA molecule.
Double-stranded chromosomes have two chromatids; normally, each one is identical to the other. The point where the two chromatids are attached is called the centromere.Chromosome Doubling vs DNA Synthesis
Overview of the Cell Cycle
During metaphase, the chromosomes have moved to the center of the cell (diagram below, photograph above). This line of chromosomes is referred to as the metaphase plate.
The structures in the diagram below are referred to as the spindle apparatus. Kinetochore microtubules are attached to the chromosomes. Polar microtubules are not attached to chromosomes but overlap each other. Asters are short microtubules that radiate from the centrosomes. The spindle apparatus can be seen on the drawing of a cell in metaphase below.
Metaphase ends when chromosomes split, thus doubling the number of chromosomes.
Telophase begins when chromosomes reach the poles of the daughter cells.
Many of the events in telophase are the reverse of prophase. The chromosomes uncoil, the nuclear membranes around daughter nuclei appear, the spindle apparatus breaks down, and the nucleolus reappears.
Cytokinesis is completed as telophase ends.
This is the non-dividing phase.
During interphase, the nucleus is visible and the chromosomes are uncoiled and invisible.
Interphase includes G1, S and G2.
Cells that permanently leave the cycle
Some cells remain permanently in G1. Examples: skeletal muscle, nerve cells
Some cells remain permanently in G2. Example: cardiac muscle
Below: Whitefish blastula X 400