Cell Division

• During cell division, DNA in the Eukaryotic cell nucleus coils tightly into structures called chromosomes.

• Each chromosome is a DNA molecule associated with proteins.

• DNA wraps tightly around these proteins, which aid the tight packing of the DNA.

• Chromosomes consist of 2 identical halves, each is called a chromatid.

• Chromatids form as DNA copies itself before cell division.

• 2 new cells each receive one of the chromatids.

• The centromere holds the two chromatids together until cell division. Centromeres are important for chromosome movement.

• Between cell divisions DNA is not as tightly packed, this way it can be read and can direct cell activity. This loosely packed DNA is called chromatin.

• Prokaryotic cells usually only have one chromosome.

• Each species has a characteristic number of chromosomes. Ex: the Fruit fly has 8

• Some species have the same number of chromosomes i.e. potato, plum, and chimpanzee all have 48

• There are 2 types of chromosomes, autosomes and sex chromosomes.

• Sex chromosomes determine what gender an organism is. It may also carry genes for other characteristics. Women have two X chromosomes, while the men have one X and one Y.

All other chromosomes are autosomes. Humans have 44 autosomes and 2 sex chromosomes.

This makes 46 total chromosomes in every human body cell.

• All cells produced by sexual reproduction have 2 copies of each autosome, they receive one from each parent.

• The two copies are called homologous chromosomes.

• They are the same size, shape, and carry out the same set of traits.

• If one contains the trait for eye color, so does the other one.

• Humans have 22 homologous pairs.

• Cells with two complete sets of chromosomes are said to be diploid.

• Diploid cells have chromosomes for each homologous pair and include sex chromosomes.

• All human cells except for the sperm cells and egg cells are diploid. This is abbreviated 2n.

• Sperm and egg cells are called haploid.

• Haploid cells contain only one complete set of chromosomes, half that of diploid cells, and is abbreviated 1n.

• When 1 sperm cell (1n) and 1 egg cell (1n) combine they make a diploid cell (2n)

• In eukaryotic cells, both the cytoplasm and the nucleus divide

• 2 types of cell division, Mitosis and Meiosis

• Mitosis occurs in the reproduction of unicellular organisms.

• Meiosis reduces the number of chromosomes in half in a new cell, which are joined together later in the cell’s life to complete the set of chromosomes during sexual reproduction.

Cell Cycle• The life of a cell is split between the time of cell

division, and the time in between, which is called interphase.

• Interphase is divided into three phases, and cell division is divided into 2.

• Cell division consists of mitosis, where the nucleus divides, and Cytokinesis, where the cytoplasm divides

Interphase• The 3 phases of Interphase are:

• G phase where the cell grow to mature size.

• S phase where DNA is copied.

• And the G phase, where the cell grows big enough and prepares for cell division.

• Cells can also enter G phase, where no growth occurs. Many human cells are in this stage.– Ex: Cells of a mature human central nervous system.

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