Chapter 10 - Cells divide for growth, repair, replacement and reproduction

Average life span of human cells

The events that occur in the cell cycle have been divided into a number of phases:

The stages of mitosis in a cell with two chromosomes

Interphase

Prophase

Two pairs of centrioles become visible early in prophase. They move to opposite ends of the cell and microtubules begin to radiate from them. At the same time, the nucleolus disappears and the nuclear membrane begins to break down. The chromatin threads of DNA become tightly coiled and can be seen as chromosomes. Coiling the long, delicate DNA molecules makes it easier to distribute the DNA to the daughter cells. Each chromosome consists of two chromatids, which are joined at a point called the centromere. The two chromatids are identical, tightly coiled DNA molecules produced from DNA replication during interphase.

Metaphase

During metaphase, the chromatid pairs line up on the equator of the spindle. The centromere of each pair is attached to a spindle fibre.

Anaphase

In anaphase, each pair of chromatids separates at the centromere. As the chromatids have become independent of each other, they are now each called chromosomes. The new chromosomes are then pulled away from one another towards opposite poles of the cell. The centromeres are still attached to the spindle fibres, and it seems that the spindle fibres pull the chromosomes apart in some way.

Telophase

During telophase, the two sets of chromosomes form tight groups at each pole of the cell. A nuclear membrane forms around each group, and a nucleolus appears in each new nucleus. The spindle fibres disappear, and the chromosomes gradually uncoil to become chromatin threads once more.

Cytokinesis

Division of the cytoplasm is called cytokinesis. A furrow develops in the cytoplasm between the two nuclei. The furrow gradually deepens until it cuts the cytoplasm into two parts, each with its own nucleus. (Note: Although the term ‘mitosis’ is commonly used to refer to cell division, it technically refers just to the division of the nucleus.)

Summary of cell division

differentiation.

cells undergo division by mitosis, different genes become activated. This makes the cells differentiate into specialised cells that can perform particular functions

Stem cells

The cells that can undergo differentiation are called stem cells.

Totipotent stem cells

have the ability to form the embryo and the membranes that will surround, support and nourish it. An example is the early embryo before the formation of the inner cell mass.

Pluripotent stem cells

are capable of giving rise to most, but not all, tissues of an organism. An example is the inner cell mass.

Multipotent stem cells

are able to give rise to cells that have a specific function. An example is blood stem cells.

First division:

The homologous pairs separate and two daughter cells form with 23 chromosomes, each with two chromatids.

Second division:

The chromatids separate, resulting in four daughter cells with 23 chromosomes, each with one chromatid.

Summary of meiosis: one diploid parent cell produces four haploid daughter cells

Second meiotic division

Comparison of mitosis and meiosis

Crossing over

An important feature of meiosis occurs during the prophase of the first meiotic division. When the homologous chromosomes are paired, the chromatids may cross, break and exchange segments. This is called crossing over and the point where two chromatids cross is called a chiasma. Crossing over can result in a new combination of alleles along the chromosome. This is called recombination. Therefore, crossing over creates new combinations of genes so that the chromosomes passed on to the offspring are not exactly the same as those inherited from the parents.

Non-disjunction

During the first division of meiosis, the homologous chromosomes pair and then separate. Sometimes one or more of the chromosome pairs may fail to separate when the cell divides. In the second meiotic division, one or more of the chromatids may fail to separate. These situations are called non-disjunction, and they result in one of the daughter cells receiving an extra chromosome and the other daughter cell lacking that chromosome

Monosomy

Monosomy is where an individual is missing a chromosome – they have only one copy instead of the normal two. Like trisomy, monosomy usually results in severe malformations and often miscarriage.

Haploid number

The number of chromosomes in a cell with only one chromosome from each homologous pair; half the diploid number

Homologous chromosomes

The pairs of chromosomes containing genes that control the same characteristics

Independent assortment

The random combination of alleles due to allele pairs separating independently of one another