Cell Division
Introduction
All living organisms grow and reproduce. Since living organisms are made of cells, it follows that cells must be able to grow and reproduce. They do this by dividing and passing on their genes to daughter cells
There are two fundamentally different kinds of cell: prokaryotic, those which lack nuclei, and eukaryotic, those which possess nuclei containing their genetic information in the form of DNA. Accordingly, there are two fundamentally different kinds of cell division: that which takes place in prokaryotes and that which takes place in eukaryotes.
Division in Prokaryotic Cells
Prokaryotes, usually known as bacteria, being for the most part single-celled, are simpler organisms than eukaryotes. They are able to divide by binary fission, in which each cell simply splits in two to produce two genetically identical daughter cells. Simple multicellular organisms, such as hydra, can reproduce by budding, a process in which a daughter organism is formed in miniature on the side of the parent organism and splits off to become autonomous.
Reasons for Cell Division in Eukaryotes
One of the most conspicuous structures in eukaryotic cells is the nucleus, which controls the activity of the cell through the use of the genetic material, DNA. nuclear division combined with cell division allows cells to reproduce themselves. Division in eukaryotic cells occurs for one of two purposes
Growth - For a diploid zygote to grow into a multicellular diploid adult, the daughter cells must have the same number of chromosomes as the parent cell. This type of division is known as mitosis.
Sexual Reproduction - If an organism’s life cycle contains sexual reproduction then there must be a point in the life cycle when the number of chromosomes is halved so that the gametes will contain only one set of chromosomes. If this were not to happen then the number of chromosomes would double at each generation. This type of division is known as meiosis.
Two types of division in Eukaryotic Cells
1. Mitosis
Mitosis is a form of nuclear division that produces two genetically identical daughter nuclei, each containing the same number of chromosomes as the parent nucleus. Although the process is continuous, it is usually divided into four main stages for convenience.
Prophase
During early prophase, the centriole replicates and the chromosomes start to coil up, becoming shorter and thicker. They become visible because they stain more intensely.
During late prophase, the centrioles move to opposite poles of the nucleus and the nucleolus and nuclear envelope break up.
Metaphase
Each centriole reaches a pole and helps to organize the production of the spindle microtubules
The chromosomes line up across the equator of the spindle which they are attached to by means of their centromeres.
Each chromosome splits at the centromere and the component chromatids are pulled apart.
Anaphase
The chromatids move to opposite poles, centromeres first, pulled along by the microtubules
Telophase
The chromatids reach the poles of the spindle and uncoil again.
The spindle begins to break down and cytokinesis – the division of the cytoplasm and cell into two by constriction from the edges of the cell
Cell division can take place now that nuclear division has occurred. During interphase, the period between cell divisions, the chromatids will replicate themselves as will the centrioles.
2. Meiosis
Unlike mitosis, meiosis involves two divisions, called meiosis I and meiosis II. meiosis I is a reduction division, resulting in the formation of two daughter nuclei each with half the number of chromosomes of the original parent. In meiosis II, the chromosomes behave as in mitosis, so that each of the two haploid daughter nuclei divides again. meiosis therefore results in the formation of four haploid daughter nuclei. The full complement of chromosomes is restored when a male gamete fertilizes an egg. Half the chromosomes - and therefore half of the genes - of the fertilized egg are received from each parent.
Early Prophase I - As mitosis early prophase
Middle Prophase I
Homologous chromosomes pair up in a process known as synapsis. Each pair is called a bivalent
centrioles move to opposite poles of the nucleus
Late Prophase I
Nuclear envelope breaks up
Crossing over of chromatids may occur, combining paternal and maternal DNA
Nucleolus breaks up
Metaphase I
Spindle formed as in mitosis
bivalents line up across equator of spindle attached by centromeres
Anaphase I
centromeres do not divide, unlike mitosis
Whole chromosomes move towards opposite ends of the spindle, centromeres first, pulled by microtubules
Telophase I
chromosomes reach poles of spindle
Nuclear envelope and nucleolus reform
Cytokinesis occurs to produce two daughter cells each with 23 chromosomes.
Prophase II
Nuclear envelope and nucleolus disperse
centrioles replicate and move to opposite poles of the cell
Metaphase II
chromosomes line up separately along the equator of the spindle
Anaphase II
The centromeres divide and the spindle microtubules pull the chromatids to opposite poles of the cell
Telophase II - As mitosis telophase except with the formation of four haploid daughter cells rather than two diploid cells