As we grow, why don’t the cells just get bigger instead of sub-dividing?
Cells are limited in size because the outside (cell membrane) must transport food and oxygen to the parts inside. As a cell gets bigger, the outside is unable to keep up with the inside, because the inside grows at a faster rate.. This can be represented by what is called the surface to volume ratio, or S: V, or S/V. In a cell that is one unit in size, the surface area is 6 square units and the volume is 1 cubic unit. The ratio then is 6/1. As a cell gets larger, this ratio gets smaller, meaning the cell membrane cannot supply the inside with what it needs to survive.
The main functions of cell division (mitosis) are growth and repair. Some cells once fully formed do not undergo cell division, such as nerve cells and muscle cells. These do not re-grow or repair once they are mature.
However, to enable growth in size from the new-born to adult phase, all of the cells must undergo cell division. This is true of all biological life. The same principle counts for body or tissue repair, such as when a cut or injury of tissue or broken bones occur – the cells along the edges of the cut undergo mitosis to repair the injury. Through inter-cell communication, growth factors (protein structures) stop the division process through a code referred to as contact inhibition. If these proteins are altered by mutation, the contact inhibition controls are lost, and the cells will continue to divide. This results in tumours or cancer.
The process of cell division is described though 2 types, called mitosis and meiosis.
Mitosis
Definition: Mitosis is a part of the cell cycle in which chromosomes in a cell nucleus are separated into two identical sets of chromosomes, and each set ends up in (with?) its own nucleus.
It is the process whereby the cell duplicates all its contents, including its chromosomes, to form 2 identical cells, called daughter cells. Because this process is so critical, mitosis is carefully controlled by a number of genes. Mistakes made during mitosis can result in changes in the DNA, which can potentially lead to genetic disorders.
After mating, when the male sperm fertilise the female ovum, a single cell forms that consist of 50% of genes provided by the mother, and 50% by the father. New life starts as a single cell. As this cell starts duplicating, it will receive codes from the DNA in the nucleus to grow new protein structures (The process of conforming genes into proteins) which will eventually grow into different types of tissues and organs.
The process of mitosis is divided ito 5 phases:
Meiosis
Meiosis is a process whereby the chromosome number is divided in half, to form sperm and egg cells. When the sperm and egg cells unite again during conception, the male sperm cell will again contribute 50 % of the chromosomes and the female egg cell will contribute the other 50 % of the chromosomes so that the new formed cell which develops into a new embryo of life, will contain the full count of chromosomes. The process of meiosis can be divided into 9 stages. The process of meiosis can be divided into 9 stages. These stages are further divided into the first time the cell divides (Meiosis I) and the second time it divides (Meiosis II).
For example in Beef cattle (Bos taurus), the chromosome count is 60, of which 30 chromosomes are supplied by the female egg cell ( thefemale gamete or ovum) and 30 chromosomes by the male sperm cell (the male gamete), to form a new cell (called a zygote) with the full 60 chromosomes that will develop through a process of mitosis into a new embryo.
The process of meiosis also allows for genetic variation through a process of DNA shuffling, whilst the cells are dividing.
During meiosis, one cell divides twice to form 4 daughter cells. Each of the 4 daughter cells contain only 50% of the chromosomes count of the original parent cell. These cells are referred to as being haploid. The haploid number is the number of chromosomes within the nucleus of a cell that constitutes one complete chromosomal set. This number is commonly abbreviated as n, where n stands for the number of chromosomes. The haploid number will be different for different organisms. In cattle, the haploid number is expressed as n=30. Haploid cattle cells have 1 set of 30 chromosomes:
Each of the 4 daughter cells further contains 2 of each type of the parent chromosome, which refer to the term that they are diploid. In other words: the diploid number, e.g. 60 in cattle, is the normal chromosomal complement of an organism’s somatic cells, that are having paired sets of chromosomes in a cell or cell nucleus.In diploid organisms that reproduce sexually, one set of chromosomes is inherited from each parent.
The process of meiosis
Meiosis 1
Meiosis 2
