Cells were discovered in 1665 by the English scientist and inventor …
Cells were discovered in 1665 by the English scientist and inventor
The concept that cells are the basic units of life became embodied in a
theory called the cell theory:
•cells form the building blocks of living organisms
•cells arise only by the division of existing cells
Cell division starts with thedivision of the nucleus. There are two forms of nuclear division: mitosis and meiosis.
Mitosis: two identical daughter cells.
Meiosis: four different daughter cells.
• cells contain inherited information which controls their activities
• the cell is the functioning unit of life; metabolism (the chemical reactions of life) takes place in cells
• given suitable conditions, cells are capable of independent existence
A typical animal cell:
•the cell has a cell surface membrane which encloses the cell contents
•the contents consist of a central ball-shaped nucleus surrounded by material called cytoplasm
•the nucleus contains a fibrous material called chromatin
•this condenses to form chromosomes during cell division
•chromatin contains DNA, the material which controls the various activities inside the cell structures. They have been described as the “power-houses” of the cell because
•scattered within the cytoplasm are mitochondria, small rod-like they supply energy.
•smaller dots within the cytoplasm are particles of stored food. Many consist of glycogen, which is a food storage polysaccharide.
A typical plant cell:
•most plant cells have a large sap-filled cavity called the vacuole. Sap is awatery fluid containing salts and sugars. The vacuole surrounded by amembrane called the tonoplast.
•the cytoplasm contains starch grains, the food storage products of plants pigments used in photosynthesis. Chlorophyll, which is green, is the main
•many plant cells have chloroplasts in the cytoplasm. These contain thepigment. Chloroplasts occur only in the parts of plants exposed to light – thegreen parts. They are absent from underground structures such as roots.
3 main stages the cell cycle:
During interphase, the cell grows, carries out its functions, andreplicates its DNA. After the DNA is replicated, new protein becomes attachedto it. The chromosome now consists of two strands called sister chromatidswhich contain identical genetic information. Sister chromatids are joined atsome point along their length by a centromere. These become visible under thelight microscope only during mitosis. Typically, interphase lasts for about 90per cent of the cell cycle.
Nuclear division takes place during mitosis. The chromatids containingreplicated DNA are separated from each other and are redistributed aschromosomes in the nuclei of the two new daughter cells.
In cell division (also called cytokinesis) the cytoplasm divides to form
two daughter cells.