Edexcel IGCSE Biology 复习笔记 3.2.5 Cell Division

• Mitosis is defined as nuclear division giving rise to genetically identical cells
• This type of cell division is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction and is known as mitosis
• Most body cells have two copies of each chromosome
• We describe these cells as diploid
• When cells divide their chromosomes double beforehand (also known as duplication)
• This ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)

The process of cell division by mitosis

Process

• Just before mitosis, each chromosome in the nucleus copies itself exactly (forms x-shaped chromosomes)
• Chromosomes line up along the centre of the cell where cell fibers pull them apart
• The cell divides into two; each new cell has a copy of each of the chromosomes

Importance

• All cells in the body (excluding gametes) are produced by mitosis of the zygote
• Mitosis is important for replacing cells e.g, skin cells, red blood cells and for allowing growth (production of new cells e.g. when a zygote divides to form an embryo)

Occurs in

• Growth: mitosis produces new cells
• Repair: to replace damaged or dead cells
• Asexual reproduction: mitosis produces offspring that are genetically identical to the parent

• Meiosis is a type of nuclear division that gives rise to cells that are genetically different
• It is used to produce the gametes (sex cells)
• The number of chromosomes must be halved when the gametes (sex cells) are formed
• Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)
• This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
• It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell
• After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete
• We describe gametes as being haploid - having half the normal number of chromosomes
• Because of this double division, meiosis produces four haploid cells

The process of cell division by meiosis to produce haploid gamete cells

Process

• Each chromosome makes identical copies of itself (forming X-shaped chromosomes)
• First division: chromosomes pair up along the centre of the cell, recombination occurs and then cell fibres will pull the pairs apart, each new cell will have one of each recombinant chromosome pair
• Second division: chromosomes will line up along the centre of the cell, cell fibres will pull them apart (as with mitosis)
• A total of four haploid daughter cells will be produced

Importance

• Production of gametes e.g. sperm cells and egg cells, pollen grains and ovum
• Increases genetic variation of offspring
• Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others

Comparing Mitosis & Meiosis Table