Cell division is a key part in the life of any organism. It is a regulated process and occurs through either mitosis or meiosis.
The Cell Cycle
"Know that the cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis."
Cells divide through mitosis on a regular basis to produce two identical daughter cells which can be used for growth and repair. The cycle consists of three main stages:
The process achieves three things:
- Growth of the zygote into an adult
- Repair and replacement of cells
- Asexual reproduction
"Understand what happens to genetic material during the cell cycle, including the stages of mitosis."
The cell cycle results in the formation of two new genetically identical daughter cells.
- G1 Phase - a period of rapid growth producing more organelles, nucleotides and histones.
- S Phase - the cells DNA is duplicated forming double stranded chromatins (chromosomes).
- G2 Phase - next phase of growth in which everything is duplicated so that the cell can start to divide.
This is when the nucleus divides, the process is split into 4 stages.
- Chromosomes condense
- Nuclear envelope disappears
- Centrioles move to opposite sides
- Spindle fibres form and attach to the centromere
- Chromatids are pulled to the centre
- Line up along the cell equator
- Chromosomes divide at the centromere
- And the sister chromatids separate
- The spindle fibres contracts and chromatids are pulled to opposite sides of the cell
- Nuclear membrane reforms around each set of chromosomes
- Thee chromosomes start to elongate
It is in this stage that cell actually sperate. In cells with a cell wall there is one additional stage.
Small gaps will remain and these are known as the plasmodesta. They allow transport and communication between cells.
- The cytoplasm splits and the nuclear membrane is pinched inwards
- In plants the cell wall must also reform
- Spindle fibres remain and direct the golgi to the centre of the cells
- Centrioles move to opposite sides, cellulose is released and these fuse together to form a new cell wall
Growth, Repair & Asexual Reproduction
"Understand how mitosis contributes to growth, repair and asexual reproduction."
Growth is a permanent increase in the cell number, size or mass. Measuring growth is not a straight forward process as mass varies with water intake. To overcome this problem scientists usually compare growth using a dry mass. However, this kills the animal so further growth cannot be measured.
Patterns of Growth
- Continuous growth occurs throughout life with most growth taking place at the beginning and continuing until maturity.
- Discontinuous growth occurs inconsistently. For example, insects grow through a series of moults, shedding their exoskeleton.
This occurs when mitosis results in one parent dividing into two genetically identical clones. This can be achieved in a few ways:
- binary fission
- runner stems and roots
"Understand how meiosis results in haploid gametes, including the stages of meiosis."
This results in 4 daughter cells which are genetically different. It forms gametes which are haploid (n) and contain half the necessary genetic information.
- The first division is the same as that of meiosis except that the chromosomes do not divide at the centromere.
- This results in two diploid (2n) cells.
- These two cells then divide again to form four gametes which are haploid (n).
"Understand that meiosis results in genetic variation through recombination of alleles, including independent assortment and crossing over."
Meiosis always results in genetic variation. This variation is introduced in two ways:
- During anaphase, chromosome pairs are pulled randomly into each cell
- This results in each cell containing a different mixture of paternal and maternal chromosome
Crossing Over (Recombination)
- During prophase I, homologous pairs of chromosomes line up
- Sections can break off and rejoin
- This again results in changes to the genome resulting in variation
- The point of each break on the chromosome is known as the chiasmata
"Understand what chromosome mutations are, as illustrated by translocations."
These mutations occur when part of a chromosome breaks off and becomes reattached in the wrong place. There are 5 different types of chromosome mutation:
If the translocations are balanced, the individual is usually healthy. However, if the translocations are unbalanced the individual may have serious health problems.
- Translocation - the joining of part of a chromosome to another, non-homologous chromosome.
- Deletion - occurs when part of a chromosome breaks off and is lost during cell division.
- Duplication - when extra copies of a gene are formed in a chromosome.
- Inversion - when part of the chromosome is reversed and the reinserted.
- Isochromosome - occurs when the centromere does not divide properly.
"Understand how non-disjunction can lead to polysomy, including Down’s syndrome, and monosomy, including Turner’s syndrome."
This occurs when chromosomes fail to separate correctly resulting in cells having too many or not enough chromosomes.
Either of these situations can be referred to as aneuploidy. This just means the wrong number of chromosomes.
- Monosomy - this is lacking a chromosome (Turner's Syndrome).
- Polysomy - this is having an extra chromosome (Down's Syndrome)