Cell Divison- Mitosis
Mitosis involves 6 stages:
Interphase, Prophase, Metaphase, Anaphase, Telophase and Cytokinesis Mitosis produces genetically identical copies of cells with the same number of chromosomes, used for growth, repair and asexual reproduction The daughter cells are diploid because they have one two copies of each chromosome and the pairs are homologous ie have the same gene sequence (Both parts of the chromosome which is separated have the same info)
Chromosome- contains one molecule of DNA
DNA- is made up of genes ans codes for proteins
Daugther Cells- the two gentically identical cells which are formed from one nucleus
Chromatin- DNA wrapped around histone proteins
Chromatid- a strand of a chromosome
Cell Divison- Mitosis, Interphase
1. Interphase has two sections:
- Growth occurs
- Cell decides whether or not to divide- If it does not divide it will differentiate and become specialised to provide replacemant for damaged cells
- If it does divide changes occur in the chromatin (DNA wrapped around histone proteins) which allow it to be copied
- The chromatin is copied- DNA replication
- Each chromosome thus becomes a pair of identical chromatids held together at the centromere
- A second shorter growth phase in which the cell prepares to divide by genertaing all the materials it needs eg mRNA (needed to condense the information in the chromosomes)
Cell Divison- Mitosis, Prophase
At the end of G2, when the cell is ready to divide Prophase begins
- The divison of centrioles- (tiny structures made of rings of microtubules in pairs) They replicate and then each pair begins to to move to opposite ends of the cell
- From here they will act to organise the spindel apparatus which organises chromosome separation
- The nuclear envelope also dissagregates into tiny vesicles which remain in the cytoplasm until reused to form the daughter cell nuclear membrane
- Spiralization of the chromatin occurs which cause the chromosomes to become thicker and shorter and so become visible to a light microscope.
- Towards the end of prophase each chromosome can be seen to consist of a pair of chromatids lying side-by-side joined at a centromere.
Cell Divison- Mitosis, Metaphase & Anaphase
Metaphase is the thrid stage to occur
- Once the centrioles have reached the poles of the cells they begin to organise the spindle
- Some extend from pole-to-pole , other become linked with the centromeres of each chromosome
- Then once each centromere becomes linked to both poles motor proteins create tension in the spindle and reposition the chromosomes
- All centromeres reposition into the equatorial plane of the cell
Anaphase is the fourth stage
- Once the tension in the spindle has been created there is a concerted enzyme catalayse spliting of the centromeres
- This meas that the sister chromatids become pulled apart
- The spindle fibres continue to shorten , pulling the sister chromatids towards the poles.
- They assume a 'V' shape because the centromeres lead.
Cell Divison- Mitosis, Telophase and cytokinesis
- As the chromosomes reach the poles of the cell a nuclear membrane reforms
- Within this the chromosomes decondense so that they're no longer visible to the light microscope
- The spindle apparatus is dismantled
Next in plant cells cytokinesis occurs:
- A furrow around the equatorial plane gets deeper due to a ring of fillaments which are linked to integral membrane proteins
- The ring constricts causing an hour-glass shape and then eventually the waste pinches off intirely- creating two separate daughter cells
This process doesnt happen in plants because of cellulose in the cell wall which cannot constrict .
- A cell plate formation occurs- the ER and Golgi apparatus synthesis fresh middle lamella which is layed down across the equatorial plane
- New primary cellulose wall assesmbles on either side and eventually the 2 daughter cells separate
- Except for thins strands of cytoplasm through the plasmodesmata which allows sharing of resources such as water mineral ions and sugars.