Cell division

what does mitosis produce?

produce 2 diploid daughter cells that are genetically identical to the parent cell

what does meiosis produce?

produces 4 genetically different haploid cells

what does diploid and haploid mean?

diploid — two complete sets of chromosomes present in each cell of an organism

haploid — one complete chromosomes present in each cell of an organism

what are the 5 stages of cell cycle? and what do they entail?

Interphase:

1. G1 → growth phase → cellular content except chromatin duplicates

2. S phase → DNA synthesis, amount of DNA doubled

3. G2 → growth phase and final preparation, cells double. check duplicated chromatin for any error and make necessary repairs

Mitosis → mitosis (PMAT) (division of nucleus)

C phase → cytokinesis (division of cytoplasm)

what does prophase entail

1. chromatin coils, condenses and thickens to form chromosomes

2. centrioles move to opposite poles of nucleus

3. spindle fibers emerge from centrioles in animal cells / spindle fibres form independently in plant cells

4. nucleolus disappears and nuclear membrane breaks down

what is the function of the spindle fibres

to immobilise chromosomes to enable equal split of sister chromatids during anaphase

what does metaphase entail

1. chromatids move and line up along the equator of the spindle fibre or cell

2. each chromatid’s centromere attached to the spindle fibre

what does anaphase entail

1. centromere holding each pair of chromatids together starts to divide

2. spindle fibre condenses, shortens and pull chromatids apart to opposite poles of the cell

3. each chromatid of the chromosome move to the two poles, centromere first

what does telophase entail

1. chromatids reach the poles and can be regarded as chromosomes

2. nucleus reappears and nuclear membrane forms round each group of chromosomes

3. chromosomes uncoil forming chromatin

what does cytokinesis entail in animal cells

1. furrow formed and as furrow deepens, cell surface membrane on each side joins up and 2 separate cells are formed

what does cytokinesis entail in plant cells

vesicles produced by the Golgi apparatus gather at the equators, each containing carbohydrates such as cellulose. they fuse together, forming cell wall between the two separate cells

importance of mitosis

1. growth and repair of tissues by identical cells, after the previous one is destroyed or damaged

2. restore the original number of chromosomes in each daughter cell and ensure daughter and parent cell are genetically identical

3. asexual reproduction, resulting in offspring tat are identical to the parents

4. growth of organisms as the number of cells making up tissues of multicellular organism increases as the organism grows. new cells must be identical to the existing ones

where do meiosis occur

1. gonads — testes and ovary

2. anther — flowering plant

3. ovules

what are the additional steps of prophase I in meiosis?

1. homologous pairing of chromosomes

2. swapping of chromatic material of non-sister chromatids (crossing over)

3. the point at which the exchange of chromatid material is called the chiasma (pl: chiasmata)

what does metaphase I of meiosis entail?

1. homologous chromatids moved up and lined up along the equatorial plane of teh spindle fibre in 2 rows

2. 2 chromosomes of each pair face opposite poles of the cell

3. each chromatid’s centromere is attached to the spindle fibre

what does anaphase I of meiosis entail

1. centromere holding each pair of chromatids together divide

2. spinle fibre condenses, shortens ad pull chromatids apart

3. each chromosome of the homologous pair move to the 2 poles, centromere first

4. each pole receives the same number of chromosomes

what does telophase I of meiosis entail

• Chromosomes reach the opposite poles of the spindle

• Nuclear membrane forms round each group of haploid chromosomes.
  

• Unlike in mitosis, here the chromosomes stayed in their condensed form and meiosis II followed on straight away.
  

• Cytokinesis occurs produced 2 haploid daughter cells.

prophase II, M II, A II

New spindle fibers develop at perpendicular to the old spindle fiber

• Separate chromosomes arrange themselves on the equatorial plane of the spindle.

• Each chromosome attached to the spindle fiber by means of its centromere

centromere divides. spindle fiber condenses, shortens and pull chromatids apart. each chromatid of the chromosome move to the 2 poles, centromere first

telophase II

• Chromatids now reach the poles and can be regarded as chromosomes

• Nucleolus reappears and nuclear membrane forms round each group of chromosomes.

• Chromosome uncoil forming chromatins

• Cytokinesis occurs result in the formation of 4 haploid

cells, each with a different genetic make up.

what contributes to genetic variation?

1. crossing over of homologous chromosomes

   1. Chromosome twisted around each other 🡪 creates tension 🡪 cause breaks along length of chromatids.

   2. During crossing over, corresponding fragments of chromatids may swap over.

   3. This “cutting and sticking” of chromatids means that genetic material is exchanged

   4. This give rise to genetic variation in the gametes to be formed.

2. Random assortment during metaphase I

   1. independent assortment of homologous chromosomes so that we get different combination of maternal and paternal chromosomes at end of metaphase I 

3. fertilisation

   1. During fertilization any male gametes can combine with any female gametes

4. mutation

   1. Sudden changes or spontaneous changes in genes and chromosome numbers can lead to new variation

what are chromomses that have underwent crossing over called, in meiosis?

recombinant chromosomes