D2.1 Cell and Nuclear Division Flashcards
What is cytokinesis?
- The splitting of cytoplasm in a parent cell between daughter cells.
- In animal cells:
– Actin and myosin proteins form a contractile ring at the equator –> forms cleavage furrow.
– As the proteins contract, they pull the plasma membrane to the center eventually separating the cell into two daughter cells. - In plant cells:
– Vesicles line up at the equator and fuse to form two cell membranes. –> forms cell plate
– This expands outwards to the sides of the cell creating a cell wall separating the new daughter cells.
Equal cytokinesis
Both daughter cells are the same size and must receive at least one mitochondrion. In plants, they have a chloroplast.
Unequal cytokinesis
- Happens when there is unequal sharing of the parent cell’s resources.
- Nutrients are donated from the smaller daughter cells to the larger one that is meant to survive and grow.
- Eg. budding in yeast cells and oogenesis in humans
Define mitosis
Nuclear division results in two identical, diploid daughter cells.
Define meiosis
A nuclear division that results in haploid daughter cells.
What is the role of mitosis and meiosis in eukaryotes?
- Nuclear division is needed before cell division to avoid the production of anucleate cells.
- Mitosis maintains the chromosome number and genome of cells.
- Meiosis halves the chromosome number and generates genetic diversity.
What happens before mitosis and meiosis?
DNA replication occurs during interphase which produces chromatids held together by a centromere.
Outline DNA condensation
- DNA is wrapped around histones which helps organize DNA. –> chromatin.
- The DNA is coiled, then supercoiled so that the coils are stacked on top of each other to form a compact pair of chromatids.
- The condensed structure ensures that different DNA molecules can be transported at once rather than being spread out.
Explain the phases of mitosis (same process as 2nd division of meiosis)
- Prophase:
- Nucleolus disappears.
- Chromatin condenses to form chromosomes.
- Mitotic spindle forms and kinetochores attach to it.
- Centrosomes move to opposite poles of the cell as a result of lengthening microtubules. - Metaphase:
- Chromosomes move to the equator of the cell.
- Centromeres align on the plate.
- The chromosomes move as a result of the action of the spindle.
- The centrosomes are at opposite poles. - Anaphase:
- Sister chromatids separate at kinetochores
- Move to opposite sides of the cell due to motor proteins (actin and myosin) pushing microtubules in opposing directions. - Telophase:
- Set of chromosomes at each pole.
- Nuclear membrane reforms.
- Nucleoli reappears.
- Spindle fibres break down.
- Cell is ready for cytokinesis.
What kind of division is meiosis?
A reduction division that results in haploid daughter cells, so that when fertilization occurs, the 46 number of chromosomes is preservation.
Outline the process of meiosis
- DNA replication
- First division of meiosis separates homologous pairs (Interphase + PMAT + cytokinesis)
- Second division separates sister chromatids (PMAT)
Down syndrome
Caused by an extra copy of the 21st chromosome hence, the name trisomy 21. Arises from non-disjunction.
How is meiosis a source of variation?
Crossing over:
- Sister chromatids are homologous but not identical, some alleles of the exchanged genes are likely to be different therefore, new combinations are produced.
Random orientation of bivalents:
- Each homologous pair forms a bivalent on the equator of the cell. The homologous chromosomes attach to spindle microtubules from different poles. The pole it attaches to is decided by chance because the orientation of bivalents is random in this stage.
Importance of cell division
Medicine, tissue repair, growth
Explain the 1st division of meiosis (division of homologous pairs)
- Prophase I:
- Nuclear membrane dissolves, centrosomes move and spindles formed.
- Chromosomes condense –> becoming visible
- Homologous chromosomes pair with their homolog, aligned gene by gene
- Sister-chromosomes of non-homologous pairs cross over at the chiasmata –> forms daughter chromosomes with new combination of alleles
- Microtubules start to attach to the two kinetochores of each homolog. - Metaphase I:
- Homologous pairs of sister chromatids align at the equator (each chromosome facing each pole).
- This assortment of homologous chromosomes is random.
- Both chromatids of one homolog are attached to microtubules at the kinetochore from one pole, those of the other homolog are attached to microtubules from the opposite pole. - Anaphase I:
- Spindle microtubules contract and pull the homologous pairs apart, causing the homologs to move towards opposite poles.
- One chromosome from each bivalent moves to either of the 2 poles
- This process halves the chromosome numbers to become haploid
- Sister chromatid cohesion persists at the centromere, causing chromatids to move as a unit - Telophase I:
- Sister chromatids have reached opposite poles and remain together
- Each half of the cell has a complete haploid set of duplicated chromosomes, each composed of two sister chromatids
- Cytokinesis occurs
- New nuclear membrane forms as the cell membrane pinches towards the centre
- A cleavage furrow in the membrane will fuse at a central point
