Mitosis and Meiosis Flashcards
Describe and explain the significance of the mitotic cell cycle.
Production of genetically identical daughter nuclei with same number and type of chromosomes and the same alleles so that genetically identical daughter cells can be produced for:
- Growth
> increase number of cell by producing more cells genetically identical to existing ones - Regeneration and cell replacement
> damaged cells replaced by cells genetically identical to original ones, retaining the same function, - Asexual reproduction
> produces genetically identical offspring, e.g. vegetative propagation in plants.
Describe the need for regulation in mitosis.
Need for regulation
1.
The cell cycle is tightly regulated as it is important for normal growth & development. Regulation is at certain control points known as G1, G2 and M checkpoints which determine if the cell cycle can proceed.
-If DNA is damaged, halted at G1, preventing cells from entering S phase
-If DNA is damaged/all chromosomes are not replicated proplery, halted at G2, preventing cells from entering mitosis, providing opportunity for repair and stopping proliferation of damaged cells
-If all chromosomes not attached to spindle fibres from both poles, halted at M preventing anaphase, prevent incorrect separation of sister chromatids.
2.
Cancer occurs when the dysregulation of checkpoints of cell division occur or cells escape the cell cycle control mechanism that normally regulates their growth. This leads to uncontrolled division of cells (i.e. tumour formation) and possibly cancer.
Describe, explain significance of interphase.
-Cell replicates its DNA to prepare for nuclear division.
G1: Intensive cellular synthesis of organelles, RNA, protein, ATP etc.
S: DNA replication occurs where DNA Molecules replicate so that DNA content of cell doubles
G2: intensive cellular synthesis of organelles, spindle proteins, ATP, etc.
Describe what happens in prophase I.
1.Chromatin condenses to form chromosomes. Each chromosome
comprises 2 sister chromatids joined at the centromere
2. Synapsis occurs > homologous chromosomes pair up to form
bivalents.
3. Crossing over occurs between non-sister chromatids of
homologous chromosomes, forming chiasmata. Exchange of
corresponding alleles on non-sister chromatids occurs.
4. Centrioles move to opposite poles and spindle fires start to form.
5. Nucleolus disappears & nuclear envelope disintegrates into vesicles.
Describe what happens in Metaphase I.
- Homologous chromosomes align in pairs at the metaphase plate.
i.e. independent assortment occurs (the orientation of 1 pair of
homologues is independent of other pairs) - Each chromosome is attached to the kinetochore microtubules
from the pole it faces.
Describe what happens in Anaphase I.
- Homologous chromosomes / homologues separate to opposite
poles - Each homologue is pulled by a shortening kinetochore
microtubule.
(* No division of centromere here.) - Non-kinetochore microtubules elongate & slide in opposite
directions > elongate the cell.
Describe what happens in telophase I.
- Each pole now has a haploid set of chromosomes.
- Chromosomes decondense to form chromatin.
- Spindle fibres disintegrate.
- Nuclear envelope reforms & nucleolus reappears.
Describe what happens in cytokinesis.
- Animal cells: Cell membrane invaginates towards the equator of
the cell, forming a cleavage furrow. The cleavage furrow deepens
and is pinched into 2 > 2 daughter cells produced - Plant cells: Fluid-filled vesicles appear in the middle of the cell and
coalese to form a cell plate, separating the 2 daughter cells
Explain significance of meoitic cell cycle.
For maintenance of chromosome number in every generation
Reduction division: production of 4 haploid gametes from 1 diploid parent cell
Chromosome number halved so that the chromosome number can be restored upon fertilisation
> Chromosome number of species remains the same after many generations
Describe, explain how events like crossing overs and independent
assortment generate genetic variations
- Crossing over between non-sister chromatids of homologous chromosomes results in new combinations of alleles on chromatids. (& eventually a variety of offspring)
- Independent assortment of homologous chromosomes at the metaphase plate & their subsequent separation during metaphase l & anaphase I respectively &
Random orientation of non-identical sister chromatids of each chromosome at the metaphase plate & their subsequent separation during metaphase Il and anaphase I respectively
-> results in gametes with different combinations of maternal & paternal chromosomes. (& eventually in a variety of offspring)
3.
Random fusion of gamete
-> during sexual reproduction/fertilisation results in offspring with a variety of genotypes & possibly phenotypes (& hence a variety of
offspring)
Explain what are homologous chromosomes.
-Carry the same genes controlling the same inherited characteristsics at the same loci but may not have the same alleles.
-one of maternal, other of paternal origin - similar in size, shape, centromere position and staining pattern
-Sister chromatids are from DNA replication, thus genetically identical i.e. same alleles
Describe what are haploid/diploid cells.
Haploid: contains one complete set of chromosomes
Diploid cell: contains two complete set of chromosomes i.e. exists as homologous pairs. Each set is from one parent.
