Molecules, Cells and Variation - 1.5 +1.6 Flashcards
Mitosis
- Type of nuclear division that produces genetically identical daughter cells.
- During, DNA replicates in the parent cell, and is distributed so each daughter cell is identical.
- Only source of variation in the cells is via mutations
Why is mitosis important?
For growth and repair of tissues, and in asexual reproduction.
Somatic cell
Any cell in an organism other than the reproductive cells. During mitosis, the chromosome homologous pairs line up separately from each other.
Stages of mitosis
Interphase Prophase Metaphase Anaphase Telophase
Interphase
Represents the non-dividing cell and includes the stages G1, S and G2. Cell is carrying out its normal cellular functions. Towards the end, the cell prepares for cell division by carrying out a number of processes:
- DNA content is doubled via DNA replication.
- Other cell organelles replicate
- RNA and protein synthesis also occurs.
- ATP content increases, as cell division is active
Prophase
1) Chromatin organises into chromosomes. Each chromosome now consists of two genetically identical sister chromatids,
2) Each chromosome condenses by spiralisation.
3) Centrioles move to opposite poles of the cell. Nucleoli and nuclear membrane break down.
Why does condensation of chromosomes occur during prophase?
Prevents tangling during cell movements.
Metaphase
1) Centrioles at the poles form a spindle across the cell.
2) Chromosomes move to equator of cell and attaches to spindle via its centromere, with its sister chromatids orientated towards opposite poles of the cell.
3) There is no association between members of each homologous pair on the spindle
What do spindle fibers consist of?
Microtubules
Not present in plant cells
Anaphase
1) Centromere splits and sister chromatids separate.
2) Sister chromatids are pulled to opposite poles of the cell by contraction of the spindle microtubules.
3) Numerous mitochondria supply energy for this
Telophase
1) Nuclear membranes reform around chromatin and organelles.
2) Sister chromatids represent chromosomes of each new cell, identical in genetic content to each other and to the parent cell.
3) Two distinct cells form as cytoplasmic cleavage occurs and a cell membrane forms between the cells
Cytokinesis
- Last stage of the cell cycle before entering G1 this is cell division to produce two daughter cells
- Organelles move towards the poles of the cell.
- Cell membrane constricts at the equator of the cell
What occurs in the final stages of plant cell mitosis?
A cell plate forms where new cell wall material is organised to form a new primary cell wall that eventually fuses with the old cell walls separating the two cells.
Importance of meiosis
- Ensures haploid gametes are produced from a diploid cell. Diploid number restores during fertilisation.
- Ensures constant number of chromosomes at each generation.
- Produces genetic variation in gametes.
Process of meiosis
- A cell divides twice, DNA replication occurs only once.
- Number of chromosomes is reduced from 2n to n.
- A cell produces 4 cells which are genetically varied.
- Cells produced usually function as gametes.
What processes in meiosis produce genetically different daughter cells?
- Independent segregation of homologous chromosomes
- Genetic recombination by crossing over
Genetic recombination
Exchange of genetic material either between multiple chromosomes or between different regions of the same chromosome
What occurs during the first meiotic division?
Homologous chromosomes pair, then separate so one member from each pair enters the gamete. This is independent from the separation of another pair. So, the chromosomes randomly associate in a gamete.
