Cell Division Flashcards
What happens during gap (G1 and G2) phase?
The cell:
- Builds up a large store of energy
- Manufactures proteins
- Synthesises organelles
-G1 phase occurs after cytokinesis, longest phase in the cell cycle
Describe what happens during synthesis
- DNA replication occurs
- Amount of DNA content in the cell is doubled
- DNA cannot be seen clearly because DNA is still loosely packed in chromatin
Describe what happens during early prophase.
- Chromatin threads become thicker and shorter (condenses) forming distinct chromosomes
- Nucleolus disappears
- In animal cells, the two pairs of centrioles move to opposite ends of the cell
Describe what happens in the cell during metaphase.
- Mitotic spindle fully forms
- Chromosomes line up at the equator of the spindle.
Describe what happens during late prophase.
- The nuclear envelope breaks down
- Microtubules develop from each pair of centrioles to form the spindle fibres
- The microtubules attach to the centromeres of the chromosomes and move the chromosome towards the centre of the cell.
What happens during anaphase?
-The sister chromatids of each chromosome are pulled apart by
the spindle fibres / microtubules shortening
-The separated sister chromatids of each chromosome move to
opposite poles of the spindle
- Poles of cell pushed further
apart. Cell elongates. - Lengthening of microtubules to elongate the cell
Describe what happens during telophase
- The chromatids reach their respective poles of the spindle:
- Telophase is the REVERSE of prophase:
- Nucleolus reappears
- Nuclear envelope reforms to form two daughter nuclei
- Chromosomes uncoil to become long & thin chromatin threads
- Spindle fires disintegrate
Describe the process of cytokinesis.
-In animal cells, a cleavage furrow forms and pinches the cell in
two, producing two daughter cells
-In plant cells, a cell plate forms, growing outwards until it
touches the parental walls forming two separate cells
What are homologous chromosomes?
-A diploid cell has two sets of chromosomes, one derived from each parent
-Any two chromosomes that
determine the same characteristics are called homologous chromosomes
-Although homologous chromosomes determine the same characteristics, they need not be identical.
- They can contain different alleles of the same gene
e. g. gene for eye colour has alleles for blue and black iris
Why is mitosis important?
-Production of genetically identical cells is required for:
- Growth of an organism
- Repair of worn-out parts of tissue, wound healing and replacement of dead/damaged cells
- Asexual reproduction
Why must DNA replication and mitosis be precisely controlled?
- To ensure that all daughter cells are genetically stable
- Prevent gene mutation - may lead to uncontrolled division of cells - cancer
What are the causes of cancer?
- Age - chances increase with age
- Chemicals - carcinogenic chemicals e.g. tar
- Radiation - e.g. too much exposure to UV radiation in sunlight causes skin cancer
- Viruses e.g. Cancer of the cervix caused by papilloma virus
- Genetic factor
What happens during Meiosis I - Prophase I?
-Same things that happen during mitosis
-Unlike Mitosis:
-Synapsis occurs, where homologous
chromosomes pair up, forming a
bivalent
-Crossing over between chromatids
of homologous chromosome may
occur
-The point where they cross over is
called a chiasma (plural: chiasmata)
This results in new combination of
alleles along the chromosomes
What happens during Metaphase I?
-Meiotic spindle is now fully formed
-Bivalents arrange themselves at the equator of the spindle, aided by the spindle fibres
-The arrangement of chromosomes of each bivalent is completely independent of the orientation of other bivalents
-Centromeres of chromosomes are attached to individual spindle
fibres
What happens during Telophase I and cytokinesis I?
-Chromosomes reach opposite poles
-Spindle fibres disintegrate
-In animal cells, nuclear envelope and nucleolus reforms and cytokinesis occurs to form two haploid (n) daughter cells
-Most plant cells do not undergo Telophase I and move straight to
Metaphase Il
