Chapter 6 - Cell Cycle Flashcards
What is the cell cycle?
An ordered sequence of events that takes place in cells, resulting in division of the cell, and the formation of two genetically identical daughter cells.
Explain interphase.
- cells do not divide continuously.
- it is a long period of growth and carrying out separate functions. Cell spends majority of its time in this phase.
- takes place between devisions. (Not a stage in division).
During interphase: - dna replicated + checked for errors in nucleus.
- protein synthesis occurs in cytoplasm.
- mitochondria grow + divide.
- chloroplasts grow + divide (in plant cells).
- normal metabolic processes occur (e.g respiration).
What are the 3 stages of interphase?
G1- first growth phase. Proteins from which organelles are synthesised are produced. Organelles replicate. Cell increases in size.
S- synthesis phase. Dna replicated in nucleus.
G2- second growth phase- cell continues to increase in size, energy stores increased. Duplicated dna checked for errors.
What is the mitotic phase?
The period of cell devision. Has 2 stages:
- Mitosis- nucleus divides.
- Cytokinesis- cytoplasm divides to form 2 cells.
What is G0 ?
The phase where the cell leaves the cycle, either temporarily or permanently.
Reasons for this:
1. differentiation- cell becomes specialised to a particular function and is no longer able to divide.
2. DNA may be damaged- a damaged cell cannot divide and enters a period of permanent cell arrest (G0).
3. As you age, the number of cells in your body increases. Growing numbers of senescent cells have been linked to diseases (cancer/arthritis).
Some cell types that enter G0 can be stimulated to go back to cell cycle and divide again. (Eg. Lymphocytes)
What must be controlled about the cell cycle?
It is vital to ensure that the cell only divides when:
- it has grown to the right size.
- the replicated dna is error free.
- chromosomes are in their correct positions (during mitosis).
This ensures the fidelity of cell division- that 2 daughter cells are created from the parent cell.
What are check points?
The control mechanisms of the cell cycle.
Monitor and verify whether processes at each stage have been completed accurately before progressing to next stage.
What is the first checkpoint and its role?
G1 checkpoint:
- checkpoint at the end of G1 phase (before S phase).
- checks for; cell size, nutrients, growth factors, dna damage.
- if these are satisfied it triggers dna replication. If not it enter G0 (resting state).
What is the second checkpoint and its role?
G2 checkpoint:
- the checkpoint at the end of G2 phase. Before the start of mitotic phase.
- checks for; cell size, dna replication (without error), dna damage.
- if these are satisfied, cell initiates the molecular processes that signal the start of mitosis.
What is the final checkpoint?
Spindle assembly/metaphase checkpoint:
- at the metaphase stage of mitosis.
- checked for chromosomes to be attached to spindle fibres.
- mitosis can’t proceed until this is passed.
What is mitosis and its importance?
Mitosis is the entire process of cell devision in eukaryotic cells.
- refers to the division of the nucleus.
- ensures that both daughter cells produced are genetically identical, have an exact copy of the dna present in the parent cell and the same number of chromosomes.
- necessary for growth, replacement and repair of tissues in multicellular organisms as daughter cells have to be identical.
- also necessary for asexual reproduction.
What is asexual reproduction?
Production of genetically identical off spring from one parent in multicellular organisms such as plants, fungi and some animals. Also in single celled organisms.
Bacteria don’t have a nucleus so reproduce asexually by binary fission.
What are chromatids and the centromere?
When dna in the nucleus is replicated during interphase, each chromosome is converted into 2 identical dna molecules called chromatids.
The two chromatids are joined together at a region called the centromere. It is necessary to keep the chromatids together during mitosis so they can be precisely manoeuvred and segregated equally, one each into 2 new daughter cells.
What are the 4 stages of mitosis and how can they be viewed?
Prophase, metaphase, anaphase, telophase.
They flow seamlessly from one to another.
- can be viewed and identified using a light microscope.
- dividing cells easily obtained from growing root tips of plants. They can be treated with a chemical to allow cells to be separated and then squashed.
- stains that bind to dna used to make chromosomes clearly visible.
Explain prophase.
- Chromatin fibres begin to coil and condense to form chromosomes. Nucleolus disappears. Nuclear membrane starts to break down.
- Protein microtubules form spindle- shaped structures linking the poles of the cell. Fibres forming spindle are necessary to move chromosomes into correct positions before division.
- Two centrioles migrate to opposite poles of the cell. (In animals/some plants).
- Spindle fibres attach to specific areas on the centromeres and move chromosomes to the centre.
- By the end, nuclear envelope has disappeared.
Explain metaphase?
- chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position.
explain anaphase.
- centromeres holding together the pairs of chromatids in each chromosome divide during anaphase.
- chromatids are separated by being pulled to opposite poles by the shortening spindle fibres.
- ‘V’ shape of chromatids is because they are dragged by their centromeres through the liquid cytosol.
Explain telophase.
- the chromatids have reached the poles and are now called chromosomes.
- the 2 new sets of chromosomes assemble at each pole.
- nuclear envelope reforms around them.
- chromosomes start to uncoil, nucleolus formed.
- cytokinesis begins.
What is cytokinesis?
The actual division of the cell into two separate cells.
Begins during telophase.
Explain cytokinesis in animal cells.
- a cleavage furrow forms around the middle of the cell.
- cell surface membrane pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two new cells.
Explain cytokinesis in plant cells.
have cell walls so formation of cleavage furrow not possible.
- vesicles from the Golgi apparatus assemble in the same place as the metaphase plate.
- vesicles fuse with each other and the cell surface membrane, dividing the cell in 2.
- new sections of cell wall form along the new membrane sections.
- (if cell wall is formed before the daughter cells separated, they would immediately undergo osmotic lysis from the surrounding water.)
What is meant by the terms diploid, gametes, zygote?
Diploid - the normal chromosome number: Normal cells have 2 chromosomes of each type. One from each parent.
In sexual reproduction, two sex cells (gametes) fuse to produce a fertilised egg (zygote).
Gametes must contain half the standard (diploid) number of chromosomes.
Explain through which process gametes are formed.
Gametes are formed by another form of cell division called meiosis.
- the nucleus divides twice to form 4 daughter cells= the gametes.
- each gamete contains half of the chromosome number of the parent cell. It is haploid.
- meiosis is therefore known as reduction division.
