2.2- All cells arise from other cells Flashcards
State what the cell cycle is and outline its stages
cycle of division with intermediate growth periods
1. interphase
2. mitosis or meiosis (nuclear division)
3. cytokinesis (cytoplasmic division)
Explain why cell cycle doesn’t occur in some cells
After differentiation, some types of cell in multicellular organisms (e.g. neurons) no longer have the ability to divide
What is the difference between cell cycle and mitosis?
Cell cycle includes growth period between division; mitosis is only 10% of the cycle & refers only to nuclear division
Outline what happens during interphase
G1: cell synthesises proteins for replication e.g. tubulin for spindle fibres & cell size doubles
S: DNA replicates= chromosomes consist of 2 sister chromatids joined at a centromere
G2: organelles divide
State purpose of mitosis
Produces 2 genetically identical daughter cells for:
- Growth
- Cell replacement/ tissue repair
- Asexual reproduction
Name stages of mitosis
- Prophase
- Metaphase
- Anaphase
- Telophase
Outline what happens during prophase
- Chromosomes condense, become visible. (X-shaped: 2 sister chromatids joined at centromere)
- Centrioles move to opposite poles of cell (animal cells) & mitotic spindle fibres form
- Nuclear envelope & nucleolus break down= chromosomes free in cytoplasm
Outline what happens during metaphase
Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres
Outline what happens during anaphase
Requires energy from ATP hydrolysis
1. Spindle fibres contract = centromeres divide
2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cell (looks like ‘V’ shapes facing each other)
3. Spindle fibres break down
Outline what happens during telophase
- Chromosomes decondense, becoming invisible again
- New nuclear envelopes form around each set of chromosomes= 2 new nuclei, each with 1 copy of each chromosome
Explain procedure for a root tip squash experiment
- Prepare a temporary mount of root tissue
- Focus an optical microscope on the slide. Count total number of cells in the field of view and number of cells in a stage of mitosis
- Calculate mitotic index (proportion of cells undergoing mitosis)
Explain how to prepare a temporary root tip mount
- Place root tip in hydrochloric acid to halt cell division & hydrolyse middle lamella
- Stain root tip with a dye that binds to chromosomes
- Macerate tissue in water using mounted needle
- Use mounted needle at 45 degrees to press down coverslip & obtain a single layer of cells. Avoid trapping air bubbles.
Name 2 dyes that bind to chromosomes
- toluidine blue (blue)
- acetic orcein (purple-red)
Why is only the root tip used when calculating a mitotic index?
- Meristematic cells at root tip are actively undergoing mitosis
- Cells further from root tip are elongating rather than dividing
What are tumour suppressor genes?
Genes that code for proteins to trigger apoptosis (programmed death of damaged cells)/ slow cell cycle (e.g. p53 acts between G1 & S in interphase so damaged DNA cannot replicate)
What are proto-oncogenes?
Genes that code for proteins to stimulate cell cycle to progress from one stage to the next
How can mutation to tumour suppressor genes & proto-oncognes cause cancer?
- Tumour suppressor: no production of a protein needed to slow the cell cycle
- Proto-oncogenes: form permanently-activated oncogenes
- Disruption to cell cycle -> uncontrolled cell division -> tumour
Suggest how cancer treatments control the rate of cell division
Disrupt the cell cycle:
- prevent DNA replication
- disrupt spindle formation= inhibit metaphase/ anaphase
NB: can also damage healthy cells
How do prokaryotic cells replicate?
Binary fission:
1. DNA loop replicates. Both copies stay attached to cell membrane. Plasmids replicate in cytoplasm
2. Cell elongates, separating the 2 DNA loops
3. Cell membrane contracts & septum forms
4. Cell splits into 2 identical progeny cells, each with 1 copy of the DNA loop but a variable number of plasmids
Why are viruses classed as non-living?
They are acellular: no cytoplasm, no metabolism & cannot self-replicate
Outline how viruses repliate
- Attachment proteins attach to receptors on host cell membrane
- Enveloped viruses fuse with cell membrane or move in via endocytosis & release DNA/RNA into cytoplasm OR viruses inject DNA/ RNA
- Host cell uses viral genetic information to synthesise new viral proteins/ nucleic acid
- Components of new viral particle assemble
How do new viral particles leave the host cell?
a) Bud off & use cell membrane to form envelope
b) Cause lysis of host cell
Why is it so difficult to develop effective treatments against viruses?
Replicate inside living cells= difficult to kill them without killing host cells
