3.2.2 All cells arise from other cells Flashcards
Definition of the cell cycle
A regulated sequence of events that occurs between one cell division and the next
Is interphase part of mitosis?
No
Role of cyclins
Trigger the movement of one phase to the next through chemical signals
A cell spends most of its time in which phase?
Interphase
3 stages of interphase
- G1
- S
- G2
G1 phase
Makes RNA, enzymes and proteins required for growth
S phase
DNA in nucleus replicates (through semi conservative replication), each chromosome consists of 2 identical sister chromatids
G2 phase
Cells continued to grow, newly synthesised DNA is checked and any errors are repaired
Role of centrioles
Produce spindle fibres
During G2, what is the function of centrioles?
Duplicate and move to opposite poles of the cell
How do cancer drugs generally treat cancer?
Killing dividing cells by blocking parts of the cell cycle, cell division ceases, hence growth of cancer
How do cancer drugs disrupt the cell cycle?
- Preventing DNA replication
- Inhibiting the metaphase stage affecting spindle formation
Explain what occurs during prophase (3)
Mark in pairs
1. Chromosomes thicken/condense/become visible
AND
2. Chromosomes consist of 2 identical chromatids (called sister centromere)
- Two centrioles move towards opposite poles of the cell
AND - Spindle fibres emerge from centrioles
- Nuclear envelope breaks down into vesicles
Explain what occurs during metaphase (2)
- Chromosomes line in the middle of the cell
- They attach to spindle fibres by their centromeres
Describe what occurs during anaphase (1)
Spindle fibres contract and pull sister chromatids to opposite poles of the cell
Explain what occurs during telophase (3)
- Chromosomes arrive at opposite poles and uncoil
- Spindle fibres break down
- Nuclear envelope reforms around chromosomes
When does cytokinesis occur?
After telophase - final step of cell cycle
Describe what occurs during cytokinesis (2)
- Cytoplasm divides
- Forms 2 genetically identical daughter cells
Mitotic index definition?
The proportion of cell tissue undergoing mitosis
Equation for mitotic index
Number of cells with visible chromosomes/Total number of cells
How do you calculate the length/duration of mitosis
Number of cells in specific stage of mitosis/Total number of cells x Time taken for whole cycle in minutes
When preparing onion root on a slide, why is only the first 5mm of the tip of the root used?
This is were mitosis occurs
When preparing an onion root tip slide, why would a student press down firmly on the cover slip?
To ensure the specimen is a thin layer of cells
Cancer
Uncontrolled mitosis, leading to a tumour
Tumour
Abnormal mass of cells
When does uncontrollable mitosis begin?
When changes occur in genes controlling cell division, the change in base sequence is a mutation
Which gene is cancerous?
Oncogene
Most mutations result in…
- Cell death
- OR cell being destroyed by immune system
Why don’t dead cells have an impact on the body?
They’re easily replaced
Which mutations don’t result in early cell death or being destroyed by the lymphatic system?
Generations of cancerous cells
Carcinogens
Agents that may cause cancer
Examples of carcinogens
- UV light
- Tobacco tar
- Asbestos
- X-rays
Benign tumours
Bound by membrane so don’t spread
Malignant tumours
Gain a blood supply, break off, invade healthy tissue, form secondary tumours
What is the main role of drugs involved in cancer treatment?
Killing dividing cells by blocking parts of the cell cycle ceasing growth of a tumour
How do cancer drugs disrupt the cell cycle?
- Preventing DNA replication
- Inhibiting the metaphase stage of mitosis affecting spindle formation
The main issue with cancer treatments
They disrupt the cell cycle for normal cells, e.g., hair cell leading to hair loss
How do bacterial cells divide?
Binary fission
Bacterial cells do not contain nuclei, instead, what do they contain?
Circular DNA and plasmids
True or False: Bacterial cells contain spindle fibres
False - they do not contain spindle fibres
Because of no spindle fibres in binary fission, what doesn’t break down?
The nuclear envelope
Explain how bacterial cells divide to form new cells (4)
- Binary fission
- Single, circular DNA (and plasmids) undergo DNA replication (through semi-conservative replication)
- Cell membrane grows (between 2 DNA molecules and) pinches inwards, dividing the cytoplasm
- 2 daughter cells (each) containing a single copy of the circular DNA and a variable number of plasmids
If in a cell, there is only one spindle fibre, instead of two, how would this affect division?
- There would be no separation of chromatids
- Chromosomes all go to one pole of the cell
- Twice the amount of chromosomes in one daughter cells
Suggest and explain two environmental variables that could be changed to increase the growth rate of cells in mitosis (4)
Principle of marking pairs: Named environmental variable; Correct effect on growth rate;
Examples
- Increased (concentration of) glucose;
- Increased respiration;
- Increased (concentration of) oxygen;
- Increased respiration;
- Increased temperature;
- Increased enzyme activity;
- Increased (concentration of) phosphate;
- Increased ATP/DNA/RNA;
- Increased (concentration of) nucleotides;
- Increased DNA synthesis;
Describe and explain the arrangement of genetic material in prophase (2)
- Chromosomes (are) becoming visible/distinct;
- Because (still) condensing;
OR
*Accept ‘chromosomes are condensed’ for 2 marks.*
*Accept shorten or thicken for ‘condensed’*
- Chromosomes (arranged) at random/not lined up;
- Because no spindle (activity);
OR
Because not attached to spindle fibres;
Link marking points-
1 and 2
3 and 4
When preparing a root tip squash, explain why only the first 5mm of the tip is used and why it is importantly to press firmly on the cover slip (2)
- Because this is where most mitosis occurs
- To ensure a thin layer of cells for light to pass through
A scientist treated growing tips of onion roots with a chemical that stops root tips growing.
During anaphase, he found that only some of the sister chromatids were pulled to opposite poles of the cell.
Use this information to suggest how the chemical stops the growth of roots (3)
- Stopping / disrupting / spindle fibres forming / attaching / pulling;
*Ignore affects anaphase*
- Preventing separation of (sister) chromatids;
*Ignore chromosomes separate / split* *Accept chromatids split* - (So) no new cells added (to root tip);
A scientist viewed chromosomes under a microscope. These cells were undergoing mitosis.
Explain why 2 identical sister chromatids could be seen (1)
Because DNA has replicated
When preparing the cells for observation the scientist placed them in a solution that had a slightly higher (less negative) water potential than the cytoplasm. This did not cause the cells to burst but moved the chromosomes further apart in order to reduce the overlapping of the chromosomes when observed with an optical microscope.
Suggest how this procedure moved the chromosomes apart (2)
- Water moves into the cells/cytoplasm by osmosis;
*Reject water moving into chromosomes/nucleus.*
- Cell/cytoplasm gets bigger;
*Accept idea of cell/cytoplasm has greater volume/swells/expands* *Ignore references to pressure changes, turgidity and chromosomes being more dilute* *Ignore references to changing water/fluid contents of the cell* *Allow ECF for ‘nucleus expands’ but not for ‘chromosomes expand’*
When preparing the cells for observation the scientist placed them in a solution that had a slightly higher (less negative) water potential than the cytoplasm. This did not cause the cells to burst but moved the chromosomes further apart in order to reduce the overlapping of the chromosomes when observed with an optical microscope.
Suggest how this procedure moved the chromosomes apart (2)
- Water moves into the cells/cytoplasm by osmosis;
*Reject water moving into chromosomes/nucleus.*
- Cell/cytoplasm gets bigger;
*Accept idea of cell/cytoplasm has greater volume/swells/expands* *Ignore references to pressure changes, turgidity and chromosomes being more dilute* *Ignore references to changing water/fluid contents of the cell* *Allow ECF for ‘nucleus expands’ but not for ‘chromosomes expand’*
Suggest why a student, when preparing a temporary mount of an onion root tip, was instructed to ’push down hard on the cover slip, but do not push the cover slip sideways’ (2)
- Push hard – spread / squash tissue;
- Not push sideways – avoid rolling cells together / breaking chromosomes.
*Neutral – to see cells clearly*
A student viewed chromosomes under an optical microscope.
He concluded that the cell was in anaphase.
Give two reasons why the student was able to conclude this (2)
- Chromosomes / chromatids are (in two groups) at poles of spindle / at ends of spindle;
*Do not accept ‘ends of cell’*
- V-shape shows that (sister) chromatids have been pulled apart at their centromeres / that centromeres of (sister) chromatids have been pulled apart.
Using the information below, calculate the percentage of cells in which chromosomes are visible and would consist of a pair of chromatids joined together. Show your working. (2)
Interphase = 872
Prophase = 74
Metaphase = 18
Anaphase = 10
Telophase = 8
74 + 18 / 982 = 9.4%
A student used a tip of garlic root to observe cells with visible chromosomes, he calculated the percentage of cells with visible chromosomes.
Later, a different set of results was obtained when the count was repeated on another occasion with different garlic root.
Give two differences for the difference in results (2)
- Genetic differences / types of garlic;
- Time of day;
- Chance;
- Age of root tip;
- Water availability;
- Temperature;
- Nutrient availability;
*(environmental factors = 1 but cannot be awarded in addition to a named environmental factor)*
A student prepared to observe plant root undergoing mitosis.
Why did he put the plant roots in acid? (1)
To stop mitosis
Describe the structure of DNA and the structure of a chromosome (6)
- Polymer of nucleotides;
- (Nucleotide) consists of deoxyribose,
phosphate and an organic/nitrogenous base; - Phosphodiester bonds (between nucleotides);
- DNA double helix held by H bonds
OR
2 strands held by H bonds; - (Hydrogen bonds/pairing) between adenine,
thymine and cytosine, guanine; - DNA is associated with histones/proteins;
- (During mitosis/when visible) chromosome
consists of two chromatids joined at a
centromere; - Accept
‘polynucleotide’ - Accept ‘phosphoric
acid’ for phosphate - Ignore bases
identified with letters
(A, T, G, C) - Reject adenosine
and cysteine
Accept correctly
annotated diagram(s)
for equivalent marking
points
Mitosis is important in the life of an organism. Give two reasons why (2)
Growth / increase in cell number
Repair cells / tissue / organs / body
Genetically identical cells
Reject produces
Asexual reproduction / cloning
Allow example
A biologist wanted to examine plant tissue in mitosis.
She observed 200 cells and counted those in mitosis.
Explain why the biologist chose to examine 200 cells (1)
(Ensures) representative (sample);
Accept find some cells in mitosis/not in interphase and more reliable
Improves reliability is not worthy of credit
Describe viral replication (4)
- Attachment proteins bind to receptors;
- (Viral) nucleic acid enters cell;
Ignore references to DNA/RNA inserted into genome
OR
Reverse transcriptase makes DNA from RNA
- Cell produces (viral) proteins/capsids/enzymes;
- Virus assembled and released (from cell);
